The Project Gutenberg eBook of The story of the Universe, Volume I (of 4)
This ebook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this ebook or online
at www.gutenberg.org. If you are not located in the United States,
you will have to check the laws of the country where you are located
before using this eBook.
Title: The story of the Universe, Volume I (of 4)
told by great scientists and popular authors
Editor: Esther Singleton
Release date: October 12, 2024 [eBook #74571]
Language: English
Original publication: New York: P.F. Collier and Son
Credits: John Campbell, and the Online Distributed Proofreading Team at https://www.pgdp.net (This book was produced from images made available by the HathiTrust Digital Library.)
*** START OF THE PROJECT GUTENBERG EBOOK THE STORY OF THE UNIVERSE, VOLUME I (OF 4) ***
TRANSCRIBER’S NOTE
Italic text is denoted by _underscores_.
Footnote anchors are denoted by [number], and the footnotes have been
placed at the end of each chapter.
A superscript is denoted by ^x, for example Pi^2 or 3^h.
A subscript is denoted by _{x}, for example L_{2}.
Basic fractions are displayed as ½ ⅓ ¼ etc; other fractions are shown
in the form a/b, for example 1/200 or 95/729. A few fractions were of
the form a-b in the original book; these have been changed to a/b for
consistency.
Some minor changes to the text are noted at the end of the book.
[Illustration: The Zodiacal Light]
THE STORY OF
THE UNIVERSE
_Told by Great Scientists
and Popular Authors_
COLLECTED AND EDITED
_By_ ESTHER SINGLETON
Author of “Turrets, Towers and Temples,” “Wonders of Nature,”
“The World’s Great Events,” “Famous Paintings,” Translator
of Lavignac’s “Music Dramas of Richard Wagner”
_FULLY ILLUSTRATED_
VOLUME I
THE
STARRY
SKIES
P. F. COLLIER AND SON
NEW YORK
COPYRIGHT 1905
BY P. F. COLLIER & SON
PREFACE
In the following pages I have endeavored to present a comprehensive
and general view of the material side of the universe. Instead of
trying myself to tell the story of the universe, I have gone to the
works of acknowledged weight and authority in this line of research
and selected from them extracts of a popular character, especially
those that are entertaining as well as merely instructive. The
average reader is frequently repelled from the study of the sciences
by the dry treatment adopted by those who try to instruct him. He
cares little for laws, theories, or affinities; and he can not help
being bored by attempts to make him understand classifications with
their long lists of words manufactured from the names of modern
celebrities or non-entities and roots from dead languages. I have
therefore kept constantly in mind the person who seeks entertaining
knowledge, and not the scientific specialist. I have tried to avoid
all technicalities wherever possible.
Of late years, in fact ever since the foundation of the British
Association, there has been a constantly increasing interest in the
wonders of nature; and the specialist has responded to this popular
interest in his scientific labors by speaking in language that an
intelligent child can comprehend. People as a rule prefer to read of
the habits, instincts, intelligence, and movements of animals and
plants, rather than of their organs and structure. Thus the study of
Natural History has received a great impetus from the writings of
such men as Darwin and Lubbock; and Astronomy has been rendered more
attractive to the lay reader by Flammarion, Gore, Proctor, and Ball.
Every traveler who returns from remote or hitherto unknown Arctic or
Torrid Zones has something fresh to tell us of the phenomena and life
of our universe, which adds fresh stimulus to the popular interest in
the Natural Sciences.
The Story of the Universe naturally falls under the following four
heads:
First, the bodies moving in infinite space, including stars, dark and
lucid, planets, nebulæ, comets, and meteors.
Second, the Earth, considered as a separate world and the only one of
which we have precise detailed knowledge. In this chapter we learn
of the past of our globe from the evidence afforded by the rocks of
which its crust is composed. The varying conformations of its present
surface are described, as is its atmospheric envelope and attendant
phenomena. The ocean and its movements and depths are likewise fully
considered.
Third, the Earth’s Garment—its flora. In this chapter we are told
of the wonders and beauties of plant-life, its development and
distribution.
Fourth, the Earth’s Creatures. Here we have a general view of animal
life, from the mighty mammoth to the fairy fly: even the beings
visible only to the microscope are not forgotten. Special attention
is also paid to man, from his origin to the present day.
I have made the selections from authentic editions of the writings of
the scientists; and have taken no liberties with the text, with the
exception of occasional cutting.
In the Introduction I have given a short sketch of the development of
the Natural Sciences, from the dawn of written history to the present
day.
E. S.
NEW YORK, _March, 1905_.
INTRODUCTION
The knowledge of the Natural Sciences among the Greeks and Romans
was derived principally from the Egyptians and Babylonians. The
Phœnicians in their voyages, also, necessarily paid considerable
attention to Astronomy. Their Cynosura consisted of the tail of
the Little Bear, by which they steered. The great names in Greek
Astronomy are Aratus, Hipparchus, and Ptolemy.
From the fancies of Astrology, in which the early Arabs largely
indulged, and which, though discountenanced by Mahomet himself, have
never been wholly abandoned by their descendants, a not unnatural
transition, led to the study of Astronomy. Under the patronage of
the Abbaside Caliph Al-Mamun (813-833 A. D.) this science made rapid
progress.
Astronomy was zealously studied in the famous schools of Bagdad and
Cordova.
The _Almagest_, or System of Astronomy, by Ptolemy, was translated
into Arabic by Alhazi and Sergius as early as 812. In the Tenth
Century, Albaten observed the advance of the line of the apsides
in the earth’s orbit; Mohammed-ben-Jeber-al-Batani, the obliquity
of the ecliptic; Alpetragius wrote a theory of the planets; and
Abul-Hassan-Ali, on astronomical instruments. The obliquity of the
ecliptic, the diameter of the earth, and even the precession of
the equinoxes, were then calculated with commendable accuracy; and
shortly after, Abul-Mezar’s _Introduction to Astronomy_ and his
_Treatise on the Conjunction of the Planets_, with the _Elements_
of Al-Furjanee (though this last author was largely indebted to the
Egyptian labors of Ptolemy), proved that the caliph’s liberality
had been well bestowed. But Al-Batinee, a native of Syria (879-920
A. D.), surpassed all his predecessors in the nicety alike of his
observations and computations. Geber, at Seville, constructed (1196
A. D.) the first astronomical observatory on record; and Ebn-Korrah
in Egypt proved by his example that the Arabs could be even better
astronomers than the Greeks.
Ulug Bekh, grandson of the great Tamerlane, was a diligent observer.
He established an academy of astronomers at Samarcand, the capital of
his dominions, and constructed magnificent instruments. Ulug Bekh,
too, made a catalogue of the fixed stars—the only one that had been
compiled since that of Hipparchus, sixteen centuries previously.
Gradually, by their intercourse with civilized nations, the Arabian
conquerors were themselves subjected to the humanizing influence of
letters, and, after 749 A. D., or during the reign of the Abassides,
literature, arts, and sciences appeared, and were generously fostered
under the splendid sway, first of Almansor (754-775), and afterward
of the celebrated Harun-al-Raschid (786-808). Learned men were now
invited from many countries and remunerated for their labors with
princely munificence; the works of the best Greek, Syriac, and old
Persian writers were translated into Arabic, and spread abroad in
numerous copies. The Caliph Al-Mamun, who reigned from 813 to 833,
offered to the Greek emperor five tons of gold and a perpetual treaty
of peace on condition that the philosopher Leo should be allowed to
give instruction to the former. Under the same Caliph the famous
schools of Bagdad, Basra, Bokhara, and Kufa were founded, and large
libraries were collected in Alexandria, Cairo, and Bagdad. The school
of Cordova in Spain soon rivaled that of Bagdad, and in the Tenth
Century the Arabs were everywhere the preservers and distributers of
knowledge.
Pupils from France and other European countries repaired to Spain in
great numbers, to study mathematics and medicine under the Arabs.
There were fourteen academies, with many preparatory and upper
schools, in Spain, and five very considerable public libraries; that
of the Caliph Hakem containing, as is said, more than 600,000 volumes.
In Geography, History, Philosophy, Medicine, Physics, and Mathematics
the Arabians rendered important services to science; and the Arabic
words still employed in science—such as algebra, alcohol, azimuth,
zenith, nadir, with many names of stars, etc. (see _The Arabian
Heavens_, pages 106-120 of Vol. I)—remain as indications of their
influence on the early intellectual culture of Europe. But Geography
owes most to them during the Middle Ages. In Africa and Asia, the
boundaries of geographical science were extended, and the old Arab
treatises on geography and works of travels in several countries by
Abulfeda, Edrisi, Leo Africanus, Ibn Batuta, Ibn Foslan, Ibn Jobair,
Albiruni the astronomer, and others, are still interesting.
The structure of the earth received little attention from the
ancients; the extent of its surface known was limited, and the
changes upon it were neither so speedy nor violent as to excite
special attention. The only opinions deserving to be noticed are
those of Pythagoras and Strabo, both of whom observed the phenomena
which were then altering the surface of the earth, and proposed
theories for explaining the changes that had taken place in
geological time. The first held that, in addition to volcanic action,
the change in the level of sea and land was owing to the retiring of
the sea; while the other maintained that the land changed its level,
and not the sea, and that such changes happened more easily to the
land below the sea because of its humidity.
From the fall of the Roman empire, during the Dark Ages, the physical
sciences were neglected. In the Tenth Century, Avicenna, Omar, and
other Arabian writers commented on the works of the Romans, but added
little of their own.
Geological phenomena attracted attention in Italy in the Sixteenth
Century, the absorbing question then being as to the nature of
fossils; only a few maintained that they were the remains of animals.
Two centuries elapsed before the opinion was generally adopted.
Aristotle was the first who collected, in his work _On Meteors_,
the current prognostics of the weather. Some of these were derived
from the Egyptians, who had studied the science as a branch of
Astronomy, while a considerable number were the result of his own
observation. The next writer upon this subject was Theophrastus, one
of Aristotle’s pupils, who classified the opinions commonly received
regarding the weather under four heads, viz., the prognostics of
rain, of wind, of storm, and of fine weather. The subject was
discussed purely in its popular and practical bearings, and no
attempt was made to explain phenomena whose occurrence appeared so
irregular and capricious. Cicero, Virgil, and a few other writers
also wrote on the subject; but the treatise of Theophrastus contains
nearly all that was known down to comparatively recent times. Partial
explanations were attempted by Aristotle and Lucretius, but their
explanations were vague, and often absurd.
In this dormant condition meteorology remained for ages, and no
progress was made till proper instruments were invented for making
real observations with regard to the temperature, the pressure, the
humidity, and the electricity of the air.
Solomon spoke of “trees, from the cedar in Lebanon even to the hyssop
that springeth out of the wall.” There is reason also to believe
that Zoroaster devoted some attention to plants, and that this
study early engaged some of the philosophers of Greece. The oldest
botanical work which has come down to us is that of Theophrastus, the
pupil of Aristotle, who flourished in the fourth century B. C. His
descriptions of plants are very unsatisfactory, but his knowledge
of their organs and of vegetable physiology may well be deemed
wonderful. It was not, indeed, till after the revival of letters in
Western Europe, that it was ever again studied as it had been by him.
About four hundred years after Theophrastus, in the First Century
of the Christian era, Dioscorides of Anazarbus, in Asia Minor—a
herbalist, however, rather than a botanist—described more than 600
plants in a work which continued in great repute throughout the
Middle Ages.
About the same time, the elder Pliny devoted a share of his attention
to Botany, and his writings contain some account of more than 1,000
species, compiled from various sources and mingled with many errors.
Centuries elapsed without producing another name worthy to be
mentioned. It was among the Arabians that the science next began to
be cultivated, about the close of the Eighth Century. The greatest
name of this period is Avicenna. Among the Arabs, Botany, like
Chemistry, was chiefly studied as subsidiary to medicine; but as an
adjunct to the old herbal pharmacopœia, it received close attention.
The principal mercurial and arsenical preparations of the _materia
medica_, the sulphates of several metals, the properties of acids and
alkalies, the distillation of alcohol—in fine, whatever resources
chemistry availed itself of up to a very recent date—were, with
their practical application, known to Er-Razi and Geber. In fact,
the numerous terms borrowed from the Arabic language—for instance,
alcohol, alkali, alembic, and others—with the signs of drugs and the
like, still in use among modern apothecaries, remain to show how
deeply this science is indebted to Arab research.
Aristotle seems to have been the first to study Zoology. Some of
the groups he established still retain their place in the most
modern classifications. His two great sections of the Animal Kingdom
consisted of Enanima (red blood) and Anima (having a circulation of
colorless fluid). Ælian and Pliny wrote on the subject, but they
indulged largely in fables. There was little advance in the science
during the Dark and Middle Ages. The _Bestiaries_ were written for
the sake of moral teaching, and the animals had to behave with
that end in view. Albertus Magnus is the only famous name in this
department before the revival of learning.
The shining light of the Thirteenth Century was Roger Bacon. His
_Opus Majus_ is “at once the Encyclopædia and the Novum Organum
of the Thirteenth Century.” In this, besides other branches of
scientific research, he devotes a rapid examination to questions of
Climate, Hydrography, Geography, and Astrology. Scientific research,
however, was out of date, and from the educated world Roger Bacon
received small recognition. His writings earned only a prison from
his own Order, and he died, in his own words, “unheard, forgotten,
buried.”
The Revival of Learning, commonly known as the Period of the
Renaissance, naturally entailed renewed interest in the sciences as
well as the arts. Green gives a comprehensive view of it:
“The last royalist had only just laid down his arms when the little
company who were at a later time to be known as the Royal Society
gathered round Wilkins at Oxford. It is in this group of scientific
observers that we catch the secret of the coming generation. From
the vexed problems, political and religious, with which it had so
long wrestled in vain, England turned at last to the physical world
around it, to the observation of its phenomena, to the discovery of
the laws which govern them. The pursuit of physical science became
a passion; and its method of research, by observation, comparison,
and experiment, transformed the older methods of inquiry in matters
without its pale. In religion, in politics, in the study of man and
of nature, not faith but reason, not tradition but inquiry, were to
be the watchwords of the coming time. The dead-weight of the past
was suddenly rolled away, and the new England heard at last and
understood the call of Francis Bacon.
“Bacon had already called men with a trumpet-voice to such studies;
but in England at least Bacon stood before his age. The beginnings
of physical science were more slow and timid there than in any
country of Europe. Only two discoveries of any real value came
from English research before the Restoration; the first, Gilbert’s
discovery of terrestrial magnetism in the close of Elizabeth’s
reign; the next, the great discovery of the circulation of the
blood, which was taught by Harvey in the reign of James. Apart from
these illustrious names England took little share in the scientific
movement of the continent; and her whole energies seemed to be
whirled into the vortex of theology and politics by the Civil
War. But the war had not reached its end when a little group of
students were to be seen in London, men ‘inquisitive,’ says one of
them, ‘into natural philosophy and other parts of human learning,
and particularly of what hath been called the New Philosophy,...
which from the times of Galileo at Florence, and Sir Francis Bacon
(Lord Verulam) in England, hath been much cultivated in Italy,
France, Germany, and other parts abroad, as well as with us in
England.’ The strife of the time indeed aided in directing the
minds of men to natural inquiries. ‘To have been always tossing
about some theological question,’ says the first historian of the
Royal Society, Bishop Sprat, ‘would have been to have made that
their private diversion, the excess of which they disliked in the
public. To have been eternally musing on civil business and the
distresses of the country was too melancholy a reflection. It
was nature alone which could pleasantly entertain them in that
estate.’ Foremost in the group stood Doctors Wallis and Wilkins,
whose removal to Oxford, which had just been reorganized by the
Puritan Visitors, divided the little company into two societies.
The Oxford society, which was the more important of the two,
held its meetings at the lodgings of Dr. Wilkins, who had become
Warden of Wadham College, and added to the names of its members
that of the eminent mathematician Dr. Ward, and that of the first
of English economists, Sir William Petty. ‘Our business,’ Wallis
tells us, ‘was (precluding matters of theology and state affairs)
to discourse and consider of philosophical inquiries and such
as related thereunto, as Physick, Anatomy, Geometry, Astronomy,
Navigation, Statics, Magnetics, Chymicks, Mechanicks, and Natural
Experiments: with the state of these studies, as then cultivated
at home and abroad. We then discoursed of the circulation of the
blood, the valves in the _venæ lacteæ_, the lymphatic vessels,
the Copernican hypothesis, the nature of comets and new stars,
the satellites of Jupiter, the oval shape of Saturn, the spots
in the sun and its turning on its own axis, the inequalities and
selenography of the moon, the several phases of Venus and Mercury,
the improvement of telescopes, the grinding of glasses for that
purpose, the weight of air, the possibility or impossibility of
vacuities, and Nature’s abhorrence thereof, the Torricellian
experiment in quicksilver, the descent of heavy bodies and the
degree of acceleration therein, and divers other things of like
nature.’
“The other little company of inquirers, who remained in London, was
at last broken up by the troubles of the Second Protectorate; but
it was revived at the Restoration by the return to London of the
more eminent members of the Oxford group. Science suddenly became
the fashion of the day. Charles was himself a fair chymist, and
took a keen interest in the problems of navigation. The Duke of
Buckingham varied his freaks of riming, drinking, and fiddling by
fits of devotion to his laboratory. Poets like Dryden and Cowley,
courtiers like Sir Robert Murray and Sir Kenelm Digby joined the
scientific company to which in token of his sympathy with it the
King gave the title of ‘The Royal Society.’ The curious glass toys
called Prince Rupert’s drops recall the scientific inquiries which,
with the study of etching, amused the old age of the great cavalry
leader of the Civil War. Wits and fops crowded to the meetings
of the new society. Statesmen like Lord Somers felt honored at
being chosen its presidents. Its definite establishment marks the
opening of a great age of scientific discovery in England. Almost
every year of the half century which followed saw some step made
to a wider and truer knowledge. Our first national observatory
rose at Greenwich, and modern astronomy began with the long
series of astronomical observations which immortalized the name
of Flamsteed. His successor, Halley, undertook the investigation
of the tides, of comets, and of terrestrial magnetism. Hooke
improved the microscope, and gave a fresh impulse to microscopical
research. Boyle made the air-pump a means of advancing the science
of pneumatics, and became the founder of experimental chymistry.
Wilkins pointed forward to the science of philology in his scheme
of a universal language. Sydenham introduced a careful observation
of nature and facts which changed the whole face of medicine. The
physiological researches of Willis first threw light upon the
structure of the brain. Woodward was the founder of mineralogy. In
his edition of Willoughby’s _Ornithology_, and in his own _History
of Fishes_, John Ray was the first to raise zoology to the rank
of a science; and the first scientific classification of animals
was attempted in his _Synopsis of Quadrupeds_. Modern botany began
with his _History of Plants_, and the researches of an Oxford
professor, Robert Morison; while Grew divided with Malpighi the
credit of founding the study of vegetable physiology. But great as
some of these names undoubtedly are, they are lost in the lustre
of Isaac Newton. Newton was born at Woolsthorpe in Lincolnshire,
on Christmas Day, in the memorable year which saw the outbreak of
the Civil War. In the year of the Restoration he entered Cambridge,
where the teaching of Isaac Barrow quickened his genius for
mathematics, and where the method of Descartes had superseded the
older modes of study. From the close of his Cambridge career his
life became a series of great physical discoveries. At twenty-three
he facilitated the calculation of planetary movements by his theory
of Fluxions. The optical discoveries to which he was led by his
experiments with the prism, and which he partly disclosed in the
lectures which he delivered as mathematical professor at Cambridge,
were embodied in the theory of light which he laid before the Royal
Society on becoming a Fellow of it. His discovery of the law of
gravitation had been made as early as 1666; but the erroneous
estimate which was then generally received of the earth’s diameter
prevented him from disclosing it for sixteen years; and it was not
till the eve of the Revolution that the _Principia_ revealed to the
world his new theory of the Universe.”
Ever since the Fifteenth Century, when Copernicus revived the ancient
theory of Pythagoras that the planets revolved around the sun (a
theory left in an imperfect state and demonstrated later by Kepler,
Galileo, Newton, and others) astronomical research has progressed
steadily. It must be remembered, however, that _De Revolutionibus
Orbium_, which met with great opposition, contained nothing regarding
the laws of motion, for these had not been as yet discovered, and
Saturn marked the boundaries of the Solar System. Copernicus assigned
the “fixed stars” to a sphere, as in Ptolemy’s heavens (see page 331).
The great Danish astronomer, Tycho Brahe, whose idea of the Solar
System is represented on page 343, was his opponent. Brahe, however,
a devoted student, a man of wealth, the favorite of kings and
princes, and the proud possessor of the Castle of Uraniberg (City of
the Heavens), an observatory equipped with fine instruments and built
for him by Frederick II, King of Denmark, on the island of Hueen, and
after his death the protégé of Rudolph II at Benatek, near Prague,
contributed greatly to the advancement of the science by means of
his discoveries, computations, solar and lunar tables, and catalogue
of stars. He, like Copernicus, placed the “fixed stars” in an outer
sphere. His observations on the planets were made to prove the truth
of his system. This mass of observations was used instead by Johann
Kepler, who had been his assistant at the Benatek Observatory, to
prove Copernicus’s theory. Of Kepler, the discoverer of the three
famous laws, who gave a complete theory of solar eclipses, calculated
the transits of Mercury and Venus, and made numerous discoveries in
optics and general physics, Proctor says:
“Kepler was not merely an observer and calculator; he inquired
with great diligence into the physical causes of every phenomenon,
and made a near approach to the discovery of that great principle
which maintains and regulates the planetary motions. He possessed
some very sound and accurate notions of the nature of gravity,
but unfortunately conceived it to diminish simply in proportion
to the distance, although he had demonstrated that the intensity
of light is reciprocally proportional to the surface over which
it is spread, or inversely as the square of the distance from the
luminous body.”
Great names follow in rapid succession. One of Kepler’s
contemporaries was Galileo Galilei, the discoverer of the “three laws
of motion” and the relation of time and space in falling bodies, the
first to apply the newly invented telescope to the observation of the
heavens and the discoverer of four satellites of Jupiter (named by
him the “Medeiran Stars” in honor of his patron). He also detected
spots on the sun’s disk, the phases of Venus, and irregularities on
the moon’s surface, and declared the Milky Way to be composed of a
countless tract of separate stars.
When we remember the limited power of the telescope of the age, we
can but marvel, not at how little, but how much was known regarding
the starry skies.
During this period, numerous observers rendered great service to
Astronomy, and other scientists were engaged in making useful
drawings, charts, maps, tables, and catalogues of stars.
To this period also belongs John Bayer of Augsburg, who published a
description of the constellations with maps upon which the stars were
marked with the letters of the Greek Alphabet—a convenient method
that was universally adopted and is still in use. Other names include
Gassendi, Riccioli, Grimaldi, and Hevelius—the latter a rich citizen
of Dantzig, who had a fine observatory of his own, where he worked
for forty years. His drawings and descriptions of the moon, his
researches on comets, which he still believed moved in parabolas, and
his celestial charts engaged most of his attention.
The Dutch astronomer Huygens (born in 1629) is famous for his
improvements in the telescope use of the pendulum clock and
developments in the machinery of astronomical instruments. He
discovered the ring of Saturn and four of his satellites. Edmund
Halley, an English astronomer (born in 1656), also took a great
interest in the telescope, and went to Dantzig to settle a
controversy between Robert Hooke and Hevelius regarding the best
glasses for use in astronomical observations; for Hevelius still
worked with the ancient instruments, while Hooke believed in the lens.
Halley revived the ancient idea that comets belonged to the Solar
System, and predicted that the comet of 1681 would return to its
perihelion in 1759. This was the first prediction of its kind
verified.
During the last quarter of the Seventeenth Century, the telescope
assumes importance and two great observatories begin their work.
In 1670 the Paris Observatory, of which Cassini was made director,
was finished, and five years later the Greenwich Observatory, where
Flamsteed was installed as royal astronomer.
Of Cassini, Lalande remarks that under him Astronomy underwent
revolutions, and in France he was regarded as the “creator of the
science.” Cassini discovered that Saturn’s ring was double and found
four satellites of Jupiter.
Flamsteed’s observations on planets, satellites, comets, “fixed
stars,” and his catalogue of 2,884 stars were valuable contributions
to science; and his _Historia Cœlestis_ is said to have “formed a new
era in sidereal astronomy.”
Flamsteed was succeeded by Halley, particularly famed for his
investigations of comets. The next great astronomical event was the
discovery of Uranus by Sir William Herschel in 1781. Sir William
Herschel also discovered two more of Saturn’s satellites, and began
the great work of resolving the Milky Way and other clusters into
swarms of suns, single stars into double and triple stars, inquiries
into the mysteries of the nebulæ, and in every way enlarging the
general conception of the sidereal universe.
To the end of the Eighteenth and beginning of the Nineteenth
Centuries belongs the brilliant French astronomer and mathematician
Laplace, who published in 1799-1808 his _Mécanique Céleste_, in which
he announced his Nebular Hypothesis (described on page 433 of Vol.
II. The discoveries of the Planetoids are described on pages 396-403,
and that of Neptune in 1846 on pages 430-432). The latest important
additions to the Solar System are the discovery by Prof. Barnard of
Jupiter’s Fifth Satellite in 1892 and Saturn’s Ninth by Prof. W.
H. Pickering in 1904. The discovery even of a Seventh Satellite of
Jupiter has just been announced from the Lick Observatory.
It would be impossible to mention the names of the astronomers
whose work from the middle of the last century to its closing years
has been distinguished in various fields. Space only permits brief
mention of the new methods of research by means of the spectroscope
and celestial photography. With the first the name of the English
astronomer, William Huggins, is identified and has yielded most
important and startling information regarding the composition of
heavenly bodies, and with the application of the photographic
telescope these new methods have created a revolution in astronomical
observation.
It may be interesting to gain a slight idea of the numbers of stars
revealed by the camera by referring to Sir Robert Ball:
“If we take a position on the equator, from whence, of course, all
the heavens can be completely seen in the lapse of six months,
the number of stars that can be reckoned with the unaided eye
will, according to Houzeau, amount to about six thousand. If we
augment our unaided vision by a telescope of even small dimensions,
such as three inches in diameter, the number of stars in the
Northern Hemisphere alone is upward of three hundred thousand. We
may assume that the Southern Hemisphere has an equally numerous
star-population, so that the entire multitude visible with this
optical aid is about six hundred thousand. Thus we see that the
use of a telescope small enough to be carried in the hands suffices
to multiply the lucid stars one-hundredfold. Great telescopes no
doubt soon show us that the hundreds of thousands are only the
brighter members of a host of millions, and now we receive the
assurance of photography that the telescopic stars are only the
more conspicuous members of that vast universe. Mr. Roberts indeed
declares that the multitudes of stars on the photographic plate
grow with each increase of exposure to such a degree that it would
almost seem as if the plate would be a wellnigh continuous mass of
stars if the operations could be sufficiently protracted.”
Naturally the past years have witnessed the making of new catalogues
and maps of stars, and many valuable computations of parallaxes,
etc. Some of the results obtained by these new methods are described
in the chapters on the Nebulæ and Swarms of Suns, The Great Nebula
of Orion, and The Colored, Double, Multiple, Binary, Variable, and
Temporary Stars in Vol. I. From this brief survey of the progress
of Astronomy the fact will be appreciated, therefore, that all the
discoveries and researches have resulted in a larger conception of
the universe, and the Solar System sinks into insignificance in the
vast ocean of stars and suns.
The study of the Earth’s crust and its contents divested of
superstition dates from the end of the Seventeenth Century. Nicolaus
Steno (1638-1687), a Dane, devoted himself to geology, and in 1669
observed successive layers of strata. He is called “the father of
Palæontology.” In 1680 Leibnitz proposed the theory that the Earth
was originally in a molten state. The classification of strata was
begun about the middle of the Eighteenth Century. The views of James
Hutton (1788), who returned to the theories advanced by Ray (a return
to the views of Pythagoras), were continued by Sir Charles Lyell.
Geology and Palæontology have progressed side by side. Among the
most famous investigators are Cuvier, Dawson, Marsh, Owen, Huxley,
Agassiz, De Blainville, Kaup, Sir Roderick Murchison, Boyd Dawkins,
Sir William Flower, R. Lydekker, and E. D. Cope.
To the review of the new developments of meteorology and the science
of probabilities by Sir Ralph Abercromby, on pages 784-792 of Vol.
II, it is only necessary to add that the interest in meteorological
research developed greatly after Torricelli’s discovery in 1643 of
weight and pressure in the atmosphere led to the perfection of the
barometer and the development of the thermometer and hygrometer, both
in the Seventeenth Century. The theory of trade-winds George Hadley
announced in the _Philosophical Transactions_ for 1735. Dalton’s
_Meteorological Essays_, published in 1793, and Dr. William Charles
Wells’s _Theory of Dew_, published in 1814, attracted much attention.
Regarding the inquiries into the laws of light by Snell, Newton,
Descartes, Thomas Young, and Sir George Airy, the reader is referred
to the chapter on The Rainbow in Vol. II, by John Tyndall, with whose
researches in the latter half of the Nineteenth Century every one is
more or less acquainted.
Little need be said here regarding the history of Botany, which is
reviewed on pages 984-1000 of Vol. II. We may add, however, that one
of the first to revive this study was Otto Brunsfels, whose _Historia
Plantarum Argentorati_ was published in two folio volumes with cuts
in Strasburg in 1530. He had many followers on the Continent and
in England. During the revival of learning, chairs of Botany were
founded in the universities; botanic gardens were established in many
places (the Jardin des Plantes was founded in 1626); and botanists
began to travel to remote countries to search for unknown flora.
To the Seventeenth Century belong the names of Dr. Turner, “the
father of English Botany”; Robert Morison, professor of Botany at
Oxford; John Ray, Nehemiah Grew, Malpighi, Henshaw, and Robert Hooke.
The two latter were among the first to employ the newly invented
microscope to the study of this science. It may be mentioned in
passing, that Huygens is said to have taken from Holland to England
microscopes about the size of a grain of sand, and that the first
microscope consisting of a combination of lenses is attributed
to Jansen, a spectacle-maker of Holland. Hooke, whom Herschel
calls “the great contemporary and almost the rival of Newton,”
gave a tremendous impetus to Microscopy, and practically laid the
foundation of Histology or the Inner Morphology of Plants, due to
Grew and Malpighi. Schleiden undertook to explain the mysteries of
cell formation in 1838, further investigated by Schwann, and is now
known as the Schleiden-Schwann theory. Nägeli and Von Mohl continued
researches on this line. To the contents of the cell Von Mohl gave
the name _protoplasm_.
In 1849, Hofmeister began investigations into the life-histories of
plants, since when the study of Vegetable Physiology has progressed
side by side with Chemistry. To Darwin great subjects are due:
the cross-fertilization of plants, their reproduction, and their
relations to insects and their movements. It may be mentioned,
however, that in 1693 Ray attempted to explain the movements of
leaves, tendrils, and petals by physical and mechanical laws.
Since the middle of the Nineteenth Century, the branches of Botany
that have been particularly studied are Vegetable Physiology and
Pathology, Inner Morphology, and Fossil Botany—and the discoveries
made have naturally had an effect upon the classification of
vegetable life.
According to Agassiz:
“We must come down to the last century, to Linnæus, before we find
the history taken up where Aristotle had left it, and some of his
suggestions carried out with new freshness and vigor. Aristotle
had already distinguished between genera and species; Linnæus took
hold of this idea, and gave special names to other groups, of
different weight and value. Besides species and genera, he gives
us orders and classes—considering classes the most comprehensive,
then orders, then genera, then species. He did not, however,
represent these groups as distinguished by their nature, but only
by their range; they were still to him, as genera and species had
been to Aristotle, only larger or smaller groups, not founded upon
and limited by different categories of structure. He divided the
animal kingdom into six classes: Mammalia, Birds, Reptiles, Fishes,
Insects, and Worms.”
Linnæus’s classification was, therefore, the first attempt to
group animals; but until Cuvier there was no great principle of
classification. In 1707 Buffon succeeded in making Zoology, which had
been regarded as a most uninteresting study, popular and respected.
He also had the idea of collecting all the known facts of scientific
investigation and arranging them systematically. Buffon was ridiculed
as a scientist by his contemporaries, Hevelius, Diderot, D’Alembert,
and Condillac, who opposed his explanations of natural phenomena.
Buffon’s _Histoire Naturelle Générale et Particulière_ is his most
important work. A complete edition in thirty-six volumes appeared in
Paris in 1749-1788. Although it is said to “have made an epoch in
the study of the natural sciences” in Buffon’s day, it now possesses
little scientific value.
Cuvier’s classification has never been overthrown. His original
investigations in various departments of science, and particularly
that of fossil vertebrate animals, opened up new fields of study. His
talents with both pen and pencil contributed largely to making that
branch of science popular.
Lamarck, Cuvier’s contemporary, divided the animal kingdom into
Vertebrates and Invertebrates. Lamarck, like Geoffroy Saint-Hilaire,
was a believer in the theory of evolution, which was opposed by
Cuvier.
Lamarck turned from the study of Meteorology to that of Botany,
and later again to that of Zoology. In 1793 he became professor of
the natural history of the lower classes of animals in the Jardin
des Plantes. His theories have greatly influenced modern science,
particularly that of the “Variation of Species,” which was set forth
in his _Philosophie Zoologique_ (two vols., Paris, 1809) and other
works. Lamarck’s _Histoire des Animaux sans Vertèbres_ (seven vols.,
Paris, 1815-22) is his greatest work.
Karl Ernst von Baer, the Russian naturalist, a pupil of Döllinger in
Würzburg, devoted himself chiefly to the study of embryology and made
valuable discoveries.
Passing by many illustrious names, we come to that of Sir Richard
Owen, of whom it has been said that “from the sponge to man, he has
thrown light over every subject he has touched.” His work in the
Hunter Museum, his descriptions and restorations of extinct birds and
animals, and his original works on every branch of animal life, form
an enormous contribution to the progress of science. He promulgated
the advanced views of John Hunter, the great physiologist and
surgeon, of whose famous museum of more than ten thousand specimens,
illustrative of anatomy and natural history, he became curator.
Three names shine with especial lustre upon the Nineteenth
Century—Darwin, Huxley, and Spencer. The theory of evolution first
appeared in De Maillet’s work, _Telliamed_, published in 1758, but
written in 1735. More than thirty writers before Darwin treated
this theory, among whom were Erasmus Darwin, Goethe, Lamarck, and
Geoffroy Saint-Hilaire. Largely owing to the opposition of Cuvier,
it never succeeded until it was revived by Charles Darwin, who,
after twenty-one years of work, published his results in 1858 in the
_Journal of the Linnæan Society_, followed in the next year by _The
Origin of Species by Means of Natural Selection_ (see pages 1482-1512
of Vol. IV).
“The lifeless earth,” says Sir Robert Ball, “is the canvas on
which has been drawn the noblest picture that modern science has
produced. It is Darwin who has drawn this picture. He has shown
that the evolution of the lifeless earth from the nebula is but
the prelude to an organic evolution of still greater interest and
complexity. He has taken up the history of the earth at the point
where the astronomer left it, and he has made discoveries which
have influenced thought and opinion more than any other discoveries
that have been made for centuries.”
The neglected department of Marine Zoology the Nineteenth Century has
made particularly its mission to investigate, but space only permits
mention of four names: Edward Forbes, Lord Kelvin (Sir Wyville
Thomson), Ernst Heinrich Haeckel, and the Prince of Monaco.
The first, whom Lord Kelvin considers “the most accomplished
and original naturalist of his time,” was a pupil of Geoffroy
Saint-Hilaire, Jussieu, and De Blainville. He is regarded as the
originator of the use of the dredge for collecting specimens and
the first who undertook the systematic study of Marine Zoology with
reference to the distribution of fauna. In 1859 his _Natural History
of the European Seas_ appeared after his death.
One of the most important investigators in this line is Prof.
Haeckel, famous for his studies of the lower class of marine animals.
He is also distinguished for his researches in other branches of
Zoology and Palæontology, and was one of the first followers of
Darwin in Germany.
Entomology has also made enormous progress during the Nineteenth
Century. At the end of the Seventeenth Century, Ray estimated the
number of insects throughout the world at 10,000 species! The great
entomologists of the Eighteenth Century include Linnæus, De Geer, and
Fabricius. Next follow Latreille, Kirby and Spence, and a host of
distinguished scientists in Europe and the United States, of whom Sir
John Lubbock (Lord Avebury) heads the list. A comparatively new line
of investigation is that of the Chalcididæ (see Fairy Flies, pages
1449-1458, in Vol. IV).
ESTHER SINGLETON.
ILLUSTRATIONS
The Zodiacal Light _Frontispiece_
Chart of the Northern Constellations _Opposite p._ 73
Belt and Sword of Orion ” 121
Nebula in the Constellation Cygnus ” 169
Sun’s Surface and Sun Spot ” 217
Portion of the Moon’s Disk ” 265
Nine Views of the Hour-Glass Sea on Mars ” 313
Twelve Views of Jupiter ” 361
Three Views of Saturn ” 409
CONTENTS
THE HEAVENS. Amédée Guillemin 25
SPACE. Richard A. Proctor 33
EXTENT OF THE SIDEREAL HEAVENS. Sir Robert S. Ball 42
THE STARS. Amédée Guillemin 53
THE LUCID STARS. J. E. Gore 60
THE CONSTELLATIONS. Camille Flammarion 70
THE ARABIAN HEAVENS. Ludwig Ideler 106
ASTRONOMY WITHOUT A TELESCOPE. J. E. Gore 120
THE MILKY WAY. Richard A. Proctor 133
THE MAGELLANIC CLOUDS—ZODIACAL LIGHT—STAR GROUPS.
Amédée Guillemin 147
THE NEBULÆ AND SWARMS OF SUNS. J. E. Gore 154
THE GREAT NEBULA OF ORION. Sir Robert S. Ball 176
COLORED, DOUBLE, MULTIPLE, BINARY, VARIABLE, AND
TEMPORARY STARS. J. E. Gore 187
A WORLD ON FIRE—NOVA PERSEI. Alexander W. Roberts 228
TELESCOPES. A. Fowler 238
METEORS. Sir Robert S. Ball 266
COMETS. Sir John Herschel 282
LIFE IN OTHER WORLDS. J. E. Gore 307
THE SUN—WHAT WE LEARN FROM IT. Richard A. Proctor 316
MERCURY. William F. Denning 353
THE PLANET VENUS. Camille Flammarion 358
THE EARTH AS A PLANET. Élisée Réclus 364
THE MOON. Thomas Gwyn Elger 376
MARS. Agnes M. Clerke 385
THE PLANETOIDS. Camille Flammarion 396
JUPITER. Agnes M. Clerke 403
SATURN. Agnes M. Clerke 415
URANUS AND NEPTUNE. William F. Denning 426
THE STORY OF THE UNIVERSE
THE HEAVENS.—AMÉDÉE GUILLEMIN
What are the heavens? Where the shores of that limitless ocean; where
the bottom of that unfathomable abyss?
What are those brilliant points—those innumerable stars, which, never
dim, shine out unceasingly from the dark profound? Are they sown
broadcast—orderless, with no other bond save that which perspective
lends to them? Or, if not immovable, as we have so long imagined, if
not golden nails fixed to a crystal vault, whither are they bound?
And, finally, what are the parts assigned to the sun, our earth,
and all the earths attendant on the glorious orb of day in this
tremendous concert of celestial spheres—this sublime harmony of the
universe?
These are magnificent problems of which the most fertile imagination
would have in vain attempted the solution, if, for the greater glory
of the human mind, astronomy—first born of the sciences—had not at
length come to our aid.
How wonderful is the power of man! Chained down to the surface of the
earth, an intelligent atom on a grain of sand lost in the immensity
of a space, he invents instruments which multiply a thousand-fold
his vision, he sounds the depths of the ether, gauges the visible
universe, and counts the myriads of stars which people it; next,
studying their most complicated movements, he measures exactly their
dimensions and the distances of the nearest of them from the earth,
and next deduces their masses; then, discovering in the seeming
disorder of the stellar groupings real bonds of union, he at last
evolves order from apparent confusion.
Nor is this all. Rising by a supreme flight of thought to the most
abstract speculations, he discovers the laws which regulate all
celestial movements, and defines the nature of the universal force
which sustains the worlds.
Such are the fruits of the unceasing labors of twenty generations
of astronomers. Such the result of the genius and of the patient
perseverance of men who have devoted themselves for two thousand
years to the study of the phenomena of the heavens. The Chaldean
shepherds were, they say, the first astronomers. We can well believe
it. Dwelling in the midst of vast plains, where the mildness of the
seasons permitted them to pass the night in the open air, where the
clear sky unfolded before them perpetually the most glorious scenes,
they ought to have been, and they were, contemplative astronomers.
And all of us would be what they were did not the rigor of our
climate and our variable atmosphere so often prevent us observing the
heavens; and did not, moreover, the turmoil and cares of civilized
life deprive us of the necessary leisure.
Nothing is more fitted to elevate the mind toward the infinite than
the pensive contemplation of the starry vault in the silent calm of
night. A thousand fires sparkle in all parts of the sombre azure of
the sky. Varied in color and brilliancy, some shine with a vivid
light, perpetually changing and twinkling; others, again, with a
more constant one—more tranquil and soft; while very many only send
us their rays intermittently, as if they could scarce pierce the
profundity of space.
To enjoy this spectacle in all its magnificence, a night must be
chosen when the atmosphere is perfectly pure and transparent—one
neither illuminated by the moon, nor by the glimmer of twilight or of
dawn. The heavens then resemble an immense sea, the broad expanse of
which glitters with gold dust or diamonds.
In presence of such splendor, the senses, mind and imagination are
alike inthralled. The impression gathered is an emotion at once
profound and religious, an indefinable mixture of admiration, and of
calm and tender melancholy. It seems as if these distant worlds, in
shining earthward, put themselves in close communication with our
thoughts.
At a first glance at the starry firmament the stars seem pretty
regularly distributed; nevertheless, look at that whitish, undecided,
vapory glimmer which girdles the heavens as with a belt. It is the
Milky Way.[1] As we approach the borders of this star-cloud in our
inspection, the stars appear more and more crowded together, and
most of them so small that the eye can scarcely distinguish them.
The accumulation of stars in the direction of the Milky Way is more
especially visible when we examine the heavens with the aid of a
powerful telescope.
The Milky Way itself is nothing more than an immensely extended zone
of stars, that is, of suns, since each star, from the most brilliant
to the faintest, is a sun.
Here, then, is an immense group, a gigantic assemblage of worlds,
which seems to embrace all the universe, if it be true that the
greater number of the scattered stars situated out of the Milky Way
nevertheless form part of it. In reality, this multitude of millions
of suns is divided into numerous and distinct groups, and those into
others still more restricted in number, each composed of two or three
suns.
What breadth of space does each of these groups occupy? What is
the measure of the space which holds them all? The most powerful
imagination in vain attempts to answer these questions intelligibly;
here numbers fail us.
Let us add—a fact well proved, and one which will seem strange to
many—
Our sun himself is a star of the Milky Way.
In examining attentively every part of the starry vault, a keen eye
perceives here and there whitish spots resembling little clouds. One
would say they were so many patches detached from the Milky Way, from
which, however, they are often very distinct and very distant. The
telescope discovers by thousands those cloud-patches, these—to give
them their astronomical name—_Nebulæ_.
It was formerly imagined that each of these star-clouds was nothing
more than an accumulation of stars, very close together, and very
numerous—so many Milky Ways lying outside our own, and for the most
part so distant that the most powerful instruments were able only
to distinguish a confused glimmering. One of the most important
observations of modern times, however, has shown that many of
these nebulæ, including the most glorious one in our northern
hemisphere—that in the sword-handle of Orion—are but masses of
glowing gases.
Others, again, of these cloud-like masses—cloud-like by reason of
their distance—show us, faintly shining on a background of apparent
nebulæ, brilliant stars, larger no doubt, or more brilliant, than
their fellows, and some of these objects called “Star-Clusters,”
which are nearest to us, are among the most glorious objects revealed
to us by our telescopes.
Let us attempt now to conceive what fearful distances separate these
archipelagoes of worlds from our own!
Unfathomable abysses whose unspeakable depths the most powerful
telescopes increase indefinitely! Profound, endless, bottomless, but
lighted up by millions of suns!
Such appears to us the universe from the natural observatory where we
are placed. But to obtain a more complete idea of its constitution,
of the infinite variety of its members, we must descend from those
regions, where the sight and mind are lost, to a group, nearer to
us, and therefore more accessible to the investigations of man—to
that group, or system, of which the earth forms part.
Of this the sun is the centre.
Round this focus of light and heat, but at various distances, revolve
more than a hundred secondary bodies—Planets, some of which are
accompanied by smaller ones—Satellites. Not self-luminous, they
would be invisible to us, if the light, which they receive from the
sun, were not reflected toward the earth, making them also appear
as luminous points spread over the celestial vault like so many
stars. Such would be the appearance of the earth seen in space, at a
distance sufficiently great.
A common character distinguishes all the celestial bodies that form
part of this group—the Solar System—from the multitude of other
stars. For while the suns, composing what is called the Sidereal
Universe, are situated at distances seemingly infinite, the bodies
composing the group of which we speak are relatively much nearer the
earth, are, in fact, our neighbors.
What results from this double fact? Two very simple consequences,
easily understood.
The first is, that the stars do not undergo any sensible change
of position in the starry vault. Their distance is such that they
appear actually at rest in the depths of space; hence the term _Fixed
Stars_—now abandoned, because a minute and elaborate study of their
relative positions has established the fact that the stars really do
move in the remote regions of the heavens. The apparent immobility
of which we have spoken, and which is one of their characteristics,
is evidenced by the uniformity of appearance preserved for
centuries by the artificial groups of stars, to which the name of
Constellations has been given.
Now, it is otherwise with the bodies that revolve round our sun: they
are near enough to the earth to allow of their displacements in space
being perceived in short intervals of time. Traveling, by virtue of
their proper motions along the starry vault, distances which appear
greater as their own distance from us is less, these bodies received
at the outset the name they have since retained—_Planets_, or
Wandering Stars.
It is thus that, when we stand in the middle of an extensive plain,
we judge distant objects—those that border the horizon—to be
immovable; while we instantly perceive the slightest change of place
in the near ones. It is true that when we ourselves move, the real
movements become complicated with the apparent movements, but the
former must be distinguished, if we wish to have an exact idea of the
actual course traveled. This complication of the apparent movements
of the planets—a necessary consequence of the movement of the
earth—is one of the most striking testimonials to the reality of the
latter; but it must also be added, that this was precisely the stone
of stumbling of ancient astronomy until the time—and that not long
ago—when the real movements were made known. Movements of rotation,
movements of revolution, around the common centre, the duration of
these movements, distances, forms and dimensions, distribution of
light and heat, all change in passing from one planet to another. And
yet, marvelous thing, the same laws govern, all in such a way that
the unity of plan is not less marked than the astonishing variety of
the phenomena.
One circumstance common to all the bodies of the solar system
forcibly strikes the imagination. It is, that these enormous
masses—these globes, many of which are much heavier than the earth,
and lastly, the earth itself—are not only suspended in space, but
move through the ether with velocities truly stupendous.
Imagine yourself a spectator, standing immovable in space. A luminous
body appears in the distance, little by little you see it approach
and increase in size; its immense circumference, which exceeds a
hundred thousand leagues, is in rapid rotation, which makes each
point on its periphery travel through nine miles a second. The globe
itself passes before you, carried through space with a velocity
twenty-four times greater than that of a cannon-ball. In such a way
Jupiter would appear to you traveling in its orbit. This headlong
course would banish it forever to the most remote regions of the
visible universe, if it were not subdued and held by the powerful
attraction of a globe a thousand times larger than its own—by the
sun himself. Not only does astronomy show, by undeniable proofs, the
reality of these marvelous movements—not only has she arrived at the
knowledge of their invariable constancy, at least during thousands of
centuries; but she has found in their very rapidity the cause of the
stability of all the celestial bodies.
If there is difficulty in imagining such masses freely circulating in
the ether, how much more are we impressed when we consider that these
rapid movements are not confined to the planets; and when we look
upon the sun with all his retinue as moving in an orbit yet unknown,
himself attracted no doubt by a more powerful sun, or by a group
of suns! All the stars which by reason of their infinite distances
appear immovable, move in different directions; and we shall see
later, that if these movements are performed with extreme slowness,
the slowness is apparent only. In reality, these are the most rapid
celestial movements that we know of.
Thousands of centuries will be necessary before these immense
sidereal voyages are accomplished. Their vast periods are to the
length of our year what the dimensions of the earth are to the
distances of the stars; and, according to the happy expression of
Humboldt, they make of the universe an eternal timekeeper. Thus,
in the contemplation of celestial phenomena, the idea of infinite
duration impresses itself on the mind with the same irresistible
power as the idea of the infinity of space.
FOOTNOTES:
[1] Via Lactea. It is also called the Galaxy, from the Greek word for
the same thing.
SPACE.—RICHARD A. PROCTOR
Although astronomy tells us in the clearest words of the vast depths
of space which surround our earth on all sides, we are not thereby
enabled to realize their enormous extension. It is not merely that
the unknown depths beyond the range of our most powerful telescopes
are inconceivable, but that the parts of space which we can examine
are on too large a scale for us to conceive their real dimensions. It
is hardly going too far to say that our powers of actual conception
are limited to the extent of space over which the eye _seems_ to
range in the daytime. Of course, in the daytime, at least in clear
weather, there is one direction in which the eyesight ranges over a
distance of many millions of miles—namely, where we see the sun. But
the sense of sight is not cognizant of that enormous distance, and
simply presents the sun to us as a bright disk in the sky, or perhaps
rather nearer to us than the sky. Even the distance of the sky itself
is underestimated. A portion of the light we receive from the sky
on a clear day comes from parts of the atmosphere distant more than
thirty or forty miles from us; but the eye does not recognize the
fact. The blue sky seems a little further off than the clouds, but
not much; the light clouds of summer seem a little, but not much,
further off than the heavier clouds of a winter sky; a cloud-covered
winter sky seems a little further off than heavy rain-clouds. The
actual varieties of distance among clouds of various kinds are not
much more clearly discerned than the actual varieties of distance
among the heavenly bodies. The estimate formed of the distance of a
cloud-covered sky overhead probably amounts to little more than a
mile, and it is very doubtful whether the mind presents the remotest
depths of a blue sky overhead at more than two miles. Toward the
horizon the distance seems greater, and probably on a cloudy day
the sky near the horizon is unconsciously regarded as at a distance
of about five miles, while blue sky near the horizon may be regarded
as lying at a distance of six or seven miles, the arch of a blue sky
seeming to be far more deeply curved than that of a cloud-covered sky.
It is to distances such as these that the mind unconsciously refers
the celestial bodies. We know that the moon is about 2,000 miles in
diameter, but the mind refuses to present her to us as other than
a round disk much smaller than those other objects in sight which
occupy a much larger portion of the field of vision. The sun can not
be conceived to exceed the moon enormously in size, seeing that he
appears no larger; and all the multitude of stars are judged by the
sight to be mere bright points of light in reality as they appear to
be.
How, then, can we hope to appreciate the vastness of space whereof
astronomy tells us? To the student of science attempting to conceive
the immensities of whose existence he is assured, the same lesson
might be taught in parable which the child of St. Augustine’s vision
taught the Numidian theologian. As reasonably might an infant hope to
pour the waters of ocean into a hollow, scooped with his tiny fingers
in the sand, as man to picture in his narrow mind the length and
breadth and depth of the abysses of space in which our earth is lost.
Yet, as a picture of a great mansion may be so drawn on a small scrap
of paper as to convey just ideas of its proportions, so may the great
truths which astronomy has taught us about the depths of space be so
presented that just conceptions may be formed of the proportions of
at least those parts of the universe which lie within the range of
scientific vision, though it would be hopeless to attempt to conceive
their real dimensions.
When we learn that a globe as large as our earth, suspended beside
the moon, would seem to have a diameter exceeding hers nearly four
times, so that the globe would cover a space in the heavens about
thirteen times as large as the moon covers, we form a just conception
of the size of the moon as compared with the earth, though the mind
can not conceive such a body as the moon or the earth really is.
When, in turn, we are told that if a globe as large as the earth, but
glowing as brightly as the sun, were set beside the sun, it would
look a mere point of light, we not only learn to picture rightly
to ourselves how largely the sun exceeds the earth, but also how
enormous must be the real distance of the sun.
Another step leads us to a standpoint whence we can form a correct
estimate of the vast distance of the fixed stars; for we can learn
that so enormous is the distance of even the nearest fixed star,
that the tremendous space separating the earth from that star sinks
in turn into the merest point, insomuch that if a globe as bright as
the sun had the earth’s orbit as a close-fitting girdle, then this
glorious orb (with a diameter of some 184,000,000 of miles) would
look very much smaller than such a globe as our earth would look at
the sun’s distance—would, in fact, occupy but about one-fortieth
part of the space in the sky which she, though she would then look a
mere point, would occupy if viewed from that distance.
But there is a way of viewing the immensities of space which, though
not aiding us indeed to conceive them, enables the mind to picture
their proportions better than any other. The dimensions of the
earth’s path around the sun sink into insignificance beside those
of the outermost planets; but these in their turn dwindle into
nothingness beside those of some among the comets. From the path of
these comets, if only sentient and reasoning beings could trace out
in a comet’s company those mighty orbits, and could have for the
duration of their existence not the brief span of time which measures
the longest human life, but many circuits of their comet home around
the same ruling orb (as we live during many circuits of our globe
around the sun), the dimensions of the star-depths, which even to
scientific insight are all but immeasurable, would be directly
discernible. Not only would the proportions of that mighty system be
perceived, whose fruits and blossoms are suns and worlds, but even
the gradually changing arrangement of its parts could be discerned.
Some comets, indeed, do not travel around the sun, but flit from
sun to sun on journeys lasting millions of years, paying each sun
but a single visit. A being inhabiting such a comet, and having
these interstellar journeys as the years of his existence, so that
he could live through many of them, would have a wonderful insight
into the economy of the stellar system. If his powers of conception
as far exceeded ours as the range of his travels and the duration
of his existence, he would be able to recognize the proportions of
a large part of the stellar universe as clearly as we recognize the
proportions of the solar system.
But leaving these wonderful wanderers, whose journeys are as far
beyond our powers of conception as the immensity of the regions
of star-strewn space, we may find, among the comets belonging to
the sun’s domain, bodies whose range of travel would give their
inhabitants far clearer views of the architecture of the heavens than
even the profoundest terrestrial astronomer can possibly obtain.
Such a comet as Halley’s, for instance, though one of comparatively
limited range in space, yet travels so far from the sun that, from
the extreme part of its path, it sees the stars displaced nearly
twenty times as much (owing to its own change of position) as they
are from the earth on opposite sides of her comparatively narrow
orbit. And the length of this comet’s year, if it indicated the lives
of all creatures traveling along with it, would suggest a power of
patiently watching the progress of changes lasting not a few of our
years only, but for centuries. Seventy-five or seventy-six years
elapse between each return of this comet to the sun’s neighborhood,
and one who should have lived during sixty or seventy circuits of
this body around its mighty orbit would have been able to watch the
rush of stars, with their velocities of many miles per second, until
visible displacements had taken place in their positions.
This, however, is as nothing compared with the mighty range in space
and the enormous period of the orbit of the great comet of the year
1811. This comet is, on the whole, the most remarkable ever known.
It was visible for nearly seventeen months, and though it did not
approach the sun within 100,000,000 miles, and was therefore not
subject to that violence of action which has caused enormous tails to
be thrown out from comets which have come within a few million miles
of him, or even within less than a quarter of his own diameter, it
flourished forth a tail 120,000,000 of miles in length. Its orbit
has, according to the calculations of the astronomer Argelander, a
space exceeding the earth’s distance from the sun 211 times, and thus
surpassing even the mighty distance of Neptune fully seven times.
It occupies in circuiting this mighty path no less than 3,065 of
our years (with a possible error either way of about forty-three
years). So that, according to Bible chronology, this comet’s last
appearance probably occurred during the rule of the Judge Tola,
son of Puah, son of Dodo, over the children of Israel, though it
may have occurred during the rule of his predecessor Abimelech, or
during that of his successor Jair.[2] During one-half of the enormous
interval between that time and 1811 the comet was rushing outward
into space, reaching the remotest part of its path somewhere about
the year 278 (A. D.), and from that time to 1811 it was on its
return journey. It is strange to think, however, that though the
remotest part of its path lay 211 times further from the sun than
the earth’s orbit, yet even this mighty path, requiring more than
3,000 years for a single circuit, can not be said to have carried the
comet into the star-depths. If the earth were to shift its position
by the some enormous amount, the nearest fixed star would have
its apparent position changed only by about an eighth part of the
apparent diameter of the sun or moon, or by about one-quarter of the
distance separating the middle star of the Bear’s tail from its close
companion.
But this fact of itself is most strikingly suggestive of the vast
distance of the stars. For consider what it means. Imagine the
middle star of the Bear’s tail to be the really nearest of all the
stars instead of lying probably twenty or thirty times further away.
Conceive a comet belonging to that sun after making its nearest
approach to it to travel away upon an orbit requiring 3,000 years
for each circuit. _Then_ (supposing that star equal to our sun in
mass) the comet, though rushing away from its sun with inconceivable
velocity during 1,500 years, would, at the end of that vast
period, seem to be no further away than one-fourth of the distance
separating the sun from its near companion. Look at the middle star
of the Bear’s tail on any clear night, and on its small satellite,
remembering this fact, and the awful immensity of the star-depths
are strongly impressed upon the mind. But the observer must not fail
to remember that the star really is many times more remote than we
have here for a moment supposed, and that such a comet’s range of
travel would be proportionately reduced. Moreover, many among the
stars are doubtless hundreds, even thousands, of times still further
away.
Let us turn lastly to the amazing comet of the year 1744. We find
that though it had the longest period of any which has ever been
assigned to a comet as the result of actual mathematical calculation,
yet its range in space would scarcely suffice to change the
position of the stars in such sort that the aspect of the familiar
constellations would be materially altered. Euler, the eminent
mathematician, calculated for this comet a period of 122,683 years,
which would correspond, I find, to a distance of recession equal to
2,469 times the distance of the earth from the sun, or about eighty
times the distance of Neptune. Yet this is but little more than
twelve times the greatest distance of the comet of 1811. Probably
the actual range of such an orbit from the middle star of the Bear’s
tail would be equal in appearance to the range described above on
the supposition that the star is no further from us than the nearest
known star (Alpha Centauri). That is, such a comet, if it could be
seen and watched during a period of about 122,000 years, would seem
to recede from the star to a distance equal to about one-fourth the
space separating it from its close companion, and then to return to
the point of nearest approach to its ruling sun.
Such are the immensities of star-strewn space! The journey of a comet
receding from the sun with inconceivable velocity during hundreds
of thousands of years carries it but so small a distance from him
compared with the distance of the nearest star as scarcely to change
the appearance of the celestial landscape; and yet the distances
separating the sun from the nearest of his fellow suns are but as
hairbreadths to leagues when compared with the proportions of the
scheme of suns to which he belongs. These distances, though so mighty
that by comparison with them the inconceivable dimensions of our
own earth sink into utter nothingness, do not bring us even to the
threshold of the outermost court of that region of space to which the
scrutiny of our telescopes extends. Yet the whole of that region is
but an atom in the infinity of space.
FOOTNOTES:
[2] It might be suggested that the appearance of this blazing comet
among the stars drove the more superstitious of the Israelites at
that time to the worship of star-gods, as we read how, during the
Judgeship of Jair, they “served Baalim and Ashtaroth, and the gods
of Syria and the gods of Moab, and the gods of the Philistines, and
forsook the Lord and served not Him.” To a people like the Jews, who
seem to have been in continual danger of returning to the Sabaistic
worship of their Chaldean ancestors, the appearance of a blazing
comet may have been a frequent occasion of backsliding.
EXTENT OF THE SIDEREAL HEAVENS.—SIR ROBERT S. BALL
Of all the discoveries that have ever been made in science there
are two which especially baffle our powers of comprehension. They
lie at the opposite extremes of nature. One relates to objects
which are infinitely small, the other relates to objects which are
almost infinitely great. The microscope teaches us that there are
animals so minute that if a thousand of them were ranged abreast
they would easily swim without being thrown out of line through the
eye of the finest cambric needle. Each of those minute creatures is
a highly organized number of particles, capable of moving about,
of finding and devouring its food, and of behaving in all other
respects as becomes an animal as distinguished from an unorganized
piece of matter. The mind is capable of realizing the structure of
these little creatures, and of fully appreciating their marvelous
adaptation to the life they are destined to lead. If these animals
excite our astonishment by reason of their extreme minuteness,
there is an appeal made to conceptions of an entirely different
character when we learn the lessons which the telescope teaches. As
the microscope reveals the excessively minute, so does the telescope
disclose the sublimely great. In each case myriads of objects are
submitted to our astonished view, but while the microscope brings
before us creatures of which countless millions could swim about
freely in a thimbleful of water, the telescope conducts our vision
to uncounted legions of stars, many of them millions of times larger
than the earth.
The grandest truth in the whole of nature is conveyed in that first
lesson in astronomy which answers the question: What are the stars?
This is a question that a child will ask, and I have heard of a
child’s pretty idea that the stars were little holes in the sky to
let the glory of heaven shine through. The philosopher will replace
this explanation by another hardly less poetical, which will enable
us to form some more adequate notion of the real magnificence of the
universe. Each star that we see is, it is true, only a glittering
little point of light, but that is merely because we are a long way
from it. An electric light which will dazzle your eye when quite
close will be reduced to an agreeable illumination if it is at a
little distance, will become a faint light a mile away, and at no
great distance will become altogether invisible. We must remember
that out in space there is plenty of room—there are no bounds; and
therefore when we see light glistening in the far distant depths we
can not at once conclude that the light is a faint one because it
appears to us to be faint. It may be that the light is only faint
because it comes from such a tremendous distance. In fact, the
brightest light conceivable could be reduced to the insignificance of
a small star if only it were removed sufficiently far.
The most intense light we know of comes, of course, from the
light which rules by day, from our sun himself. The sun pours his
unrivaled beams around us in all directions with prodigal abundance,
notwithstanding his enormous distance of ninety-three millions of
miles. Let me describe an experiment with respect to our sun, an
experiment, it is needless to say, which could never be performed,
but the results to which it leads us are none the less certain.
Astronomers have demonstrated them in many other ways.
Suppose that the sun were gradually to be moved away further and
further into space; suppose that by this time to-morrow the great
luminary should be twice as far as it is now, and the next day should
be three times as far, and the day after that four times, and so on
until in a year’s time we should find that the sun was 365 times
the distance from us that it is at present. Let us now trace the
changes which we should see in the brilliancy of our orb of day.
When he had reached double his distance from us, we should find that
the light had decreased to a quarter of its present amount, and the
heat which we derived from his beams would have decreased in the same
proportion. In ten days we should find that the light had become so
feeble as to be only one-hundredth part of that which we enjoy now.
The apparent size of the sun would also be steadily decreasing, for
as the distance of a body increases its apparent dimensions diminish.
Sometimes the diminution of apparent size with distance is well
illustrated on a clock tower. You would hardly believe that the hands
and face of a clock like that at Westminster were so large until you
happen to see a man cleaning or repairing it, when he appears a mere
pigmy in comparison with the mighty dial which points out the hours.
In a similar way with every increase of distance, the apparent size
of the sun would decline, and in the lapse of a year the sunlight
would be reduced to a feeble twilight. The sun itself would remain
visible for many years, even if it were steadily moving away, though
its lustre would continually decline, and its size would continually
diminish, until at last it would have shrunk to the insignificance of
a small point of light, still visible as a glittering object, but too
minute to enable any definite form to be perceived.
Further still, the sun might recede until it passed beyond the reach
of vision of the unaided eye; the telescope would, however, be able
to pursue the retreating luminary until at last it sank into the
depths of space beyond the reach of any instrument whatever.
This little argument will prepare us for an explanation of the stars.
They merely appear to us to be points of light of varying degrees
of brightness, but we have seen that our own sun might be reduced
in lustre to that of the very dimmest of the stars if only it were
removed sufficiently far. If, therefore, the stars are at a great
enough distance from our system, it may indeed be that they also are
suns, possibly equaling, or possibly even surpassing, our own sun in
magnificence.
Here is indeed an imposing suggestion. Can it be that the host of
stars which adorn our midnight sky are actually suns themselves of an
importance comparable with that of our own? This is a great thought,
and we desire to test it by every means in our power. You will see
from the reasoning I have given that the whole question turns simply
on one point, and that is: How far off are the stars?
The tiniest point of light that is just seen as a glimmer in the
mightiest of telescopes may be indeed a sun as great, or indeed
a million times greater, than our sun, if only that star be
sufficiently far off. To find the distance of a star is a problem
which taxes the utmost powers of the painstaking astronomer; every
refinement of skill in making his measurements and of care in the
calculation of his observations have to be lavished on the operation.
Alas! it but too often happens that the astronomer’s labors prove
to be futile. The surveying navigator often has to mark on his
chart that no bottom could be found in the depths of the sea. His
appliances would not work, or work reliably, in those ocean abysses;
so, too, the astronomer, when he tries to sound the depths of space
to the distances of the stars, has also to mark, generally speaking,
“No bottom here,” as the result of most of his investigations. When
this is the case we know for certain that the star on which his
calculations have been made must be a gorgeous sun, because we are
assured of the greatness of its distance, even though we have not
been able to find out what that distance was. There are, however,
some few places through the sky where the astronomer’s sounding line
can, so to speak, touch bottom; there are a few stars of which we do
know the distance, and the result is not a little significant. Were
our sun to be withdrawn from us to a distance so great as that of
the very nearest of the stars, our magnificent ruler and benefactor
would certainly have lost all his splendor; he would, in fact, have
shrunk to the similitude of a little star not nearly so bright as
many of those which we see over our heads every night. Imagine the
sun’s light subdivided into two hundred thousand parts, each of which
would give us only a feeble illumination, and then imagine that each
of these parts was again divided into two hundred thousand parts
more, and it is one of these last fragments that would represent the
miserable lustre which the sun would then display.
From these considerations we can enunciate the magnificent truth
which astronomy discloses to us. I do not think that in the whole
range of nature there is any thought so magnificent or so imposing as
that which teaches us to regard every star of every constellation
as a sun. We can not indeed assert that they are all so great as
our sun, but we can affirm with certainty that many of them are far
greater and far more splendid. Considering that our sun presides
over a system of worlds of which the earth is one, that it gives
light and heat to those worlds, and guides them in their movements,
it would greatly enlarge our conceptions of the universe if we were
assured that there was even one more sun as large and as splendidly
attended as is our own. But now we find that not only is there one
additional sun, but that they teem in uncounted thousands through
space. Look, for example, on the next fine night at the Great Bear,
the best known of all our northern constellations, and there you see
seven stars forming the well-known feature. Figure in your mind’s eye
each one of those stars in the likeness of a majestic sun, as big,
warm, and bright as our sun, and look at other parts of the sky and
repeat the process with the other constellations, and your conception
of the magnificence of the starry system will begin to assume proper
proportions. But this is only the first step, you must next look at
the smaller stars, and reflect that they, too, are also suns, only
much further off as a general rule than the brighter stars, though
this is by no means invariably the case. Thus your estimate of the
number of suns in the universe will rise to thousands, but you will
not stop there, you will get a telescope to help you, and, to your
extreme delight and wonder, you will find that there are hosts of
stars—too faint to be visible to the eye, but which the telescope
will immediately disclose. You will get a more powerful instrument,
and then you will perceive that the stars are to be numbered by tens
of thousands, and even by millions, and with every fresh accession
of power in your telescope fresh troops and myriads of suns are
revealed. Suns in clusters, suns strewn thickly here and sparsely
there, so as to give us the notion that the only limit to the number
we can see is the power of the telescopes we are using. Attempts at
actual numeration are futile, for who can tell the number of the
stars?
We can, however, form an estimate, and by taking samples, so to
speak, of the sky here and other samples there, we have been enabled
to learn the overwhelming fact that our universe does contain at the
very least one hundred millions of suns.
In discussing the extent of the visible universe, it must always
be borne in mind that the further a source of light is from us the
fainter is the illumination which we receive from it. Suppose that
a star which just lies on the limits of naked-eye visibility were
somehow to be transported to a distance which is twice as great,
then the lustre of that star would be diminished to one-fourth of
its original amount. It would, therefore, be of course invisible to
the unaided eye, but could still be easily perceived by a telescope.
Indeed, the very word _telescope_ means an instrument for looking at
objects a long way off, and the effect of the telescope is to reduce
the apparent distance of the object.
The bulk of a grain of sand as compared with the bulk of a football
may illustrate the space accessible to our eyes when compared with
the space accessible to one of the great telescopes. The larger
of these spaces has a thousand times the diameter of the others;
therefore, the relative quantities of these spaces are to be
obtained by multiplying 1,000 by 1,000 and by 1,000 again. Thus we
finally learn that the amplitude of our vision is augmented to one
thousand million times its original extent by the use of our greatest
telescopes. It need, therefore, be no matter for surprise that the
number of stars visible through our great telescopes or recorded on
the sensitive films of photographic plates should number scores of
millions. In fact, it would sometimes seem surprising that the number
of telescopic stars is not even greater than it actually appears to
be. If we are able to explore one thousand million times as much
space, we might expect that the number of objects disclosed would be
also increased about a thousand million-fold, but this is certainly
not the case. The truth seems to be that our sun is but one star of
a mighty cluster of stars; we happen to lie near the middle of the
cluster, and the rest of the stars belonging to it form what we know
as the Milky Way. There are, of course, other clusters scattered
through the heavens, some of them, perhaps, as great as that body
of stars which forms the Milky Way. Owing to our residence in this
cluster we see the neighboring suns in multitudes, and thus we
receive the impression that the solar system lies in an exceptionally
rich part of the universe in as far as the distribution of stars is
concerned.
On the outskirts of the universe lie those faintest and dimmest of
objects which we can just perceive through our greatest telescopes.
We know that many of the stars around us would still remain visible
in great instruments, even though they were removed a thousand times
as far off. Among the myriads of faint stars which we see from
our observatories, there may be many, indeed there must be many,
which are fully a thousand times as distant as the bright stars
which twinkle in our comparative neighborhood. We thus obtain some
conception of the stupendous distance at which the outskirts of the
universe are situated.
There are different ways of illustrating this point, but I think the
simplest, as well as the most striking, is that which is founded on
the velocity of light. It is a remarkable fact that the beautiful
star known as Vega[3] has a distance from us so tremendous that its
light must have taken somewhere about eighteen years to travel hither
from thence. Notwithstanding that the light dashes along with such
inconceivable speed that it will cover 185,000 miles in every second,
notwithstanding that a journey at this pace will complete the entire
circuit of this globe seven or eight times between two successive
ticks of the clock, the light will, nevertheless, take eighteen years
to reach our eye from the time it leaves Vega. We do not, therefore,
see the star as it is at present; we see it as it was eighteen years
ago. For the light which this evening enters our eyes has been all
that time on its journey. Indeed, if Vega were actually to be
blotted out from existence it would still continue to shine out as
vividly as ever for eighteen years before all the light on its way
had reached us.
We have been led to the belief that among the more distant stars in
the universe there must be many which are fully a thousand times as
far from us as is Vega, hence we arrive at the startling conception
that the light they emit has been on its journey for 18,000 years
before it reached us. When we look at those lights to-night we are
actually viewing them as they were 18,000 years ago. In fact, those
stars might have totally vanished 17,000 years ago, though we and our
descendants may still see them glittering for yet another thousand
years.
We shall realize a little more fully what this reasoning involves if
we suppose that astronomers dwelt on such a star, and that they had
eyes and telescopes sufficiently keen not only to discern our little
earth, but even to scrutinize its surface with attention. Let us
suppose that the stellar astronomers looked at England: do you think
they would see a network of railways joining mighty and populous
cities, furnished with immense manufactories and with countless
institutions? Such would be the England of to-day. But from the
distance at which these astronomers are situated light takes 18,000
years for its journey, and, therefore, what they would see would be
England as it was 18,000 years ago. To them England would even now
appear as a country mainly covered with forests inhabited by bears
and wolves, and totally void of any trace of civilization. This
illustration will, at all events, serve to convey some conception
of the distance at which the outskirts of our visible universe are
plunged in the depths of space.
FOOTNOTES:
[3] Vega is the brightest star in the Lyre and is nearly always at
night directly overhead in our latitude.—E. S.
THE STARS.—AMÉDÉE GUILLEMIN
No sight is at once so awe-inspiring and so grand as that of the
heavens on a beautiful night. If care be taken to choose as a
standpoint for observation an open place, such as a plain or the
summit of a hill on land, or, again, the open sea, and if the
atmosphere, somewhat charged with dew, possesses all its transparency
and purity, we shall see thousands of luminous points twinkling
in all directions, accomplishing slowly and together their silent
march. The contrast of the obscurity which reigns on the surface of
the earth with the brightness of that resplendent vault gives an
indefinite depth to the celestial ocean that deepens over our heads.
But let us here leave the magnificence of the spectacle to study it
in its most minute details.
Let us commence with the appearances. A characteristic common to
all the stars is an incessant and very rapid change of brightness,
which has received the name of _scintillation_. This is accompanied
by variations of color equally rapid, due to the same cause as the
successive disappearances and reappearances. All stars scintillate,
whatever may be their brilliancy, at least in our temperate regions.
But the intensity of this luminous movement is not the same in all,
and it varies, moreover, both with the degree of purity of the sky,
the elevation of the stars above the horizon, and the temperature of
the night.
According to Arago, scintillation is due to the difference of
velocity of the various colored rays traversing the unequally warm,
unequally dense, unequally humid atmospheric strata. Thus, in
tropical regions, where the atmospheric strata are more homogeneous,
scintillation is rarely observed in stars the elevation of which
above the horizon is more than 15°, or the sixth of the distance of
the horizon from the zenith. “This circumstance,” says Humboldt,
“gives to the celestial vault of these countries a particularly calm
and soft character.”
Another specific character of the stars is that their diameters are
without appreciable dimensions. To the naked eye, this distinction
would be insufficient, since, the moon and the sun excepted, the
most considerable planets have not sensible diameters. But, while
the magnifying power of optical instruments shows us the principal
planets under the form of clearly defined disks, the most powerful
glasses only show a star as a luminous point. The distance which
separates us from these bodies is so great that there is nothing to
astonish us in such a result.
Wollaston affirms that the apparent diameter of the most brilliant
star in the heavens, Sirius, is not more than the fiftieth part of
a second of an arc. But let us hasten to say that this result still
leaves a good margin as to the real dimensions of the star, since, at
the distance of Sirius, an apparent diameter would represent a real
diameter of 11,000,000 miles; that is, twelve times the diameter of
our sun.
Let us add, lastly, that the absence of appreciable dimensions does
not suffice to distinguish absolutely the stars from the planets,
since a certain number of the latter, as we have before seen, appear
in telescopes only as simple luminous points. Let us come, then, to a
permanent specific characteristic, the knowledge of which will always
prevent us from confounding a star with one of the known or unknown
bodies which form part of our solar group. This characteristic is as
follows:
The stars, properly so called, preserve among themselves—nearly
enough for our present purpose—the same relative distances. They
form, then, on the celestial vault apparent groups, the configuration
of which is nearly invariable. Centuries must elapse to show a change
of form, unless we employ extremely delicate measures. A planet, on
the contrary, moves rapidly across these groups, to such a degree
that, in the interval of a night, or at most of a few nights, this
displacement is very perceptible; hence the old denomination of
_fixed stars_, in opposition to the _wandering_ ones, or planets.
We must be careful, however, to guard against assigning to this word
a rigidity which it does not possess, for the stars really move with
a velocity not inferior to that which animates the members of our
system. Their immense distance is the only cause of their apparent
immobility, which vanishes when precise observations, embracing a
sufficient interval of time—some years, for example—are made.
A fact which strikes every one is the great diversity of brightness
in the stars which people the heavens. All degrees of intensity
are remarked, from the resplendent light of Sirius to the scarcely
perceptible glimmer of those hardly visible to the naked eye.
Whence arises this difference of brightness? This question we can not
answer for any star in particular, but it is easy to imagine that it
may result from various circumstances, such as their less or greater
distance, the real and various dimensions of the bodies, and, lastly,
the intrinsic brightness of the light peculiar to each. However this
may be, astronomers without regard to the unknown causes which may
influence the intensity of the stellar light, have divided stars
into _classes_ or _magnitudes_; and when we speak of a star of the
first, second, or fifth magnitude, it is understood that this way of
speaking refers only to the apparent brightness, and that nothing
is affirmed either as to the real dimensions or distance, or even
intrinsic brightness.
Besides, as the stars, arranged in the order of their brightness,
would form a progression decreasing by imperceptible degrees, the
classes adopted are themselves conventional and arbitrary. The first
six magnitudes comprise all stars visible to the naked eye. But the
use of the most powerful telescopes brings to view stars of feebler
light, descending to the sixteenth and seventeenth magnitudes. In
truth, the progression has no inferior limit: it extends more and
more in proportion as the progress of the optician’s art increases
the penetrating power of our instruments.
To gain an idea of the respective intensities of the light emitted by
the stars of the first six magnitudes, following the scale adopted
by astronomers, the accompanying illustration (Fig. 1), should be
inspected; in it the stars are figured by disks, the surfaces of
which are in proportion to their brilliancy.
But, we repeat, it must not be thought that the stars ranked in the
same class are, on that account, of the same brightness. Thus the
light of Sirius is estimated at four times the star Alpha Centauri;
but both, nevertheless, are included by astronomers in the number of
the stars of the first magnitude.
[Illustration: Fig. 1.—Relative Brilliancy of Stars of the first Six
Magnitudes]
We here give the names of the twenty most brilliant stars of the two
hemispheres which it is usual to consider as forming the first class.
They are here arranged in the order of their brightness:
1. Sirius
2. Eta Argus
3. Canopus
4. Alpha Centauri
5. Arcturus
6. Rigel
7. Capella
8. Vega
9. Procyon
10. Betelgeuse
11. Achernar
12. Aldebaran
13. Beta Centauri
14. Alpha Crucis
15. Antares
16. Altair
17. Spica
18. Fomalhaut
19. Beta Crucis
20. Pollux
Lastly, Regulus, a bright star in the constellation of the Lion, is
also ranked by some astronomers in the first magnitude, while others
only admit in this class the first seventeen stars in the above list.
These divergences are of no importance.
In proportion as the scale of brilliancy or magnitude is descended,
the number of the stars contained in each class rapidly increases.
The number of second magnitude stars in the heavens is about 65;
of the third, about 200; of the fifth, 1,100; and of the sixth
magnitude, 3,200. Adding these numbers together, we obtain a few over
5,000 stars of the first six magnitudes, and these comprise very
nearly all those that can be seen with the naked eye.
The smallness of this number nearly always astonishes those who have
not tried to form an exact estimate of the number of stars which
shine in the celestial vault on the most favorable nights.
The aspect of the multitude of sparkling points which are scattered
over the sky makes us disposed to believe that they are innumerable,
and to be counted, if not by millions, at all events by hundreds of
thousands. This is, nevertheless, an illusion. All observers who have
taken the trouble to make an exact enumeration of the stars visible
to the naked eye have arrived at a maximum of 3,000 as the mean
number which can be observed in every part of the heavens, visible at
the same time, at the same place; this, of course, is but half of the
entire heavens.
Argelander has published an exact catalogue of the stars visible on
the horizon of Berlin during the course of the year. This catalogue
comprises 3,256 stars. According to Humboldt, there are 4,146 visible
on the horizon of Paris in the whole course of the year; and as this
number increases in proportion as we approach the Equator, that is to
say, in proportion as the double movement of the earth unfolds to us
during a year a more extensive portion of the heavens, 4,638 stars
are already visible to the naked eye on the horizon of Alexandria.
We repeat, the maximum number is comprised between 5,000 and 6,000
stars for the entire heavens, including those seen by the most
piercing and most accustomed eyes in the best nights for observation.
When the atmosphere is lit up by the moon, or by twilight, or, as
happens in the great centres of population, by the illumination
of the houses and streets, the lowest magnitude stars are effaced
altogether, and the number of those visible is consequently much more
limited. We may add in conclusion, that the more the scintillation,
the more easy it is to distinguish very faint stars.
A word now on the number of stars that can be seen with the help of
the telescope. Here we shall find the numbers which our imagination
had erroneously led us to believe are visible to the naked eye.
According to the illustrious director of the Observatory of
Bonn—Argelander—the seventh magnitude comprises nearly 13,000 stars;
the eighth, 40,000; and, lastly, the ninth, 142,000. The calculations
of Struve give the total number of stars visible in the entire
heavens by the aid of Sir William Herschel’s 20-foot reflector as
more than 20,000,000. But, without doubt, these approximate numbers
are much below the real ones. It will be seen, besides, that the
richness of the heavens in stars is very unequal. The bright zone
known under the name of the Milky Way alone contains, according to
Herschel, 18,000,000.
THE LUCID STARS.—J. E. GORE
The term “lucid” has been applied to the stars visible to the naked
eye, without optical aid of any kind.[4] Many people think that the
number of stars visible in this way is very large. But in reality the
number visible to the naked eye is comparatively small. Some persons
are, of course, gifted with very keen eyesight—“miraculous vision”
it is sometimes called—and can see more stars than others; but to
average eyesight the number visible in this way, and which can be
individually counted, is very limited. The famous Hipparchus formed
a catalogue of stars in the year 127 B. C. This presumably contained
all the most conspicuous stars he could see in his latitude, and
it includes only 1,025 stars. Al-Sûfi, the Persian astronomer, in
his _Description of the Fixed Stars_, written in the Tenth Century,
describes the positions of only 1,018 stars, although he refers to a
number of other faint stars, of which he does not record the exact
places. Pliny thought that about 1,600 stars were visible in the sky
of Europe.
In modern times, however, a considerable number of fainter stars
have been recorded as visible to the naked eye. The famous German
astronomer, Heis, who had keen eyesight, records the positions of
3,903 stars north of the Equator, and 1,040 between the Equator and
20 degrees south declination, or a total of 4,943 stars between the
North Pole and 20 degrees south of the Equator. This would, I find,
give a total of about 7,366 stars for both hemispheres if the stars
were equally distributed. Behrmann, in his _Atlas of Southern Stars_,
between 20 degrees south declination and the South Pole, shows 2,344
stars as visible to the naked eye. This would give a total of 7,124
for both hemispheres. The actual number seen by Heis and Behrmann
in both hemispheres is 4,943 + 2,344, or 7,287 stars. The Belgian
astronomer, Houzeau, published a catalogue and atlas of the stars in
_both_ hemispheres, made from his own observations in Jamaica and
South America, and finds a total of 5,719 stars in the whole sky.
As all these observers had good eyesight, we may take a mean of the
above results as the total number visible to the naked eye in the
whole star sphere. This gives 6,874 stars, or in round numbers we
may say that there are about 7,000 stars visible to average eyesight
in both hemispheres. This gives, of course, about 3,500 stars to one
observer at the same time at any point on the earth’s surface.
As the whole star sphere contains an area of 41,253 square degrees,
we have an average of one star to six square degrees. In other words
there is, _on an average_, one lucid star in a space equal to about
thirty times the area covered by the full moon! This result may seem
rather surprising considering the apparently large number of stars
visible to the naked eye on a clear night, but the fact can not be
denied. The stars are not, of course, equally distributed over the
surface of the sky, but are gathered together in some places, and
sparsely scattered in others, and this may perhaps help to give the
impression of a greater number than there really are.
That the stars are of various degrees of brightness was recognized
by the ancient astronomers. Ptolemy divided them into six classes,
the brightest being called first magnitude, those considerably
fainter the second, those much fainter still the third, down to the
sixth magnitude, which were supposed to be the faintest just visible
to the naked eye on a clear moonless night. Ptolemy only recorded
whole magnitudes, but Al-Sûfi, in the Tenth Century, divided these
magnitudes, for the first time, into thirds. Thus a star slightly
less than an average star of the second magnitude he called 2—3,
that is nearer in brightness to 2 than to 3; one a little brighter
than the third he recorded as 3—2, or nearer to 3 than to 2, and so
on. This method has been followed by Argelander, Behrmann, Heis, and
Houzeau, but in the photometric catalogues of Harvard, Oxford, and
Potsdam the magnitudes are measured in decimals of a degree. This has
been found necessary for greater accuracy, as the heavens contain
stars of all degrees of brightness.
The term “magnitude” means the ratio between the light of a star of
a given magnitude and that of another exactly one magnitude fainter.
This ratio has been variously estimated by different astronomers,
and ranges from 2.155, found by Johnson in 1851, to 3.06, assumed by
Pierce in 1878. The value now universally adopted by astronomers
is 2.512 (of which the logarithm is 0.4). This number is nearly a
mean of all the estimates made, and agrees with the value found by
Pogson in 1854 by means of an oil flame, and by Rosen with a Zöllner
photometer in 1870. It simply means that an average star of the first
magnitude is 2.512 times the brightness of a star of the second
magnitude; a star of the second, 2.512 times brighter than one of the
third, and so on. This makes a star of the first magnitude just 100
times brighter than one of the sixth.
There are several stars brighter than an average star of the first
magnitude, such as Aldebaran. These are Sirius, which is nearly 11
times brighter than Aldebaran (according to the revised measures at
Harvard); Canopus, the second brightest star in the heavens, and
about two magnitudes brighter than Aldebaran; Arcturus, Capella,
Vega, Alpha Centauri, Rigel, Procyon, Alpha Eridani, Beta Centauri,
and Alpha Orionis. Al-Sûfi rated 13 stars of the first magnitude,
visible at his station in Persia, and Halley enumerates 16 in the
whole sky. According to the Harvard photometric measures, there are
13 stars in both hemispheres brighter than Aldebaran, which is rated
1.07.
As average stars of the different magnitudes the following may
be taken as examples, derived from the Harvard measures: First
magnitude, Aldebaran and Spica; second magnitude, β Aurigæ and β
Canis Majoris; third magnitude, ι Aurigæ and β Ophiuchi; fourth
magnitude, θ Herculis and ε Draconis; and fifth magnitude, ρ Ursæ
Majoris and ω Sagittarii. Stars of about the sixth magnitude are,
of course, numerous, and lie near the limit of naked-eye vision for
average eyesight, although on clear moonless nights still fainter
stars may be “glimpsed” by keen-eyed observers.
The stars have been divided into groups and constellations, now
chiefly used for the purpose of reference, but in ancient times they
were associated with the imaginary figures of men and animals, etc.
The origin of these constellation figures is doubtful, but they
are certainly of great antiquity. Ptolemy’s constellations were 48
in number, but different writers from the First Century B. C. give
different numbers, ranging from 43 to 62. Bayer’s _Uranometria_,
published in 1603, contains 60, 12 new constellations in the Southern
Hemisphere having been added by Theodorus to Ptolemy’s original 48.
The figures representing the constellations were originally drawn on
spheres, or celestial globes, as they are now called. The ancient
astronomers attributed the invention of the sphere to Atlas. It seems
certain that a celestial sphere was constructed by Eudoxus in the
Fourth Century B. C. Strabo speaks of one made by Krates about the
year 130 B. C., and according to Ovid, Archimedes had constructed
one at a considerably earlier period. None of these ancient spheres
has been preserved. There is, however, in the Vatican a fragment in
marble of a Græco-Egyptian planisphere, and a globe in the museum
of Arolsen, but these are of much later date. Our knowledge of the
original constellation figures is derived from the accounts given by
Ptolemy and his successors, and from a few globes which only date
back to the Arabian period of astronomy. Among the Arabian globes
still existing the most famous is one made of copper, and preserved
in the Borgia Museum at Velletri in Italy. It is supposed to have
been made by a person called Caisar, who was executed by the Sultan
of Egypt in A. D. 1225. The most ancient of all is one discovered
some years ago at Florence. It is supposed to date back to A. D.
1081, and to have been made by Meucci. There is also one in the
Farnese Museum at Naples, made in A. D. 1225. Of modern celestial
globes the oldest is one made by Jansson Blaeu in 1603. This gives
all the constellations of the Southern Hemisphere as well as the
Northern.
Ptolemy’s figures of the constellations were restored by the famous
painter Albert Dürer of Nuremberg in 1515. The figures on modern
globes and maps have been copied from this restoration. Dürer’s maps
are now very rare.
In 1603, an atlas was published by Bayer. This was the first atlas
to show the southern sky, and the first to designate the brightest
stars by the letters of the Greek alphabet.[5] Flamsteed published
an atlas in 1729. Maps and catalogues of the lucid stars have been
published in recent times by Argelander, Behrmann, Heis, Houzeau,
Proctor, and others. Of these Heis’s is, perhaps, the most reliable,
at least so far as accurate star magnitudes are concerned. Houzeau
shows _both_ hemispheres, all the stars had been observed by himself
in Jamaica and South America. Behrmann’s maps are confined to the
Southern Hemisphere, between the South Pole and 20 degrees south
of the Equator. The maps of the _Uranometria Argentina_, made at
Cordoba in the Argentine Republic, show all the southern stars to the
seventh magnitude, but many of these are beyond the reach of ordinary
eyesight.
It is a well-known fact that the planets Venus and Jupiter are
bright enough to form shadows of objects on a white background. It
has also been found that the brightest stars, especially Sirius, are
sufficiently brilliant to cast shadows. Kepler stated that a shadow
was formed by even Spica, but I am not aware that this has been
confirmed by modern observations.
There are some remarkable collections or clusters of stars visible to
the naked eye, of these the Pleiades are probably the best known. To
ordinary eyesight 6 stars are visible, but Möstlin, Kepler’s tutor,
is said to have seen 14 with the naked eye, and some observers in
modern times have seen 11 or 12. Other naked-eye clusters are the
Hyades in Taurus, called _Palilicium_ by Halley, and the Præsepe,
or Bee-Hive in Cancer. Of larger groups, the Plow or Great Bear,
Cassiopeia’s Chair, and Orion are probably known to most people.
Many of the lucid stars are double, that is, consist of two
components, but most of these are only visible in powerful
telescopes. There are, however, a few objects visible to the naked
eye as double, and these have been called “naked-eye doubles,”
although not strictly double in the correct sense of the term.
Ptolemy applied the term double to the star ν Sagittarii, which
consists of two stars separated by a distance of fourteen minutes of
arc, or about half the apparent diameter of the moon. According to
Riccioli, Van der Hove saw two naked-eye doubles, one in Capricornus,
5 to 5½ minutes distant, and the other in the Hyades, 4½ or 5 minutes
apart. The one in Capricornus was probably α, and the one in the
Hyades θ Tauri. The middle star in the tail of the Great Bear, or
handle of the Plow, has near it a small star, Alcor, which to many
eyes is distinctly visible without optical aid. The famous Belgian
astronomer, Houzeau, who seems to have had excellent sight, saw the
star χ Tauri double, and 51 and 56 Tauri separated, also ι Orionis,
and others.
Many of the stars are variable in their light, and several hundred of
these curious and interesting objects are now known to astronomers.
In a few of these the light changes may be followed with the naked
eye. It is an interesting question whether any of the lucid stars
have disappeared or changed in brightness since the early ages of
astronomical observations. Al-Sûfi failed to find seven of Ptolemy’s
stars, and Ulug Bekh, comparing his observations with the catalogues
of Ptolemy and Al-Sûfi, announced twelve cases of supposed
disappearance. Some of these may, however, be due to errors of
observation. Montanari, writing in 1672, mentions two stars as having
disappeared, namely β and γ of the constellation Argo, but these
stars are now visible in the positions originally assigned to them.
In a careful examination of Al-Sûfi’s description of the stars
written in the Tenth Century, and a comparison with modern estimates
and measures, I have found several very interesting cases of apparent
change in the brightness of the lucid stars. Al-Sûfi was an excellent
and careful observer, and as a rule his estimates agree well with
modern observations. We can therefore place considerable reliance on
his estimates of star magnitudes. _The Story of Theta Eridani_ has
been well told by Dr. Anderson, and there seems to be no doubt that
this southern star, which is now only of the third magnitude, was a
bright star of the first magnitude in Al-Sûfi’s time! The following
are other interesting cases of apparent change which I have met with
in my examination of Al-Sûfi’s work. The Pole Star was rated third
magnitude by both Ptolemy and Al-Sûfi, but it is now of the second
magnitude, or a little less. The star γ Geminorum was rated third
magnitude by Ptolemy and Al-Sûfi, or equal to δ Geminorum, but γ is
now of the second magnitude, and its great superiority in brightness
over δ is noticeable at a glance. Another interesting case is that
of ζ and ο Persei, two stars which lie near each other, about seven
degrees north of the Pleiades. Al-Sûfi distinctly describes these
stars as _both_ of the 3—4 magnitude; but Argelander, Heis, and the
photometric measures at Harvard agree in making ζ about one magnitude
brighter than ο. The stars being close are easily compared, and their
present great difference in brightness is very noticeable. This
is one of the most remarkable cases I have met with in Al-Sûfi’s
work, and strongly suggests variation in ο, as ζ is still about the
same brightness as Al-Sûfi made it. The identity of the stars is
beyond all doubt, as Al-Sûfi describes their positions very clearly,
and says there is no star between them and the Pleiades, a remark
which is quite correct for the naked eye. The remarkable decrease
in brightness of β Leonis (Denebola) since Al-Sûfi’s time has been
considered in my paper on _Some Suspected Variable Stars_. That it
was a bright star of the first magnitude is fully proved by the
observations of Al-Sûfi and Tycho Brahe. These were careful and
accurate observers, and they could not have been mistaken about a
star of the first magnitude. β Leonis is now fainter than an average
star of the second magnitude, and there can be no reasonable doubt
that it has faded considerably since the Tenth Century.
There are some other discrepancies between Al-Sûfi’s observations
and modern estimates, but the above are perhaps the most remarkable.
With reference to lucid stars not mentioned by Al-Sûfi, he has not,
I think, omitted any star brighter than the fourth magnitude in that
portion of the sky visible from his station. There are, however, a
number of stars between the fourth and sixth magnitudes which he does
not mention. Of these the brightest seem to be ε Aquilæ, ρ and μ
Cygni, and ζ Coronæ Borealis.
With reference to the distribution of the lucid stars in the sky
there seems to be a well-marked tendency to congregate on the Milky
Way. It is a remarkable fact that of the 15 brightest stars in the
heavens, no less than 11 lie on or near the Milky Way, although the
space covered by the Galaxy does not exceed one-fifth or one-sixth of
the whole sky. From a careful enumeration of the stars in or near the
Milky Way which I made some years ago, I found that of stars brighter
than the fourth magnitude there are 118 on the Milky Way out of a
total of 392, or about 30 per cent. From the Southern catalogue known
as the _Uranometria Argentina_, Colonel Markwick, F.R.A.S., found 121
out of 228 stars to fourth magnitude, or a percentage of 53 per cent.
These results seem to show some intimate relation between the lucid
stars and the Galaxy.
FOOTNOTES:
[4] Except concave spectacles used by short-sighted persons.
[5] This custom has since prevailed. The following are the letters
and their names:
α Alpha η Eta ν Nu τ Tau
β Beta θ Theta ξ Xi υ Upsilon
γ Gamma ι Iota ο Omicron φ Phi
δ Delta κ Kappa π Pi χ Chi
ε Epsilon λ Lambda ρ Rho ψ Psi
ζ Zeta μ Mu σ Sigma ω Omega
THE CONSTELLATIONS.—CAMILLE FLAMMARION
The earth is forgotten, with its small and ephemeral history. The
sun himself, with all his immense system, has sunk in the infinite
night. On the wings of inter-sidereal comets we have taken our flight
toward the stars, the suns of space. Have we exactly measured, have
we worthily realized the road passed over by our thoughts? The
nearest star to us reigns at a distance of 275,000 times 37 millions
of leagues—that is to say, at ten trillions[6] of leagues (about
twenty-five billions of miles); out to that star an immense desert
surrounds us, the most profound, the darkest, and the most silent of
solitudes.
The solar system seems to us very vast, the abyss which separates our
world from Mars, Jupiter, Saturn, and Neptune appears to us immense;
relatively to the fixed stars, however, our whole system represents
but an isolated family immediately surrounding us: a sphere as vast
as the whole solar system would be reduced to the size of a simple
point if it were transported to the distance of the nearest star. The
space which extends between the solar system and the stars, and which
separates the stars from each other, appears to be entirely void of
visible matter, with the exception of nebulous fragments, cometary
or meteoric, which circulate here and there in the immense voids.
Nine thousand two hundred and fifty systems like ours (bounded by
Neptune), would be contained in the space which isolates us from the
nearest star!
If a terrible explosion occurred in this star, and if the sound could
traverse the void which separates it from us, this sound would take
more than three millions of years to reach us.
It is marvelous that we can perceive the stars at such a distance.
What an admirable transparency in these immense spaces to permit the
light to pass, without being wasted, to thousands of billions of
miles! Around us, in the thick air which envelops us, the mountains
are already darkened and difficult to see at seventy miles; the least
fog hides from us objects on the horizon. What must be the tenuity,
the rarefaction, the extreme transparency of the ethereal medium
which fills the celestial spaces!
Let us suppose ourselves, then, on the sun nearest to ours. From
there our dazzling furnace is already lost like a little star, hardly
recognizable among the constellations: earth, planets, comets sail in
the invisible. We are in a new system. If we thus approach each star
we find a sun, while all the other suns of space are reduced to the
rank of stars. Strange reality!—the normal state of the universe is
night. What we call day only exists for us because we are near a star.
The immense distance which isolates us from all the stars reduces
them to the state of motionless lights apparently fixed on the vault
of the firmament. All human eyes, since humanity freed its wings
from the animal chrysalis, all minds since the minds have been, have
contemplated these distant stars lost in the ethereal depths; our
ancestors of Central Asia, the Chaldeans of Babylon, the Egyptians
of the Pyramids, the Argonauts of the Golden Fleece, the Hebrews
sung by Job, the Greeks sung by Homer, the Romans sung by Virgil—all
these earthly eyes, for so long dull and closed, have been fixed
from age to age on these eyes of the sky, always open, animated, and
living. Terrestrial generations, nations and their glories, thrones
and altars have vanished: the sky of Homer is always there. Is it
astonishing that the heavens were contemplated, loved, venerated,
questioned, and admired even before anything was known of their true
beauties and their unfathomable grandeur?
Better than the spectacle of the sea calm or agitated, grander than
the spectacle of mountains adorned with forests or crowned with
perpetual snow, the spectacle of the sky attracts us, envelops
us, speaks to us of the infinite, gives us the dizziness of the
abyss; for, more than any other, it seizes the contemplative mind
and appeals to it, being the truth, the infinite, the eternal,
the all. Writers who know nothing of the true poetry of modern
science have supposed that the perception of the sublime is born of
ignorance, and that to admire it is necessary not to know. This is
assuredly a strange error, and the best proof of it is found in the
captivating charm and the passionate admiration which divine science
now inspires, not in some rare minds only, but in thousands of
intellects, in a hundred thousand readers impassioned in the search
for truth, surprised, almost ashamed at having lived in ignorance of
and indifference to these splendid realities, anxious to incessantly
enlarge their conception of things eternal, and feeling admiration
increasing in their dazzled minds in proportion as they penetrate
further into Infinitude. What was the universe of Moses, of Job,
of Hesiod, or of Cicero, compared to ours! Search through all the
religious mysteries, in all the surprises of art, painting, music,
the theatre, or romance, search for an intellectual contemplation
which produces in the mind the impression of truth, of grandeur, of
the sublime, like astronomical contemplation! The smallest shooting
star puts to us a question which it is difficult not to hear; it
seems to say to us, What are we in the universe? The comet opens its
wings to carry us into the profundities of space: the star which
shines in the depths of the heavens shows us a distant sun surrounded
with unknown humanities who warm themselves in his rays. Wonderful,
immense, fantastic spectacles, they charm by their captivating beauty
and transport into the majesty of the unfathomable the man who
permits himself to soar and wing his flight to Infinitude.
Nel ciel che più della sua luce prende
Fu’ io, e vidi cose che ridire
Né sa, né può qual di lassù discende.
“I have ascended into the heavens, which receive most of His light,
and I have seen things which he who descends from on high knows not,
neither can repeat,” wrote Dante in the first canto of his poem on
“Paradise.” Let us, like him, rise toward the celestial heights, no
longer on the trembling wings of faith, but on the stronger wings
of science. What the stars would teach us is incomparably more
beautiful, more marvelous, and more splendid than anything we can
dream of.
[Illustration: Chart of the Northern Constellations
Showing the principal Stars of the first five magnitudes visible to
the naked eye]
Among the innumerable army of stars which sparkle in the infinite
night, the gaze is especially arrested by the most brilliant lights
and by certain groups which vaguely present a mysterious bond between
the worlds of space. These groups have been noticed at all epochs,
even among the rudest races of men, and from the earliest ages of
humanity they have received names, usually derived from the organic
kingdom, which give a fantastic life to the solitude and the silence
of the skies. Thus were early distinguished the seven stars of the
North, or the Chariot, of which Homer speaks; the _Pleiades_, or the
“_Poussinière_”; the giant _Orion_; the Hyades in the head of Taurus;
_Boötes_, near the Chariot or Great Bear. These five groups were
already named more than 3,000 years ago, and so were the brightest
stars of the sky, _Sirius_ and _Arcturus_, etc.
The epoch of the formation of the constellations is unknown, but we
know that they were established successively. The centaur Chiron,
Jason’s tutor, has the reputation of having first divided the sky on
the sphere of the Argonauts. But this is mythology; and, besides,
Job lived before the epoch at which Chiron is supposed to have
flourished, and Job had already spoken of Orion, the Pleiades, and
the Hyades 3,000 years ago. Homer also speaks of these constellations
in describing the famous shield of Vulcan. “On its surface,” says
he, “Vulcan, with a divine intelligence traces a thousand varied
pictures. He represents the earth, the heavens, the sea, the
indefatigable sun, the moon at its full, and all the stars which
wreath the sky: the Pleiades, the Hyades, the brilliant Orion, the
Bear, which they also call the Chariot, and which revolves round the
pole; this is the only constellation which does not dip into the
ocean waves” (_Iliad_, chapter xviii.).
Several theologians have affirmed that it was Adam himself, in
the terrestrial paradise, who gave their names to the stars; the
historian Josephus assures us that it was not Adam, but his son Seth,
and that in any case astronomy was cultivated long before the Deluge.
This nobility is sufficient for us.
Attentive observation of the sky also noticed from the beginning
the beautiful stars _Vega_ of the Lyre, _Capella_ of Auriga,
_Procyon_ of the Little Dog, _Antares_ of the Scorpion, _Altair_
of the Eagle, _Spica_ of the Virgin, the _Twins_, the _Chair_ of
Cassiopeia, the Cross of the White _Swan_, stretched in the midst of
the _Milky Way_. Although noticed at the epoch of Hesiod and Homer,
these constellations and stars were probably not yet named, because
doubtless men had not yet felt the necessity of registering them for
any application to the calendar, to navigation, or to voyages.[7]
At the epoch when the maritime power of the Phœnicians was at its
apogee, about 3,000 years ago, or twelve centuries before our era, it
was the star β of the Little Bear which was the nearest bright star
to the pole, and the skilful navigators of Tyre and Sidon (O purpled
kings of former times! what remains of your pride?) had recognized
the seven stars of the Little Bear, which they named the Tail of the
Dog, _Cynosura_; they guided themselves by the pivot of the diurnal
motion, and during several centuries they surpassed in precision all
the mariners of the Mediterranean. The Dog has given place to a Bear,
doubtless on account of the resemblance of the configuration of these
seven stars to the seven of the Great Bear, but the tail remains long
and curled up, in spite of the nature of the new animal.
Thus the stars of the North at first served as points of reference
for the first men who dared to venture on the seas. But they served
at the same time as guides on the mainland for the nomadic tribes who
carried their tents from country to country. In the midst of savage
nature, the first warriors themselves had nothing but the Little Bear
to guide their steps.
Imperceptibly, successively, the constellations were formed. Some
groups resemble the names which they still bear, and suggested their
denomination to the men of ancient times, who lived in the midst
of nature and sought everywhere for relations with their daily
observations. The Chariot; the Chair; the Three Kings, also named
the Rake; Jacob’s Staff and the Belt of Orion; the Pleiades, or the
Hen and Chickens; the Arrow (Sagitta); the Crown; the Triangle; the
Twins; the Dragon; the Serpent; and even the Bull, the Swan, the
Giant Orion, the Dolphin, the Fishes, the Lion, Water and Aquarius
(the Water-bearer), etc., have given rise to the analogy. These
resemblances are sometimes vague and far-fetched, like those we
find in the clouds; but it appears much more natural to admit this
origin than to suppose, with the classic authors, that these names
were suggested by the concordance between the seasons or the labors
of the fields and the presence of the stars above the horizon. That
the name of the Balance (Libra) was given to the constellation of
the equinox because then the days are equal, seems to us more than
questionable; that Cancer (the Crab) signifies that the sun goes back
to the solstice, and that the Lion has for its object to symbolize
the heat of summer, and Aquarius the rain and inundations, appears
to us no less imaginary. However, they have also had other origins.
Thus, the Great Dog Sirius certainly announced the rising of the
Nile and the dog-days (which remain in our calendar as a fine type
of anachronism). Poetry, gratitude, the deification of heroes,
mythology, afterward transferred to the sky the names of personages
and sovereigns—Hercules, Perseus, Andromeda, Cepheus, Cassiopeia,
Pegasus; later, in the Roman epoch, they added the Hair of Berenice
and Antinous; later still, in modern times, they added the Southern
Cross, the Indian, the Sculptor’s Workshop (Cœlum), the Lynx, the
Giraffe (Camelopardus), the Greyhounds (Canes Venatici), the Shield
of Sobieski, and the little Fox (Vulpecula). They even placed in the
sky a Mountain, an Oak, a Peacock, a Swordfish, a Goose, a Cat, a
Crane, a Lizard, and a Fly, for which there was no necessity.
This is not the place to describe and draw in detail all these
constellations, with their more or less strange figures. The
important point for us here is to form a general idea.
The sky remains divided into provinces, each of which continues to
bear the name of the primitive constellation. But it is important
to understand that the positions of the stars themselves, as we see
them, are not absolute, and that the different configurations which
they may show us are only a matter of perspective. We already know
that the sky is not a concave sphere on which brilliant nails could
be attached; that it is not a species of vault; that an immense
infinite void envelops the earth on all sides, in all directions. We
know also that the stars, the suns of space, are scattered at all
distances in the vast immensity. When, therefore, we remark in the
sky several stars near each other, that does not imply that these
stars form the same constellation, that they are on the same plane,
and at an equal distance from the earth. By no means; the arrangement
which they assume to our eyes is but an appearance caused by the
position of the earth relatively to them. This is a mere matter of
perspective. If we could leave our world, and transport ourselves
to a point in space sufficiently distant, we should see a variation
in the apparent arrangement of the stars so much the greater as
our station of observation were more distant from where we are at
present. A moment’s reflection is sufficient to convince us of this
fact, and save us from insisting further on this point.
Once these illusions are appreciated at their true value, we can
begin the description of the figures with which the ancient mythology
has constellated the sphere. A knowledge of the constellations is
necessary for the observation of the heavens and for the researches
which a love of the sciences and curiosity may suggest; without it
we find ourselves in an unknown country, of which the geography has
not been made, and where it would be impossible to know our exact
position. Let us make, then, this celestial geography; let us see how
to find our way, in order to read readily in the great book of the
heavens.
There is a constellation which everybody knows; for greater
simplicity we will begin with it. It will serve us well as a point of
departure from which to go to the others, and as a point of reference
to find its companions. This constellation is the _Great Bear_, which
has also been named the _Chariot of David_.
It may well boast of being celebrated. If, notwithstanding its
universal notoriety, some of our readers have not yet made its
acquaintance, the following is a description by which they may
recognize it.
[Illustration: Fig. 2]
Turn yourself toward the north—that is to say, opposite to the point
where the sun is found at noon. Whatever may be the season of the
year, the day of the month, or the hour of the night, you will always
see there a large constellation formed of seven fine stars, of which
four are in a quadrilateral, and three at an angle with one side; all
are arranged as we see in Fig. 2.
You have all seen it, have you not? It never sets. Night and day it
watches above the northern horizon, _turning slowly_ in twenty-four
hours round a star of which we shall speak directly. In the figure of
the Great Bear, the three stars of the extremity form the tail, and
the four in the quadrilateral lie in the body. In the Chariot, the
four stars of the quadrilateral form the wheels, and the other three
the pole, the horses, or the oxen. Above the second of these latter
stars, ζ, good sight distinguishes quite a little star named Alcor,
which is also called the Cavalier. It serves to test the power of the
sight. Each star is designated by a letter of the Greek alphabet: α
and β mark the first two stars of the quadrilateral, γ and δ the two
following, ε, ζ, η, the three of the pole. Arabic names have also
been given to these stars, which we will pass in silence, because
they are generally obsolete, with the exception, however, of that of
the second horse—Mizar. With reference to the Greek letters, many
persons think that it would be preferable to suppress them and to
replace them by numbers. But this would be impossible in the practice
of astronomy; and, moreover, inevitable confusion would result, on
account of the numbers which the stars bear in the catalogues.
The Latins gave to plowing oxen the name of _triones_; instead
of speaking of a chariot and three oxen, they came to call them
the seven oxen (_septemtriones_). From this is derived the word
septentrion, and there are now doubtless but few persons who, in
writing this word, know that they are speaking of seven oxen. It is
the same, however, with many other words. Who remembers, for example,
in using the word _tragedy_, that he speaks of a song of a goat:
_tragôs-ode_?
Let us go back to Fig. 2. If we draw a straight line through the two
stars marked α and β which form the right side of the square, and
produce it beyond α to a distance equal to five times that from β
to α, or to a distance equaling that from α to the end of the tail,
η, we find a star a little less brilliant than at the extremity of
a figure similar to the Great Bear, but smaller and pointing in
the opposite direction. This is the _Little Bear_, or the _Little
Chariot_, also formed of seven stars. The star to which our line
leads us—that which is at the tip of the tail of the Little Bear, or
at the end of the pole of the Little Chariot—is the _polar star_.
[Illustration: Fig. 3]
The polar star enjoys a certain fame, like all persons who are
distinguished from the common, because, among all the bodies which
scintillate in the starry night, it alone remains motionless in the
heavens. At any moment of the year, by day or by night, when you
observe the sky, you will always find it. All the other stars, on the
contrary, turn in twenty-four hours round it, taken as the centre of
this immense vortex. The pole star remains motionless at the pole of
the world, from whence it serves as a fixed point to navigators on
the trackless ocean, as well as to travelers in the unexplored desert.
[Illustration: Fig. 4]
In looking at the pole star, motionless in the midst of the northern
region of the sky, we have the south behind us, the east to the
right, the west to the left. All the stars turn round the pole star
in a direction contrary to that of the hands of a watch; they should,
then, be recognized according to their mutual relations rather than
by reference to the cardinal points.
On the other side of the pole star, with reference to the Great
Bear, is found another constellation which we can also recognize
at once. If from the middle star, δ, we draw a line to the pole,
and produce this line by the same distance (see Fig. 3), we arrive
at _Cassiopeia_, formed of five principal stars arranged somewhat
like the strokes of the letter M. The little star χ, which completes
the square, gives the constellation the form of a _chair_. This
group assumes all possible positions in turning round the pole; it
is found sometimes above, sometimes below, sometimes to the right,
and sometimes to the left; but it is always easily recognized, for,
like the preceding group, it never sets, and is always opposite to
the Great Bear. The pole star is the axle round which both these
constellations turn.
[Illustration: Fig. 5 Fig. 6]
If, now, we draw from the stars α and δ of the Great Bear two lines
through the pole, and produce them beyond Cassiopeia, we come to the
Square of Pegasus (see Fig. 4), which shows a line of three stars
somewhat similar to the tail of the Great Bear. These three stars
belong to _Andromeda_, and lead to another constellation, _Perseus_.
The last star of the Square of Pegasus is, as we see, the first
(α) of Andromeda; the three others are named γ, α, and β. To the
north of β of Andromeda is found, near a little star, ν, an oblong
nebula, which can be distinguished with the naked eye. In Perseus,
α, the brightest—on the prolongation of the three principal stars of
Andromeda—appears between two others less brilliant, which form with
it a concave arc very easy to distinguish. This arc serves us for a
new alignment. Producing it in the direction of δ, we find a very
brilliant star of the first magnitude; this is _Capella_ (the Goat).
Forming a right angle with this prolongation toward the south we come
to the _Pleiades_ (Fig. 5). Not far from that is a variable star,
_Algol_, or the _Head of Medusa_, which varies from the second to the
fourth magnitude[8] in 2 days, 20 hours, 48 minutes, 51 seconds. We
may add, that in this region the star γ of Andromeda is one of the
most beautiful double stars (it is even triple).
[Illustration: Fig. 7 Fig. 8]
If, now, we produce beyond the Square of Pegasus (Fig. 6) the curved
line of Andromeda, we reach the Milky Way, and we meet in these parts
Cygnus, like a cross; the Lyre, where Vega shines (Fig. 7); the
Eagle, and Altair (not Atair, as it is sometimes written) with two
companions (Fig. 8).
Such are the principal constellations visible in the circumpolar
regions on one side; we shall make a fuller acquaintance with them
directly. While we are tracing the lines of reference let us still
have a little patience and finish our summary review of this part of
the sky.
[Illustration: Fig. 9]
Look now at the side opposite to that of which we have just spoken.
Let us return to the Great Bear. Producing the tail along its curve,
we find at some distance from that a star of the first magnitude,
_Arcturus_ (Fig. 9), or α of Boötes. A little circle of stars which
we see to the left of Boötes constitutes the _Northern Crown_ (Corona
Borealis). In the month of May, 1866, there was seen shining there
a fine star, the brightness of which lasted only fifteen days. The
constellation of Boötes is traced in the form of a pentagon. The
stars which compose it are of the third magnitude, with the exception
of Arcturus, which is of the first. This is one of the nearest to the
earth; at least, it is one of a small number whose distance has been
measured. It shines with a beautiful golden yellow color. The star
ε, which we see above it, is _double_—that is to say, the telescope
resolves it into two distinct stars, one yellow, the other blue.
[Illustration: Fig. 10.]
This technical description is far from the poetry of Nature; but it
is especially important here to be clear and precise. Let us suppose
ourselves, however, under the starry vault on a beautiful summer’s
night, splendid and silent, and let us consider that each of these
points which we seek to recognize is a world, or rather a system of
worlds. Look at this equilateral triangle (Fig 10); it permits us
to cast our eyes successively on three important suns: Vega of the
Lyre, Arcturus of Boötes, and the pole star, which watches above the
solitudes of our mysterious North Pole. Many martyrs of science have
died looking at it! In twelve thousand years our descendants will
see the Lyre at the pole, ruling the harmony of the heavens.
The stars which are near the pole, and which have for that reason
received the name of circumpolar stars, are distributed in the
groups which have just been indicated. I earnestly invite my readers
to profit by fine evenings, and try to find for themselves these
constellations in the sky.
We have here the principal stars and constellations of the Northern
Hemisphere, the North Pole being at the centre of the circle. We come
now in the order of our description to the twelve constellations of
the zodiacal belt, which makes the circuit of the sky, inclined at
23° to the Equator, and of which the ecliptic, the apparent path of
the sun, forms the centre line.
The name of zodiac, given to the zone of stars which the sun
traverses during the course of the year, comes from ζώδια,
_animals_, an etymology which is due to the species of figures
traced on this belt of stars. Animals, in fact, predominate in these
figures. The entire circumference of the sky has been divided into
twelve parts, which have been named the twelve signs of the zodiac;
our ancestors called them the “houses of the sun,” or “the monthly
abodes of Apollo,” because the day star visits them each month, and
returns every spring to the beginning of the zodiacal city. Two
memorable Latin verses of the poet Ausonius present to us these
twelve signs in the order in which the sun travels through them, and
this still appears the easiest method of learning them by heart.
Sunt _Aries_, _Taurus_, _Gemini_, _Cancer_, _Leo_, _Virgo_,
_Libraque_, _Scorpius_, _Arciteneus_, _Caper_, _Amphora_, _Pisces_;
or, in English, the Ram ♈︎, the Bull ♉︎, the Twins ♊︎, the Crab
♋︎, the Lion ♌︎, the Virgin ♍︎, the Balance ♎︎, the Scorpion ♏︎,
the Archer ♐︎, Capricornus ♑︎, Aquarius ♒︎, and the Fishes ♓︎. The
signs placed beside these names are a vestige of the primitive
hieroglyphics which described them: ♈︎ represents the horns of the
Ram, ♉︎ the head of the Bull; ♒︎ is a stream of water, etc.
If we now know our northern sky, if its most important stars are
sufficiently noted down in our mind, with the reciprocal relations
which they preserve among themselves, we have no more confusion to
fear, and it will be easy to recognize the zodiacal constellations.
This zone may be of use to us as a line of division between the north
and the south. Here is a description of it:
The Ram, which, moving in front of the herd, and regulating, so to
say, the march, opens the series. This constellation has in itself
nothing remarkable; the brightness of its stars indicates the
base of one of the horns of the leader of the sheep; it is but of
the second magnitude. After the _Ram_ comes the Bull. Admire on a
fine winter’s night the charming Pleiades which scintillate in the
ether; not far from them shines a fine red star—this is the _eye_
of the Bull—Aldebaran, a star of the first magnitude and one of
the finest of our sky. We now arrive at the Twins, whose heads are
marked by two fine stars of the second magnitude, situated a little
above a star of the first magnitude—_Procyon_, or the Little Dog;
_Cancer_, or the Crab, a constellation very little conspicuous (its
most visible stars are but of the fourth magnitude, and occupy the
body of the animal); the _Lion_, a fine constellation, marked by a
star of the first magnitude, _Regulus_, by one of the second, β,
and by several others of the second and third magnitudes arranged
in a trapezium; the _Virgin_, indicated by a very brilliant star
of the first magnitude; _Spica_, situated in the neighborhood
of a star, also of the first magnitude, Arcturus, which is found
on the prolongation of the tail of the Great Bear; the _Balance_
(Libra), indicated by two stars of the second magnitude, which
would exactly resemble the Twins if they were nearer to each
other; the _Scorpion_, a remarkable constellation; a star of the
first magnitude, of a fine red color, marks the _Heart_ (Antares),
in the middle of two stars of the third magnitude, above which
are three bright stars arranged in a diadem; _Sagittarius_, the
Archer, of which the arrow, indicated by three stars of the
second and third magnitudes, is pointed toward the tail of the
Scorpion; _Capricornus_, a constellation not conspicuous, which
is recognized by two stars of the third magnitude very near each
other, and representing the base of the horns of the hieroglyphic
animal; _Aquarius_, indicated by three stars of the third magnitude
arranged in a triangle, of which the most northern occupies a
point on the equator; _Pisces_, the _Fishes_, composed of stars,
barely conspicuous, of the third to fourth magnitudes, situated to
the south of a large and magnificent quadrilateral—the Square of
Pegasus—of which we have already spoken.
We have now enumerated the zodiacal constellations in the order of
the direct motion (from west to east) of the sun, moon, and planets
which traverse them. They marked at the epoch of their formation,
the monthly passage of the sun into each of them. The distribution
of the stars in figurative groups was the first truly hieroglyphical
writing; it was engraved on the firmament in indelible characters.
The zodiac has played a great part in the ancient history of every
nation—in the formation of the calendar, in the appointment of
public festivals, and in the constitution of eras. The zodiac of
Denderah, discovered by the French _savants_ in Egypt at the end of
the Eighteenth Century, was at first believed to have an antiquity
of 15,000 years; but it is now proved that it is necessary to deduct
from that number of years half the cycle of precession—that is to
say, nearly 13,000 years—which brings down the date of this sculpture
to 2,000 years before our epoch; and this in fact corresponds with
the evidence of archæology. It is remarkable that all the ancient
zodiacs and calendars which have been preserved to us begin the year
with the constellation of the Bull, as we have already noticed. The
zodiac of the Elephanta Pagoda (Salsette) has at the head of the
procession the sign of the sacred Bull, the ox Apis, Mithra—of which
the promenade of the fat ox, which is still performed in the environs
of Paris, is a vestige. The ceiling of a sepulchral chamber at Thebes
shows the Bull at the head of the procession. The zodiac of Esne,
the astronomical picture discovered by Champollion in the Ramesseum
of Thebes, carries us back to the same origin, between two and three
thousand years before our era; Biot supposes the date of this to be
the year 3285, the vernal equinox passing through the Hyades on the
forehead of Taurus. Father Gaubil has proved that from ancient times
the Chinese have referred the beginning of the apparent motion of the
sun to the stars of Taurus; and we have a Chinese observation of the
star η of the Pleiades as marking the vernal equinox in the year 2357
before our era. Hesiod sings of the Pleiades as ruling the labors of
the year, and the name of Vergilia, which the ancient Romans gave
them, associates them with the beginning of the year in spring.
[Illustration: Fig. 11]
Without entering into any details of the different zodiacs which have
been preserved to us from the most ancient and diverse nations, a
glance at those which are reproduced here will lead us to appreciate
the part which they have played in ancient religions. Several
zodiacal signs have become veritable gods. The zodiac represented by
Fig. 11 was engraved, in the Thirteenth Century, on an Arabic magic
mirror, and dedicated to the sovereign prince Aboulfald, “Victorious
Sultan, Light of the World,” if we are to believe the bombastic
inscription which encircles it. Fig. 12 shows an ancient Hindoo
zodiac. Fig. 13 shows a Chinese zodiac stamped upon a talisman, even
now in use. The twelve signs differ from ours; they are: the Mouse,
the Cow, the Tiger, the Rabbit, the Dragon, the Serpent, the Horse,
the Ram, the Ape, the Hen, the Dog, and the Pig. Fig. 14 represents
a Chinese medal, on which we see the constellation _Teou_, the
Great Bear[9] (which they call the Bushel), the Serpent, the Sword,
and the Tortoise. This is a talisman intended to give courage; it
appears that it is in great demand among the Chinese, and is as well
circulated as the medals of the Immaculate Conception are in France.
[Illustration: Fig. 12.—Ancient Hindoo Zodiac]
Of all the zodiacal constellations, that of the Bull has played the
principal rôle in ancient myths; and in this constellation it was the
sparkling cluster of the Pleiades which appears to have regulated
the year and the calendar among all the ancient nations. The Mosaic
deluge itself, referred to 17 Athir (November), in commemoration of
an important inundation, had its date coincident with the appearance
of the Pleiades.[10]
[Illustration: Fig. 13.—Chinese Zodiac, from a Talisman
Fig. 14.—Chinese Medal, showing the Great Bear]
But we forget the stars. If our descriptions have been carefully
followed, the reader will now know the zodiacal constellations as
well as those of the north. There remains but little to do to know
the entire sky. But there is an indispensable addition to be made to
what precedes. The circumpolar stars are perpetually visible above
the London horizon; at any time of the year when we wish to observe
them it is sufficient to turn to the north, and we shall always
find them, either above the pole star or below it, to one side or
the other, and always maintaining among themselves the relations
which we have employed to find them. The stars of the zodiac do not
resemble them from this point of view, for they are sometimes above
the horizon, sometimes below. It is necessary, then, to know at what
epoch they are visible. For this purpose it will be sufficient to
remember the constellation which is found in the middle of the sky at
_nine o’clock in the evening_ on the first day of each month—that,
for example, which crosses at that moment a line descending from the
zenith to the south. This line is the _meridian_, of which we have
already spoken; all the stars cross it once a day, moving from east
to west—that is to say, from left to right. In indicating each of the
constellations which pass at the hour indicated, we also give the
centre of the visible constellations.
On January 1 Taurus passes the meridian at 9 o’clock in the
evening; notice Aldebaran, the Pleiades. On February 1 the Twins
(Gemini) are not yet there; we see them a little to the left. March
1, Castor and Pollux have passed; Procyon to the south, the little
stars of the Crab (Cancer) to the left. April 1, the Lion, Regulus.
May 1, β of the Lion, Berenice’s Hair. June 1, Spica of the Virgin,
Arcturus. July 1, the Balance (Libra), the Scorpion. August 1,
Antares, Ophiuchus. September 1, Sagittarius, Aquila. October 1,
Capricornus, Aquarius. November 1, Pisces, Pegasus. December 1,
Aries, the Ram.
Our general review of the starry sky must now be completed by the
stars of the southern heavens.
Below Taurus and Gemini, to the south of the zodiac, you notice the
giant Orion, who raises his club toward the forehead of the Bull.
Seven brilliant stars are here distinguished; two of them, α and
β, are of the first magnitude; the five others are of the second
magnitude, α and γ mark the shoulders, κ the right knee, β the left
knee; δ, ε, ζ mark the belt or girdle. Below this line is a luminous
train of three stars, very near each other; this is the Sword.
Between the western shoulder and Taurus is seen the Shield, composed
of a row of small stars. The head is marked by a little star (λ) of
the fourth magnitude.
On a fine winter’s night turn toward the south, and you will
immediately recognize this giant constellation. The four stars,
α, γ, β, κ, occupy the angles of a great quadrilateral. The three
others, δ, ε, ζ, are crowded in an oblique line in the middle of this
quadrilateral; α, at the northeast angle, is named _Betelgeuse_ (not
Beteigeuse, as some books print it); β, at the southeast angle, is
called _Rigel_.
The line of the Belt, produced both ways, passes to the northeast
near _Aldebaran_, the Eye of the Bull, which we know already, and to
the southeast near _Sirius_, the finest star of the sky, which we
shall soon consider.
It is during the fine nights of winter that this constellation shines
in the evening above our heads. No other season is so magnificently
constellated as the months of winter. While nature deprives us of
certain enjoyments in one way, it offers us in exchange others no
less precious. The marvels of the heavens present themselves from
Taurus and Orion in the east to Virgo and Boötes on the west. Of
eighteen stars of the first magnitude which are counted in the whole
extent of the firmament, a dozen are visible from nine o’clock to
midnight, not to mention some fine stars of the second magnitude,
remarkable nebulæ, and celestial objects well worthy of the attention
of mortals. It is thus that nature establishes a harmonious
compensation, and while it darkens our short and frosty days of
winter, it gives us long nights enriched with the most opulent
creations of the sky.
The constellation of Orion is not only the richest in brilliant
stars, but it conceals for the initiated treasures which no other is
known to afford. We might almost call it the California of the sky.
To the southeast of Orion, on the line of the Three Kings, shines
the most magnificent of all the stars, _Sirius_, or α of the
constellation of the Great Dog. This star of the first magnitude
marks the upper eastern angle of a great quadrilateral, of which the
base near the horizon of London, is adjacent to a triangle. This
constellation rises in the evening at the end of November, passes
the meridian at midnight at the end of January, and sets at the end
of March. It played the greatest part in Egyptian astronomy, for
it regulated the ancient calendar. It was the famous Dog Star; it
predicted the inundation of the Nile, the summer solstice, great
heats and fevers; but the precession of the equinoxes has in 3,000
years moved back the time of its appearance by a month and a half,
and now this fine star announces nothing, either to the Egyptians
who are dead or to their successors.
The _Little Dog_, or Procyon, is found above the Great Dog and
below the Twins (Castor and Pollux), to the east of Orion. With the
exception of α Procyon, no brilliant star distinguishes it.
_Hydra_ is a long constellation, which occupies a quarter of the
horizon, under Cancer, the Lion, and the Virgin. The head, formed of
four stars of the fourth magnitude, is to the left of Procyon, on
the prolongation of a line drawn from that star to Betelgeuse. The
western side of the great trapezium of the Lion, like the line from
Castor and Pollux, points to α, of the second magnitude. This is the
Heart of Hydra; we remark the asterisms of the second class, Corvus
the Crow, and Crater the Cup.
_Eridanus_, _Cetus_, _Piscis Australis_, and the _Centaur_ are the
only important constellations which remain to be described. We
find them, in the order which we have indicated, to the right of
Orion. Eridanus is a river composed of a train of stars winding
from the left foot of Orion and losing itself below the horizon.
After following long windings, it ends with a fine star of the first
magnitude, α Eridani, or Achernar. This is the river into which
Phaeton fell when he unskilfully directed the Chariot of the Sun. It
was placed in the sky to console Apollo for the death of his son.
To find the Whale (Cetus), we may notice below the Ram a star of the
second magnitude which forms an equilateral triangle with the Ram and
the Pleiades; this is α of Cetus, or the Jaw; α, μ, ξ, and γ form
a parallelogram which represents the head. The base, α, γ, may be
produced to a star of the third magnitude, δ, and to a star of the
neck marked ο. This star is one of the most curious in the heavens.
It is named the Wonderful, _Mira Ceti_. It belongs to the class
of variable stars. Sometimes it equals in brightness stars of the
second magnitude, sometimes it becomes completely invisible.[11] Its
variations have been followed since the end of the Sixteenth Century,
and it has been found that they are reproduced periodically every 331
days on the average. The study of these singular stars presents us
with curious phenomena.
Lastly, the constellation of the Centaur is situated below Spica
of the Virgin. The star θ, of the second magnitude, and the star
ι, of the third, mark the head and the shoulder. This is the only
part of this figure which rises above our horizon. The Centaur
contains the _nearest star_ to us (α) of the first magnitude, the
distance of which is about twenty-five billions of miles. The feet
of the Centaur touch the _Southern Cross_, formed of four stars of
the second magnitude, always hidden below our horizon. It reigns in
silence above the icy solitudes of the Southern Pole, where ships
proceed only with difficulty. Further on, at the centre of the other
hemisphere, is the southern celestial pole, which is not marked by
any remarkable star.
It was from this region, Dante relates, that, having visited hell,
inclosed in the centre of the earth, he went to the Mountain of
Purgatory, and from there to the Heights of Paradise. These beautiful
dreams have disappeared in the sunshine of modern astronomy.
We will complete these descriptions by a little astronomical
chronology, which is not without interest. From a careful examination
of the most ancient historical sources of classical astronomy, the
following is the order in which the constellations appear to have
been noticed, formed, and named, beginning with the most ancient:
Most Ancient Reference
The Great Bear _Job_ (ch. xxxviii. ver. 32)
(Seventeenth Century before our
era), _Homer_ (Ninth Century).
Orion _Job_ (ch. ix. ver. 9), _Homer_,
_Hesiod_.
The Pleiades (the Hyades) _Job_ (ch. xxxviii. ver. 31),
_Homer_, _Hesiod_.
Sirius and the Great Dog _Hesiod_ mentions it. _Homer_
calls Sirius the Star of Autumn.
Aldebaran (Taurus) _Homer_, _Hesiod_.
Boötes, Arcturus _Job_ (ch. xxxviii. ver. 32),
_Homer_, _Hesiod_.
The Little Bear _Thales_ (Seventh Century),
_Eudoxus_, _Aratus_.
Draco (the Dragon) _Eudoxus_ (Fourth Century),
_Aratus_ (Third Century).
The Man on his Knees, or Hercules _Id._
The Branch and Cerberus[12] _Id._
Corona Borealis _Id._
Ophiuchus or Serpentarius _Id._
The Scorpion _Id._
Virgo and Spica _Eudoxus_ (Fourth Century),
_Aratus_ (Third Century)
Gemini (the Twins) _Id._
Procyon _Id._
Cancer (the Crab) _Id._
Leo (the Lion) _Id._
Auriga (the Charioteer) _Id._
Capella (the Goat, the Kids) _Id._
Cepheus _Id._
Cassiopeia _Id._
Andromeda _Id._
Pegasus (the Horse) _Id._
Aries (the Ram) _Id._
The Triangle _Id._
Pisces (the Fishes) _Id._
Perseus _Id._
Lyra _Id._
The Bird, or Cygnus (the Swan) _Id._
Aquila (the Eagle) _Id._
Aquarius _Id._
Capricornus _Id._
Sagittarius _Id._
Sagitta (the Arrow) _Id._
Delphinus (the Dolphin) _Id._
Lepus (the Hare) _Id._
Argo (the Ship) _Id._
Canobus (afterward written Canopus) _Id._
Eridanus _Id._
Cetus (the Whale) _Id._
Piscis Australis (the Southern Fish) _Id._
Corona Australis _Id._
The Altar _Id._
The Centaur _Id._
The Wolf (Lupus) _Id._
Hydra _Id._
Crater (the Cup) _Id._
Corvus (the Crow) _Id._
Libra (the Balance) _Manetho_ (Third Century B. C.)
_Geminus_ (First Century) B. C.).
The Hair of Berenice[13] _Callimachus_, _Eratosthenes_
(Third Century).
Feet of the Centaur _Hipparchus_ (First Century B. C.).
Propus (η of Gemini) _Hipparchus._
The Manger and Donkeys _Id._
The Little Horse (Equuleus) _Id._
The Head of Medusa _Id._
Antinous[13] Under the Emperor Adrian
(130 A. D.).
The Peacock (Pavo) _John Bayer_, 1603.
Toucan _Id._
Grus (the Crane) _Id._
Phœnix _Id._
Doradus _Id._
The Flying Fish _Id._
Hydrus _Id._
Chamæleon _Id._
The Bee (Musca) _Id._
The Bird of Paradise (Apus) _Id._
Triangulum Australis _Id._
The Indian (Indus) _Id._
The Giraffe (Camelopardus) _Bartschius_, 1624.
The Fly (Musca) _Id._
The Unicorn (Monoceros) _Id._
Noah’s Dove (Columba) _Id._
The Oak of Charles II _Halley_, 1679.
The Southern Cross (already
seen by the ancients) _Augustine Royer_, 1677.
The Great and Little Cloud
(Magellanic Clouds) _Hevelius_, 1690.
The Fleur de Lys _Id._
The Greyhounds (Canes Venatici) _Id._
The Fox and Goose (Vulpecula
et Anser) _Id._
The Lizard (Lacerta) _Id._
The Sextant of Urania (Sextans) _Id._
The Little Lion (Leo Minor) _Hevelius_, 1690.
The Lynx _Id._
The Shield of Sobieski _Id._
The Little Triangle _Id._
Mount Mænalus _Flamsteed_, 1725.
The Heart of Charles II (α Canum
Venaticorum) _Id._
The Sculptor’s Workshop (Sculptor) _Lacaille_, 1752.
The Chemical Furnace (Fornax) _Id._
The Clock (Horologium) _Id._
The Rhomboid Reticule (Reticulum) _Id._
The Engraver’s Pen _Id._
The Painter’s Easel (Pictor) _Id._
The Compass (Circinus) _Id._
The Air Pump (Antlia) _Id._
The Octant (Octans) _Id._
The Compass and Square _Id._
The Telescope (Telescopium) _Id._
The Microscope (Microscopium) _Id._
The Table Mountain (Mensa) _Id._
The Reindeer _Lemonnier_, 1774.
The Solitaire (Indian Bird) _Id._
Le Messier _Lalande_, 1776.
The Bull of Poniatowski _Poczobut_, 1877.
The Honors of Frederick _Bode_, 1786.
The Harp of the Georges _Hell_, 1789.
The Telescope of Herschel _Bode_, 1787.
The Electrical Machine _Id_, 1790.
The Printer’s Workshop _Id._
The Mural Quadrant _Lalande_, 1795.
The Air Balloon _Id._, 1798.
The Cat _Id._, 1799.
Such are the constellations, ancient and modern, venerable or recent,
into which the celestial sphere has been divided. The ancient names
are respectable and respected, on account of their relations,
known or unknown, with the origins of history and religion; the new
ones must be ephemeral. It is useful to know them, because several
stars celebrated under different titles have for their principal
designation their position in these asterisms; but what we should
wish would be to see them disappear.[14]
Many other substitutions have, however, been attempted. I have in my
library a splendid folio of the year 1661, containing twenty-nine
engraved plates, illuminated in gold and silver, among which are
two which represent the sky delivered from the pagans and peopled
with Christians. Instead of divinities more or less virtuous, in
place of animals of forms more or less fantastic, we behold the
elect—apostles, saints, popes, martyrs, sacred persons of the
Old and New Testament—seated in the celestial vault, clothed in
rich costumes of all colors, embroidered with gold, and carefully
installed in the place of all the pagan heroes who for so many ages
reigned in the sky.
The author of this metamorphosis was named Jules Schiller, and it
was in the year 1627 that he introduced it, coupling his name with
that of John Bayer. He began his dissertation by showing how the
pagan constellations are opposed to Christian opinion and even to
common-sense. He quoted the Fathers of the Church who expressly
disapprove of them: Isodorus, who treats them as diabolical;
Lactantius, who condemns the corruption of the human race; Augustine,
who sends their heroes to hell, etc.
These constellations formed by chance, in the course of ages,
without a fixed object; their inconvenient size, the uncertainty of
their boundaries; the complicated designations, for which it was
sometimes necessary to exhaust whole alphabets; the bad taste with
which observers have introduced into the southern sky the frigid
nomenclature of instruments used in science alongside mythological
allegories—all these accumulated defects have often suggested plans
of reform for the stellar divisions, and even the banishing of all
configuration. But ancient customs are difficult to overcome, and it
is very probable that, except the recently named groups, which we may
now suppress, the venerable constellations will always reign.
Such are the provinces of the sky. But these provinces are of no
intrinsic value; the important point for us is to make acquaintance
with the inhabitants.
FOOTNOTES:
[6] The French trillion is equivalent to the English billion, or a
million times a million (1,000,000,000,000).—J. E. G.
[7] The Chinese had designated them all, it is true, at the same
epoch, but their groups as well as their denominations are absolutely
different from ours, and do not appear to have exercised any
influence on the foundations of astronomical history. It was another
world, other methods, other inspirations, as if Asia and Europe
formed two distinct planets. A distinguished author, M. Schlegel,
published in 1875 a Chinese Uranography, which is composed of 670
asterisms, and of which he believes he can trace back the origin to
17,000 years before our era. His argument is not convincing, and
it seems to me that the origin of the astronomy of the Celestial
Empire can not be very much anterior to the reign of the Emperor
Hoang-Ti—that is to say, to the Twenty-seventh Century before our
era—and would go back at furthest to the time of Fou-Hi that is to
say, to the Twenty-ninth Century. It was about the same epoch—the
Twenty-eighth Century before our era—that the Egyptians, observing
Sirius, the early rising of which announced the inundation of the
Nile, formed their canicular year of 365 days.
[8] More correctly, from 2.3 magnitude to 3.5 magnitude.—J. E. G.
[9] The author possesses in the Museum of the Observatory at
Juvisy a Japanese executioner’s sword, on the guard of which this
constellation is engraved. Was it believed that the souls of executed
criminals were sent there?
[10] See _Astronomical Myths, based on Flammarion’s History of the
Heavens_. By J. F. Blake. London, 1876.
[11] That is, to the naked eye; it never descends below the tenth
magnitude, and always remains visible in a 3-inch telescope.—J. E. G.
[12] A constellation wrongly attributed by Arago and others to
Hevelius. It is found on the sphere of Eudoxus.
[13] Constellations incorrectly attributed to Tycho Brahe. The first
is given by Eratosthenes, the second dates from the Emperor Adrian.
[14] Especially those which are absolutely superfluous, and occupy
places stolen from the ancient constellations, like the Heart of
Charles II, the Fox and Goose, the Lizard, the Sextant, the Shield
of Sobieski, Mount Mænalus, the Reindeer, the Solitaire, the
Messier, the Bull of Poniatowski, the Honors of Frederick, the Harp,
the Telescope, the Mural Circle, the Air Balloon, the Electrical
Machine, the Printer’s Workshop, and the Cat. I know, however, with
reference to this last animal, that Lalande wrote: “I love cats! I
adore cats! I may be pardoned for having placed one in the sky after
my sixty years of assiduous labors.” But the illustrious astronomer
had no necessity for this plea in order that his name should remain
inscribed in letters of gold on the tablets of Urania. The Heart of
Charles II is but the flattery of a courtier; the Shield of Sobieski,
the Bull of Poniatowski, should fall from the sky; the Messier is but
a play on words which makes the celestial flocks guarded by a pastor
whose name is the same as that of the prolific hunter of comets,
Messier. As for the Honors of Frederick, they usurp an unmerited
place, for, in order to make room for them, Andromeda has been
obliged _to draw in her arm, which she had stretched out there for
three thousand years_.
THE ARABIAN HEAVENS.—LUDWIG IDELER
The majority of Arabic star-names mentioned by Kazwini owe their
origin to the astronomy of the Greeks. For instance, to the latter
belong El-dschediain, the two Kids (Hædi); El-ma’lef, the Manger;
El-hhimârain, the two Asses; Kalb el-ased, the Lion’s Heart;
El-sumbela, the Ears; El-zubênâ, the two Claws. Others indicate the
positions of the stars in the Greek constellations as Râs el-tinnîn,
Dragon’s Head; Râs el-hhauwâ, Head of the Snake Man; Râs el-dschêthi,
Head of the Kneeling (Hercules); Dseneb el-dedschâdsche, the Hen’s
Tail (Swan’s); Dseneb el-dschedi, Goat’s Tail (Wild-goat); Dseneb
Kaitos, Whale’s Tail; Fom el-hhût, Jaw of the (southern) Fish;
Ridschl el-dschebbâr, Giant’s Foot (Orion), etc. Still others,
such as Khebd el-ased, Dafîra el-ased, El-dsirâ el-mebsûta, and
el-mekbûda, El-nethra, El-dschebha, El-zubra, Sâk el-ased, Adschaz
el-ased, refer to the Arabic Lion, which is a caricature of the Greek
one.
Now if we separate these and many similar expressions from the
astronomical nomenclature of the Arabs, there remains a class of
star-names that present sufficient internal evidence to show plainly
that they are indigenous to Arabia. It is worth while taking the
trouble to collect and compare them. We shall in this way obtain a
clearer idea of the sky that was altogether peculiar to this people.
In the first place, a large number of names of animals attracts our
notice. In the vicinity of the North Pole, a shepherd (El-râï, Gamma
in Cepheus), accompanied by his dog (Khelb el-râï, Zeta in Cepheus),
is pasturing a herd of sheep (El-firk and El-agnâm, Alpha, Beta, Eta,
and smaller stars in Cepheus), to which group also seem to belong
two calves (El-ferkadain, Beta and Gamma in the Little Bear), a
she-goat (El-anâk, Zeta in the Great Bear), a he-goat (El-tais in the
Dragon), a young he-goat (El-dschedi, Alpha in the Little Bear), four
mother-camels, a camel-foal, and a single camel pasturing by itself
(El-awaîd, El-raba, and El-râfid, collectively on the head of the
Dragon).
Various predatory animals are slinking around this herd, two Jackals
(El-dsîbain, Zeta and Eta in the Dragon), which are specially
stalking the camel-foal; a male-hyena (El-dsîch, Iota in the Dragon)
and many other she-hyenas (El-dibâ, Beta, Gamma, Delta and Mu in
Boötes), and other she-hyenas with their young (Aulâd el-dibâ, Theta,
Iota, Kappa, Lambda, and others in the same figure).
In the neighborhood of the two jackals (two stars in the Dragon) bear
the name of their claws (Adhfâr el-dsîb).
Another shepherd (El-râï, Alpha in Ophiuchus) pastures his sheep
(El-agnâm, small stars in the region of Hercules’s Club) on a mead
(El-rauda), which is defended on the side of the above-mentioned
hyenas by two hurdles (Nasak schâmi and Nasak jemêni, rows of stars
in Hercules and in the upper part of the Snake), and is open in the
direction of the shepherd’s two dogs (Khelb el-râï, Alpha in Hercules
and Beta in Ophiuchus).
A third shepherd and a third herd are to be found further to the
south in the Milky Way. The latter was represented as a river in
which four animals (camels or sheep) are drinking, while four others
(El-naâîm el-sâdira, Zeta, Sigma, Tau, and Phi, in the Archer), are
going away from it after having quenched their thirst. Lambda in the
Archer was regarded as their shepherd (Râï el-naâïm).
Yet another shepherd was signified by the star Beta in Orion (Rigel).
He was called Râï el-dschauzâ, the shepherd in the Dschauzâ, or
Nut-region, _i. e._, in the region of Orion, which is splendid with
many conspicuous stars. The herd which he was given to pasture are
probably the “Thirst-quenched Camels” (El-nihâl), which were regarded
as being the stars Alpha, Beta, Gamma, and Delta, in the Hare in the
vicinity of the Milky Way.
Besides these groups of animals, there are several others scattered
over the heavens. The three pairs of stars standing close together
at the feet of the Great Bear were likened to the footmarks of a
gazelle. They were called the Gazelle’s Springs, or Hoofs (Kafzât
el-dhibâ or Dhufra el-gizlân). Naturally the animal itself was
regarded as being in the neighborhood of its tracks. On the one hand,
Omicron, Pi, Rho, Sigma, A and d, on the head of the Great Bear,
and on the other, as it appears, the stars of the Little Lion were
included under the name Gazelle (El-dhibâ). The latter group also
appears under the names the Gazelles and their Young (El-dhibâ w’
aulâdhâ).
The five stars of the Virgin, Beta, Eta, Gamma, Delta, and Epsilon,
were looked upon as so many yelping dogs (El-auwâ); Alpha and Beta
in the Archer as a pair of birds peculiar to Arabia (El-suradain);
Alpha (Fomalhaut) in the Southern Fish and Beta (Diphda) in the Whale
as two Frogs (El-difda el-awel and El-difda el-thâni); four stars
in the Great Dog and the Dove and as many Monkeys (El-kurûd), and
the two bright stars of the latter constellation as a pair of Ravens
(El-ag’riba).
All the creatures so far mentioned are familiar to the Arabs, the
camel most of all. Just as their language is rich in words which
refer to this useful animal, so also it plays the chief rôle in their
astronomical nomenclature. We have already met with some camel-groups
in the Arabian heavens. We find two more in the Bull and in the Crow.
The brightest star in the Hyades has the name of “the Large Camel”
(El-fenîk or El-fetîk), the others are called “the Small Camels”
(El-kilâs or El-kalâjis). The four principal stars of the Crow were
regarded as so many male-camels (El-adschmâl), analogous to the
above-mentioned four female-camels in a similar figure at the head of
the Dragon.
Just as frequently do we come across the ostrich in the Arabian
heavens. The Southern Crown bears the name of the Ostrich Nest (Udha
el-naâm), to which two pairs of ostriches (El-dhalîmain, Lambda
and Mu in the Archer) appear to belong. A second ostrich-nest was
formed from a number of stars in the upper part of Eridanus. In the
neighborhood are five hen-ostriches (El-naâmât, Zeta, Eta, Theta,
Tau, and Upsilon) in the belly of the Whale, and somewhat further
away are two male birds (El-dhalîm, Alpha in the Southern Fish and
Alpha in the River). The latter have a number of young ostriches
(El-rijâl stars in the Phœnix) between them. Ostrich eggs (El-baid),
or their shells (El-kaid), are represented by small stars in the
vicinity of the nest.
Besides the groups, we also find various isolated animals in
the starry heavens of the Arabs. Among these is the Black Horse
(El-dschaun, Epsilon in the Great Bear), perhaps belonging to the
neighboring Governor (El-kâïd, Eta, in the same constellation); the
beast of prey (Anâk el-ard, Gamma in Andromeda); the Male Camel
(El-fahl), which was represented by Canopus and the Dog running in
front of Sirius (El-khelb, Beta in Canis Major). This nomenclature,
borrowed from the animal kingdom, to which must be added the Maidens
(El-adsâra Omicron, Eta, Delta, and Epsilon in Canis Major); the
Outrider and the Man-riding-behind (El-fawâris and El-ridf, Delta,
Gamma, Epsilon, Zeta, and Alpha in the Swan); this nomenclature, I
say, is peculiar in that only one star was always used to distinguish
one animal.
The Arabs with so lively an imagination saw in the skies sheep,
camels, ostriches, but without being led to it by the resemblance
of the contour of the entire star group, as was the case of the
designers of the Greek heavens. They therefore had no animal
figures proper, but only animal names, such as the She-goat, the two
He-goats, and the two Asses of the Greeks. On two occasions, however,
it happened that more than one star was given to one animal. When the
eight stars of the Archer, which were represented under the figure of
only four animals at pasture going to and returning from drinking,
were regarded by some as two ostriches, this does not seem to be an
exception to the rule, but a misunderstanding instead, caused by the
resemblance of two words (Naâïm and Naâm). The case is probably the
same with the four stars, Delta, Pi, Rho, and Epsilon, in the Dragon
which are called the He-goat by a very late Arab astronomer; for a
star-name given by the lexicographer, Firuzabadi, would argue that
analogy held true here also.
The two unmistakable cases to which I refer are those of the falling
and flying Eagle (El-nesr el-wâki and El-nesr el-tâïr), the former
of which was made up of three stars in the form of an equilateral
triangle, and the latter of three standing in a straight line (Alpha,
Epsilon, and Zeta of the Lyre, Alpha, Beta and Gamma of the Greek
Eagle).
We need not take into consideration in this connection either the
Arabic Lion or the complete Horse, since most probably both owe their
origin to false interpretations of later grammarians.
It is quite different with a second class of Arabic star-names
which signify inanimate objects. These have to do with real forms
throughout, which, however, for the most part consist of only a few
stars after the manner of the Greek Arrow and Triangle. To these
belong El-chibâ, the tent of the Arab nomads resting on three or
four supports. One of these was represented by three stars of the
Charioteer (Lambda, Mu, and Sigma), and another by the four chief
stars of the Crow.
El-athâfi, the three stones which the nomadic Arab placed under his
pot or kettle in the form of an equilateral triangle to form the
hearth. Every triad of stars standing in a similar figure might be
called an Athâfi; for instance, Delta, Epsilon, and Rho in the Ram,
and the three on the head of Orion, which were actually likened to
one of these. In just so many words, however, the only stars that
occur under this name are Alpha, Epsilon, and Zeta in the Lyre, and
Sigma, Tau, and Upsilon in the Dragon.
El-kidr, the Pot, a ring of stars in the vicinity of the last Athâfi,
which was formed from a number of small stars of Cepheus and the Swan.
El-midschdah, the wooden twirling-stick (spit). A kitchen utensil of
similar triangular form was represented by the Hyades. The name in
course of time came to be restricted to the chief star of this group.
El-fekka, the sounding plate with the broken rim, or Kas’a
el-masâkhîm, the Beggar’s-dish. This name was given to the stars of
the Northern Crown, which stand in a circle open toward the northeast.
El-mîzân, the Scale-beam, an appropriate name for three stars in a
straight line. The ancient Arabs used it for Theta, Eta, and Delta in
the Eagle; the modern ones use it to distinguish the three stars on
the Belt and the three on the Sword of Orion, the former of which, on
account of their equal distance from each other, are called the true
scale-beam, and the latter the false one, on account of the unequal
intervals.
El-dsirâ, the Ell, a term which may fitly be applied to every pair of
conspicuous stars standing a certain distance from one another. It
was used for the two pairs of stars on the head of the Twins and in
the Little Dog.
El-ma’lef, the Manger—the name of the stars of the Cup which stand in
a circular form. The more familiar Manger in the Crab belongs to the
Greek Heavens.
El-kubba, the Traveling-tent, drawn by camels of the Arab’s female
apartment. This name was given by some to the stars of the Southern
Crown, while others, as has already been remarked, regard it as an
Ostrich Nest.
El-zaurak, the Boat, was represented by the chief stars of the
Phœnix. El-delv, the Well-Bucket, represented by the Square of
Pegasus, occurred more frequently than any other, as is shown by
the star-names relating to it—El-ferg, El-arkûwa, El-khereb, and
Elnaâïm. Elna’sch, the Bier, was applied to the well-known quadrangle
in the Great and Little Bear. The term particularly signifies
the death-bier, and taken in this sense each of the two biers is
accompanied by three mourning women—Benât—biers and mourners combined
are called Benât na’sch, literally Daughters of the Bier, _i. e._,
belonging to the Bier.
El-salîb, the Cross: one of these was referred to under the four
stars on the head of the Dragon, which others regarded as four mother
camels. A second was found in the stars of the Dolphin.
El-serîr, El-khursi, El-arsch, various kinds of Thrones. One, named
Serîr benât na’sch, was represented by seven stars standing in the
form of a bow on the head of the Great Bear, which were also called
El-hhûd, the Pond. Two other thrones under the names Khursi, or Arsch
el-dschauzâ, were distinguished under four stars of Eridanus, and
four in the Hare, and yet another, named Arsch el-simâkh el-a’zal, in
the stars of the Crow.
El-nidâm and El-nedm, every set of things arranged in a row,
especially the Pearl Necklace, which was the name given to the four
stars 1, 2, 3, 4, and Phi of the Whale standing in a straight line,
and the three on Orion’s Belt. Synonymous with this, among words
taken in their common acceptation is El-nasak, a name used for two
rows of stars in the upper part of the Snake and Hercules, which also
has a picturesqueness about it, since the two rows were regarded
as hurdles around the meadow on which the above-mentioned shepherd
pastures his flock.
El-fikrat, El-fekâr, and El-kelâda, the Brooch: the first of these
appears as the name of the stars on the vertebra of the Scorpion’s
tail; the second, for Orion’s Belt; the third, for stars of the
Archer. El-dschauzâ, the Nuts, and El-lekat, the Golden-grains or
Spangles. The former name was used for the stars of Orion and the
neighboring Twins collectively, the latter merely for those on
Orion’s Sword. Finally, to this class belongs El-khaf el-chadîb and
El-khaf el-dschedsmâ, the Dyed and the Mutilated Hand, which figures
were represented by the five chief stars of Cassiopeia and the five
better known on the head of the Whale. Several of these figures, as
we have seen, appear at more than one place in the sky. Hence arose,
for astronomers at least, the necessity for distinguishing epithets.
Thus the Cross on the head of the Dragon was called “the falling,”
the Tent in the Crow “the southern,” one of the Biers “the smaller,”
the other, “the greater”; one of the thrones in the vicinity of
Orion, “the front”; the other, “the back.”
When these distinctions are wanting, as in the case of the Athâfis,
it is probably because the astronomers only made use of the one in
the Dragon. Ulug Bekh does not name the other in the Lyre; Kazwini
also states that it only occurred in the speech of the common people.
There is still a third and very numerous class of genuine Arabic
star-names, which, borrowed neither from animate nor inanimate
objects, are consequently names that do not represent any figures.
They owe their origin to many circumstances, the majority of which
are lost to us. I will content myself with mentioning only a few of
them whose origin is not shrouded in doubt.
The small star over the middle of the Great Bear’s tail is called
El-suhâ, the Forgotten, the Lost, because it is only noticeable to
a sharp eye; also El-saidak, the Touchstone (test-stone), because
by it the eyesight was tested; Arcturus, Hâris el-semâ, the Warder
of the Heavens, because it is never entirely lost in the rays of
the sun; Capella, Rakîb el-thorejâ, the Watchman of the Pleiades,
because it rises at the same time as they do; Alpha (Aldebaran),
in the Bull, Hhâdi el-nedschm, the Driver of the Seven Stars; also
El-tâbi and El-debarân, the Follower, because it rises immediately
after that constellation; Beta (Denebola) in the Lion, El-serfa,
the Breaker-up (Upsetter), because at its rising and setting in the
morning twilight the hot and cold weather change; Alpha (Ras Alhague)
in the Watersnake; El-ferd, the Isolated, because it is situated in a
starless region, etc. Besides this, among this class we must include
the Su’ûd, or fortunate stars, four of which are in Pegasus, two in
the Wild Goat, and four in the Waterman.
It will already have been noticed that in this nomenclature single
stars frequently appear under several names. Thus the stars of the
Crow are sometimes called El-adschmâl, the Camels; sometimes El-chibâ
el-jemêni, the Southern Tent; sometimes Arsch el-simâkh el-a’zal,
the figure of the throne in the neighborhood of Spica—three quite
different names which express so many various notions and have also
so many separate authors.
Who were the originators of this nomenclature as a whole?
The Arabs, and particularly the nomad Arabs. To prove this we have
only to cast a glance at the names in the first two classes.
The inhabitants of the northern part of the Arabian peninsula, the
so-called “desert” and “stony” Arabia, for the most part, lead a
nomadic kind of life.
The country is a treeless and waterless plain covered with naked
rocks and sand-drifted hills, on which lie scattered single oases
watered by springs and glorified with a luxuriant vegetation. On
these the Arabs camp with their herds, and do not leave them until
the provender is consumed, or until more powerful tribes force them
to depart. They call themselves Bedâvi (Bedouins), that is, Scenitæ,
Nomads, as they were called by the Greeks. These nomads, cut off
from all intercourse with the world around them, who have never
been subjugated by a foreign power, have preserved their character
and their customs unchanged for several thousand years. Their most
important occupation is cattle-breeding. Besides this, they follow
the chase, or war upon their enemies, regarding as such all those
not belonging to their race or who are not under their protection.
They dwell in tents. Several families are under a Schech and several
Schechs generally under an Emîr, who rules over the whole tribe.
The majority of these nomadic Arabs were Sabians, or Star-worshipers,
before the adoption of Islam. History has preserved for us the
names of several tribes who paid divine honors to single planets,
or conspicuous fixed stars. No wonder that they should have fallen
into such idolatry! The dust raised by the desert wind, which, as a
rule, only blows during the day, and the heat of the sun compel them
to pasture their herds and to undertake their hostile expeditions
during the night. Leisure and necessity bid them gain information
by directing their gaze at the starry sky, which is presented to
them in a splendor of which we in our northern regions can scarcely
form any idea. Since, therefore, the aborigines must have noticed
at an early period that the nearly regular succession of changes in
their climate took place in conformity with the annually recurring
phenomena of the fixed stars, they ascribed to the latter a divine
power. Thus originated the worship of the stars; and this once
established, no other motives were needed to induce them to devote
their constant attention to the starry skies. One result of this was
that they applied proper names to the most conspicuous stars and
groups of stars which were borrowed partly from the animal world
around them, partly from their simple household effects, partly from
various qualities and circumstances which they noticed in the stars.
One tribe selected one name; another, another; and so it came to pass
that one star, or group of stars, frequently bears more than one
name. When, on the other hand, stars no less bright bear no names at
all, the probable reason is that only fragments of the astronomical
nomenclature of the Arab nomads have come down to us.
After this terminology had been transmitted by oral tradition,
and especially by folk-songs, for hundreds, perhaps thousands, of
years in its original condition, it was combined into an entirely
heterogeneous mass—that variegated mixture which we find in the works
of Kazwini, Ulug Bekh, and others.
When the Arabs in their fanatic zeal for the spread of Mohammed’s
doctrines had conquered a great part of Asia, Africa, and Europe,
and established in the heart of the ancient world a mighty empire,
they adopted from the Greeks, with whom they had now come in
contact, their astronomy among other sciences, and with it the Greek
constellations and their method of distinguishing the stars according
to their position in the figures.[15]
Their astronomers now generally discriminated between the two classes
of names in attributing the one to the Arabs, the other to the
astronomers.
Abdelrahman Sufi, in the preface to his work on the constellations,
says there are two kinds of heavens to become acquainted with—that of
the astronomers and that of the Arabs. In the work itself he first
describes the constellations used by the astronomers, _i. e._, the
Greek ones, and then the old constellations of the Arabs. Kazwini in
every case mentions a genuine Arabic star-name when he speaks of the
Arabic, which is the case with almost every constellation.
Our early astronomers had very false notions of this relation of
the nomadic heavens of the ancient Arabs to the mythological one
of the Greeks adopted by their descendants. Schickard, in his
_Astroscopium_, says: “Instead of the Dragon the Arabs depict two
wolves and five dromedaries.” He means the two jackals and the family
of camels which the nomads represented under the five stars on the
head of the Dragon. The Arab astronomers drew the Greek dragon on
their charts and globes just as we do. They only looked on the old
jackals and camels as _names_ for some of its stars. In Golius and
Hyde we find a more correct view of the case.
FOOTNOTES:
[15] Already in the ancient book of _Job_, whose hero has quite the
characteristics of a Nomadic Emir, we find some astronomical terms
whose analogy with the true Arabic star-names is unmistakable. See
_Job_, ix. 9; xxxviii. 31, 32.
ASTRONOMY WITHOUT A TELESCOPE.—J. E. GORE
It must be remembered that astronomy was studied ages before the
invention of the telescope, and that the ancient astronomers gained,
without any optical assistance, a considerable amount of knowledge
respecting the heavenly bodies.
Let us first consider the stars visible to the naked eye. The number
of these down to the sixth magnitude—about the faintest that average
eyesight can see—is, for both hemispheres, about 6,000. The number,
therefore, visible at _one_ time from any given place is about
3,000. Possibly double this number might be seen by those gifted
with exceptionally keen eyesight; but even this is a comparatively
small number, scattered as it is over so large an area. Those who do
not possess the power of effective enumeration estimate the number
visible to the naked eye as considerably greater than is really
the case. This is partly due to the irregular distribution of the
lucid stars over the celestial vault, and partly to the effect which
the aspect of the starry sky produces on the imagination; the fact
of the stars increasing in number as they diminish in brightness
inducing us to suspect the presence of points of light which we do
not actually see. An attempt to count those visible with _certainty_
in any selected portion of the sky will, however, convince any
intelligent person that the number, far from being large, is really
very small, and that the idea, which some entertain, of a countless
multitude is merely an optical illusion, and a popular fallacy
which has no foundation in fact. Of course, the number visible in
telescopes is very considerable. Perhaps with the largest telescopes
100,000,000 could be seen; but even this large number is very far
from being “countless.” The present population of the earth is about
1,400,000,000, or about fourteen times the number of the _visible_
stars!
The first thing to be done in studying the heavens with the naked
eye is to learn the positions and names of the brighter stars; and
from these the fainter ones may easily be identified by means of
a star atlas. Those who study the stars in this way have probably
a more intimate knowledge of the starry heavens than professional
astronomers, who generally find the stars—at least the fainter
ones—by referring to a catalogue of stars, and then setting their
telescope to the place indicated by the figures given in the
catalogue. Although the famous astronomer Sir William Herschel
possessed several large telescopes, he also studied the stars with
the naked eye, and it is related of this great observer that he could
without hesitation identify any star he could see in this way by its
name, letter, or number! Such an exhaustive knowledge of the heavens
is, of course, very rare; but an acquaintance with all the brighter
stars can easily be acquired by any person of ordinary intelligence.
The “Plow,” or Great Bear,[16] is familiar to most people. This
remarkable group of seven stars will be found very useful in
identifying some of the brighter stars. The two stars furthest
from the “tail” are called “pointers,” as they point nearly to the
Pole Star, or star to which the axis of the earth nearly points. I
say “nearly,” for the Pole Star is not _exactly_ at the pole, but
distant from it about three diameters of the moon. The northern of
these stars is known to astronomers by the Greek letter Alpha and
the southern as Beta. The others, following the order of the figure,
are known by the letters Gamma, Delta (the faintest of the seven),
Epsilon, Zeta, and Eta.[17] Now, if the curve formed by the three
stars in the tail, Epsilon, Zeta, and Eta, is continued on, it
will pass near a very bright star. This is Arcturus (Alpha of the
constellation Boötes), one of the brightest stars visible. Again, if
we draw an imaginary line from Gamma to Beta, and produce it, it will
pass near another bright star. This is Capella (Alpha of Auriga, “the
Charioteer” referred to by Tennyson).
Again, if we draw a line from Delta to Beta, and produce it, it will
pass near the tolerably bright stars, Castor and Pollux (Alpha and
Beta of the constellation Gemini, or the Twins), the northern of
the two being Castor. Another line from Delta to Gamma produced will
pass near a bright star called Regulus (Alpha of Leo, the Lion).
Another line from Beta to Eta will pass near a group called Corona
Borealis, or the Northern Crown.
[Illustration: Constellation of Orion, showing the Belt and Sword]
On the opposite side of the Pole Star from the Plow, a group of five
conspicuous stars will be found, forming a figure shaped somewhat
like a W. This is Cassiopeia’s Chair. Commencing with the most
westerly of the five, these stars are known as Beta, Alpha, Gamma,
Delta, and Eta. Like the stars of the Plow, those of Cassiopeia’s
Chair may be used to find other stars. For instance, a line drawn
from Beta to Alpha passes close to a star known as Gamma in
Andromeda; and the same line produced in the opposite direction will
pass a little north of the bright star Vega (Alpha Lyræ), one of the
brightest stars in the northern heavens. A line from Gamma to Alpha
produced will pass through the well-known “Square of Pegasus.”
To the east of Vega lies Cygnus, or the Swan, a well-known northern
constellation. It may be recognized by the long cross formed by its
principal stars, Alpha, Beta, Gamma, Delta, and Epsilon; Alpha, or
Deneb, being the most northern and brightest, and Beta the most
southern and faintest of the five.
To the southeast of Cassiopeia’s Chair lies the constellation
Perseus, distinguished by its well-known festoon, or curve, of
stars. South of this lies the constellation Taurus or the Bull,
which contains the well-known groups or clusters, the Pleiades and
the Hyades. The Pleiades form perhaps the most remarkable group of
stars in the heavens, and are easily found, when above the horizon.
To ordinary eyesight the cluster consists of six stars. Some persons
gifted with exceptionally keen eyesight have, however, seen eleven
or twelve. A map of the Pleiades made in the sixteenth century
shows eleven stars very correctly. This was drawn, of course, from
observations made with a measuring instrument, but without the aid of
a telescope. The observer (I think it was Möstlin, Kepler’s tutor)
must have possessed wonderfully sharp eyesight. The Hyades form a
V-shaped figure, and contain the bright reddish star Aldebaran.
South of Taurus and Gemini will be found the splendid constellation
of Orion, perhaps the most brilliant group of stars visible in
either hemisphere. A remarkable quadrilateral figure is formed by
its four stars, Betelgeuse (Alpha) and Gamma[18] on the north, and
Rigel (Beta) and Kappa on the south. Of these Betelgeuse and Rigel
are bright stars of the first magnitude. Betelgeuse is distinctly
reddish and also slightly variable in its light. Rigel is a beautiful
white star. In the middle of the quadrilateral are three stars of the
second magnitude, nearly in a straight line, known as Delta, Epsilon,
and Zeta, Delta being the northern of the three. These form Orion’s
“belt.” South of these are three faint stars, also in a straight
line, forming the “sword” of Orion. Surrounding the central star
of the “sword” is “the great nebula of Orion,” one of the finest
objects in the heavens. It is barely visible to the naked eye, but
may be seen with a good opera-glass.
To the southeast of Orion will be found Sirius, the brightest star
in the heavens. It is the chief star of the constellation Canis
Major, or the Great Dog, and has been well termed “the monarch of the
skies,” from its great brilliancy.
The bright star Regulus, referred to above, is situated in a
remarkable group of stars shaped like a sickle, and known as “the
Sickle in Leo.” Regulus lies at the extremity of the handle. Leo is
well placed for observation in April and May.
The famous group called the Southern Cross forms a conspicuous object
in the southern heavens. It has formed a subject of interest since
the earliest ages of antiquity. Its component stars, are, however,
not so brilliant as some suppose, the two brightest being between the
first and second magnitudes, the next of the second, and one between
the third and fourth magnitudes. Near the Southern Cross are two
bright stars known as Alpha[19] and Beta of the Centaur.
Among the stars are many objects known as “double stars.” These
consist of two stars very close together, but which appear to
the naked eye only as single stars. Some are triple, and even
quadruple. Of these double stars there are now about 10,000 known
to astronomers, but they are only visible with a telescope. Some,
indeed, are so close that the highest powers of the very largest
telescopes are necessary to see them as anything but single stars.
Of the naked-eye stars there are, however, some apparently so close
that they present very much the appearance of real double stars as
seen in a telescope. These, although not recognized by astronomers
as double stars, have been termed “naked-eye doubles.” Houzeau found
that the brighter the stars are the easier it is to separate them;
and that for small stars, about 15′ of arc, or half the moon’s
apparent diameter, is about the limit below which the naked eye can
not see a faint star double.
[Illustration: Fig. 15.—Constellation of the Great Bear]
Of the “naked-eye doubles,” perhaps the most remarkable is Mizar,
the middle star in the “tail” of the Great Bear. Close to it is a
small star, sometimes called “Jack on the Middle Horse.” It was
known to the ancient astronomers as Alcor, or “the test,” as it was
_then_ considered a test of excellent eyesight. Whether it has really
brightened seems doubtful, but at present it is perhaps visible to
_ordinary_ eyesight. Some, however, fail to see it, while to others
with keener vision it seems as plain as the proverbial “pike-staff.”
The star Alpha Capricorni consists of two stars which, although
closer than Mizar and Alcor, are more equal in brightness, and may
be easily seen with the naked eye on a clear night. Nu Sagittarii
may also be seen double in this way. Theta Tauri, in the Hyades, is
another object which some eyes can see distinctly double; also Kappa
Tauri, a little to the north of the Hyades; Omicron Cygni, a little
to the west of Alpha Cygni (Deneb), is another example. On a very
fine night two stars may be seen in Iota Orionis, the most southern
star in the “sword.” Near Gamma Leonis, one of the brightest stars in
the “sickle,” is a star of the sixth magnitude, which some can see
without optical aid.
The most severe test is, however, Epsilon Lyræ, the northern of two
small stars which form a little triangle with the brilliant Vega.
This, to some eyes, appears double. The famous German astronomer
Bessel is said to have seen it at thirteen years of age. To most
people, however, it will perhaps appear only elongated. This is a
very remarkable star, as each of the components is seen to be a close
double when examined with a good telescope; and between the pairs are
several fainter stars.
Among those interesting objects, the variable stars, are several
which may be well observed without optical assistance. Of these
may be mentioned Algol, of which all the fluctuations of light
may be easily observed with the naked eye; Mira Ceti, which may be
well observed when at its brightest; Lambda Tauri, a variable star
of the Algol type; Betelgeuse (Alpha Orionis), which is slightly
variable; Zeta Geminorum, a fourth magnitude star, which varies
about three-quarters of a magnitude in a period of about ten days;
R. Hydræ, which is visible to the naked eye at maximum; Beta Lyræ,
period about thirteen days; Eta Aquilæ, period about seven days;
and Delta Cephei, which varies about one magnitude in a period of a
little over five days. Of all these stars useful observations may be
made without optical assistance of any sort.
Observations, and even discoveries, of new or “temporary” stars
may also be made with the naked eye. This occurred in the case of
the “temporary” stars of 1572, 1604, 1670, 1866, and 1870, but, of
course, these were bright objects at the time of their discovery.
Hind’s “new star” of 1848 in Ophiuchus was, however, only of the
fifth magnitude when it appeared, and it might have escaped detection
with the naked eye. A star of this magnitude might, however, be
easily detected by an observer who is familiar with the principal
stars of a constellation.
The Milky Way may, perhaps, be better seen with the naked eye than
with any instrument, although an opera-glass brings out well, in some
places, its more delicate details. A mere passing glance might lead
a casual observer to suppose that the Galaxy stretched as a band of
nearly uniform brightness across the heavens. But good eyesight,
careful attention, and a clear sky will soon disclose numerous
details previously unsuspected; streams and rays of different
brightness, intersected by rifts of darkness, and interspersed with
spots and channels of comparatively starless spaces. An excellent
drawing of the Milky Way—the result of five years’ observations
with the naked eye alone—has recently been completed by Dr. Otto
Boeddicker at Lord Rosse’s observatory in Ireland. This beautiful
picture is exquisitely drawn, and shows a wonderful amount of
detail. A writer in the _Saturday Review_ of November 30, 1889,
says: “His maps are in many respects a completely new disclosure.
Features barely suspected before come out in them as evident and
persistent; every previous representation appears, by comparison,
_structureless_.” This shows what can be done with the naked eye in
the study of this wonderful zone.
Among the nebulæ and clusters there are not many objects visible to
the naked eye. A hazy appearance about the middle star in Orion’s
“sword” indicates the presence of the “great Nebula,” one of the
finest objects in the heavens. The “great Nebula in Andromeda,” aptly
termed “the Queen of the Nebulæ,” is distinctly visible to the naked
eye on a very clear night. It lies near the four and a half magnitude
star, Nu Andromedæ (a few degrees north of Beta Andromedæ), and may
be well seen in the early evening hours in the month of January, when
it is high in the sky. It somewhat resembles a small comet. This
nebula was known long before the invention of the telescope, and it
was described by one of the earlier astronomers as resembling “a
candle shining through horn,” a not inapt description.
Of star clusters visible without optical aid may be mentioned the
double cluster Chi Persei, which appears to the eye as a luminous
spot in the Milky Way; the cluster known as 35 Messier, a little
north of Eta Geminorum, just visible to the naked eye on a very clear
night; and there are others in the Southern Hemisphere, notably the
globular cluster known as Omega in the Centaur, which shines as a
hazy star of the fourth magnitude. Among the clusters may perhaps
be included the Præsepe, or the “Beehive,” in Cancer, which has a
nebulous appearance to the naked eye.
Coming now to the Solar System, the sun and moon, of course, first
attract attention. Cases of sun-spots visible to the naked eye are
recorded, but, of course, spots of such enormous size are of rare
occurrence. Of lunar detail little can be seen without a telescope of
some sort, but the larger markings are sufficiently distinct to good
eyesight to convince the observer that they do not alter perceptibly,
thus showing clearly that the moon always turns the same side to the
earth.
Of the planets, nothing of their appearance in the telescope can,
of course, be seen with the naked eye, but it is easy to identify
the brighter planets. Mercury, owing to its proximity to the sun,
is rarely visible in Europe and North America, but when favorably
situated, it may sometimes be detected near the sun shortly after
sunset or a little before sunrise. Notwithstanding the difficulty of
seeing it, it was well known to the ancients, an observation of the
planet dating back to 264 B. C. It is easier, however, to see in more
southern latitudes, and I have frequently observed it as bright as
a star of the first magnitude in the clear air of the Punjab sky. I
have also seen it on several occasions in Ireland, and the Rev. S. S.
Johnson, F.R.A.S., tells me he has seen it with the naked eye no less
than one hundred times in the south of England. The brilliant planet
Venus can hardly be mistaken when seen in the morning or evening sky.
When at its brightest it considerably exceeds Jupiter and Mars, and
far surpasses Sirius, the brightest star in the heavens.
If a very bright planet is seen rising at sunset, it can not be
Venus, which is never seen beyond a limited distance from the sun.
The observer may, therefore, conclude with certainty that the
planet is either Jupiter or Mars. The latter, which occasionally
rivals Jupiter in brilliancy, may be easily distinguished from the
“giant planet” by its distinctly reddish color. Saturn shines with
a yellowish light, and is never so bright as Mars or Jupiter when
at their brightest. The planet Uranus is just visible to the naked
eye, and may be found without optical assistance when its position is
accurately known.
Some observers think that they can see the crescent of Venus with
the naked eye when the planet is in that phase, but this seems
very doubtful. Cases have been recorded of one or two satellites
of Jupiter having been seen with the unaided eyesight, but few are
gifted with such keen vision.
Occultations of bright stars may be well seen with the naked eye,
especially when they pass behind the moon’s _dark_ limb, and as the
disappearance of a star is practically instantaneous, really valuable
observations may be made without a telescope, by merely noting the
exact time at which the star vanishes.
Most of the comets discovered by astronomers are small and faint, and
only visible in good telescopes. At intervals, however, a brilliant
visitor appears on the scene, and its path among the stars may be
watched from night to night with the naked eye. Before the invention
of the telescope, bright comets were watched in this way, and their
course recorded so carefully that it has been found possible to
calculate their orbits with some approach to accuracy. In these days
of large telescopes and instruments of almost mathematical precision,
such a method of observation is, of course, superseded; but we may
still watch the movements of a bright comet with interest, and note
its apparent path across the sky with pleasure and profit. Shooting
stars and fire-balls may be best observed with the naked eye, and
the excellent work done in this way by Mr. W. F. Denning, F.R.A.S.,
should encourage others to take up this interesting branch of
astronomy.
Another object which may be well seen with the naked eye—indeed, it
may best be observed in this way—is the Zodiacal Light. This is a
lenticular or cone-shaped beam of light, which makes its appearance
at certain times of the year, above the eastern horizon before the
dawn, and above the western horizon after sunset, when the sky is
clear and the moon absent. In the tropics it is much more easily
seen, the twilight being shorter, and I have often observed it in
India shining with great brilliancy.
From the above sketch my readers will see how much may be learned of
astronomy without optical assistance of any kind, and I hope that
those who do not possess a telescope will use their eyes instead, and
thus gain some knowledge of the wonders and beauties of the starry
heavens. The knowledge thus gained will stimulate their curiosity and
will give them keener interest in reading books which describe the
still greater wonders revealed by the telescope.
FOOTNOTES:
[16] Also known as the Dipper and Charles’s Wain.—E. S.
[17] The Arabian names Dubhe (Alpha), Merak (Beta), Phecda (Gamma),
Megrez (Delta), Alioth (Epsilon), Mizar (Zeta), and Alkaid (Eta).
—E. S.
[18] Bellatrix.
[19] This is the nearest star to the earth.—E. S.
THE MILKY WAY.—RICHARD A. PROCTOR
To those who rightly appreciate its meaning the Milky Way is the most
magnificent of all astronomical phenomena. However opinions may vary
as to the configuration of the star-streams composing this object, no
doubt now exists among astronomers that the Milky Way consists really
of suns, some doubtless falling short of our own sun in brilliancy,
but many probably surpassing it. Around these suns, we may fairly
conceive, there revolve systems of dependent orbs, each supporting
its myriads of living creatures. We have afforded to us a noble theme
for contemplation in the consideration of the endless diversities of
structure, and of arrangement, which must prevail throughout this
immensity of systems.
The Galaxy traverses the constellation Cassiopeia. Thence it throws
off a branch toward Alpha Persei (Mirfak), prolonged faintly toward
the Pleiades. The main stream, here faint, passes on through Auriga,
between the feet of Gemini and the Bull’s horns, over Orion’s club
to the neck of Monoceros. Thence, growing gradually brighter, the
stream passes over the head of Canis Major, in a uniform stream,
until it enters the prow of Argo, where it subdivides. One stream
continues to Gamma Argus, the other diffuses itself broadly, forming
a fan-like expanse of interlacing branches, which terminate abruptly
on a line through Lambda and Gamma Argus. Here there is a gap beyond
which the Milky Way commences in a similar fan-shaped grouping,
converging on the brilliant (and in other respects remarkable)
star Eta Argus. Thence, it enters the Cross by a narrow neck, and
then directly expands into a broad, bright mass, extending almost
to Alpha Centauri. Within this mass is a singular cavity known as
the Coal-Sack. At Alpha Centauri the Milky Way again subdivides, a
branch running off at an angle of 20°, and losing itself in a narrow
streamlet. The main stream increases in breadth, until, “making
an abrupt elbow,” it subdivides into one continuous but irregular
stream, and a complicated system of interlacing streams covering the
region around the tail and following claw of Scorpio. A wide interval
separates this part of the Galaxy from the great branch on the
northern side, terminating close on Beta Ophiuchi.
The main stream, after exhibiting several very remarkable
condensations, passes through Aquila, Sagitta, and Vulpecula to
Cygnus. In Cygnus there is a “confused and patchy” region marked by a
broad vacancy, not unlike the Coal-Sack. From this region there is
thrown off the offset to Beta Ophiuchi, already mentioned; the main
stream is continued to Cassiopeia.
[Illustration: Fig. 16.—The Midnight Sky, with Milky Way]
There only remains to be noticed “a considerable offset or
protuberant appendage,” thrown from the head of Cepheus directly
toward the pole. Galileo was the first to prove, though earlier
astronomers had entertained the notion, that the Milky Way was
composed of a vast number of stars crowded closely together. But no
attempt was made to offer a theory of its structure until, in 1754
Thomas Wright, in his _Theory of the Universe_, propounded views
closely according with those entertained later by Sir W. Herschel.
Wright, having examined a portion of the Galaxy with a reflecting
telescope, only one foot in focal length, came to the conclusion that
our sun is in the midst of a vast stratum of stars; that it is when
we look along the direction in which this stratum extends that we see
the zone of light constituting the Milky Way; and that as the line of
sight is inclined at a greater and greater angle to the mean plane
of the stratum, the apparent density of the star-grouping gradually
diminishes.
But it is to Sir W. Herschel, and the supplementary labors of Sir
J. Herschel, that we owe the more definite views now commonly
entertained respecting the Via Lactea. The elder Herschel, whose
nobly speculative views of nature were accompanied by practical
common-sense, and a wonderful power of patient observation, applied
to the heavens his celebrated method of gauging. He assumed as a
first principle, to be modified by the results of observation, that
there is a tolerable uniformity in the distribution of stars through
space. Directing his twenty-foot reflector successively toward
different parts of the heavens, he counted the number of stars which
were visible at any single view. The field of view of this reflector
was fifteen minutes in diameter, so that the portion of the sky
included in any one view was less than one-fourth of that covered by
the moon. He found the number of stars visible in different parts
of the heavens in a field of view of this size to be very variable.
Sometimes there were but two or three stars in the field;[20]
indeed, on one occasion he counted only three stars in four fields.
In other parts of the heavens the whole field was crowded with
stars. In the richer parts of the Galaxy as many as four hundred or
five hundred stars would be visible at once, and on one occasion he
saw as many as five hundred and eighty-eight. He calculated that
in one-quarter of an hour 116,000 stars traversed the field of his
telescope, when the richest part of the Galaxy was under observation.
Now, on the assumption above named, the number of stars visible when
the telescope was pointed in any given direction was a criterion of
the depth of the bed of stars in that direction. Thus, by combining
a large number of observations, a conception—rough, indeed, but
instructive—might be formed of the figure of that stratum of stars
within which our sun is situated.
Sir J. Herschel, during his residence at the Cape of Good Hope,
carried out an extensive series of observations of the southern
heavens. Applying his father’s methods of gauging with a telescope
of equal power, he obtained a result agreeing, in a most remarkable
manner, with those obtained by Sir William Herschel. It appeared,
however, that the Southern Hemisphere is somewhat richer in stars
than the Northern—a result which has been accepted as indicating that
our system is probably somewhat nearer the southern than the northern
part of the galactic nebula. Moreover, Sir J. Herschel was led to
believe that the sidereal system forms a cloven flat ring rather than
a disk.
I think no one who has attentively examined the glories of Orion,
the richly jeweled Taurus, the singular festoon of stars in Perseus,
and the closely set stars of Cassiopeia, but must have felt that
the association of splendor along this streak of the heavens is
not wholly accidental. The stars here seem to form a system, and a
system which one can hardly conceive to be wholly unconnected with
the neighboring stream of the Milky Way. But in the southern portion
the arrangement is yet more remarkable and significant. From Scorpio,
over the feet of the Centaur, over the keel of Argo, to Canis Major,
there is a clustering of brilliant stars, which it seems wholly
impossible not to connect with the background of nebulous light.
It is noteworthy, also, that this stream of stars merges into the
stream commencing with the group of Orion, already noticed. Nor is
this all. It is impossible not to be struck by the marked absence of
bright stars in the region of the heavens between Algol, Crux, and
Corvus. One has the impression that the stars have been attracted
toward the region of the stream indicated, so as to leave this space
comparatively bare.
Now, this last circumstance would appear less remarkable if the
paucity of stars here noticed were common also in parts of the
heavens far removed from the Milky Way. But this is not the case.
Beyond this very region, which we find so bare of stars, we come to a
region in which stars are clustered in considerable density, a region
including Crater, Corvus, and Virgo, with the conspicuous stars
Algores, Alkes, and Spica. But what is very remarkable, while we can
trace a connection between the stream of bright stars over the Milky
Way and the stream of nebulous light in the background, it is obvious
that the two streams are not absolutely coincident in direction.
The stream lies on one side of the Milky Way near Scorpio, crosses
it in the neighborhood of Crux, and passes to the other side along
Canis Major, Orion, and Taurus. Does the stream return to the Milky
Way? It seems to me that there is clear evidence of a separation
near Aldebaran, one branch curving through Auriga, Perseus, and
Cassiopeia, the other proceeding (more nearly in the direction
originally observed) through Aries (throwing out an outlier along the
band of Pisces), over the Square of Pegasus, and along the streams
which the ancients compared to water from the urn of Aquarius (but
which in our modern maps are divided between Aquarius and Grus).
The stream-formation here is very marked, as is evident from the
phenomenon having attracted the notice of astronomers so long ago.
But modern travels have brought within our ken the continuation of
the stream over Toucan, Hydrus, and Reticulum (the two latter names
being doubtless suggested by the convolutions of the stream in this
neighborhood). Here the stream seems to end in a sort of double loop,
and it is not a little remarkable that the Nubecula Major lies within
one loop, the Nubecula Minor within the other. It is also noteworthy
that from the foot of Orion there is another remarkable stream
of stars, recognized by the ancients under the name of the River
Eridanus, which proceeds in a sinuous course toward this same region
of the Nubeculæ.
Having thus met with evidence—striking at least, if not decisive—of
a tendency to aggregation into streams, let us consider if, in any
other parts of the heavens, similar traces may not be observable. We
traced a stream from Scorpio toward Orion, and so round in a spiral
to the Nubeculæ. Let us now return to Scorpio, and trace the stream
(if any appear) in the contrary direction. Now, although over the
Northern Hemisphere star-streams are not nearly so marked as over the
Southern, yet there appears a decided indication of stream-formation
along Serpens and Corona over the group on the left hand of Boötes
to the Great Bear. A branch of this stream, starting from Corona,
traverses the body of Boötes, Berenice’s Hair, the Sickle in Leo,
the Beehive in Cancer, passing over Castor and Pollux in Gemini,
toward Capella. A branch from the feet of Gemini passes over Canis
Minor, along Hydra (so named doubtless from the obvious tendency to
stream-formation along the length of this constellation), and so to
the right claw of Scorpio.
One other remarkable congeries of stars is to be mentioned. From the
northern part of the Milky Way there will be noticed a projection
toward the North Pole from the head of Cepheus. This projection
seems to merge itself in a complex convolution of stars, forming the
ancient constellation Draco, which doubtless included the ancient
(but probably less ancient) constellation Ursa Minor. After following
the convolutions of Draco, we reach the bright stars Alwaid and
Etanin (Beta and Gamma) of this constellation, and thence the stream
passes to Lyra, where it seems to divide into two, one passing
through Hercules, the other along Aquila, curving into the remarkable
group Delphinus.
The streams here considered include every conspicuous star in the
heavens. But the question will at once suggest itself, whether we
have not been following a merely fanciful scheme, whether all these
apparent streams might not very well be supposed to result from
mere accident. Now, from experiments I have made, I am inclined to
believe that in any chance distribution of points over a surface, the
chance against the occurrence of a single stream as marked as that
which lies (in part) along the back of Grus, or as the curved stream
of bright stars along Scorpio, is very great indeed; I am certain
that the occurrence of _many_ such streams is altogether improbable.
And wherever one observes a tendency to stream-formation in objects
apparently distributed wholly by chance, one is led to suspect,
and thence often to detect, the operation of law. I will take an
illustration, very homely perhaps, but which will serve admirably to
explain my meaning. In soapy water, left in a basin after washing,
there will often be noticed a tendency to the formation of spiral
whorls on the surface. In other cases there may be no definite
spirality, but still a tendency to stream-formation. Now, in this
case, it is easy to see that the curved bottom of the basin has
assisted to generate streams in the water, either circulating in one
direction or opposing and modifying each other’s effects, according
to the accidental character of the disturbance given to the water in
the process of washing.[21] Here, of course, there can be no doubt
of the cause of the observed phenomena; and I believe that in every
case in which even a single marked stream is seen in any congeries
of spots or points, a little consideration will suggest a regulating
cause to which the peculiarity may be referred.
It is hardly necessary to say that, if the stream-formation I
have indicated is considered to be really referable to systematic
distribution, the theory of a stratum of stars distributed with any
approach to uniformity, either as respects magnitude or distance,
must be abandoned. It seems to me to be also quite clear that the
immense extent of the Galaxy, as compared with the distances of the
lucid stars from us, could no longer be maintained. On this last
point we have other evidence, which I will briefly consider.
First, there is the evidence afforded by clusterings in the Milky
Way. I will select one which is well known to every telescopist,
namely, the magnificent cluster on the sword-hand of Perseus. No
doubt can be entertained that this cluster belongs to the galactic
system, that is, that it is not an _external_ cluster: the evidence
from the configuration of the spot and from the position it occupies
is conclusive on this point. Now, within this spot, which shows
no stars to the naked eye, a telescope of moderate power reveals a
multitude of brilliant stars, the brightest of which are of about
the seventh magnitude. Around these there still appears a milky
unresolved light. If a telescope of higher power be applied, more
stars are seen, and around these there still remains a nebulous
light. Increase power until the whole field blazes with almost
unbearable light, yet still there remains an unresolved background.
“The illustrious Herschel,” says Professor Nichol, “penetrated, on
one occasion, into this spot, until he found himself among depths
whose light could not have reached him in much less than 4,000 years;
no marvel that he withdrew from the pursuit, conceiving that such
abysses must be endless.” It is precisely this view that I wish to
controvert. And I think it is no difficult matter to show at least
a probability against the supposition that the milky light in the
spot is removed at a vast distance behind the stars of the seventh
magnitude seen in the same field.
The supposition amounts, in fact, to the highly improbable view that
we are looking here at a range of stars extending in a cylindrical
stratum directly from the eye—a stratum whose section is so very
minute in comparison with its breadth that, whereas the whole
field within which the spot is included is but small, the distance
separating the nearest parts of the group from the furthest is
equivalent to the immense distance supposed to separate the sphere
of seventh magnitude stars from the extreme limits of our Galaxy.
And the great improbability of this view is yet further increased
when it is observed that within this spot there is to be seen a very
marked tendency to the formation of minor streams, around which the
milky light seems to cling. It seems, therefore, wholly improbable
that the cluster really has that indefinite longitudinal extension
suggested by Professor Nichol. In fact, it becomes practically
certain that the milky light comes from orbs really smaller than the
seventh magnitude stars in the same field, and clustering round these
stars in reality as well as in appearance.
The observations applied to this spot may be extended to all clusters
of globular form; and where a cluster is not globular in form,
but exhibits, on examination, either (1) any tendency within its
bounds to stream-formation, or (2) a uniform increase in density as
we proceed from any part of the circumference toward the centre,
it appears wholly inconceivable that the apparent cluster is not
really a cluster, but a long range of stars extending to an enormous
distance directly from the eye of the observer. When, in such a case,
many stars of the higher magnitudes appear within the cluster, we
seem compelled to admit the probability that they belong to it; and,
in any case, we can not assign to the furthest parts of the cluster
a distance greatly exceeding (_proportionally_) that of the nearest
parts.
Of a like character is the evidence afforded by narrow streams and
necks within the Galaxy itself. If we consider the convolutions over
Scorpio, it will seem highly improbable that in each of these we
see, not a real convolution or stream, but the edge of a _roll_ of
stars. For instance, if a spiral roll of paper be viewed from any
point taken at random, the chances are thousands to one against its
appearing as a spiral _curve_, and, of course, the chance against
several such rolls so appearing is very much greater. The fact that
we are assumed to be not very far from the supposed mean plane of the
Milky Way would partly remove the difficulty here considered, if it
were not that the thickness and extent of the stratum, as compared
with the distances of the lucid stars, must necessarily be supposed
very great, on the assumption of any approach to uniformity of
distribution.
Evidence pointing the same way is afforded by circular apertures
in the Galaxy, or indeed by apertures of other forms. Another
peculiarity of these cavities is also noticeable; whereas on the
borders of every one there are many lucid stars, or in some cases two
or three very bright stars, _within_ the cavity there is a marked
paucity of stars. This phenomenon seems to indicate a much closer
connection between the brighter stars and the milky light beyond
than is supposed on the stratum theory. One can hardly conceive the
phenomenon to be wholly accidental.
There are some other points on which I fain would dwell, but space
will not permit me. I will merely note that there are peculiarities
in the distribution of red double and multiple stars, in the position
in which temporary stars have made their appearance, and in the
distribution of nebulæ, which seem very worthy of notice.
One point, however, immediately connected with my subject remains
to be mentioned. I have traced streams of stars _more_ conspicuous
than those forming the Milky Way. We have also evidence of streams
of light yet more delicate and evanescent than the light of our own
Galaxy. In Sir John Herschel’s great work on the southern skies,
he notes the frequent recurrence of “an exceedingly delicate and
uniform dotting, or _stippling_, of the field of view by points of
light too small to admit of any one being steadily or fully examined,
and too numerous for counting, were it possible so to view them.”
In thirty-seven places he detected this remarkable and significant
phenomenon; a phenomenon so faint that he says, “The idea of illusion
has continually arisen subsequently”; an idea well befitting the
modesty of the philosophic observer, but which those who appreciate
Sir John Herschel’s skill as an observer will be very unwilling to
accept. As Professor Nichol remarks, “It is enough to read from
Herschel’s notebook—‘I feel satisfied the stippling is no illusion,
for its dark mottling moves with the stars as I move the tube to and
fro’—to feel convinced that the phenomenon is real.” Now a remarkable
fact connected with those observations is, that when Sir J. Herschel
marked down in a star-chart the places in which he had detected
this nebulous appearance, he found that, “with the exception of
_three_ which appeared outlying and disconnected, they formed several
_distinct but continuous streams_.”
FOOTNOTES:
[20] Field means the actual space covered by the lens.—E. S.
[21] Sometimes a singular regularity of curvature is noticed, and
a spiral is formed closely resembling in configuration some of the
great spiral nebulæ, as drawn by Lord Rosse, so that one is tempted
to see in the centrifugal tendency of the disturbed water, and the
centripetal effects caused by reflection from the basin’s surface,
causes which may in some sense illustrate the laws operating in wider
domains of space.
THE MAGELLANIC CLOUDS—ZODIACAL LIGHT—STAR GROUPS.—AMÉDÉE GUILLEMIN
When we look on the region of the celestial vault which surrounds
the South Pole, we can not help being struck with the contrast
presented by the small quantity of stars which it contains, with the
brilliant zone which borders the Milky Way, from Orion and Argo to
the Centaur, passing by the Southern Cross. One solitary star of the
first magnitude, Achernar, more distant from the pole than are the
beautiful stars of the Centaur and of the Cross, shines in this part
of the sky.
But even this circumstance renders the singular aspect of the two
nebulous spots, which seem two detached pieces of the great galactic
zone, still more striking. These half-stellar, half-nebulous systems,
unequal in magnitude and brightness, but easily seen with the naked
eye on a clear, moonless night, are situated, one, the larger and
more brilliant, between the pole and Canopus, in the constellation
of Doradus; the other, the smaller and less brilliant, ordinarily
visible during the full moon, in Hydrus, between Achernar and the
pole.
Both are known by astronomers and navigators under the name of “Cape
Clouds,” or again, “Magellanic Clouds.” And, to distinguish them, we
have again the Great Cloud (_Nebecula Major_) and the Small Cloud
(_Nebecula Minor_).
The Clouds of Magellan are distinguished from all other nebulæ by
their great apparent dimensions, and by their physical structure;
this last character distinguishes them from most of the branches and
offshoots of the Milky Way, with which, we may also add, they do not
appear connected in any way.
The Great Cloud extends over a space which embraces not less than
forty-two square degrees—about two hundred times the apparent surface
of the lunar disk. The Small Cloud occupies in extent four times less
than the other; according to Humboldt, it is surrounded “with a kind
of desert,” where, it is true, shines the magnificent stellar cluster
of Toucan. If the exterior aspect of these two remarkable nebulæ,
and their situation in a celestial region poor in stars, give to the
southern sky a peculiar appearance, their real structure makes them
one of the wonders of the heavens.
In the Great Cloud, Herschel has counted 582 single stars, among
which one only is of the fifth magnitude; six others are of the order
immediately inferior, and would doubtless be visible to the naked eye
if their light were not effaced by the general glare.
In the Small Cloud, the single stars are proportionally more
numerous, since 200 have been counted, among which three are of the
sixth magnitude, while it only includes thirty-seven of the nebulæ
and seven star-clusters. These immense aggregations, the elements of
which are themselves swarms of suns, remind us of the largest, in
appearance at least, of all the clusters which the eye contemplates
in the depths of the sky—the Milky Way.
In the evenings, about the time of the vernal equinox—in March and
April, when in our climate the twilight is of short duration—if we
examine the horizon toward the west, a little after sunset, we may
perceive a faint light that rises in the form of a cone among the
starry constellations.
This is what astronomers call the Zodiacal Light. Those unfamiliar
with it, or little accustomed to the ordinary aspect of the sky,
might confuse the glimmering either with the Milky Way or with
the ordinary twilight, or even with an aurora. But, with a little
attention, it is impossible to mistake it.
The triangular form of this luminous cone, its elevation and its
inclined position to the horizon, make it a thing apart, and one
eminently deserving particular mention.
As the days lengthen, and with them the duration of twilight, the
Zodiacal Light disappears; it becomes invisible, at least in our
climate. But it may again be seen in the morning, in the east, about
the time of the autumnal equinox, in September and October, when
the dawn has an equally short duration—again, however, to disappear
during the period of long nights and long twilights.
It is needless to add that the sky must be clear and the night
moonless for observations of the Zodiacal Light to be possible.
Among the explanations that have been given, the most probable one
is that which likens the Zodiacal Light to a flattened nebulous ring
surrounding the sun at some distance. It is to be remarked that the
direction of the axis of the cone, or of the pyramid, prolonged
below the horizon, always passes through the sun.
It was believed at first that this direction precisely coincided with
the solar equator; but it seems more certain that it coincides with
the plane of the earth’s orbit, or the ecliptic.
Now, what is the nature of this luminous mass? Must it be considered
as a zone of vapors thrown off by the sun, when in the process of
consolidation, when our central star passed from a nebulous state to
that of a condensed fluid sphere? This was the opinion of Laplace.
Another hypothesis, also connected with the first, is that the
Zodiacal Light is formed of myriads of solid particles, analogous
to the aerolites, possessing a general movement, but traveling
separately around the focus of our solar world. The light of the
ring would be thus produced by the accumulation of this multitude of
brilliant points, reflecting toward us the light borrowed by each of
them from the sun.
This explanation accounts for the intensity of the Zodiacal Light at
different epochs; it would suffice to admit that the condensation of
the particles or the density of the ring is not the same throughout
its extent, and that its movement of circulation round the sun
presents successively different parts to the earth. In this case, it
becomes a question whether this lenticular ring of matter is distinct
from the zone of aerolites.
Lastly, some astronomers regard the Zodiacal Light as a vaporous ring
which belongs to the earth, surrounding it at some distance. But
this is an opinion which appears somewhat wild, and is utterly at
variance with observation.
Are the stars that are visible to the naked eye spread orderless on
the celestial vault? or is there not between those apparently most
closely connected some real or physical connection which requires us
to rank them in natural groups?
These questions have been already partly solved by what is known of
the double and multiple star systems. Soon, exploring the regions of
the sky visible by means of the telescope, we shall have to pass in
review a multitude of stellar associations, in which suns are found
so compact and so numerous, and the form of the groups so regular,
that it is impossible to deny their reciprocal dependence.
But long before the discovery of these islands, these archipelagos
as worlds, scattered with such astonishing profusion over the
infinite, the naked eye had already distinguished a certain number of
groups, the stars composing which were so near together that it was
impossible to doubt their physical connection.
Such, for example, is the group of the Pleiades. Such, again, are
the groups known under the names of the Hyades, of Præsepe, and of
Berenice’s Hair. All are visible to the naked eye, and good eyes
distinguish without difficulty the principal stars of the first-named
groups. The Pleiades are situated in the constellation of the Bull,
which we can distinguish so easily to the northwest of Orion and
Aldebaran.
Of about eighty stars which form the group of the Pleiades, six are
visible without the help of telescopes. Formerly, the Latin poet
tells us, seven were counted, which may be held to prove that one
of them is variable, and has diminished in brightness, or else has
disappeared.
The most brilliant, Alcyone, is of the third magnitude; Electra and
Atlas are of the fourth; Merope, Maïa, and Taygete of the fifth.
Three others again have received particular names, although they are
below the limit of ordinary vision; these are Pleione, Celeno, and
Asterope, from the sixth to the eighth magnitude. All the others are
only visible by the aid of a telescope; but with an ordinary glass
it is possible to distinguish a large number. The Pleiades are known
under the name of the Hen-coop, doubtless because Alcyone appears in
the group as a hen surrounded with her chickens.
The Hyades, which are near the Pleiades, form a less numerous and
more scattered group. The bright light of Aldebaran, which is, as
is known, of the first magnitude, renders them more difficult to
distinguish with the naked eye.
They appear in the rainy season. Hence their name of Hyades, from the
Greek word which signifies to rain.
The connection of the stars which compose this group is not so
striking as in the case of the Pleiades. Nevertheless, it seems
difficult to admit that they are quite independent of each other’s
attraction. In examining the position of these two groups in the
vicinity of the Milky Way, and observing that both are situated
in the prolongation of a branch of the great zone, we are almost
entitled to consider them as two clusters of stars, belonging to the
immense stellar stratum which surrounds us, and in the midst of which
the sun himself is placed.
In Berenice’s Hair, most of the stars are visible to the naked eye,
and are perfectly distinguished in the sky, a little to the east of
the Lion. No very brilliant star in the vicinity inconveniences the
eye by effacing their light.
The next group is situated in the Crab, and is known under the
name of Præsepe: it is visible to the unassisted sight; but it is
impossible to distinguish the separate stars without the help of
a telescope. Nevertheless, an instrument of moderate power easily
separates them.
The groups which we have just described form a transition between
the stars scattered over the celestial vault and the more condensed
clusters, the undefined aspect of which caused them formerly to be
designated under the general name of nebulæ.
Doubtless, if we could place ourselves in space, and contemplate from
a sufficiently distant standpoint the whole of the stars which appear
to us isolated, we should see them condensed into one or several
distinct groups, analogous to those of the Pleiades; while, were we
to penetrate into the midst of one of those compact clusters, we
should see the stars of which it is formed separated and scattered
over the celestial vault in such a way as to give it the aspect of
our own heavens.
THE NEBULÆ AND SWARMS OF SUNS.—J. E. GORE
We will now consider the nebulæ, properly so called, that is to say,
objects which the spectroscope shows to consist of glowing gas. These
are sometimes large and irregular in form, like the great nebula
in the “Sword” of Orion, sometimes with spiral convolutions, and
sometimes of a definite shape, like the planetary and annular nebulæ.
Of the large and irregular nebulæ, one of the most remarkable is that
known as “the great nebula in Orion.” It surrounds the multiple star,
Theta Orionis. It is a curious fact that it escaped the searching eye
of Galileo, although he gave special attention to the constellation
of Orion, for even with a good opera-glass a nebulous gleam is
distinctly visible round the central star of the “Sword.” The nebula
seems to have been discovered by Cysat, a Swiss astronomer, in the
year 1618, and it was sketched by Huygens in 1656. It has been called
the “fish-mouth” nebula, from the fancied resemblance of the centre
portion to the mouth of a fish. A number of small stars are visible
over the surface of the nebula, and at one time Lord Rosse thought it
showed indications of resolution into stars when examined with his
giant telescope; but this is now known to have been a mistake, for
Dr. Huggins finds, with the spectroscope, that it consists of nothing
but glowing gas.
The brightest line in the nebular spectrum—the “chief nebular
line,” as it is called—has not yet been identified with that of any
terrestrial substance.
Mr. W. H. Pickering and Dr. Max Wolf have photographed another
nebula surrounding the star Zeta Orionis—the southern star of the
“Belt,” which seems to be connected with the nebula in the “Sword”;
and Professor Barnard, using the “lens of a cheap oil lantern” of
1½ inches aperture and 3½ inches focal length, has photographed “an
enormous curved nebulosity” stretching over nearly the whole of the
constellation of Orion, and involving the “great nebula.”
Professor Keeler found, with the spectroscope, that the Orion nebula
is apparently receding from the earth at the rate of nearly eleven
miles a second, but this motion may be, in part at least, due to the
sun’s motion in space in the opposite direction. Professor Pickering
considers that the parallax of the nebula is probably not more than
0.″003, which corresponds to a thousand years’ journey for light!
In the southern constellation, Argo is a magnificent nebula, somewhat
similar in appearance to the great nebula in Orion. It surrounds the
famous variable star Eta Argûs. It is sometimes spoken of as the
“keyhole” nebula, owing to a curious opening of that shape near its
centre. It was carefully drawn by Sir John Herschel at the Cape of
Good Hope in the years 1834-38. It lies in a very brilliant portion
of the Milky Way, and Sir John Herschel thus describes it: “It is
not easy for language to convey a full impression of the beauty and
sublimity of the spectacle which the nebula offers as it enters the
field of view of a telescope, fixed in right ascension, by the
diurnal motion, ushered in as it is by so glorious and innumerable
a procession of stars, to which it forms a sort of climax, and in
a part of the heavens otherwise full of interest,” and he adds:
“In no part of its extent does this nebula show any appearance of
resolvability into stars, being, in this respect, analogous to the
nebula of Orion. It has, therefore, nothing in common with the Milky
Way, on the ground of which we see it projected, and may therefore
be, and not improbably is, placed at an immeasurable distance behind
that stratum.” Sir John Herschel’s conclusion as to its physical
constitution has been fully confirmed by the spectroscope, which
shows it to consist of luminous gas. As in the Orion nebula, there
are numerous stars scattered over it. Some of these may possibly have
a physical connection with the nebula, while others may belong to the
Milky Way. The nebula is of great extent, covering an apparent space
about five times the area of the full moon, and its real dimensions
must be enormous. It was photographed by Mr. Russell, director of
the Sydney Observatory, in July, 1890, and the photograph shows that
“one of the brightest and most conspicuous parts of the nebula”—the
swan-shaped form near the centre of Herschel’s drawing—has “wholly
disappeared,” and its place is now occupied by “a great, dark oval.”
Mr. Russell first missed the vanished portion of the nebula in
the year 1871, while examining it with a telescope of 11½ inches
aperture, and the photograph now confirms the disappearance, which
is very remarkable, and shows that changes are actually in progress
in these wonderful nebulæ, changes which may be detected after a
comparatively short interval of time.
Smaller than the nebula in Argo, but somewhat similar in general
appearance, is that known as 30 Doradus, which forms one of the
numerous and diverse objects which together constitute the greater
Magellanic Cloud. Sir John Herschel drew it carefully at the Cape
of Good Hope, and describes it as “one of the most singular and
extraordinary objects which the heavens present,” and he says “it is
unique even in the system to which it belongs, there being no other
object in either nubecula to which it bears the least resemblance.”
It is sometimes called the “looped nebula,” from the curious openings
it contains. One of these is somewhat similar to the “key-hole”
opening in the Argo nebula. Near its centre is a small cluster of
stars, and scattered over the nebula are many faint stars, of which
Sir John Herschel gives a catalogue of 105, ranging from the ninth to
the seventeenth magnitude. I do not know whether this nebula has been
examined with the spectroscope, but its appearance would suggest that
it is gaseous. It is remarkable as being the only object of its class
which is found outside the zone of the Milky Way.
Among the nebulæ of irregular shape, although its spectrum is said to
be not gaseous, may be mentioned that known as the “trifid nebula,”
or 20 Messier. It lies closely north of the star 4 Sagittarii in a
magnificent region of the heavens. In the drawing made by Sir John
Herschel at the Cape of Good Hope, the principal portion consists of
three masses of nebulous matter separated by dark “lanes” or “rifts.”
Near the junction of the three “rifts” is a triple star. A beautiful
drawing of this nebula has also been made by Trouvelot. It agrees
fairly well with that of Sir John Herschel, but shows more detail.
Among other gaseous nebulæ may be mentioned that called by Sir John
Herschel the “dumb-bell” nebula. It lies a little south of the sixth
magnitude star 14 Vulpeculæ, and was discovered by Messier in 1779,
while observing Bode’s comet of that year. In small telescopes it
has the appearance of a dumb-bell, or hour-glass, but in larger
telescopes the outline is filled in with fainter nebulous light,
giving to the whole an elliptical form. Several faint stars have
been seen in it, but these probably belong to the Milky Way, as Dr.
Huggins finds the spectrum gaseous. Dr. Roberts has photographed it,
and he thinks that “the nebula is probably a globular mass of nebular
matter which is undergoing the process of condensation into stars,
and the faint protrusions of nebulosity in the _south following_
and _north preceding_ ends are the projections of a broad ring of
nebulosity which surrounds the globular mass. This ring, not being
sufficiently dense to obscure the light of the central region of the
globular mass, is dense enough to obscure those parts of it that are
hidden by the increased thickness of the nebulosity, thus producing
the ‘dumb-bell’ appearance. If these inferences are true, we may
proceed yet a step, or a series of steps, further, and predict that
the consummation of the life-history of this nebula will be its
reduction to a globular cluster of stars.”
Among the gaseous nebulæ may also be included those known as “annular
nebulæ.” These are very rare objects, only a few being known in the
whole heavens. The most remarkable is that known as 57 Messier, which
lies between the stars Beta and Gamma Lyræ, south of the bright star
Vega. It was discovered by Darquier, at Toulouse, in 1779, while
following Bode’s comet of that year. Lord Rosse thought it resolvable
into stars, and so did Chacornac and Secchi, but no stars are
perceptible with the great American telescopes, and Dr. Huggins finds
it to be gaseous. The central portion is not absolutely dark, but
contains some faint nebulous light. Examined with the great telescope
of the Lick Observatory, Professor Barnard finds that the opening of
the ring is filled in with fainter light “about midway in brightness
between the brightness of the ring and the darkness of the adjacent
sky. The aperture was more nearly circular than the outer boundary
of the nebula, so that the ends of the ring were thicker than the
sides.” The entire nebula was of a milky color. A central star,
noticed by some observers, was usually seen by Professor Barnard, but
was never a conspicuous object. He found the extreme dimensions of
the nebula about 81″ in length by about 59″ in width, or more than
double the apparent area of Jupiter’s disk. It has been beautifully
photographed by Dr. Roberts, and he says “the photograph shows the
nebula and the interior of the ring more elliptical than the drawings
and descriptions indicate; and the star of the _following_ side is
nearer to the ring than the distance given. The nebulosity on the
_preceding_ and _following_ ends of the ring protrudes a little, and
is less dense than on the _north_ and _south_ sides. This probably
suggested the filamentous appearance which Lord Rosse shows. Some
photographs of the nebula have been taken between 1887 and 1891, and
the central star is strongly shown on some of them, but on others it
is scarcely visible, which points to the star being variable.” On a
photograph taken by MM. Androyer and Montaugerand of the Toulouse
Observatory, with an exposure of nine hours (in multiple exposures),
about 4,800 stars are visible on and near the nebula in an area of
three square degrees.
Another object of the annular class will be found a little to the
southwest of the star Lambda Scorpii. It is thus described by Sir
John Herschel: “A delicate, extremely faint, but perfectly well
defined, annulus. The field crowded with stars, two of which are
on the nebula. A beautiful, delicate ring of a faint, ghost-like
appearance, about 40″ in diameter in a field of about 150 stars,
eleven and twelve magnitude and under.”
Near the stars 44 and 51 Ophiuchi is another object of the annular
class, which Sir John Herschel describes as “exactly round, pretty
faint, 12″ diameter, well terminated, but a little cottony at the
edge, and with a decided darkness in the middle, equal to a tenth
magnitude star at the most. Few stars in the field, a beautiful
specimen of the planetary annular class of nebula.”
The Planetary Nebulæ form an interesting class. They were so named
by Sir William Herschel from their resemblance to the disks of the
planets, but, of course, much fainter. They are generally of uniform
brightness, without any nucleus or brighter part in the centre. There
are numerous examples of this class, one of the most remarkable
being that known as 97 Messier, which is situated about two degrees
southeast of Beta Ursæ Majoris—the southern of the two “pointers” in
the Plow. It is of considerable apparent size, and even supposing its
distance to be not greater than that of 61 Cygni, its real dimensions
must be enormous. Lord Rosse observed two openings in the centre
with a star in each opening, and from this appearance he called it
the “owl nebula.” One of the stars seems to have disappeared since
1850, and a photograph recently taken by Dr. Roberts confirms the
disappearance.
Another fine object of the planetary class is one which lies close
to the pole of the ecliptic. Webb saw it “like a considerable star
out of focus.” Smyth found it pale blue in color. Dr. Huggins finds
a gaseous spectrum, the first discovery of the kind made. Professor
Holden, observing it with the great Lick telescope, finds its
structure extraordinary. He says it “is apparently composed of rings
overlying each other, and it is difficult to resist the conviction
that these are arranged in space in the form of a true helix,” and he
ranks it in a new class which he calls “helical nebulæ.”
A somewhat similar nebula lies a little to the west of the star Nu
Aquarii. Secchi believed it to be in reality a cluster of small
stars, but Dr. Huggins finds its spectrum gaseous. A small nebula
on each side gives it an appearance somewhat similar to the planet
Saturn, with the rings seen edgewise. The great Lick telescope shows
it as a wonderful object—“a central ring lies upon an oval of much
fainter nebulosity.” Professor Holden says “the color is a pale
blue,” and he compares the appearance of the central ring “to that of
a footprint left in the wet sand on a sea beach.”
About two degrees south of the star Mu Hydræ is another planetary
nebula, which Smyth describes as resembling the planet Jupiter in
“size, equable light and color.” Webb saw it of “a steady, pale blue
light,” and Sir John Herschel, at the Cape of Good Hope, speaks of
its color as “a decided blue—at all events, a good sky-blue,” a
color which seems characteristic of these curious objects. Although
Sir William Herschel, with his large telescopes, failed to resolve
it into stars, Secchi thought he saw it breaking up into stars with
a “sparkling ring.” Dr. Huggins, however, finds the spectrum to be
gaseous, so that the luminous points seen by Secchi could not have
been stellar.
Sir John Herschel, in his _Cape Observations_, describes a planetary
nebula which lies between the stars Pi Centauri and Delta Crucis.
He says it is “perfectly round, very planetary, color fine blue ...
very like Uranus, only about half as large again, and blue.... It
is of the most decided independent blue color when in the field by
itself, and with no lamplight and no bright star. About 10′ north of
it is an orange-colored star, eighth magnitude. When this is brought
into view, the blue color of the nebula becomes intense ... color, a
beautiful rich blue, between Prussian blue and verditer green.”
There are some rare objects called “nebulous stars.” The star Epsilon
Orionis—the centre star of Orion’s Belt—is involved in a great
nebulous atmosphere. The triple star Iota Orionis is surrounded by a
nebulous haze. The star Beta in Canes Venatici is a 4½ magnitude star
surrounded by a nebulous atmosphere.
The term elliptical nebulæ has been applied to those of an elliptical
or elongated shape. This form is probably due in many cases to the
effect of perspective, their real shape being circular, or nearly so.
Perhaps the most remarkable object of this class is the well-known
“nebula in Andromeda,” known to astronomers as 31 Messier. It can be
just seen with the naked eye, on a clear moonless night, as a hazy
spot of light near the star Nu Andromedæ, and it is curious that it
is not mentioned by the ancients, although it must have been very
visible to their keen eyesight in the clear Eastern skies. It was,
however, certainly seen so far back as 905 A. D., and it is referred
to as a familiar object by the Persian astronomer, Al-Sûfi, who wrote
a description of the heavens about the middle of the Tenth Century.
Tycho Brahe and Bayer failed to notice it, but Simon Marius saw it
in December, 1612, and described it “as a light seen from a great
distance through half-transparent horn plates.” It was also observed
by Bullialdus, in 1664, while following the comet of that year. It
has frequently been mistaken for a comet by amateur observers in
recent years. Closely northwest of the great nebula is a smaller
one discovered by Le Gentil in 1749, and another to the south,
detected by Miss Caroline Herschel in 1783. The great nebula is of
an elliptical shape and considerable apparent size. The American
astronomer, Bond, using a telescope of 15 inches aperture, traced
it to a length of about four degrees, and a width of two and a half
degrees. A beautiful photograph taken by Dr. Roberts in December,
1888, shows an extension of nearly two degrees in length, and about
half a degree in width, or considerably larger than the apparent
size of the full moon. Bond could not see any symptom of resolution
into stars, but noticed two dark rifts or channels running nearly
parallel to the length of the nebula. In Dr. Roberts’s photograph
these rifts are seen to be really dark intervals between consecutive
nebulous rings into which the nebula is divided. Dr. Roberts says:
“A photograph which I took with the 20-inch reflector on October 10,
1887, revealed for the first time the true character of the great
nebula, and one of the features exhibited was that the dark bands,
referred to by Bond, formed parts of divisions between symmetrical
rings of nebulous matter surrounding the large diffuse centre of the
nebula. Other photographs were taken in 1887, November 15; 1888,
October 1; 1888, October 2; 1888, December 29; besides several
others taken since, upon all of which the rings of nebulosity are
identically shown, and thus the photographs confirm the accuracy of
each other, and the objective reality of the details shown of the
structure of the nebula.” Dr. Roberts adds: “These photographs throw
a strong light on the probable truth of the _Nebular Hypothesis_, for
they show what appears to be the progressive evolution of a gigantic
stellar system.”
The largest telescopes have hitherto completely failed to resolve
this wonderful object into stars. Dr. Huggins, however, finds that
the spectrum is _not_ gaseous, so that if the nebula really consists
of stellar points, they must be of very small dimensions.
The question may be asked, What is the probable size and distance of
this wonderful nebula? and could it be an external universe?
The temporary star which appeared near the nucleus of the nebula in
August, 1885, was of the seventh magnitude. I find that our sun,
if placed at the distance indicated by a parallax of 1/200th of a
second, would be reduced to a star of about the eleventh magnitude,
or four magnitudes fainter than the temporary star appeared to
us. That is to say, the star would have been—with the assumed
distance—about forty times brighter than the sun. With any greater
distance, the star would have been proportionately brighter, compared
with the sun. This seems improbable, and tends to the conclusion
that the nebula is _not_ an external galaxy, but a member of our
own sidereal system, a system which probably includes all the stars
and nebulæ visible in our largest telescopes. Dr. Common, indeed,
suggests that it may be comparatively near our system. He says:
“It is difficult to imagine that such an enormous object, as the
Andromeda nebula must be, is not very near to us; perhaps it may be
found to be the nearest celestial object of all beyond the Solar
System. It is one that offers the best chance of the detection of
parallax, as it seems to be projected on a crowd of stars, and there
are well-defined points that might be taken as fiducial points for
measurement,” and he adds: “Apart from the great promise this nebula
seems to give of determining parallax, there is a fair presumption
that in the course of time the rotation of the outer portion may
perhaps be detected by observation of the positions of the two outer
detached portions in relation to the neighboring stars.”
The spiral nebulæ are wonderful objects, and were discovered by the
late Lord Rosse with his great six-foot telescope. Their character
has been fully confirmed by photographs taken by Dr. Roberts. One
of the most remarkable of these extraordinary objects is that known
as 51 Messier. It lies about three degrees southwest of the bright
star Eta Ursæ Majoris—the star at the end of the Great Bear’s tail.
It was discovered by Messier while comet-hunting on October 13,
1773. Telescopes of moderate power merely show two nebulæ nearly
in contact, but Lord Rosse saw it as a wonderful spiral, and his
drawing agrees fairly well with a photograph taken by Dr. Roberts in
April, 1889. The nebula has also been photographed by Dr. Common. Dr.
Roberts says: “The photograph shows both nuclei of the nebula to be
stellar, surrounded by dense nebulosity, and the convolutions of the
spiral in this as in other spiral nebulæ are broken up into star-like
condensations with nebulosity around them. Those stars that do not
conform to the trends of the spiral have nebulous trails attached
to them, and seem as if they had broken away from the spirals.” A
tendency to a spiral structure in the smaller nebula is also visible
on the original negative. Dr. Huggins finds that the spectrum is
_not_ gaseous.
The nebula known as 99 Messier is of the spiral form. It lies on the
borders of Virgo and Coma Berenices, near the star 6 Comæ. In large
telescopes it somewhat resembles a “Catherine wheel.” D’Arrest and
Key thought it resolvable into stars. It has been photographed by M.
Von Gothard.
Among the clusters and nebulæ, we may class the Magellanic Clouds,
or Nubeculæ in the Southern Hemisphere, as they consist of stars,
clusters, and nebulæ.
Among the so-called nebulæ are many objects which, when examined
with telescopes of adequate power, are seen to be resolved into
myriads of small stars; their comparative isolation from surrounding
objects impresses us forcibly with the idea that they form, as it
were, families of stars connected by some physical bond of union. Of
these clusters, as they are called, we have naked-eye examples in
the Pleiades and the “Bee-Hive” in Cancer. Others may be partially
seen with a good opera-glass or binocular, but most of them require
telescopes of considerable power to view them to advantage. They
are of various forms and of all degrees of condensation. Some are
comparatively large and irregular, others small and compressed, with
the component stars densely crowded. Many are of such uniform shape
as to have received the name of globular clusters. These have been
aptly termed “balls of stars,” and are among the most interesting
objects in the stellar heavens.
The most remarkable object of this class visible in the Northern
Hemisphere is that known as 13 Messier. It lies between the tolerably
bright stars Zeta and Eta Herculis, nearer the latter star. It may be
seen with an opera-glass as a hazy-looking star of about the sixth
magnitude, with a star on each side of it. Examined with a powerful
telescope, it is resolved into numerous small stars. Sir William
Herschel estimated them at 14,000, but the real number is probably
much less. Assuming the average magnitude of the components at twelve
and a half, I find that an aggregation of 14,000 stars of this
brightness would shine as a star of about the second magnitude, or a
little fainter.
Another object of the globular class, but less resolvable, is that
known as 92 Messier, which lies between the stars Eta and Iota in
Hercules, nearer the latter. Sir William Herschel’s telescopes showed
it as seven or eight minutes of arc in diameter. It is considerably
brighter at the centre. The larger components are easily visible in
moderate-sized telescopes, but even Lord Rosse’s giant instrument
failed to resolve the central blaze. There is no doubt, however,
that it consists wholly of small stars, as the unerring eye of
the spectroscope shows a stellar spectrum, similar to that of the
neighboring 13 Messier.
[Illustration: Fig. 17.—The Region of Boötes and Hercules]
Another fine example of the globular class is 5 Messier, which lies
closely north, preceding the fifth magnitude star, 5 Serpentis. It is
considerably compressed at the centre. Sir William Herschel counted
200 stars, but failed to resolve the central nebulosity. Messier, its
discoverer, found it visible with a telescope only one foot long.
Another fine object is 3 Messier, in Boötes. Admiral Smyth describes
it as “a brilliant and beautiful globular aggregation of not less
than 1,000 small stars.” It is beyond the power of small telescopes,
but it was resolved by Buffham, even in the centre, with a 9-inch
reflector.
Numerous fine examples of the globular class are found in the
Southern Hemisphere, which indeed seems to be richer in these
marvelous objects than the northern sky. Of these the most
interesting are those known as Omega Centauri and 47 Toucani. Omega
Centauri, from its great apparent size—about two-thirds of the
moon’s diameter—and its visibility to the naked eye, may perhaps be
considered as the most remarkable object of its kind in the heavens.
It shines as a hazy star of the fourth magnitude, and I have often
so seen it in the Punjab sky. Its large size and globular form are
clearly visible in a binocular field-glass, but, of course, its
component stars are far beyond the reach of such an instrument. Sir
John Herschel, observing it with his large telescope at the Cape of
Good Hope, found it a “truly astonishing object. All clearly resolved
into stars of two magnitudes, viz., thirteen and fifteen, the larger
lying in lines and ridges over the smaller;... the larger form rings
like lace-work on it.” If we take the average magnitude of the
components at thirteen and a half, the apparent brightness of the
cluster would imply that it contains about 15,000 stars.
[Illustration: The Great Nebula in the Constellation Cygnus]
The other wonderful cluster is that known as 47 Toucani, which lies
close to the smaller Magellanic Cloud. It is smaller in apparent
size than Omega Centauri, but Dr. Gould, observing it at Cordoba,
speaks of it as “one of the most impressive and perhaps the grandest
of its kind in either hemisphere,” and he estimates its magnitude at
four and a half, as seen with the naked eye. It is thus described by
Sir John Herschel: “A most magnificent globular cluster. It fills
the field with its outskirts, but within its more compressed part I
can insulate a tolerably defined, circular space, of 90″ diameter,
wherein the compression is much more decided, and the stars seem to
run together, and this part, I think, has a pale pinkish or rose
color, which contrasts, evidently, with the white light of the rest.
The stars are equal, fourteen magnitude, immensely numerous and
compressed.... Condensation in three distinct stages.... A stupendous
object.” Sir John Herschel’s drawing of this cluster reminds one of a
swarm of bees, and perhaps suggested to Tennyson the lines:
“Clusters and beds of worlds, and bee-like swarms
Of suns and starry streams.”
There are other interesting specimens of the globular class in the
Southern Hemisphere, but not of such large apparent dimensions as
those already described. Of these may be mentioned 22 Messier, which
lies about midway between the stars Mu and Sigma Sagittarii. It is
described by Sir John Herschel as a fine globular cluster, with stars
of two magnitudes, namely eleven or twelve, and fifteen or sixteen,
the larger being visibly reddish, and he suggested that it consists
of “two layers, or one shell over another.” Owing to the comparative
brightness of the larger components, this cluster forms a good object
for small telescopes. I saw the brighter stars well with a 3-inch
refractor in the Punjab sky, but, of course, the greater portion of
the cluster has a nebulous appearance in a telescope of this size.
Between Alpha and Beta Scorpii there is a condensed globular cluster.
With small telescopes it very much resembles a telescopic comet, but
with larger instruments its true character is revealed. Sir William
Herschel considered it “the richest and most condensed mass of stars
in the firmament.” In May, 1860, a “temporary star” of the seventh
magnitude suddenly appeared in the centre, almost blotting out the
cluster by its superior light. The star faded away before the end
of June of the same year, and has not been seen with any certainty
since. It has been suggested that this temporary star lay _between_
the cluster and the earth, but it seems to me much more probable that
the outburst took place _in_ the cluster itself, and that it was
possibly caused by a collision between two of the component stars,
or by a swarm of meteors rushing with a high velocity through the
cluster.
The beauty and sublimity of the spectacle presented by these globular
clusters, when viewed with a powerful telescope, is such as can not
be adequately described, and it has been said that when seen for the
first time, “few can refrain from a shout of rapture.” The component
stars, although distinctly visible as points of light, defy all
attempts at counting them, and seem literally innumerable. Placed
like a mass of glittering diamond-dust on the dark background of
the heavens, they impress us forcibly with the idea that if each of
these lucid points is a sun, the thousands which seem massed together
in so small a space must be in reality either relatively close and
individually small, or else the system of suns must be placed at a
distance almost approaching the infinite.
The distance of these globular clusters from the earth is, however,
certainly very great. Attempts to accurately determine their position
in space have not been attended with success. As the component
stars are at practically the same distance from the eye, we have
no comparison stars to measure from, and their exact distance,
therefore, remains unknown. We may, however, estimate their probable
distance with some show of plausibility. We may assume that the stars
of the Hercules cluster would, if concentrated in a point, shine
as a star of about the fourth magnitude. As the components are of
about the twelfth and thirteenth magnitudes, this would imply that
the cluster consists of about 2,500 stars. With the data assumed, we
may therefore conclude that the components of the Hercules cluster
are suns of comparatively small size, separated by considerable
distances, but apparently massed together by the effect of distance.
Among less condensed star clusters there are many interesting
objects. The Pleiades have been already referred to. On a photograph
of this remarkable group, taken at the Paris Observatory, over 2,000
stars can be counted of all degrees of brilliancy, from those
visible without optical aid down to points of light so faint as
to be invisible to the eye in the telescope with which they were
photographed. Here we have a cluster of probably larger size than
that in Hercules, probably at a greater distance from the earth, and
with its larger components of considerably greater mass than our sun.
Near the bright star Pollux, I see a small cluster of stars of
about the seventh and eight magnitudes, which, with a binocular
field-glass, very much resembles the Pleiades as seen with the naked
eye. A smaller cluster (known as 39 Messier) may be seen near the
star Pi Cygni.
The well-known Chi Persei may be also seen with an opera-glass, but
a telescope is necessary to show the component stars to advantage,
and the larger the telescope the greater the number of faint stars in
these wonderful objects.
The cluster known as 35 Messier, a little north of the star Eta
Geminorum, is visible in an opera-glass, but a small telescope
is required to see the component stars. A well-marked clustering
tendency is visible among the brighter stars of the group, two,
three, four, and sometimes five stars being grouped together in
subordinate collections. Admiral Smyth says: “It presents a gorgeous
field of stars from the ninth to the sixteenth magnitude, but with
the centre of the mass less rich than the rest. From the small stars
being inclined to form curves of three or four, and often with a
large one at the root of the curve, it somewhat reminds one of the
bursting of a sky-rocket.” This tendency to “stream” formation in
the components of star clusters is also well marked in a photograph
of the cluster 38 Messier (kindly sent to me by MM. Henry of the
Paris Observatory). It was described by Webb as “a noble cluster
arranged in an oblique cross,” and Smyth says: “The very unusual
shape of this cluster recalls the sagacity of Sir William Herschel’s
speculations upon the subject, and very much favors the idea of an
attractive power lodged in the brightest part. For although the form
is not globular, it is plainly to be seen that there is a tendency
toward sphericity, by the swell of the dimensions as they draw near
the most luminous part, denoting, as it were, a stream or tide of
stars, setting toward the centre.”
Sir W. Herschel, speaking of a compressed cluster in Perseus, says
“the large stars are arranged in lines like interwoven letters,” and
Webb says “it is beautifully bordered by a brighter foreshortened
pentagon.”
Observing with a 3-inch telescope in India, I noticed a beautiful
cluster of stars, about 4° north of Gamma and Upsilon Scorpii,
resembling in shape a bird’s foot, with remarkable streams of stars.
This cluster is visible to the naked eye as a star of about the fifth
magnitude.
Although these loosely associated star clusters do not show such
evidence in favor of family connection as the more closely compacted
globular clusters, still we can hardly escape from the conviction
that their apparent aggregation is really due to some physical bond
of union, and not merely the result of a fortuitous scattering of
stars at different distances in the line of sight.
THE GREAT NEBULA OF ORION.—SIR ROBERT S. BALL
The telescope, ever an ally in the study of the heavens, is in this
part of the science absolutely indispensable. In other branches of
astronomy we can learn something without its aid. Indeed, many great
astronomical discoveries were made long before the telescope was
invented. But ere this memorable event in the history of science
it was impossible for us to know anything of the existence of the
nebulæ. It is indeed true that there is one of these objects which
can be just detected by the naked eye. It lies in the constellation
of Andromeda, where, on a clear and dark night, a faint spot of
light can just be discerned by a good eye. But a mere glimpse gives
us really no adequate notion of the true character of the object.
It might only, so far as the naked eye discloses its nature, be a
cluster of stars like that we have already discerned in Perseus,
or like the similar group that, under the name of the Beehive, is
comparatively familiar in the constellation of Cancer. With the
single exception of the nebula in Andromeda, all the objects so
called are entirely telescopic, yet how important a constituent the
nebulæ form in the contents of the heavens will be shown by a look at
some of the lists of these objects. There are now several thousands
of nebulæ known, and their positions in the sky, as well as the
details of their appearances, are set forth in the catalogues.
The most glorious constellation of stars in the firmament is
undoubtedly that of Orion. This splendid group is seen in the
south during the winter months, and toward the close of January it
is situated in a very convenient position for observing early in
the evening. The group is specially characterized by the number
of unusually bright stars which it includes, and the three stars
in the centre, forming the so-called Belt of Orion, is as well
known a celestial figure as the sky contains. Directly under the
belt are three much smaller stars nearly in a line, which points
straight upward to the middle star of the belt. These three lower
stars are usually known as the sword handle of Orion, this being
the position which they occupied in the fanciful old sketches of
the constellation. The three stars of the sword handle of Orion are
plunged in the Great Nebula. This object can not be seen by the
unassisted eye, though doubtless around the central star a little
haziness is perceptible, and even the slightest telescopic aid will
suffice to indicate that the central star of the sword handle is
attended by a surrounding glow of light, which renders it quite
unlike other stars. This can indeed be sufficiently shown with an
ordinary opera-glass, one glance through which will awaken in the
beholder a keen desire to study the object under more favorable
conditions. But to do justice to the object, telescopes of large
power are desirable.
To realize fully the magnificence of the Great Nebula, the observer
who is being introduced to the object for the first time should not,
strange to say, direct the telescope at the nebula; the instrument
should rather be pointed at the heavens, just a little to the
west of the nebula. The clock driving the equatorial should not be
started, and the observer should take his seat and look through the
eye-piece before the nebula has entered the field. He will see, no
doubt, a few stars on the black background, which gradually pass in
procession across his field of view. This is merely the ordinary
diurnal journey of the heavens, by which all the objects move slowly
from east to west; I ought rather to say _appear_ to move, for, of
course, the motion on the heavens is only apparent, the fact being
that it is the earth which is turning round.
After the observer’s eye for a minute or so has become familiarized
with the dark aspect of the heavens under ordinary circumstances,
he will begin to perceive on the eastern side (it will appear in
the telescope no doubt as on the western side) a faint dawn of
light. Gradually there will steal across his field of view a sort
of ghostlike luminosity that is in marked contrast to the darkness
in the rest of the field; as the seconds move on, this object will
disclose itself until the full splendor of the Great Nebula comes
into view; then the entire field will be filled with the light,
and then it will gradually advance and gradually pass away again
to emphasize the contrast between the brilliance of the nebula and
the darkness of the sky. Unless this method is adopted, the full
interest of a telescopic view of the Great Nebula is not attained,
for when the entire field is full of the glow the beginner will
hardly recognize the nebula. He will be apt to think that the fainter
part of the field he sees is the ordinary groundwork of the sky, and
this illusion can only be dispelled by enabling him to witness the
actual contrast in the way I have described. The central portions
of the nebula are, however, so brilliant and so wonderfully marked
with interesting detail, that even a small instrument will suffice to
reveal much of its beauties.
In the centre of the nebula is the star known to astronomers at
Theta Orionis, the most prominent star of the sword handle. To the
eye this looks like an ordinary star, but the telescope speedily
dispels that notion. Theta Orionis is found to consist of four, or
rather six, stars all so close together that the unaided eye fails
to distinguish them separately. A structure so complex gives to this
star quite a special, indeed a unique, interest, wholly apart from
the marvelous nebula of which it is the focus. We must dwell a little
on the peculiarities of this star. We are familiar with stars which
are called double; there are indeed some ten thousand objects so
designated known to astronomers and duly registered in catalogues.
Many of these double stars are objects of extreme telescopic beauty;
sometimes they offer to our admiration a delightful contrast of
colors; perhaps one will be topaz color and the other bluish, or on
rare occasions a pair of emerald gems will be seen with an invisible
band of mutual connection. Sometimes triple stars are found, in which
three stars are obviously in alliance; but multiple stars of greater
complexity are comparatively rare; and so marvelous a spectacle
as Theta Orionis, in which no fewer than six stars are obviously
an allied group, is almost unique. It is not a little remarkable
that we find the most exquisite multiple star which the sky can
show, beautifully framed or set in the centre of the grandest of
the nebulæ. Of course it might conceivably happen that the apparent
concourse of these objects was fortuitous. The actual phenomenon
could be accounted for by the belief that the Great Nebula was either
very much nearer or very much further than the multiple star, and
that they chanced to lie in the same line of sight, and had no other
connection. But to me it appears that this view is quite at variance
with every reasonable probability; that the most wondrous multiple
star should have happened to lie in line with the very centre of
the most wondrous nebula would have been a coincidence against the
occurrence of which the probabilities were almost infinite. There can
scarcely be any doubt that the multiple star and the Great Nebula are
part of the same system, and that the star is, in truth, placed in
the middle of the nebula, as it actually appears to be.
And now as to the composition of this mysterious object.
The word nebula means, of course, a little cloud, but the expression
is apt to be a misleading one. In a sense no doubt they are little,
inasmuch as the patch of the sky which a nebula covers would be small
compared with one of our ordinary clouds. Indeed, a nebula which
covered as large an apparent part of the sky as the size of the
moon would be ranked as a large object of its class, while even the
greatest of them is perhaps not more than ten or twelve times as
great. Nor is the word cloud, as applied to nebula, an appropriate
one. What we mean by a cloud is only a vast mass of watery vapor
raised by the sun from the sea, and poised aloft until such time as
it shall be again dispersed into invisible water, or until it shall
descend to the earth as rain. Such clouds are, of course, within
the limits of our atmosphere, and are rarely more than a few miles
above the earth’s surface. The light which renders clouds visible
only comes from reflected sunbeams, and consequently at night
clouds become invisible, though the astronomer is often only too
unpleasantly made acquainted with their presence by the opacity with
which they shut out the stars from his view.
Utterly different in all respects are the nebulæ. They are not masses
of watery vapor. It may no doubt possibly be that water in some form
is there, but it is not water which we see. We are looking at some
gaseous material of a bluish hue. The light with which it glows is
no reflected sunlight. The nebula is indeed indebted to no foreign
source for that weird—I had almost said ghostlike—radiance which
it gives forth. The light comes from the nebula itself. But how,
it may well be asked, should a purely gaseous substance be able to
radiate forth light? It is easy for us to comprehend how stars or
suns or comparatively solid bodies can, in virtue of their tremendous
temperature, glow with heat like red-hot or white-hot iron. It is
true that flame is gas in an incandescent state, but in flame a
vehement chemical union of oxygen with some other substance is in
progress, and this is the source of the heat and the light that
flame gives forth. We can not regard the Great Nebula in Orion as
originating in anything resembling flame.
We can, however, in our physical laboratories arrange an experiment
which seems to throw some light on the composition of the nebula.
Into a glass tube a small quantity of hydrogen gas is admitted, the
air having been previously extracted. Then, by means of two wires,
one at each end of the tube, an electric current is transmitted
through the gas. Here there is no combustion; the gas is merely the
vehicle by which the electricity flows from one pole to the other.
In doing so the gas instantly begins to glow with an intense bluish
light, and a very beautiful effect is produced, which can be renewed
or terminated at will by simply making or breaking the electric
current. It would seem as if the gas we see in the nebula were in a
condition somewhat analogous to the gas in the tube. I do not mean
that the passage of electricity through the nebula is the source of
its luminosity. There is, indeed, no ground for such a supposition.
It is the property of electricity when passing through a conductor
to warm that conductor; thus we know that if a powerful current be
transmitted through a wire of the most infusible of all metals,
platinum, the wire will not only get warm, but it may become red
hot, white hot, and even melt under the influence of the heat which
is generated. In those beautiful incandescent electric lamps which
are now happily coming into extensive use a current of electricity
flows through a filament of carbon, and kindles that exquisite
incandescence which is maintained while the current flows. It would
appear that so long as the electricity is flowing through the glass
tube its action on the gas is to impart a very high temperature. It
is in consequence of this temperature that the gas glows. Now we can
offer a reasonable account of the luminosity of the Great Nebula in
Orion. The particles of gaseous or vaporous material of which it is
formed are of an extremely high temperature, sufficient to enable
them to glow with the brilliancy which renders them visible.
It is now almost twenty years since a marvelous accession to our
knowledge of such objects as the Great Nebula in Orion was made
by Dr. Huggins. I have used our gas hydrogen as an illustration
in describing the character of the nebula, but I have now to add
that the presence of hydrogen is no mere fiction but a substantial
verity. Truly we here open up one of the most marvelous chapters
which science has to disclose. The chemist can analyze the different
substances on the earth with his test tubes, and he can tell the
elements of which they are composed. But in this old-fashioned
chemistry it was at least reasonable for the chemist to demand a
portion of the substance he was expected to analyze. Unless he were
provided with a sample, how could it be possible for him to grind
it up or submit it to the various operations of his laboratory? In
these modern days the chemist can perform operations of which his
predecessors never even dreamed. No doubt the old method is still
used—nay, is indeed at this moment cultivated with greater skill and
means than in any previous age—but side by side with the old method,
and as an invaluable supplement thereto, the new method of chemical
research, called spectrum analysis, has been created, and has already
conducted to many profoundly interesting discoveries in the most
varied branches of science.
In the application of the spectroscopic method it is not
indispensably necessary that we actually have a fragment of the
substance; all we require is a beam of light which that substance can
be made to yield when heated to a sufficiently high temperature.
When a beam of the nebular light is transmitted through the prisms,
it declares at once that the object from which that light has come
is totally different from a star like the sun. Instead of the
beautifully colored band, decked in all the glowing hues of the
rainbow, the nebular beam is seen to be composed simply of six or
seven widely separated strips. It is important to test the character
of the light in these strips. Fortunately this can be done in a way
that is completely satisfactory. We can produce artificial lights
from known sources, and observe them through the spectroscope
simultaneously with the light of the nebula.
There are in the composition of this globe some sixty or seventy
different elementary substances, and under suitable conditions
each one of these substances can afford a perfectly characteristic
spectrum. Thus the way of making the comparison with the nebula is
to try the different elements one after another, until one can be
discovered which pours forth a light that behaves under the prism
as does the light from the nebula. Pursuing this inquiry, Dr.
Huggins found that when hydrogen gas was ignited to incandescence by
the passage of electricity, it emitted light which, after passage
through the prisms, came to coincidence with one of the lines in the
spectrum of nebula; and the hydrogen character of two of the other
lines has been since demonstrated. It was thus established that
hydrogen is one of the constituents of the Great Nebula in Orion.
Further confirmation of this important discovery was forthcoming
when the photographs of the spectrum of the Great Nebula were
subsequently obtained. On these photographs lines were present which
are constituted by light of such a nature as to be wholly invisible
to the eye, though perceptible on the photographic plate. It is of
the greatest interest to discover that these invisible rays from
the nebula are also indicative of the presence of hydrogen. Thus we
obtain a beautiful confirmation of the fact that the nebula is partly
composed of glowing hydrogen.
There are, however, some remaining lines, the character of which has
not yet been ascertained.
It would be a little premature to assert that there must be some
substance in the Great Nebula not at present known to us on the
earth. This would be, no doubt, one interpretation of the facts. We
must, however, admit the possibility of another explanation. It is
frequently found that the lines yielded by an incandescent material
vary to some extent when the physical conditions of temperature and
of pressure are modified. It is, therefore, not impossible that
the unknown lines in the spectrum of the Great Nebula may be due to
some element known to us, but which has not yet been tested under
the conditions which would make it yield the particular rays we are
speaking of.
The composition of a nebula as disclosed to us by these researches
is very instructive. Here we are looking at an object which seems to
lie at the very limits of the visible universe—an object so remote
that our attempts to fathom its distance are quite unsuccessful; yet
in this inconceivably distant part of our system we find at least one
ingredient which we know well on the earth. Previous to actual trial
no one would have expected, I think, to find the Great Nebula largely
constituted from such a familiar element as hydrogen. This gas enters
into the composition of water, and is thus an element of extreme
abundance on the earth. That an element so common with us here should
also be abundant in these awfully distant regions of the universe is
one of the most astonishing facts that modern science has revealed.
As the eye follows these ramifications of the Great Nebula, ever
fading away in brightness until it dissolves in the blackness of
the sky; as we look at the multitudes of bright stars which sparkle
out from the depths of the great glowing gas; as we ponder on the
marvelous outlines of a portion of the nebula, we are tempted to
ask what the true magnitude of this object must really be. Here,
again, we have to confess that science is unable to satisfy this very
legitimate curiosity. The only means of learning the true length
and breadth of a celestial object depends upon our first having
discovered the distance from us at which the object is situated.
Unhappily we are, as I have said, entirely ignorant of what this
distance may be in the case of the Great Nebula in Orion. Our
ordinary methods of conducting such an inquiry are hardly applicable
to such an object, and its position so near the Equator introduces
fresh difficulties into the problem. We shall, however, certainly not
err on the side of exaggeration if we assert that the Great Nebula
must be many millions of times larger than that group of bodies which
we call the Solar System.
COLORED, DOUBLE, MULTIPLE, BINARY, VARIABLE AND TEMPORARY STARS.
—J. E. GORE
On a clear night a careful observer will notice a marked difference
in the colors of the brighter stars. The brilliant white or
bluish-white light of Sirius, Rigel, and Vega contrasts strongly
with the yellowish color of Capella, the deeper yellow, or orange,
of Arcturus, and the ruddy light of Aldebaran and Betelgeuse. These
colors are, however, limited to various shades of yellow and red.
No star of a _decided_ blue or green color is known, at least among
those visible to the naked eye in the Northern Hemisphere. The third
magnitude star Beta Libræ is described by Webb as of a “beautiful
pale green hue,” but probably such a tint in the light of this star
will to most people prove quite imperceptible. Dr. Gould, observing
it in the Southern Hemisphere—under, of course, more favorable
conditions—says: “There is a decidedly greenish tinge to the light
of Beta Libræ, although its color can not properly be called
conspicuous.”
Among the ruddy stars visible to the naked eye, Mu Cephei, Herschel’s
“garnet star,” is generally admitted to be the reddest, but it is
not sufficiently bright to enable its color to be well distinguished
without the aid of an opera-glass. With such an instrument, however,
its reddish hue is striking and beautiful, and very remarkable when
compared with other stars in its vicinity. Like so many of the red
stars, Mu Cephei is variable in its light, but seems to have no
regular period, and often remains for many weeks without perceptible
change. It may be seen near the zenith in the early evening hours
toward the end of October, and when in this position its ruddy color
is very conspicuous.
Among the brightest stars, Betelgeuse is perhaps the reddest, and
the contrast between its ruddy tint and the white color of Rigel in
the same constellation (Orion) is very noticeable. Like Mu Cephei,
Betelgeuse is irregularly variable in its light, but not to such
an extent, and, like the “garnet star,” it frequently remains for
protracted periods nearly constant in brightness. There are other
cases of reddish color among the naked-eye stars. Among these may
be mentioned Antares (Alpha Scorpii), Alphard (Alpha Hydræ), noted
as red by the Persian astronomer Al-Sûfi, in the Tenth Century, and
called by the Chinese “The Red Bird”; Eta and Mu Geminorum; Mu and
Nu Ursæ Majoris; Delta and Lambda Draconis; Beta Ophiuchi; Gamma
Aquilæ, and others in the Southern Hemisphere.
But it is among the stars below the limit of naked-eye vision that
we meet with the finest examples of the red stars. Some of these are
truly wonderful objects. The small star, No. 592 of Birmingham’s
Catalogue of Red Stars (No. 713 of Espin’s edition), which lies
a little south of the 5½ magnitude star 79 Cygni, was described
as “splendid red” by Birmingham, “very deep red” by Copeland and
Dreyer, and “orange vermilion” by Franks. The star 248 Birmingham,
which lies about 5° south of Gamma Hydræ, is another fine specimen.
Birmingham described it as “fine red” and “ruby”; Copeland as “brown
red”; Dreyer as “copper red”; and Espin as “magnificent blood red.”
This star is variable in light, as the estimates of magnitude range
from 6.7 to below 9. About 3° to the northeast of this remarkable
object is another highly-colored star, known as R Crateris. It is
easily found, as it lies in the same telescopic field of view with
Alpha Crateris, a 4½ magnitude star. Sir John Herschel described it
as “scarlet, almost blood-color; a most intense and curious color.”
Birmingham called it “crimson”; and Webb “very intense ruby.”
Observing it with a 3-inch refractor in India in 1875, I noted it as
“full scarlet.” It varies in light from above the eighth magnitude to
below the ninth, and has near it a star of the ninth magnitude of a
paler blue tint.
Another very red star is No. 4 of Birmingham’s Catalogue, which will
be found about 5° north, preceding the great nebula in Andromeda. It
is of about the eighth magnitude, and may be well seen with a 3-inch
refractor. Krüger describes it as “_intensiv roth_”; Birmingham as
“fine red” and “crimson”; Franks as “fine color, almost vermilion”;
and Espin as “intense red color, most wonderful.”
Another fine object is R Leporis, which forms roughly an equilateral
triangle with Kappa and Mu Leporis. This is also variable from 6½
to 8½ magnitude. It was discovered by Hind in 1845, and described
by him as “of the most intense crimson, resembling a blood-drop on
the background of the sky; as regards depth of color, no other star
visible in these latitudes could be compared with it.” Schönfeld
called it “_intensiv blutroth_,” but Dunér, observing its spectrum in
1880, gives its color as a less intense red than that of other stars.
Possibly it may vary in color as well as in light.
The variable star U Cygni, which lies between Omicron and Omega
Cygni, is also very red. Webb described it as showing “one of the
loveliest hues in the sky.” It varies from about the seventh to 11½
magnitude, with a period of about 461 days.
Another deeply colored star is the well-known variable R Leonis.
Hind says: “It is one of the most fiery-looking variables on our
list—fiery in every stage from maximum to minimum, and is really
a fine telescopic object in a dark sky about the time of greatest
brilliancy, when its color forms a striking contrast with the steady
white light of the sixth magnitude a little to the north.” This
latter star is 19 Leonis.
In the Southern Hemisphere there are some fine examples of red stars.
Epsilon Crucis, one of the stars in the Southern Cross, is very red.
Mu Muscæ is described by Dr. Gould as of “an intense orange red.”
Delta^2 Gruis is a very reddish star of about the fourth magnitude.
Pi^1 Gruis was observed by Gould as “deep crimson,” and forming a
striking contrast with its white neighbor Pi^2 Gruis, which he notes
as “conspicuously white.” The variable L_{2} Puppis is described as
“red in all its stages, and remarkably so when faint.” Miss Clerke,
observing—at the Cape of Good Hope—R Doradûs, another southern
variable, says: “This extraordinary object strikes the eye with the
glare of a stormy sunset,” and with reference to the variable R
Sculptoris, described by Gould as “an intense scarlet,” she says:
“The star glows like a live coal in the field,” a description I have
found myself very applicable to other small red stars.
An eighth magnitude star about 5° north of Beta Pictoris is noted by
Sir John Herschel, in his _Cape Observations_, as “vivid sanguine
red, like a blood-drop. A superb specimen of its class.” With
reference to a star of about 8½ magnitude in the field with Beta
Crucis, Herschel says: “The fullest and deepest maroon red; the most
intense blood-red of any star I have seen. It is like a drop of blood
when contrasted with the whiteness of Beta Crucis.”
Of stars of other colors, the asserted green tint of Beta Libræ has
already been referred to. Among the brighter stars of the Southern
Hemisphere, Theta Eridani, Epsilon Pavonis, Upsilon Puppis, and
Gamma Tucanæ are said to be decidedly blue. The wonderful cluster
surrounding the star Kappa Crucis contains several bluish, greenish
and red stars, and is described by Sir John Herschel as resembling “a
superb piece of fancy jewelry.”
Among the double stars we find many examples of colored suns. Of
these may be mentioned Epsilon Boötis, of which the colors are
“most beautiful yellow” and “superb blue,” according to Secchi;
Beta Cephei, “yellow and violet”; Beta Cygni, “golden yellow and
smalt blue”; Gamma Delphini, of which I noted the colors in 1874
as “reddish yellow and grayish lilac”; Alpha Herculis, “orange and
emerald or bluish green,” and described by Admiral Smyth as “a lovely
object, one of the finest in the heavens”; Zeta Lyræ, “pale yellow
and lilac” (Franks); and Beta Piscis Australis, of which I observed
the colors in India as white and reddish lilac.
Some distant telescopic companions to red stars have been described
as blue. This may be in some case due, partly at least, to the effect
of contrast. In others the blue color seems to be real. This has been
shown spectroscopically to be the case with the bluish companions of
Beta Cygni.
The physical cause of the difference of color is still more or less
a matter of mystery. Although we can not consider it proved that the
red stars are cooling and “dying out” suns, as has been suggested,
we may, I think, conclude that their temperature, although doubtless
very high, must be lower than that of the white stars. We know that
a bar of iron when heated to redness is not so hot as when raised to
“white heat,” and although the analogy between hot iron and stellar
photospheres may not be a perfect one, it seems probable that the
higher the temperature of a star, the whiter its color will be.
Most of the white stars, as Sirius, Vega, and those only yellow or
slightly colored, show spectra of Secchi’s first and second types,
while the great majority of the red stars exhibit banded spectra of
the third and fourth types.
To this rule there are, however, like other rules, some notable
exceptions. For instance, Aldebaran, Alpha Hydræ, Xi Cygni, and 31
Orionis, although distinctly reddish stars, show well-marked spectra
of the second or solar type. On the other hand, Rho Ursæ Majoris and
Omega Virginis, which, according to Dunér, are only slightly yellow,
have well-marked spectra of the third type.
An apparent change of color seems in some cases to be well
established. The supposed red color of Sirius in ancient times is
well known. A certain established change is found in the case of the
famous variable star Algol, which is distinctly described as red by
Al-Sûfi in the Tenth Century. It is now pure white, or nearly so, and
this is probably the best attested instance on record of change of
color in a bright star.
Schmidt’s Nova Cygni of 1876 was noted as “golden yellow” on the
night of its discovery. When it had faded to the eighth magnitude,
Dr. Copeland called it “decided red,” but when examined at Lord
Crawford’s observatory in September, 1877, its color was recorded as
“faint blue”! The new star in the Andromeda nebula was considered to
be yellowish or reddish by most observers when near its maximum, but
about a month later its color was noted as “bluish.”
Among the red and variable stars, there are many suspected cases
of color variation. Espin and other observers have noted that the
wonderful variable Mira Ceti is much less red at maximum than at
minimum. My own observations confirm this. When at its maximum
brightness, Mira does not seem to me a very highly-colored star,
while at one of its minima I noted it as “fiery red.” Possibly,
however, the great difference between its maximum and minimum
brilliancy may have an influence on estimations of its color. The
remarkable variable Chi Cygni is said to be “strikingly variable in
color.” Espin’s observations in different years show it “sometimes
quite red, at others only pale orange red.” The star Birmingham 118
was described by Schjellerup in 1863 as “decided red,” but it was
found yellow by Secchi in 1868; “bluish” by Birmingham, 1873-76; “no
longer red” by Schjellerup in March, 1876; and “white” by Franks
in 1885. Espin omits it from his revised edition of Birmingham’s
Catalogue.
Birmingham 169 was found red by Struve, blue or bluish-white by
Birmingham in 1874, and white at Greenwich in the same year. Espin
also saw it white in March, 1888. The star Birmingham 30, which lies
close to Phi Persei (54 Andromedæ), was described by Schweizer
as a “red star with a little disk” in January, 1843; Birmingham
noted it as “light red” in December, 1875; Copeland “deep red” in
January, 1876; and Dreyer “reddish” in September, 1878; but Espin,
in November and December, 1887, found it “certainly not red, and
nothing peculiar in the star’s appearance.” It might be expected
that these curious changes of color, if real, would be accompanied
by corresponding changes in the star’s spectrum. Such may be the
case, and observations in this direction would probably lead to some
interesting results.
There seems to be some law governing the distribution of the colored
stars. The white stars appear to be most numerous, as a rule, in
those constellations where bright stars are most abundant, for
instance, in Orion, Cassiopeia, and Lyra; yellow and orange stars in
large and ill-defined constellations, such as Cetus, Pisces, Hydra,
Virgo, etc. The very reddish stars are most numerous in or near the
Milky Way, and one portion of the Galaxy—between Aquila, Lyra, and
Cygnus—was termed by Birmingham “the red region in Cygnus.”
Many of the stars when examined with a good telescope are seen to be
double, some triple, and a few quadruple, and even multiple. These
when viewed with the naked eye, or even a powerful binocular, seem
to be single, and show no sign of consisting of two components.
These telescopic double stars should be carefully distinguished
from those which appear very close together with the naked eye,
and which in opera-glasses or telescopes of small power might be
mistaken for wide double stars by the inexperienced observer. These
latter stars, such as Mizar—the middle star in the tail of the Great
Bear—and its small companion, Alcor, have been called “naked-eye
doubles,” but they are not, properly speaking, double stars at all.
Telescopic double stars are far closer, and even the widest of them
could not possibly be seen double without optical aid, even by
those who are gifted with the keenest vision. Of these so-called
“naked-eye doubles,” we may mention Alpha Capricorni, which on a
very clear night may be seen with the naked eye to consist of two
stars. On a very fine night two stars may be seen in Iota Orionis,
the most southern star in Orion’s Sword. The star Zeta Ceti has near
it a fifth magnitude star, Chi, which may be easily seen with the
unaided vision. The star Epsilon Lyræ (near Vega) is a severe test
for naked-eye vision. Bessel, the famous German astronomer, is said
to have seen it when thirteen years of age. Omicron Cygni (north of
Alpha and Delta Cygni) forms another naked-eye double, and other
objects of this class may be noticed by a sharp-eyed observer.
The star Mizar, already referred to, is itself a wide telescopic
double, and it seems to have been the first double star discovered
with the telescope (by Riccioli in 1650). It consists of two
components, of which one is considerably brighter than the other. It
will give an idea of the closeness of even a “wide” telescopic double
when we say that the apparent distance between Mizar and Alcor is
nearly forty times the distance which separates the close components
of the bright star. From this it will be seen that even a powerful
binocular field-glass would fail to show Mizar as anything but a
single star. The components may, however, be well seen with a 3-inch
telescope, or even with a good 2-inch. The colors of the two stars
are pale green and white. Between Mizar and Alcor is a star of the
eighth magnitude, and others fainter. Mizar was the first double star
photographed by Bond.
The Pole Star has a small companion at a little greater distance
than that which separates the components of Mizar, but owing to the
faintness of this small star, the object is not so easy as Mizar.
The star Beta Cygni is composed of a large and small star, of which
the colors are described as “golden yellow and smalt blue.” This is
a very wide double, and may be seen with quite a small telescope.
Another fine double star is that known to astronomers as Gamma
Andromedæ. The magnitudes of the components are about the same as
those of Mizar, but a little closer. Their colors are beautiful
(“gold and blue”). This is one of the prettiest double stars in the
heavens. It is really a triple star, the fainter of the pair being
a very close double star; but this is beyond the reach of all but
the largest telescopes. The star Gamma Delphini is another beautiful
object, the components being a little more unequal in magnitude, but
the distance between them about the same as in Gamma Andromedæ. I
have noted the colors with a 3-inch telescope as “reddish yellow and
grayish lilac.” Gamma Arietis, the faintest of the three well-known
stars in the head of Aries, is another fine double star, a little
closer than Gamma Delphini. This is an interesting object, from the
fact that it was one of the first double stars discovered with the
telescope—by Hooke, in 1664, when following the comet of that year.
Another beautiful double star is Eta Cassiopeiæ, the components
being about equal in brightness to those of Gamma Delphini, but the
distance less than one-half. The colors are, according to Webb,
yellow and purple; but other observers have found the smaller star
garnet or red. This is a very interesting object, the components
revolving round each other, and forming what is called a binary star.
Another fine double star is Castor, which is composed of two nearly
equal stars separated by a distance about half that between the
components of Gamma Andromedæ. This is also a binary, or revolving
double star, but the period is long. Gamma Virginis is another fine
double star, with components at about the same distance as those of
Castor, and the colors very similar. It is also a remarkable binary
star.
Among double stars of which the components are closer than those
mentioned above, but which are within the reach of a good 3-inch
telescope—a common size with amateur observers—the following may be
noticed: Alpha Herculis, colors, orange or emerald green; the light
of this star is slightly variable. Gamma Leonis, another binary star
with a long period; colors, pale yellow and purple. Epsilon Boötis,
a lovely double star, the colors of which Secchi described as “most
beautiful yellow, superb blue.”
For observers in the Southern Hemisphere, the following fine double
stars may be seen with a 3-inch telescope: Alpha Centauri; this
famous star, the nearest of all the fixed stars to the earth, is also
a remarkable binary; its period, as recently computed by Dr. See,
is eighty-one years. Theta Eridani is a splendid pair, but closer
than Alpha Centauri. It is, however, an easy object with a 3-inch
telescope, and with a telescope of this size I noted the colors in
India as light yellow and dusky yellow. The star known as ƒ Eridani
is a very similar double to Theta, but the components are fainter. I
noted the colors in India as yellowish-white and very light green.
Of triple, quadruple, and multiple stars, there are several which may
be well seen with a small telescope. Of these may be mentioned Iota
Orionis, the lowest star in the Sword of Orion, which consists of a
bright star accompanied by two small companions. In Theta Orionis,
the middle star of the Sword, four stars may be seen forming a
quadrilateral figure, known to observers as the “trapezium.” There
are two fainter stars in this curious object, which lie in the midst
of the Orion nebula, but a somewhat larger telescope is required
to see them. Within the trapezium are two very faint stars, which
are only visible in the largest telescopes. In Sigma Orionis—a star
closely south of Zeta, the lowest star in Orion’s Belt—six stars may
be seen with a 3-inch telescope.
Double and multiple stars may be either optical or real. Optical
double stars are those in which the component stars are merely
apparently close together, owing to their being seen in nearly the
same direction in space. Two stars may _seem_ to be close together,
while, in reality, one of them may be placed at an immense distance
behind the other. Just as two lighthouses at sea may, on a dark
night, appear close together when viewed from a certain point,
whereas they may be really miles apart. In the case of double stars
it is, of course, always difficult to determine whether the apparent
closeness of the stars is real or merely optical. But when, from a
long series of observations of their relative position, we find that
one is apparently moving round the other, we know that the stars must
be comparatively close, and linked together by some physical bond
of union. These most interesting objects are known to astronomers
as binary, or revolving double stars. The probable existence of
such objects was predicted from abstract reasoning by Mitchell in
the Eighteenth Century; but the discovery of their actual existence
was made by Sir William Herschel, while engaged on an attempt to
determine the distance of some of the double stars from the earth.
Unlike the planetary orbits, which are nearly circular, at least
those of the larger planets of the Solar System, it is found that the
orbits of these double stars differ, in many cases, widely from the
circular form, in some cases, indeed, approaching in shape more the
orbit of a comet than a planet.
The binary stars are among the most interesting objects in the
heavens. The number now known probably amounts to nearly one
thousand. In most of them, however, the motion is very slow, and
in only about seventy cases has the change of position, since their
discovery, been sufficient to enable an orbit to be computed.
Savary, in 1830, was the first astronomer who attempted to compute
the orbit of a binary star, namely, the star Xi Ursæ Majoris. This
remarkable pair was discovered by Sir William Herschel in 1780, and
as the period of revolution is about sixty-one years, a considerable
portion of the ellipse had been described in 1830, when it was
attacked by Savary.
The binary star with the shortest period known at present seems to be
the fourth magnitude star Kappa Pegasi. It was discovered as a wide
double star by Sir William Herschel in 1786, the companion star being
of the ninth magnitude. In August, 1880, Mr. Burnham, the famous
American double star observer, examining the star with the 18½-inch
refractor of the Dearborn Observatory, found the brighter star to be
a very close double, with a distance between the components of only
a quarter of a second of arc. A few years’ observations showed that
this pair were in rapid motion round each other (about eleven years).
Another binary star, with a period of about the same length, is Delta
Equulei, which was discovered to be a close double by Otto Struve
in 1851. Next in order of shortness of period comes the southern
binary star Zeta Sagittarii, for which an orbit was first computed
in the year 1886 by the present writer. The orbit of this star will,
I think, require still further revision, but the period of about
eighteen years is probably not far from the truth.
Another remarkably rapid binary star is 85 Pegasi. Next in order of
rapidity of motion we have the southern binary star 9 Argûs.
The star 42 Comæ Berenices has a period of about 25¾ years, according
to Otto Struve. The orbit is remarkable from the fact that its plane
passes through, or nearly through, the earth, and is, therefore,
projected into a straight line, the companion star oscillating
backward and forward on each side of its primary.
The star Beta Delphini—the most southern of the four stars in the
“Dolphin’s Rhomb”—is also a fast-moving binary, discovered by
Burnham in 1873. Burnham thinks the period will prove to be about
twenty-eight years. The spectrum of the light of Beta Delphini
is similar to that of our sun, so that the two bodies should be
comparable in intrinsic brilliancy.
Another remarkable binary star with a comparatively short period
is Zeta Herculis. This pair have now performed three complete
revolutions since their discovery in 1782 by Sir William Herschel.
Several orbits have been computed, but Dr. See’s period of
thirty-five years is probably the best. The companion is, however,
rather faint, being only 6½ magnitude, while the primary star is of
the third.
In the case of the binary star, Eta Coronæ Borealis, it was, some
forty years ago, uncertain whether its period was forty-three or
sixty-six years, but now that two complete revolutions have been
performed since its discovery by Sir William Herschel in 1781, the
question has been finally decided in favor of the shorter period.
The brilliant star Sirius is also an interesting binary star. The
companion, which is relatively very faint—about tenth magnitude—was
discovered by Alvan Clark in 1862. The existence of some such
disturbing body was previously suspected by astronomers, owing to
observed irregularities in the proper motion of Sirius. Several
orbits, giving periods of about fifty years, have been computed.
The great brilliancy of Sirius, the brightest star in the heavens,
naturally suggests a sun of great size. Recent investigations do not
favor this idea. Its spectrum is, however, of the first type, and
the star is therefore not comparable with the sun in brilliancy. The
above result would indicate that stars of the first, or Sirian type,
are intrinsically brighter than our sun.
Sirius is about eleven magnitudes brighter than its faint companion.
This makes the light of Sirius about 25,000 times the light of
the small star. The two bodies must, therefore, be differently
constituted, and, indeed, the companion must be nearly a dark body.
If Sirius has any planets revolving round it—like those of our solar
system—they must forever remain invisible in our largest telescopes.
This remark, of course, applies to all the fixed stars, single and
double. They may possibly have attendant families of planets, like
our sun, but if so, the fact can never be ascertained by direct
observation.
The star Zeta Cancri is a well-known triple star, the close pair
revolving in a period of about sixty years. Nearly two revolutions
have now been completed since its discovery by Sir William Herschel
in 1781. All three stars probably form a connected system, but the
motion of the third star round the binary pair is very slow and
irregular.
[Illustration: Fig. 18.—System of the Double Sun Alpha Centauri]
Another interesting binary star is Xi Ursæ Majoris. As already
stated, this was the first pair for which an orbit was computed. More
than a complete revolution has now been performed since its discovery
by Sir William Herschel in 1780. The period has, therefore, been well
determined, and seems to be about sixty years.
The bright southern star, Alpha Centauri, the nearest of all the
fixed stars to the earth, so far as is known at present, is also
a remarkable binary star. It seems to have been first noticed as a
double star by Richaud in 1690.
Assuming my value of the sun’s stellar magnitude (about 27), I find
that the sun, if placed at the distance of Alpha Centauri, would
appear of about the same brightness as the star does to us. As,
according to Professor Pickering, the spectrum of Alpha Centauri is
of the second or solar type, it would seem that in mass, brightness,
and physical condition the star closely resembles our sun.
We next come to another very interesting binary star, known to
astronomers as 70 Ophiuchi. It is a very fine double star, the
magnitudes of the components being about four and six, and the
colors yellow and orange. More than a complete revolution has now
been described by the components since its discovery by Sir William
Herschel in 1779. Placed at the distance indicated by Krüger’s
parallax, I find that our sun would be reduced to a star of about
magnitude 3½, which shows that the sun and star are of about equal
brightness. The spectrum is of the solar type, according to Vogel.
A very famous binary star is that known to astronomers as Gamma
Virginis. Its history is a very interesting one. It lies close to the
celestial equator, about one degree to the south and about fifteen
degrees to the northwest of the bright star Spica (Alpha of the same
constellation), with which it forms the stem of a Y-shaped figure
formed by the brightest stars of the constellation Virgo, or the
Virgin, Gamma being at the junction of the two upper branches. The
brightness of Gamma Virginis is a little greater than an average
star of the third magnitude. Variation of light has, however, been
suspected in one or both components. The Persian astronomer, Al-Sûfi,
in his description of the heavens, written in the Tenth Century,
rates it of the third magnitude, and describes it as “the third of
the stars of _al-auvâ_, which is a mansion of the moon,” the first
and second stars of this “mansion” being Beta and Eta Virginis, the
fourth star Delta, and the fifth Epsilon, these five stars forming
the two upper branches of the Y-shaped figure above referred to.
Gamma was called _Zawiyah-al-auvâ_, “the corner of the barkers!”
perhaps from its position in the figure, which formed the thirteenth
Lunar Mansion of the old astrologers. It was also called _Porrima_
and _Postvarta_ in the old calendars. The fact that Gamma Virginis
really consists of two stars very close together seems to have been
discovered by the famous astronomer, Bradley, in 1718. The rapid
decrease in the apparent distance from 1780-1834 indicated that the
apparent orbit is very elongated, and that possibly the two stars
might “close up” altogether, and appear as a single star even in
telescopes of considerable power. This actually occurred in the year
1836, or, at least, the stars were then so close together that the
most powerful telescopes of that day failed to show Gamma Virginis as
anything but a single star. Of course, it would not have been beyond
the reach of the giant telescopes of our day. From the year 1836 the
pair began to open out again.
Another interesting binary star is Eta Cassiopeiæ. Periods ranging
from 149 to 222½ years have been found by different computers. The
most recent computation makes it about 196 years.
The bright star Gamma Leonis, situated in the well-known “Sickle in
Leo,” is also a binary star, but only a small portion of the orbit
has been described since its discovery by Sir William Herschel in
1782. Dr. Doberck finds a period of 407 years. It is remarkable for
its very high “relative brightness.” This pair forms a fine object
for a small telescope.
The star known as 12 Lyncis is a triple star, the components being
5, 6, and 7½ magnitude. The close pair forms a binary system, for
which an orbit has been computed by the present writer, who finds a
period of about 486 years. Sir John Herschel predicted in 1823 that
the angular motion of the pair would “bring the three stars into a
straight line in 57 years.” This prediction was fulfilled in 1887,
when measures by Tarrant showed that the stars were then exactly in a
straight line.
The bright star Castor is a famous double star, and has been known
since the year 1718, when it was observed by Bradley and Pond. It was
also observed by Maskelyne in 1759, and frequently by Sir William
Herschel from 1799 to 1803. Numerous orbits have been computed, with
periods ranging from 199 years by Mädler and 1,001 years by Doberck.
I find that the mass of the system of Castor is only 1/19th of the
sun’s mass, a result which would imply that the components are masses
of glowing gas! Dr. Bélopolsky has found, with the spectroscope,
that the brighter component is a close binary star with a dark
companion, like Algol. The period of revolution is about three days,
and the relative orbital velocity about 20¾ miles a second. Dr.
Bélopolsky’s observations show that the system is receding from the
earth at the rate of about 4½ miles per second.
With reference to the colors of the components of binary stars, the
following relation between color and relative brightness has been
established:
(1) When the magnitudes of the components are equal, or approaching
equality, the colors are generally the same, or similar.
(2) When the magnitudes of the components differ considerably, there
is also a considerable difference in color.
A new class of binary stars has been discovered within the last
few years by means of the spectroscope. These have been called
“spectroscopic binaries,” and the brighter component of Castor,
referred to above, is an example of the class. They are supposed to
consist of two component stars, so close together that the highest
powers of the largest telescopes fail to show them as anything but
single stars. Indeed, the velocities indicated by the spectroscope
show that they must be so close that the components must forever
remain invisible by the most powerful telescopes which could ever be
constructed by man. In some of these remarkable objects, the doubling
of the spectral lines indicates that the components are both bright
bodies, but in others, as in Algol, the lines are merely shifted from
their normal position, not doubled, thus denoting that one of the
components is a dark body. In either case, the motion in the line
of sight can be measured by the spectroscope, and we can, therefore,
calculate the actual dimensions of the system in miles, and thence
its mass in terms of the sun’s mass, although the star’s distance
from the earth remains unknown. Judging, however, from the brightness
of the star, and the character of its spectrum, we can make an
estimate of its probable distance from the earth.
The bright star Spica has also been found by the spectroscope to be a
close binary star. Vogel finds a period of four days with a distance
between the components of about 6¼ millions of miles, and assuming
that the components have equal mass and are moving in a circular
orbit, he finds the mass of the system about 2.6 times the mass of
our sun. In addition to its orbital motion, Vogel finds that Spica is
approaching the sun at the rate of over nine miles per second.
To ordinary observers, the light of the stars seems to be constant.
Even to those who are familiar with the constellations, the stars
appear to maintain their relative brilliancy unchanged. To a great
extent this is, of course, true; the great majority of the stars
remaining of the same brightness from day to day, and from year to
year. There are, however, numerous exceptions to this rule. Many
of the stars, when carefully watched, are found to fluctuate in
their light, being sometimes brighter and sometimes fainter. These
are known as “variable stars”—one of the most interesting class of
objects in the heavens. Some of these have been known for a great
number of years, and their variations having been carefully watched,
the laws governing their light changes have been well determined.
We will first consider the variable stars with long periods of
variation, as these generally show the largest fluctuations of light.
Among these, the first star in which variation of light seems to have
been noticed is the extraordinary object, Omicron Ceti, popularly
known as Mira, or the “wonderful” star. It appears to have been first
noticed by David Fabricius in the year 1596. He observed that the
star now called Omicron, in the constellation Cetus, was of the third
magnitude on April 13 of that year, and that in the following year it
had disappeared. Bayer saw it again in 1603, when forming his maps
of the constellations, and assigned to it the Greek letter Omicron,
but does not seem to have noticed the fact that it was the same star
which had been observed by Fabricius seven years previously. No
further attention seems to have been paid to it until 1638 and 1639,
when it was observed at Francker by Professor Phocylides Holwarda
to be of the third magnitude in December, 1638, invisible in the
following summer, and again visible in October, 1639. From 1648 to
1662 it was carefully observed by Hevelius, and in subsequent years
by several observers. Its variations are now regularly followed
from year to year, and it forms one of the most interesting objects
of its kind in the heavens. Its light varies from about the second
magnitude to the ninth, but its brightness at maximum is variable to
a considerable extent.
Perhaps the long period variable star next in order of interest—at
least to observers in the Northern Hemisphere—is that known as
Chi Cygni. It was discovered by Kirch in 1686. The star varies at
maximum from 4 to 6½ magnitude, and at the minimum it sinks to below
the thirteenth magnitude. At some maxima, therefore, it is easily
visible to the naked eye, and at others it is just below the limit
of ordinary vision. At the maximum of 1847, it was visible to the
naked eye for a period of 97 days. The average period is about 406
days; but according to Schönfeld—a well-known authority on the
variables—observations indicate a small lengthening of the period.
Chi Cygni is said to be “strikingly variable in color.” Espin’s
observations in different years show it “sometimes quite red, at
others only pale orange-red.” In the spectroscope, its light shows
a splendid spectrum of the third type (or banded spectrum, very
characteristic of these long period variables), in which bright lines
were observed by Espin in May, 1889.
R Leonis is another remarkable variable star, which is sometimes
visible to the naked eye at maximum. It lies closely south of the
star known as 19 Leonis. It was discovered by Koch in 1782. At the
maximum, its brightness varies from 5.2 to 7 magnitude, and at
minimum it fades to about the tenth magnitude. The mean period is
about 313 days. The star is red in all phases of its light, and forms
a fine telescopic object. Close to it are two small stars, which
form, with the variable, an isosceles triangle.
There is a very remarkable variable star in the Southern Hemisphere
known as Eta Argûs. It lies in the midst of the great nebula in Argo,
and the history of its fluctuations in light is very interesting.
Observed by Halley in 1677 as a star of the fourth magnitude, it was
seen of the second magnitude by Lacaille in 1751. After this, it must
have again faded, for Burchell found it of only the fourth magnitude
from 1811 to 1815. From 1822 to 1826 it was again of the second
magnitude, as observed by Fallows and Brisbane; but on February 1,
1827, it was estimated of the first magnitude by Burchell. It then
faded again, for on February 29, 1828, Burchell found it of the
second magnitude. From 1829 to 1833 Johnson and Taylor rated it of
the second magnitude; and it was still of this magnitude, or a little
brighter, when Sir John Herschel commenced his observations at the
Cape of Good Hope in 1834. It does not seem to have varied much in
brightness from that time until December, 1837, when Herschel was
astonished to find its light “nearly tripled.” He says: “It very
decidedly surpassed Procyon, which was about the same altitude,
and was far superior to Aldebaran. It exceeded Alpha Orionis, and
the only star (Sirius and Canopus excepted) which could at all be
compared with it was Rigel.”
From this time its light continued to increase. On the 28th December
it was far superior to Rigel, and could only be compared with Alpha
Centauri, which it equaled, having the advantage of altitude, but
fell somewhat short of it as the altitudes approached equality. The
maximum of brightness seems to have been obtained about the 2d of
January, 1838, on which night, both stars being high and the sky
clear and pure, it was judged to be very nearly matched, indeed,
with Alpha Centauri. In 1843 it again increased in brightness, and
in April of that year it was observed by Maclear to be brighter than
Canopus, and nearly equal to Sirius! It then faded slightly, but
seems to have remained nearly as bright as Canopus until February,
1850, since which time its brilliancy gradually decreased. It was
still of the first magnitude in 1856, according to Abbott, but was
rated a little below the second magnitude by Powell in 1858. Tebbutt
found it of the third magnitude in 1860; Abbott a little below the
fourth in 1861. Ellery rated it fifth magnitude in 1863, and Tebbutt
sixth magnitude in 1867. In 1874 it was estimated 6.8 magnitude at
Cordoba, and only 7.4 in November, 1878. Tebbutt’s observations from
1877-86 show that it did not rise above the seventh magnitude in
those years, and in March, 1886, it was rated 7.6 magnitude by Finlay
at the Cape of Good Hope. This seems to have been the minimum of
light, for in May, 1888, Tebbutt found that it “had increased fully
half a magnitude” since April, 1887. The star is very reddish in
color.
We will now consider the variables of short period, which are
particularly interesting objects, owing to the comparative rapidity
of their light changes. The periods vary in length from about 17¼
days down to a few hours. Perhaps the most interesting of these short
period variables, at least to the amateur observer, is the star Beta
Lyræ, which is easily visible to the naked eye in all phases of its
light. It can be readily identified, as it is the nearest bright star
to the south of the brilliant Vega, and one of two stars of nearly
the same magnitude, the second being Gamma Lyræ. The variability of
Beta Lyræ was discovered by Goodricke in the year 1784. The period is
about 12 days, 21 hours, 46 minutes, 58 seconds. Recent observations
with the spectroscope indicate that the star is a very close double
or “spectroscopic binary,” although it does not seem certain that
an actual eclipse of one component by the other takes place, as in
the case of Algol. Bright lines were detected in the star’s spectrum
by Secchi so far back as 1866. In 1883 M. Von Gothard noticed that
the appearance of these bright lines varied in appearance, and from
an examination of photographs taken at Harvard Observatory in 1891,
Mrs. Fleming found displacements of bright and dark lines in a double
spectrum, the period of which agreed fairly well with that of the
star’s light changes.
Another interesting star of short period is Delta Cephei, which is
one of three stars forming an isosceles triangle a little to the
west of Cassiopeia’s Chair, the variable being at the vertex of the
triangle, and the nearest of the three to Cassiopeia. Its variability
was also discovered by Goodricke in 1784. It varies from 3.7 to 4.9
magnitude, with a period of 5 days, 8 hours, 47 minutes, 40 seconds.
The amount of the variation is, therefore, the same as in the case
of Algol, the star’s light at maximum being about three times its
light at minimum. The observations also show that Delta Cephei is
approaching the earth at the rate of about 8¾ miles a second. The
color of the star is yellow, and it has a distant bluish companion
of about the fifth magnitude, which may possibly have some physical
connection with the brighter star, as both stars have a common proper
motion through space.
Another remarkable star of short period is Eta Aquilæ, the
variability of which was discovered by Pigott in 1784. It varies from
magnitude 3.5 to 4.7, with a period of 7 days, 4 hours, 14 minutes,
but Schönfeld found marked deviations from a uniform period. Its
color is yellow, and its spectrum, like that of Delta Cephei, of the
second or solar type.
A remarkable variable star of short period was discovered in 1888
by Mr. Paul in the southern constellation Antlia. It varies from
magnitude 6.7 to 7.3, with the wonderfully short period of 7 hours,
46 minutes, 48 seconds, all the light changes being gone through
no less than three times in twenty-four hours! It was for some
years believed that the variation was of the Algol type, but recent
measures made at the Harvard College Observatory show that it belongs
to the same class as Delta Cephei and Eta Aquilæ.
A telescopic variable with a wonderfully short period was discovered
by Chandler in 1894. It lies a little to the west of the star Gamma
Pegasi, and has been designated U Pegasi. It varies from magnitude
8.9 to 9.7, and was first supposed to be of the Algol type with a
period of about two days, but further observations showed that the
period was much shorter, and only 5 hours, 31 minutes, 9 seconds.
The remarkable rapidity of its light changes, which are gone through
four times in less than twenty-four hours, make this remarkable star
a most interesting object. Possibly there may be other stars in the
heavens with a similar rapidity of variation which have hitherto
escaped detection.
Unlike the variable stars of long period which seemed scattered
indifferently over the surface of the heavens, the great majority of
the short period variables are found in a zone which nearly coincides
with the course of the Milky Way. The most notable exceptions to this
rule are W Virginis with the comparatively long period of 17¼ days,
and U Pegasi, above described, which has the shortest known period
of all the variable stars. Another peculiarity is that most of them
are situated in what may be called the following hemisphere, that is
between 12 hours and 24 hours of right ascension. The most remarkable
exception to this rule is Zeta Geminorum.
Algol, or Beta Persei, is a famous variable star, and the typical
star of the class to which it belongs. Its name, Algol, is derived
from a Persian word, meaning the “demon,” which suggests that
the ancient astronomers may have detected some peculiarity in
its behavior. The real discovery of its variation was, however,
made by Montanari in 1667, and his observations were confirmed by
Maraldi in 1692. Its fluctuations of light were also noticed by
Kirch and Palitzsch, but the true character of its variations was
first determined by the English astronomer, Goodricke, in 1782. Its
fluctuations of light are very curious and interesting. Shining with
a constant, or nearly constant, brightness for a period of about
59 hours as a star of a little less than the second magnitude, it
suddenly begins to diminish in brightness, and in about 4½ hours
it is reduced to a star of about magnitude 3½. In other words, its
light is reduced to about one-third of its normal brightness. If we
suppose three candles placed side by side at such a distance that
their combined light is merged into one, and equal to the usual
brightness of Algol, then, if two of these candles are extinguished,
the remaining candle will represent the light of Algol at its minimum
brilliancy. The star remains at its minimum, or faintest, for only
about 15 minutes. It then begins to increase, and in about 5 hours
recovers its normal brightness, all the light changes being gone
through in a period of about 10 hours out of nearly 69 hours, which
elapse between successive minima. These curious changes take place
with great regularity, and the exact hour at which a minimum of light
may be expected can be predicted with as much certainty as an eclipse
of the sun.
Goodricke, comparing his own observations with one made by Flamsteed
in the year 1696, found the period from minimum to minimum to be
2 days, 20 hours, 48 minutes, 59½ seconds, and he came to the
conclusion that the diminution in the light of the star is probably
due to a partial eclipse by “a large body revolving round Algol.”
This hypothesis was fully confirmed in the years 1888-89 by Professor
Vogel with the spectroscope. As no close companion to Algol is
visible in the largest telescopes, we must conclude that either the
satellite is a dark body, or else so close to the primary that no
telescope could show it. Now, if the diminution in Algol’s light
is due to a dark body revolving round it, and periodically coming
between us and the bright star, it follows that both components
will be in motion, and both will revolve round the common centre
of gravity of the pair. A little before a minimum of light takes
place, the dark companion should therefore be approaching the eye,
and, consequently, the bright companion will be receding. During
the minimum there will be no apparent motion in the line of sight,
as the motion of both bodies will be at right angles to the visual
ray. After the minimum is over, the motion of the two bodies will
be reversed, the bright one approaching the eye, and the dark
one receding. Now, this is exactly what Vogel found. Before the
diminution in the light of Algol begins, the spectroscope showed that
the star is receding from the earth and after the minimum that it is
approaching the eye. That the companion is dark and not bright, like
the primary, is evident from the fact that the spectral lines are
merely shifted from their normal position and not doubled, as would
be the case were both components bright, as in the case of some of
the “spectroscopic binaries”—for example, Beta Aurigæ. Vogel found
that before the minimum of light, Algol is receding from the earth
with the velocity of 24½ miles a second, and after the minimum it is
approaching at the rate of 28½ miles a second. The difference between
the observed velocities indicates that the system is approaching
the earth with a velocity of about 2 miles a second. Knowing, then,
the orbital velocity, which is evidently about 26½ miles a second,
and assuming the orbit to be circular, it is easy, with the observed
period of revolution, or the period of light variation, to calculate
the diameter of the orbit in miles, although the star’s distance
from the earth remains unknown. Further, comparing its period of
revolution and the dimensions of the orbit with that of the earth
round the sun, it is easy to calculate, by Kepler’s third law of
motion, the mass of the system in terms of the sun’s mass, and the
probable size of the component bodies. Calculating in this way, Vogel
computes that the diameter of Algol is about 1,061,000 miles, and
that of the dark companion 830,300 miles, with a distance between
their centres of 3,230,000 miles, and a combined mass equal to
two-thirds of the sun’s mass, the mass of Algol being four-ninths,
and that of the companion two-ninths, of the mass of the sun. Taking
the diameter of the sun as 866,000 miles, and its density as 1.44
(water being unity), I find that the above dimensions give a mean
density for the components of Algol of about one-third that of water,
so that the components are probably gaseous bodies, as Hall has
already concluded.
[Illustration: Portion of the Sun’s Surface. Sunspot nearly 60,000
Miles Across]
It is a curious fact that Al-Sûfi, the Persian astronomer, in his
_Description of the Heavens_, written in the Tenth Century, speaks
distinctly of Algol as a red star (_étoile_, _brillant_; _d’un
éclat_, _rouge_), while at present it is white or at the most of a
yellow color. A similar change of color is supposed to have taken
place in the case of Sirius, but the change in Algol seems more
certain, as Al-Sûfi’s descriptions are generally most accurate and
reliable.
Stars of the Algol type of variable are very rare objects, only a
dozen or so having been hitherto discovered in the whole heavens.
Those visible to the naked eye, when at their normal brightness, are:
Algol, Lambda Tauri, Delta Libræ, R Canis Majoris, and U Ophiuchi.
A remarkable peculiarity about the variable stars in general is that
none of them has any considerable proper motion. As a large proper
motion is generally considered to indicate proximity to the earth,
we may conclude, with great probability, that the variable stars,
as a rule, lie at a great distance from our system. In other words,
it appears that the sun does not lie in a region of variable stars,
and, with the exception of Alpha Cassiopeiæ and Alpha Herculis, a
measurable parallax has not yet been found, so far as I know, for any
known variable star.
We now come to the interesting and mysterious class of objects known
as “new” or “temporary” stars. These phenomena are of very rare
occurrence, and but few undoubted examples of the class are recorded
in the annals of astronomy. Possibly in some cases they have been
merely variable stars, of irregular period and fitful variability;
but others may have been due to a real catastrophe, such as the
collision of two dark bodies in space, or, possibly, the passage of a
bright or dark body through a gaseous nebula.
The earliest temporary star of which we have any reliable
information seems to be one which is recorded in the Chinese annals
of Ma-tuan-lin, as having appeared in the year 134 B. C. in the
constellation Scorpio. Its position seems to have been somewhere
between the stars Beta and Rho of Scorpio. Pliny informs us that it
was the sudden appearance of a new star which induced the famous
astronomer Hipparchus to form his catalogue of stars, the first ever
constructed. As the date of Hipparchus’s catalogue is 125 B. C., it
seems highly probable that the new star referred to by Pliny was the
same as that recorded by the Chinese astronomer as having appeared
nine years previously.
A new star is said to have appeared in the year 76 B. C. between the
stars Alpha and Delta in the Plow, but the accounts are vague.
In 101 A. D., a small “yellowish-blue” star is said to have appeared
in the “sickle” in Leo, but its exact position is not known. In 107
A. D., a new star is mentioned near Delta, Epsilon and Eta in Canis
Major, three bright stars southeast of Sirius. In 123 A. D., another
new star is recorded by Ma-tuan-lin to have appeared between Alpha
Herculis and Alpha Ophiuchi.
The Chinese annals record that on December 10, 173 A. D., a brilliant
star appeared between Alpha and Beta Centauri in the Southern
Hemisphere. It remained visible for eight months, and is described
as resembling “a large bamboo mat!”—a curious description. There is
at present, close to the spot indicated, a known variable star—R
Centauri—of which the period seems to be long and the variation
of light irregular. Possibly an unusually bright maximum of this
variable star formed the star of the Chinese annals, or perhaps the
variable star is the remnant of the outburst which took place in the
First Century. The variable is a very reddish star, and at present
varies from about the sixth to the tenth magnitude.
A new star is recorded in the year 386 A. D. as having appeared
between Lambda and Phi Sagittarii. Near the position indicated,
Flamsteed observed a star, No. 65 of his catalogue, which is now
missing; and it has been conjectured that the star seen by Flamsteed
may possibly have been a return of the star mentioned in the Chinese
annals.
Cuspianus relates that a star as bright as Venus appeared near Altair
in 389 A. D., during the reign of the Emperor Honorius, and that he
had himself seen it. There is some doubt, however, about the exact
date, as other accounts give the year 388 or 398. The star seems to
have disappeared in about three weeks.
In the year 393 A. D., another strange star is recorded in the tail
of Scorpio. An extraordinary star is said to have been seen near
Alpha Crateris in 561 A. D. Here again a known variable and red
star—R Crateris—is close to the position indicated by the ancient
records.
The Chinese annals record a new star in 829 A. D., somewhere in the
vicinity of the bright star Procyon, and in this locality there are
several known variable stars.
The Bohemian astronomer, Cyprianus Leoviticus, mentions the
appearance of new stars in Cassiopeia in the years 945 A. D. and
1264, and it has been conjectured that perhaps these were apparitions
of Tycho Brahe’s famous star of 1572 (to be presently described),
forming a variable star with a period of over 300 years. Lynn and
Sadler, however, have shown that the supposed stars of 945 and 1264
were, in all probability, comets.
Extraordinary stars are recorded near Zeta Sagittarii in 1011 A. D.,
near Mu Scorpii in 1203, and near Pi Scorpii on July 1, 1584. It is
remarkable how many of these objects seem to have appeared in this
portion of the heavens.
A very brilliant star is mentioned by Hepidannus as having appeared
in Aries in May, 1012. He describes it as “dazzling the eye.” Other
temporary stars are mentioned in 1054 A. D., near Zeta Tauri, and in
1139 near Kappa Virginis; but the accounts of these are very vague,
and it seems by no means certain that they were really new stars.
No possible doubt, however, can be entertained with reference to the
appearance of the object which suddenly blazed out in Cassiopeia’s
Chair in November, 1572. It was called the “Pilgrim Star,” and was
observed by the famous astronomer, Tycho Brahe, who has left us a
very elaborate account of its appearance, position, etc. Although
usually spoken of as Tycho Brahe’s star, it seems to have been
really discovered by Cornelius Gemma on the evening of November 9.
That its appearance was very sudden may be inferred from Cornelius
Gemma’s statement that it was not visible on the preceding night in
a clear sky. Tycho Brahe’s attention was first attracted to it on
November 11. His description of the new star is as follows—as quoted
by Humboldt: “On my return to the Danish islands from my travels in
Germany, I resided for some time with my uncle, Steno Bille, in the
old and pleasantly situated monastery of Herritzwadt, and here I made
it a practice not to leave my chemical laboratory until the evening.
Raising my eyes, as usual, during one of my walks, to the well-known
vault of heaven, I observed with indescribable astonishment, near
the zenith in Cassiopeia, a radiant fixed star of a magnitude never
before seen. In my amazement, I doubted the evidence of my senses.
However, to convince myself that it was no illusion, and to have the
testimony of others, I summoned my assistants from the laboratory,
and inquired of them, and of all the country people that passed by,
if they also observed the star that had thus suddenly burst forth.
I subsequently heard that in Germany, wagoners and other common
people first called the attention of astronomers to this great
phenomenon in the heavens—a circumstance which, as in the case of
non-predicted comets, furnished fresh occasion for the usual raillery
at the expense of the learned. This new star I found to be without
a tail, not surrounded by any nebula, and perfectly like all other
fixed stars, with the exception that it scintillated more strongly
than stars of the first magnitude. Its brightness was greater than
that of Sirius, Alpha Lyræ, or Jupiter. For splendor, it was only
comparable to Venus when nearest to earth (that is, when only a
quarter of her disk is illuminated). Those gifted with keen sight
could, when the air was clear, discern the new star in the daytime,
and even at noon. At night, when the sky was overcast, so that all
other stars were hidden, it was often visible through the clouds, if
they were not very dense (_nubes non admodum densas_). Its distances
from the nearest stars of Cassiopeia, which throughout the whole
of the following year I measured with great care, convinced me of
its perfect immobility. Already in December, 1572, its brilliancy
began to diminish, and the star gradually resembled Jupiter, but by
January, 1573, it had become less bright than that planet. Toward the
month of November the new star was not brighter than the eleventh
in the lower part of Cassiopeia’s Chair. The transition to the
fifth and sixth magnitudes took place between December, 1573, and
February, 1574. In the following month the new star disappeared, and,
after having shone seventeen months, was no longer discernible to
the naked eye.” (The telescope was not invented until thirty-seven
years afterward.) Humboldt adds: “At its first appearance, as long
as it had the brilliancy of Venus and Jupiter, it was for two months
white, and then passed through yellow into red. In the spring of
1573, Tycho Brahe compared it to Mars; afterward he thought it nearly
resembled Betelgeuse, the star in the right shoulder of Orion. The
color for the most part was like the red tint of Aldebaran. In the
spring of 1573, and especially in May, its white color returned
(_albedinam quandam sublividam induebat, qualis Saturni stellæ
subesse videtur_). So it remained in January, 1574; being, up to the
time of its entire disappearance in the month of March, 1574, of the
fifth magnitude, and white, but of a duller whiteness, and exhibiting
a remarkably strong scintillation in proportion to its faintness.”
Ma-tuan-lin speaks of a star in 1578 “as large as the sun” (!) but
does not state its position.
The star known as P (34) Cygni is sometimes spoken of as a “Nova,”
or new star; but it is still visible to the naked eye as a star
of the fifth magnitude. It was observed of the third magnitude by
Jansen in 1600 and by Kepler in 1602. After the year 1619 it appears
to have diminished in brightness, and is said to have vanished in
1621; but it may merely have become too faint to be seen with the
naked eye. It was again observed of the third magnitude by Dominique
Cassini in 1655, and it afterward disappeared. It was again seen by
Hevelius in November, 1655. In 1667, 1682, and 1715 it is recorded
as of the sixth magnitude, and there is no further record of any
marked increase in its light. A period of about 18 years was assumed
by Pigott; but this is now disproved, and it seems probable that
the star is a variable of irregular period and fitful variability,
and not, properly speaking, a temporary star. Its present color is
yellow, and bright lines have been seen in its spectrum.
A new star of the third magnitude was observed near Beta Cygni by the
Carthusian monk Anthelmus in 1670. It remained visible for about two
years, and is said to have increased and diminished several times
before its final disappearance. Schönfeld computed its exact position
from observations made by Hevelius and Picard. Quite close to the
spot indicated, a star of the eleventh magnitude has been observed at
the Greenwich Observatory, and fluctuations of light were suspected
in this small star by Hind and others.
A very remarkable star, sometimes called the “Blaze Star,” suddenly
appeared in Corona Borealis, in May, 1866. It was first seen by
the late Mr. Birmingham, at Tuam, Ireland, about midnight on the
evening of May 12, when it was of the second magnitude, and equal to
Alphecca, “the gem of the coronet.” Its appearance must have been
very sudden, for Schmidt, the Director of the Athens Observatory,
stated that he was observing the constellation on the same evening,
about two and one-half hours previous to Birmingham’s discovery,
and observed nothing unusual. He was certain that no star, of even
the fifth magnitude, could possibly have escaped his notice. On the
following night it was seen by several observers in different parts
of the world.
A remarkable and very interesting temporary star was discovered in
1892 in the constellation Auriga.
It is a remarkable fact that the great majority of the temporary
stars appeared in or near the Milky Way. The chief exceptions to
this rule are: the star of 76 B. C., in the Plow, the star recorded
by Hepidannus in Aries, 1012 A. D., and the “Blaze Star” of 1866 in
Corona Borealis.
A WORLD ON FIRE—NOVA PERSEI.—ALEXANDER W. ROBERTS
In the small hours of the morning of 22d February, 1901, Dr.
Anderson of Bonnington, Edinburgh, saw a bright star shining in
the constellation of Perseus, where he knew no such star was ever
seen before. The circumstances connected with this discovery afford
another striking instance of how Nature keeps her secrets for her
true amateur, using the word in its highest sense.
The evening of 21st February was cloudy, and nine out of ten
astronomers would have gone to bed when there seemed little prospect
of the night clearing; but Dr. Anderson was the tenth man. At twenty
minutes to three in the morning the clouds rolled away from over
the old gray Scottish capital, and the trained eye of the patient
observer saw right in the heart of Perseus a new star. Never before
had its light, blue-white, like an unpolished diamond, shone down on
this strange earth of ours.
Next day the news of the wonderful discovery was flashed to all the
great observatories of the world, and telescopes and spectroscopes,
cameras and photometers, were directed toward the strange phenomenon,
and by testing, measuring, examining, sought to wrest its secrets
from it.
Much is still a mystery; but what has been ascertained during the
period that the rhythm of its light-waves beat upon our shores
is of great interest and importance as bearing directly on the
life-history of each individual star in the heavens, and of our own
sun and planet among them.
The first and simplest question that arises for settlement is the
date when the new star blazed forth in our terrestrial sky. The
curious reader will notice the reservation: in _our_ terrestrial
sky. When the star _actually_ burst forth into resplendent
light is another matter, as we shall discover later on. It was
certainly before Dr. Anderson was born, and probably before another
Scotsman—Ferguson by name—combined, like many another sage, counting
and watching sheep with counting and watching stars.
With regard to the date of the appearance in our sky of the new
star, Nova Persei, as it is called in astronomical literature, when
Dr. Anderson discovered it at twenty minutes to three o’clock on
the morning of 22d February, it was bright enough to be straightway
evident to a trained astronomer. In these later days of strenuous
scientific activities every portion of the sky is constantly being
examined and charted, and no sooner was the discovery of Nova Persei
announced than a searching of records began, in order to ascertain
if, at any time, the star had ever been seen before.
[Illustration: Fig. 19.—Chart Showing Position of Nova Persei]
It so chanced that on the evenings of 18th and 19th February two
photographs of the very spot where three days later the new star
appeared were taken at Harvard Observatory. On neither of these
photographs is there the slightest evidence of the star’s existence.
It was, therefore, on these dates non-existent so far as our earth
was concerned. On the evening of 20th February a well-known English
observer, Mr. Stanley Williams, had also taken a photograph of the
same portion of the sky; and again there was no trace of the star.
Mr. Williams’s photograph was taken twenty-eight hours before Dr.
Anderson saw it. Still more strange is the fact that on the evening
of 21st February three observers on the Continent testify that they
had the constellation Perseus under observation from seven o’clock
to eleven, and had the new star then been visible they could not have
failed to see it. The star, therefore, blazed out some time between
eleven o’clock and three on the night of its discovery.
Now, what does this mean? It means this: that by some cause a star,
quite dark before, or so faint that it could not be seen even by
means of a powerful telescope, in a few hours, or perhaps in a few
minutes, blazed forth as a star of conspicuous brightness. In this
brief space of time a dark and probably chill globe became a seething
mass of fire, a million times hotter than it was before. Fierce,
fervent heat lit up the orb with a glow that reached from rim to rim
of the stellar universe. We have here a catastrophe that goes beyond
our wildest conceptions: the conflagration of a world, the ruin of
a star. What guarantee have we for an assumption of this kind? What
of certitude is there in our vision of such a Day of Doom for any
part of our universe? Let us consider the salient facts regarding the
recent changes in the appearance and structure of this star. We shall
relate only those facts that are beyond controversy, as far as our
present knowledge goes.
Nova Persei did not reach its maximum brightness till the evening of
25th February, when it was probably the most conspicuous object in
the midnight sky. It was then at least six times brighter than at the
time of its discovery. After this date it began to wane slowly. At
intervals there were spurts of brightness lasting for two or three
days, as if the fires had not exhausted themselves. On the whole,
however, the light of the star waned, and by the end of the year its
enfeebled light was just bright enough to be evident to the naked
eye; twelve months after its appearance it could only be seen with
the aid of a telescope.
Now, one of the most powerful instruments of research in the
new astronomy is the spectroscope. It takes hold of the rays of
light that come to us from a star, and makes these rays reveal
the condition of things in the world they come from. One of the
spectroscopes turned on the new star in Perseus was Professor
Copeland’s magnificent instrument at Blackford Hill Observatory,
Edinburgh. Professor Copeland described the new star as “a feebly
developed” sun. As the star, however, increased in brightness the
spectroscope chronicled the fact that great physical changes were
taking place in its composition and structure. The star soon ceased
to be a feebly developed sun, for development had gone on apace with
the increase of light. Round the solid or semi-molten mass there was
rapidly aggregating an ocean of fiery gases, probably thrown up from
the nucleus.
Put simply, Nova Persei, for long ages a cold, dark, solid globe,
was in the brief space of a few days transformed from circumference
to core into a luminous, heated gaseous sphere. By what chance or
circumstance this vast change came about may be inquired into later
on. We only note here that this was the story spelled out by those
skilled in deciphering the observations recorded by the spectroscope.
In July, 1901, Professor Pickering of Harvard Observatory announced
that the star had become a nebula; that, indeed, its once solid
globe had practically dissolved into thinnest air. Not only had
its elements become molten with fervent heat, but they had become
transformed into shimmering wisps of matter more diaphanous than a
gossamer web.
Everything connected with the history of this star is of exceptional
interest; but all that had already been ascertained was completely
overshadowed by the astonishing discovery made in November, 1902,
that nebulous prominences were observed darting out from the star
with a velocity of at least 100,000 miles every second of time. These
astonishing changes have been confirmed at the two great American
observatories, the Yerkes and the Lick.
Whence and how had destruction come upon this particular star? At
one hour the star is dark, cold, solid. A few hours later this dark,
solid, cold body is a blazing world, its solid mass blown apparently
into countless fragments; from every fragment, big or little, there
pour streams of fiery vapor; for millions of miles round the star
there is a whirlpool of fire, a tempest of flame; and from end to end
of this great universe of ours the brightness of the burning star
pulsates. Three explanations have been given.
The one that naturally arises in our mind is that it was struck by
another star. Two worlds, each moving at the rate of twenty miles
a second, come into collision, and the result is the annihilation
of both. The force of their impact, changed into heat, drives their
elements into vapor. Such a catastrophe is quite possible in a
universe like ours, where stars and worlds, millions and millions
in number, sweep down the great avenues of space with a velocity far
beyond our comprehension.
We take it that when the crack of doom comes to this earth of ours
it will be in this fashion. Some great dark star will strike our sun
fair and square, and then in the twinkling of an eye, before the
inhabitants of earth know what has taken place, sun and moon and
planet will be wrapped up and dissolved in an atmosphere of fire.
We can in a certain rough way compute the increase in temperature
that would arise from the collision of two great orbs. Thus, let
us suppose that Nova Persei was moving onward through space with a
velocity of ten miles a second—a moderate velocity, be it noted, for
a star—when it collided with the body that wrought its destruction.
The impact would be terrific, and the result of it would be not
only the complete disintegration of both stars, but a sudden rise
in temperature of about five hundred thousand degrees, an increase
sufficient to vaporize the hardest adamant.
The second theory which has been suggested as explanatory not only
of Nova Persei, but of all new stars, is a modification of the
foregoing. This theory is that the new star in its flight through
space suddenly plunged into a nebula, or into some portion of
space denser than that through which it had already passed. This
explanation is not only intelligible but reasonable. If the new star
plunged into a region filled with matter even as rare as air, the
friction would immediately set the star on fire. We see the same
phenomenon every night when a meteor hustles through our atmosphere.
The meteoric rocks, with the chill of empty space in and around them,
dash into our upper air. A few seconds are ample for the practical
annihilation of most of them: in that brief space of time they have
been subjected to a heat many times greater than that of a Bessemer
furnace.
We can imagine Nova Persei as some monster meteor, a meteor larger
than the sun, plunging into a gaseous mass somewhat like our air. In
a few hours its temperature would be increased a million-fold. This
increase would fill the surrounding space with fire, and there would
be an immense and ever-increasing area at fervent heat.
To the mind of the writer this explanation has most to commend it.
It is the one that is most in harmony with the information which has
been gathered by hundreds of observers aided by the finest of modern
scientific equipment. But there are other explanations. There will
always be other explanations so long as the world lasts.
One of these explanations is of more interest than the rest, inasmuch
as it makes a link of connection between the recent terrible volcanic
eruption in the West Indies and the sudden appearance of a new star
like Nova Persei. It is suggested that Nova Persei is, or rather was,
a world somewhat like our own, only vastly larger—that is, there was
an inner core of molten matter and an outer shell of solid material.
One day, according to the explosion theory, this outer shell burst,
and the interior fires rushed hither and thither like a devouring
flood all over the stellar globe. Vast chemical changes went on as
the lambent flames turned everything solid into streams of lava.
Great electrical disturbances took place all round the star. The
whole phenomenon of Nova Persei, according to this theory, is just
the destruction of St. Pierre on a sidereal scale.
Such a doom, of course, is possible in any star or planet whose
interior is still molten. At any moment the imprisoned fires might
break their barriers and change a cold, fruitful, life-bearing earth
into a furnace; but it is far from probable that any such fate will
ever be meted out to our planet or to any other, and, at any rate,
destruction did not come to Nova Persei in this manner. No explosion
could account for an access of heat and light any way comparable to
that which was observed. Neither could any interior disruption be
violent enough to hurl the star into fragments. The gravitational
hold of the star would prevent this dismemberment. Yet during the
ages the mind of man has been irresistibly drawn to this conception
of the world’s end, so much so that perhaps, after all, our instinct
is right and our science wrong, and the vision of the Minorite Celano
of the
Dies iræ, dies illa
Solvet sæculum in favilla,
is a vision of those things that will be in the later days.
We have already touched on one strange circumstance connected
with the appearance of Nova Persei. Dr. Anderson saw it for the
first time at a few minutes to three o’clock on the morning of 22d
February—that is, the news of the strange occurrence reached our
planet then; but when did the event actually take place?
At Greenwich and at some of the other foremost observatories attempts
have been made directly and indirectly to determine the distance of
Nova Persei. And yet this distance defies measurement. The star is
so far away that we have no instruments refined enough to deal with
the problem. But we know that the sudden blazing up of Nova Persei
was over and done with before our great-grandfathers were born.
It happened more than two hundred years ago—perhaps two thousand
years ago. All this time the news was swiftly traveling earthward,
traveling on and on and on, two hundred thousand miles every second
of the clock, past star and nebula and system, never halting, never
faltering—yet it took hundreds of years to come to us; and beyond us
lie countless worlds that will not see the new star for centuries to
come. Hundreds of years hence in _their_ sky will appear suddenly
in the constellation of Perseus a strange star; it will increase in
brightness for a few days just as it did in ours; it will fade away
intermittently just as it did in ours. There is no imagination here;
only sober facts.
We may be allowed, in closing our narrative of this wonderful star,
to make one excursion into the region of imagination. As the news
of the star passes on through space, are there any beings beyond
ourselves who will take record of its appearance? It has taken
centuries to come to us. Did any other creatures in some far-off
world lift their eyes to the stars and wonder, as we do, what all
this meant? Will some mortal, like ourselves, in some remoter world,
in a day yet to come, see the sight, and have the intelligence
to say, “Lo! a new star?” We have room enough here for the most
extravagant fancy. Perhaps there is so much room that we shall lose
ourselves if we venture to stray in such directions.
TELESCOPES.—A. FOWLER
THE REFRACTING TELESCOPE.—The function of a telescope is twofold.
First, to magnify the heavenly bodies, or, what comes to the same
thing, to make them look as if they were nearer to us, so that we can
see them better. Second, to collect a much greater number of rays
of light than the unassisted eye alone can grasp, so that objects
too dim to be otherwise perceptible are brought within our range of
vision.
There are two forms of telescope, distinguished as _Refractors_
and _Reflectors_. The simplest form of refracting telescope is
exemplified by the common opera-glass, and large refractors are not
essentially different. Such instruments depend for their action upon
the formation of an image by a lens. One can easily illustrate this
by producing upon the wall of a room an inverted image of a candle
or gas flame with a spectacle lens (one adapted for a long-sighted
person), or with one of the larger lenses from an opera-glass. Having
such an image, it may be magnified by means of another lens, just
as one may magnify a photograph with an ordinary reading glass.
Technically, the lens which forms the primary image is called the
_object-glass_ of the telescope, and that which is used to magnify
this image is called the _eye-piece_. The object-glass is usually a
large lens, which is placed at one end of a tube, while the eye-piece
is a much smaller lens, placed at the other end. Means are provided
for adjusting the distance between the two lenses so as to admit of
distinct vision.
Matters are, however, not quite so simple as has been stated. There
is a very great difficulty introduced by the fact that a lens made
out of a single piece of glass gives an image which is surrounded by
fringes of color, so that some device has to be adopted in order to
destroy, as far as possible, this enemy of good definition. In the
early history of the telescope, this so-called _chromatic aberration_
was considerably reduced by making small object-glasses of very great
focal length.[22]
Lenses of 100-foot focus, however, are not easy to employ as
object-glasses, and astronomy was, therefore, greatly benefited by
Dollond’s invention of the _achromatic lens_ in 1760. This is a
compound lens, usually consisting of a double convex crown-glass
lens and a concavo-convex, or double concave, lens of flint glass.
The curvatures of the lenses, and the optical properties of the two
kinds of glass composing them, are such that the color due to one
of them is practically neutralized by that due to the other acting
in opposition. A section of such an object-glass, with the “cell” in
which it rests, is shown in Fig. 20.
[Illustration: Fig. 20.—The Achromatic Object-Glass]
In this way the focal length of the lens, and, therefore, the length
of the telescope tube, can be kept within reasonable dimensions,
while the definition is improved. There is, however, usually a little
outstanding color, due to the imperfect matching of the two lenses,
and if one looks through a large refractor, even of a good quality,
a purple fringe will be noticed round all very bright objects. This
only affects a few of the brighter objects, while millions of others
which are dimmer may be seen free from spurious color.
It may be remarked that the curved surfaces of the lenses forming
telescopic object-glasses must not be parts of spheres. If they are,
the images will be rendered indistinct by _spherical aberration_, and
the optician has to design his curves to get rid of this defect at
the same time as chromatic aberration.
A new form of telescopic objective, consisting of three lenses,
which has many important advantages, has been invented by Mr. Dennis
Taylor, of the well-known firm of T. Cooke & Sons, York, England.
Such a lens as this illustrates the perfection which the optician’s
art has now attained. Six surfaces of glass have to be so accurately
figured that every ray of light falling upon the surface of the lens
shall pass through the finest pin-hole at a distance of eighteen
times the diameter of the lens.
THE REFLECTOR.—In a reflecting telescope, the object-glass of the
refractor is replaced by a concave mirror. In order that such a
mirror may reflect all the rays from a star to a single point, its
concave surface must be part of a paraboloid of revolution, that is,
a surface produced by the revolution of a parabola on its axis. If a
spherical surface be employed, all the rays will not be reflected to
a single point and the images which it gives will be ill-defined. Yet
it is astonishing to find that the difference between a parabolic and
spherical surface, even in the case of a large mirror, is exceedingly
small. Sir John Herschel states that in the case of a mirror four
feet in diameter, and forming an image at a distance of forty feet,
the parabolic only departs from the spherical form at the edges by
less than a twenty-one thousandth part of an inch.
[Illustration: Fig. 21.—The Newtonian Reflector]
An image being formed by a mirror, it is next to be viewed with
an eye-piece just as in the case of a refracting telescope. Here
there is a little difficulty, for if the eye-piece be applied in the
direct line of the mirror, the interposition of the observer’s head
will block out the light. Several ways of overcoming this have been
devised, but the plan most generally followed is that which Newton
adopted in the first reflecting telescope which was ever constructed.
With his own hands Newton made a small reflector, 6¼ inches long and
having an aperture of 1⅓ inches, with which he was able to study
the phases of Venus and the phenomena of Jupiter’s satellites. This
precious little instrument is now one of the greatest treasures in
the collection of the Royal Society of London. The general design
of this telescope is shown in Fig. 21. The concave mirror is at the
bottom of the telescope tube, and normally it would form an image of
a star near the end of the tube. A plane mirror, however, of small
size intercepts the rays and reflects them to the side, where they
converge to a focus. This image is observed and magnified by an
eye-piece, as in the refractor. It is true that in this arrangement
the plane mirror, or _flat_, renders the central part of the
principal mirror ineffective, but the loss of light is very much less
than would be the case if the eye-piece were placed in position to
view the image centrally.
In the hands of Sir William Herschel the reflecting telescope was
greatly developed. The great telescope with which he enriched
astronomical science had a mirror four feet in diameter, and its
tube was forty feet in length. With the view of utilizing the whole
surface of the mirror and dispensing with a second reflecting
surface, the 4-foot mirror was placed at a small angle to the bottom
of the tube, so that its principal focal point was no longer at the
centre, but at the side of the tube.
In practice, however, it is found that the Herschelian form of
reflector does not give the best definition, and it is now very
seldom seen.
Among other forms, the “Cassegrain” is perhaps the most important.
During the last years this form has received a great deal of
attention, more especially in regard to its special adaptability for
photographic purposes.
In the Cassegrain telescope, the plane mirror of the Newtonian form
is replaced by a small convex mirror which is part of a hyperboloid
of revolution, its axis and focal point being coincident with those
of the primary mirror. The rays are in this way reflected back to the
mirror at the bottom of the tube, and in order that the image may be
seen, it is necessary to cut out the middle part of the mirror to
admit the eye-piece.
Although the small mirror must theoretically be hyperbolic, tolerable
definition is obtained even if it be spherical or ellipsoidal, and
its actual departure from these forms is so slight as to be beyond
detection by measurement, so that the figuring of such mirrors can
only be tested in the telescope. For photographic purposes this
telescope has the very important advantage that a short telescope is
equivalent to a very long one of the Newtonian form, or refracting
telescope, so that the image of sun, moon, or planets formed at the
focus is very large in comparison with the size of the telescope. A
modification of this form of telescope, in which the small mirror
is out of the path of the rays falling upon the larger one, and no
longer obstructing the central part, has been revived by Dr. Common,
and has become generally known as the “Skew Cassegrain.”
In reflecting telescopes the mirrors were formerly made of speculum
metal (an alloy of copper and tin), and the word _speculum_ is even
now commonly employed to signify a telescopic mirror, although it is
usual to make the mirror of glass, with the concave surface silvered
and highly polished.
One is frequently asked for an opinion as to which is the better form
of telescope, the reflector or refractor, and it is a question that
one finds some little difficulty in answering. On one point, however,
all are agreed, namely, that the reflector has the advantage in
regard to its achromatism; it is indeed perfectly achromatic, while
the so-called “achromatic” refractor is at best only a compromise.
For the rest, one can not do better than quote the evidence of Dr.
Isaac Roberts before the International Astrophotographic Congress:
“The reflector requires the exercise of great care and patience, and
a thorough personal interest on the part of the observer using it. In
the hands of such a person it yields excellent results, but in other
hands it might be a bad instrument. The reflector gives results at
least equal, if not superior, to those obtained with the refractor,
if the observer be careful of the centring, and of the polish of the
mirror, and keeps the instrument in the highest state of efficiency;
but when intrusted to an ordinary assistant the conditions necessary
for its best performance can not be so well fulfilled as the same
could be in the case of the refractor.” One great practical advantage
of the reflector is that there are fewer optical surfaces, so that
a large reflector may be obtained for the price of a much smaller
refractor.
[Illustration: Fig. 22.—The Cassegrain Reflector]
EYE-PIECES.—So far we have regarded the eye-piece of a telescope as
a simple lens, but it is evident that the spherical and chromatic
aberration of such a lens will interfere with its performance. For
occasional use, however, even a simple lens is very serviceable if
the object observed is brought to the centre of the field of view.
Compound eye-pieces are of various forms, each having certain
advantages, the desiderata being freedom from color and “flatness
of field”—that is, stars in different parts of the field are to be
equally well in focus. Those most commonly employed are the Ramsden
and Huyghenian eye-pieces. The former consists of two plano-convex
lenses of equal focal lengths, having their curved faces toward each
other, and being placed at a distance apart equal to two-thirds of
the focal length of either lens. Such an eye-piece can be used as a
magnifying-glass, and it is therefore placed outside the focal image
formed by the telescope with which it is used; on this account it is
called a _positive eye-piece_. This kind of eye-piece is not quite
achromatic, but its flat field of view gives it a special value for
many purposes.
In the Huyghenian eye-piece there are again two lenses, made of
the same kind of glass. That which comes nearest to the eye has a
focal length of only one-third that of the _field_ lens, and the
distance between the two lenses is half the sum of the focal lengths.
This form of eye-piece can not be used as a magnifying-glass in
the ordinary sense, and as the field lens must be placed on the
object-glass or mirror side of the focus, it is called a _negative
eye-piece_. The Huyghenian eye-piece is more achromatic than the
Ramsden, and is more widely used when it is only required to view the
heavenly bodies. In instruments employed for purposes of measurement,
a positive eye-piece is essential in order that the spider threads
may be placed at the focus of the telescope. The images formed by an
astronomical telescope are upside down, and neither of the eye-pieces
described reinverts them.
A special form of eye-piece is therefore used when a telescope
is employed for terrestrial sight-seeing. The desired result is
obtained by the introduction of additional lenses, but there is a
corresponding reduction of brightness.
For viewing the sun some device is necessary to reduce the quantity
of light entering the eye. To look at the sun directly, even with a
small instrument, is very dangerous. The arrangement usually adopted
is a _solar diagonal_, in which the light is reflected from a piece
of plane glass before entering the eye-piece; the piece of glass
is wedge-shaped, so that the reflection from one surface only is
effective; if the glass had parallel sides, the solar image would be
double.
MAGNIFYING POWER.—The magnifying power of a telescope depends upon
the focal length of the object-glass, or speculum, and that of the
eye-piece. Optically, it is equal to the former divided by the
latter, so that the greater the focal length of an object-glass, or
the smaller the focal length of the eye-piece, the greater will be
the magnifying power. In a given telescope, the object-glass, or
speculum, is a constant factor and the magnifying power can only
be varied by changing the eye-piece. The focal length of the Lick
telescope, for example, is about 600 inches; with an eye-piece which
is equivalent to a lens of one-inch focus, the magnifying power
would be 600; with a lens of half an inch focus, it would be 1,200,
and so on.
The magnifying power which can be effectively employed, however,
depends upon a great variety of circumstances. First, the clearness
and steadiness of the air; then there is the quality of the
object-glass, or speculum, to be considered; and also the brightness
of the object to be observed, for when the object is very dim, its
light will be spread out into invisibility if too high a power be
used.
In practice, good refractors perform well with powers ranging up to
80 or 100 for each inch in the diameter of the object-glass. Thus, on
sufficiently bright objects, a six-inch telescope will work well with
a power of about 500, while a 30-inch may be effectively employed
with powers between 2,000 and 3,000.
ILLUMINATING POWER.—It has already been pointed out that
magnification is not the only function of a telescope. As a matter
of fact, the most powerful telescopes in the world fail to produce
the slightest increase in the apparent size of a star, for even if
these objects be brought to apparently a 3,000th part of their real
distances, they are still too far away to have any visible size. But
although a star can not be magnified, it can be rendered more visible
by the telescope, for the reason that the object-glass collects a
greater number of rays than the naked eye. The pupil of the eye may
be taken to have a diameter of one-fifth of an inch; a lens one-inch
in diameter will have twenty-five times the _area_ of the pupil, and
will therefore collect twenty-five times the amount of light from
a star; a two-inch lens will grasp one hundred times, and a 36-inch
32,400 times as much light as the pupil alone. Practically all these
rays collected by the object-glass, or speculum, of a telescope can
not be brought into the eye; some are lost through the imperfect
transparency of the glass, or the imperfect reflecting power of the
speculum. Still, allowing a considerable percentage for loss, there
is an enormous concentration of light when a large telescope is
employed.
THE ALTAZIMUTH MOUNTING.—Having got a telescope, we have next to
see how it can be best supported, for unless it be a very small
instrument indeed, it will be impossible to hold it in the hand like
a spy-glass. However a telescope be mounted, provision must be made
for turning it to any part of the sky whatsoever. Very frequently
one of the axes on which the instrument turns is vertical, while
the other is horizontal. Such a stand for a telescope is called an
_altazimuth mounting_, for the reason that it permits the instrument
to be moved in altitude and in azimuth.
As a rule, one finds only small telescopes mounted in this manner.
The objection to it is that, as one continues to observe a heavenly
body, two independent movements must be given to the telescope in
order to follow the body in its diurnal movement across the heavens.
If we commence observing a star newly risen, for example, the
telescope must trace a star-like path in order to follow it as it
ascends into the heavens.
THE EQUATORIAL TELESCOPE.—A much more convenient method of setting
up a telescope is to mount it as an _equatorial_. The essential
feature of this instrument is that one of the axes of movement,
instead of being vertical, is placed parallel to the axis of the
earth. This is called the _polar axis_, and, when the telescope is
turned around such an axis, it traces out curves in the sky which are
identical with those described by the stars in their diurnal motions.
If, then, the telescope be directed to a star or other heavenly body,
it can be made to follow the object and keep it in view by a single
movement. The axis at right angles to the polar axis is called the
declination axis, and is necessary in order that the telescope may be
moved toward and from the poles so that all the heavenly bodies above
the horizon may be included in its sweep.
One very important advantage of the equatorial is that, as only one
motion is required to keep a star in view, so long as it is above
the horizon, the necessary movement may be furnished by clockwork. A
good equatorial is accordingly provided with a driving-clock, which
is regulated so that it would drive the telescope through a whole
revolution once a day. Unlike an ordinary clock, the driving-clock of
a telescope is regulated by a governor, in order that the instrument
may have a continuous and not a jerky movement.
The telescope is also provided with clamps and fine adjustments, one
each in R. A. and declination, in order that it may be under the
control of the observer. It is evident that the telescope must be
capable of moving independently of the driving-gear, so that it may
first be placed in the desired direction; when this is accomplished,
the R. A. clamp is used to put the telescope in gear with the clock.
The declination clamp is then made to fix the telescope firmly to the
declination axis. Fine adjustments in both directions are necessary,
because it is impossible to sight a large instrument with such
precision as to bring an object exactly to the centre of the field of
view.
Some of the driving-clocks fitted to equatorials are very elaborate.
As clocks regulated by governors are not such reliable timekeepers
as those regulated by pendulums, arrangements are made by which the
accuracy of a pendulum can be electrically communicated to a governor
clock. One of the best forms of electrically controlled clocks is
that devised by Sir Howard Grubb.
Another important feature of an equatorial is that it can be provided
with circles which enable the telescope to be pointed to any desired
object of known right ascension and declination. One of these is the
declination circle, attached to the declination axis and read by a
vernier fixed to the sleeve in which the axis turns; this is adjusted
so as to read 0° when the telescope points to any part of the
celestial equator, and 90° when it is directed to the pole. The other
circle is attached to the polar axis, and determines the position of
the telescope with regard to the meridian; this is called the _hour
circle_, and is divided into twenty-four hours. When the telescope
is on the meridian, the hour circle reads zero, so that its reading
in any other position gives the hour angle of the telescope. Having
given the right ascension and declination of a heavenly body which it
is desired to observe, the telescope is turned until the declination
circle reads the proper angle, and the hour circle indicates the
hour angle which is calculated for the particular moment of pointing
the telescope. [The hour angle is the difference between the right
ascension of the object and the sidereal time of observation.] In
this way it is easy to find objects of known position which are
invisible to the naked eye, and one can even pick up the planets and
brighter stars in full sunshine. Conversely, one can determine from
the circles the right ascension and declination of any object under
observation, but for various reasons only approximate results can be
obtained in this way. The chief use of the circles on an equatorial
is therefore to provide a means of pointing the telescope.
Telescopes of four inches aperture and upward are usually provided
with a smaller companion called a _finder_. This has a larger field
of view than the main telescope, so that objects which are of
sufficient brightness can readily be picked up and brought to the
centre of the finder, the adjustments being such that the object is
then also at the centre of the field of the large telescope.
There are, of course, many practical details connected with the
working of an equatorial with which space does not permit us to deal.
It may be remarked, however, that the adjustment of the polar axis is
very simply performed by first inclining it at an angle approximately
equal to the latitude of the place where it is set up, and setting
it as nearly as possible in the meridian by means of a compass or by
observations of the sun at noon. The final adjustment is then made by
a series of observations of stars of known position.
SOME OF THE WORLD’S GREAT TELESCOPES.—Thanks to the wide public
interest taken in astronomical matters, a large number of powerful
telescopes have been set up in various parts of the world. To the
British Islands belongs the honor of possessing the largest telescope
in the world. This is the giant reflector erected by Lord Rosse, in
1842, at Parsonstown, the mirror being six feet in diameter, and the
focal length sixty feet. Many very valuable observations were made
with this instrument in its early days, but of late years it seems to
have fallen into disuse. One reason may be that the mounting is not
of the most convenient form, and makes the telescope unsuitable for
photographic work.
Coming next in point of size to the Rosse telescope is the reflector
erected at Ealing by Dr. A. A. Common. The glass mirror of this
telescope is five feet in diameter, five inches thick, and weighs
more than half a ton. Dr. Common aimed specially at constructing the
largest possible telescope which could be equatorially mounted and
provided with a driving-clock, and he was only limited to an aperture
of five feet by the impossibility of obtaining a glass disk of larger
size. He has attained such great skill in this work that he was able
to produce a perfect mirror five feet in diameter in three months’
time, although no less than 410,000 strokes of the polishing machine
were required.
The telescope is of the Newtonian form, and the mounting is quite
unique. The polar axis consists of an iron cylinder, made up of
boiler plate, seven feet eight inches in diameter, and about fifteen
feet long. From the top of the cylinder, near its outer edge, two
horns, each six feet long, project outward, and the tube of the
telescope swings on trunnions attached to the ends of the horns. The
main part of the telescope tube is square, built up of steel angle
iron, and carries the mirror at its lower end; the upper part of the
tube, which carries the “flat” and eye-piece, is round, and of tinned
steel strengthened by a skeleton framework.
It is evident that such an enormous instrument as this can not be
made to travel by clockwork with the necessary uniformity without
some very efficient arrangement for reducing friction. Dr. Common’s
plan—and it is here that his instrument is unlike others—is to make
the hollow polar axis watertight, and to fix it in a tank of water.
At the bottom of the polar axis is a ball and socket joint to keep
it in position, and at the top is another bearing, which can be
adjusted so that the polar axis lies truly in the meridian. It was
found necessary to introduce nine tons of iron into the bottom of
the hollow polar axis in order to sink it to the proper angle, and
to put sufficient weight on the bearings to give stability to the
instrument. In this way the great mass is brought into the region of
manageability, and the driving-clock, which is driven by a weight of
one and a half tons, is able to do its work efficiently. Such, in
general outline, is this wonderful telescope, which, although not so
large as Lord Rosse’s famous instrument, is undoubtedly its superior
in light-grasping power and general utility, and more especially in
its adaptability for photographing the heavens.
Among other large reflecting telescopes now in use are the 4-foot
reflectors at Melbourne and Paris, and the 3-foot reflectors at South
Kensington and the Lick Observatory, California.
The largest refracting telescope yet constructed is one of forty
inches aperture for the Yerkes Observatory of the University of
Chicago. It is interesting to note here that Professor Keeler, in
his report as an expert upon the performance of the object-glass,
considers that there is “evidence for the first time that we
are approaching the limit of size in the construction of great
objectives.” Unlike a mirror, a lens can be supported only upon its
circumference, and it is the bending by its own weight that proves
detrimental to its defining power. If the lens be made thicker with a
view of overcoming this defect, the absorption of light by the glass
increases, so that there is in the end no special gain by increasing
the size.
The length of the Yerkes telescope is 62 feet, and is provided
with all accessories pertaining to astrophysical research. The
world-renowed Lick Telescope is of thirty-six inches aperture. The
story of the foundation of this monster instrument is not much
less wonderful than the telescope itself. Brought up in poor
circumstances, with few opportunities for intellectual development,
James Lick, nevertheless, amassed a fortune in business, and having
few relations, he was anxious to dispose of his wealth in such a way
as to bring him that fame which he had failed to achieve in other
directions. Although it is very probable that he had never looked
through a telescope in his life, the idea of a large telescope had
taken a very firm hold upon his mind, and, thanks to the influence of
his advisers, it was definitely announced in 1873 that Mr. Lick’s bid
for immortality was to take this form. Several sites were examined
by experts, and finally Mount Hamilton, California, 4,200 feet above
sea-level, was selected. An excellent road, twenty-six miles in
length, made at the cost of the county authorities, connects the
observatory with the nearest town, San José, thirteen miles distant.
Owing to various delays, operations were not commenced until 1880,
and five years were consumed in clearing away 72,000 tons of rocks
and in erecting the buildings.
Mr. Lick had stipulated for the erection of “a telescope superior to
and more powerful than any telescope yet made,” and Messrs. Alvan
Clark & Co. contracted to supply a lens of thirty-six inches aperture
for the sum of $50,000. It turned out, however, that it was much
easier to make such a contract than to fulfil it. To produce large
disks of optically perfect glass, even in the rough, requires the
greatest possible skill and patience, and this part of the work was
undertaken by Feil & Co. of Paris. The flint glass disk was safely
delivered in America in 1882, but the crown disk was cracked in
packing. The elder Feil having retired from business, the duty of
providing a new block of crown glass devolved upon his sons, who,
after two years spent in vain attempts, ended in bankruptcy, and it
was only through the elder Feil again resuming business that the
much-required disk was finally completed in 1885. After the lapse of
another year, the rough disks were fashioned, in the workshops of the
Clarks, into the most marvelous of telescopic lenses.
The mounting of the object-glass is worthy of the occasion. The tube
is no less than thirty-seven feet long, and four feet in diameter
in the middle part. An iron pier, thirty-eight feet high, beneath
which lie the remains of Mr. Lick, supports the equatorial head,
and a winding staircase enables the observer to reach the setting
circles. Inside the hollow pier is the powerful driving-clock which
turns the telescope to follow the heavenly bodies in their apparent
movements. Finders of six, four, and three inches diameter, rods for
the manipulation of the instrument, and all necessary accessories,
complete what must long remain one of the most perfect instruments at
the service of astronomical science. The $200,000 expended upon it
have already been amply justified by the work accomplished, while Mr.
Lick’s dream of immortality has become a reality.
The following list indicates some of the large refractors now doing
active service:
Aperture Observatory
36 inch [Lick] California.
30 ” Pulkowa, Russia.
30 ” [Bischoffeim] Nice.
28 ” Greenwich.
27 ” Vienna.
26 ” Washington.
25 ” [Newall] Cambridge.
24 ” [Lowell] Mexico.
23 ” Princeton, New Jersey.
It is right to add, however, that opinion is still greatly divided
as to whether these telescopes of large aperture really repay
the expense and labor involved in their erection and use. On the
very rare occasion when the “seeing” is practically perfect—which
occurs perhaps only a few hours in a year—it is probable that the
superiority of a large telescope is very marked, but under average
conditions there seems to be little advantage over instruments of
moderate size for many classes of observations.
Certain it is that a great deal of valuable work is done with
comparatively small telescopes, ranging from six to fifteen inches
aperture, and this in all departments of astronomical research.
Hence, some of the most active observatories do not figure in the
above list; among them may be mentioned the observatories of Harvard
College, Potsdam, Paris, Heidelberg, Cape of Good Hope, Edinburgh,
South Kensington, Stonyhurst College, and the observatory of Dr.
Isaac Roberts at Crowborough, England.
HOUSING OF EQUATORIALS.—The building which accommodates an equatorial
telescope must evidently be designed to admit of giving a clear
opening to any part of the sky. Usually this is accomplished by
making the roof, or _dome_, with a circular base, provided with
wheels, which run on rails. It is then only necessary to open a
narrow portion of the dome, extending from top to base, and to turn
the dome until this aperture is in the required direction. One of
the most elaborate domes now in existence is that built by M. Eiffel
for the great refractor of the Nice Observatory. The lower part of
the building is in the form of a square, having a side of about
eighty-seven feet and a height of about thirty feet. The dome itself
is seventy-four feet in diameter, and the moving parts alone weigh
ninety-five tons.
There are two shutters, each a little wider than half the possible
opening; these run on short rails, and are moved simultaneously
by means of an endless rope. The whole of the dome is built up of
steel angle iron, covered with very thin sheet steel. In order to
facilitate the manipulation of the dome, its great weight is buoyed
up by means of a float attached to its base and immersed in a
circular tank of water of a little greater size than the base of the
dome. If any mishap occurs with this gigantic tank, the dome rests
on wheels which run on a circular rail, so that the work need not be
interrupted. The whole arrangement is very easily turned with the aid
of a winch by one man when the dome is floating, but when resting on
the wheels several men are required at the winch.
This brief description will serve to illustrate some of the problems
which confront the possessor of a very large telescope. For smaller
instruments, the observatories follow pretty nearly the same plan,
except that it is unnecessary to provide an arrangement for floating
the dome.
The observatory which shelters a reflecting telescope need not differ
very greatly from one which contains a refractor. If the instrument
be a Newtonian, it is generally convenient to sink the polar axis
below the level of the floor in order that the observer may not be
at too great a height from the ground, and in that case, the dome,
or its equivalent, is all that is necessary. For his five-foot
reflector, Dr. Common designed an observatory which is not of the
ordinary form, but gives the necessary opening partly by means of
large shutters and partly by a revolution of the whole house. It is
not every one who is able to lay out $40,000 on such a dome as that
erected at Nice by M. Bischoffeim.
The varying position of the eye end of a telescope, when it is
turned to different parts of the sky, makes it necessary to provide
comfortable and safe seating accommodation for the observer, more
especially when the telescope is a very large one. In the case of the
Yerkes telescope, the eye-piece is thirty feet higher when observing
near the horizon than when observing near the zenith, and the
observer must necessarily follow the telescope. The most convenient
arrangement in such a case is to raise or lower the floor of the
observatory as occasion demands. The floor of the Yerkes Observatory
is seventy-five feet in diameter, and by means of electric motors
it can be given a vertical motion of twenty-two feet. A similar
arrangement was provided for the Lick telescope from the designs of
Sir Howard Grubb. With smaller instruments, observing ladders and
adjustable chairs of various forms are employed.
THE EQUATORIAL COUDÉ.—A form of equatorial telescope which has
possibly a great future before it is one introduced at Paris under
the name of the _equatorial coudé_, or elbowed telescope. Its
practical advantage is that the observer remains in a constant and
comfortable position, so that revolving domes and elevating floors,
or other arrangements serving similar purposes, are no longer
necessary. The telescope tube is of two parts of nearly equal length,
and what is ordinarily the lower half of the tube forms part of the
polar axis, while the other half is attached to it at right angles.
At the point of intersection of the two halves of the tube is a plane
mirror, and there is another mirror in front of the object-glass. If
the latter mirror were removed, such a telescope would only enable
the observer to see objects lying along the celestial equator, but
by its means objects in all parts of the heavens can be brought
within range to an observer gazing down the hollow polar axis. The
largest instrument is that at the Paris Observatory, which has an
object-glass 23½ inches in diameter for visual observations, and
another of the same size for photographic purposes.
FIXED TELESCOPES.—There is still another method of using a telescope.
The telescope itself may be fixed, and the light of the heavenly
bodies may be reflected into it by means of a mirror which is made to
revolve so as to keep pace with their movements. Foucault devised an
instrument called the _siderostat_ for this purpose, and although
it is not largely employed for telescopic observations, it is very
widely utilized for spectroscopic work, where the spectroscope is of
a kind not readily attached to a telescope.
Another instrument used for the same purpose has recently been
brought forward under the name of the _cœlostat_. This is simply a
mirror which is made to turn on a polar axis in its own plane, and
since a reflected ray of light moves through twice the angle that
the reflecting surface turns through, the mirror is made to revolve
at the rate of one revolution in two days. As the name indicates,
the whole heavens appear stationary in such an instrument, whereas
in a siderostat only one star at a time appears at rest, while its
neighbors slowly revolve round it.
PHOTOGRAPHIC TELESCOPES.—The application of photography to the study
of the heavenly bodies marks one of the greatest advances of the
present century. The instruments which are employed for this purpose
range from the ordinary tourist camera to the largest telescope.
Unlike a person sitting for a portrait, the heavenly bodies can
not be made to stand still for the purpose, and as instantaneous
photographs can only be obtained in the case of the sun and moon, it
is usually necessary to make the camera follow the stars very exactly
during the time of exposure in order that the images may fall on
precisely the same parts of the photographic plate.
Some guiding arrangement is, therefore, essential, and generally
the photographic camera or telescope is attached to an ordinary
equatorial which is driven by clockwork, or very carefully by hand
if the camera be a small one. In the guiding telescope are two
spider-threads at right angles to each other, and it is by constantly
keeping the image of a star at the intersection of these “wires” that
the operator ensures the images remaining in a constant position upon
the sensitive plate.
An ordinary portrait camera, in the hands of a skilled observer,
yields very beautiful pictures, but they are naturally on a small
scale. The field of view of such an instrument is so large that a
whole constellation may be photographed with a single exposure.
Portrait lenses of six inches aperture in the hands of Dr. Max Wolf
and Professor Barnard have given magnificent delineations of the
Milky Way, and of the extremely faint nebulosities which are to be
found in many parts of the heavens.
For many purposes, however, telescopes of greater power are required,
and here it may be remarked that the distance between the images of
any two adjacent stars will vary in direct proportion to the focal
length of the telescope. In the same way the size of the image of a
planet, the moon, or a comet, increases as the focal length of the
objective is increased.
Refracting telescopes which are employed for photography require
object-glasses which are specially “corrected” for the photographic
rays. White light is compounded of light of all colors, but it is
the blue and violet constituents which are effective in producing
photographic action on an ordinary sensitive plate. Now, an
object-glass which is intended for visual purposes is made to
focus at the same point as many as possible of the rays which
are most effective to the human eye, that is the green, yellow,
and red, and usually there is a blue or purple halo round the
images of the brighter objects, which is, however, too feeble as
a rule to interfere with visual observations. This blue halo will
evidently result in defective definition if the lens be employed for
photography. By putting the plate at the point where the blue rays
are most nearly focused, a better image is obtained; but for really
good work a photographic object-glass must be so designed that all
the blue and violet rays are brought to one and the same focus. Such
a lens will consequently be a very poor one for visual observations.
The new “photo telescopic” object-glass now manufactured by Messrs.
Cooke appears to be full of promise. In this lens all the colors of
the spectrum are brought to almost exactly the same focal point, so
that it serves equally well for photographic or visual purposes.
This difficulty in regard to achromatism does not exist in the case
of the reflecting telescope, since rays of light of every color are
reflected at precisely the same angles. For this reason reflectors,
when properly managed, give the best photographic results. Dr. Isaac
Roberts and Dr. Common are especially identified with the application
of the reflecting telescope for celestial photography. The instrument
employed by the former consists of a 20-inch reflector and a 7-inch
guiding telescope of the refracting form. The two telescopes are
mounted on the extreme ends of the declination axis of an equatorial.
Dr. Common does not employ a guiding telescope at all. The
photographic plate which he places at the focus of the reflector is
smaller than the field of view, so that by means of an eye-piece
fitted with a cross wire at the side of the dark slide, he is able
to watch a star near the edge of the field. Both eye-piece and dark
slide are attached to a frame which can be controlled by two screws
at right angles to each other. If the guiding star leaves the cross
wire through errors in driving, or other causes, the eye-piece and
dark slide are bodily moved after it by means of the adjusting
screws. This method not only has the advantage of saving the cost of
a guiding telescope, but reduces the effects of vibration consequent
upon the correction of errors by moving the whole telescope.
For photographing the sun a special instrument called a
_photoheliograph_ is usually employed. This differs only from an
ordinary photographic telescope in being provided with a secondary
magnifier, by which means the focal image formed by the object-glass
is amplified before falling upon the photographic plate. On a bright,
clear day pictures of the sun eight inches in diameter can be taken
with an exposure of about 1/500th of a second, and such a photograph
will frequently record more facts as to the state of the solar
surface than a whole day’s observation. Lenses or mirrors of very
long focus are also occasionally employed in solar photography, and
in this way a large image is obtained without the use of a secondary
magnifier.
Photographs of the moon and planets may be taken either with or
without a secondary magnifier, but in either case the exposures are
longer than for the sun.
Finally, it may be added that the sensitive plates and processes used
in astronomical photography do not differ from those employed by
ordinary photographers.
FOOTNOTES:
[22] The focal length of a lens is the distance from its centre at
which an image of a very distant object, such as the sun, is formed.
METEORS.—SIR ROBERT S. BALL
Our present knowledge as to the natural history of the shooting stars
has been mainly acquired during the last hundred years. The first
important step in the comprehension of these bodies was to recognize
that the brilliant flash of light was caused by some object which
came from without and plunged into our air. This was known at the end
of the Eighteenth Century, largely by the labors of the philosopher
Chladni in 1794.
[Illustration: A Portion of the Moon’s Disk
Where Four Mountain Ranges Meet]
Could an ordinary shooting star tell us its actual history, the
narrative would run somewhat as follows:
“I was a small bit of material, chiefly, if not entirely, composed
of substances which are formed from the same chemical elements as
those you find on the earth. Not improbably I may have had some iron
in my constitution, and also sodium and carbon, to mention only a
few of the most familiar elements. I only weighed an ounce or two,
perhaps more, perhaps less—but you could probably have held me in
your closed hand, or put me into your waistcoat pocket. You would
have described me as a sort of small stone, yet I think you would
have added that I was very unlike the ordinary stones with which you
were familiar. I have led a life of the most extraordinary activity;
I have never known what it was to stay still; I have been ever on
the move. Through the solitudes of space I have dashed along with a
speed which you can hardly conceive. Compare my ordinary motion with
your most rapid railway trains; my journey will be done ere the best
locomotive ever built could have drawn the train out of the station.
Pit me against your rifle bullets, against the shots from your
one-hundred-ton guns; before the missile from the mightiest piece
of ordnance ever fired shall have gone ten yards I have gone 1,000
yards. I do not assert that my speed has been invariable—sometimes it
has been faster, sometimes it has been slower; but I have generally
done my million miles a day at the very least. Such has been my
career, not for hours or days, but for years and for centuries,
probably for untold ages. And the grand catastrophe in which I
vanished has been befitting to a life of such transcendent excitement
and activity; I have perished instantly, and in a streak of splendor.
In the course of my immemorial wanderings I have occasionally passed
near some of the great bodies in the heavens; I have also not
improbably in former years hurried by that globe on which you live.
On those occasions you never saw me, you never could have seen me,
not even if you had used the mightiest telescope that has ever been
directed to the heavens. But too close an approach to your globe was
at last the occasion of my fall. You must remember that you live on
the earth buried beneath a great ocean of air. Viewed from outside
space, your earth is seen to be a great ball, everywhere swathed
with this thick coating of air. Beyond the appreciable limits of the
air stretches the open space, and there it is that my prodigious
journeys have been performed. Out there we have a freedom to move of
which you who live in a dense atmosphere have no conception. Whenever
you attempt to produce rapid motion on the earth, the resistance of
your air largely detracts from the velocity that would be otherwise
attainable. Your quick trains are impeded by air, your artillery
ranges are shortened by it. Movements like mine would be impossible
in air like yours.
“And this air it is which has ultimately compassed my destruction.
So long as I merely passed near your earth, but kept clear of that
deadly net which you have spread, in the shape of your atmosphere,
to entrap the shooting stars, all went well with me. I felt the
ponderous mass of the earth, and I swerved a little in compliance
with its attraction; but my supreme velocity preserved me, and I
hurried past unscathed. I had many narrow escapes from capture
during the lapse of those countless ages in which I have been
wandering through space. But at last I approached once too often to
the earth. On this fatal occasion my course led me to graze your
globe so closely that I could not get by without traversing the
higher parts of the atmosphere. Accordingly, a frightful catastrophe
immediately occurred. Not to you; it did you no harm; indeed, quite
the contrary. My dissolution gave you a pleasing and instructive
exhibition. It was then, for the first time, that you were permitted
to see me, and you called me a shooting star or a meteor.
“When from the freedom of open space I darted into the atmosphere,
I rubbed past every particle of air which I touched in my impetuous
flight, and in doing so I experienced the usual consequence of
friction—I was warmed by the operation.
“You can readily comprehend the immense quantity of heat that will
have been produced ere friction could deprive me of a speed of twenty
miles a second. That heat not merely warmed me, but I rapidly became
red-hot, white-hot, then I melted, even though composed of materials
of a most refractory kind. Still friction had much more to do, and
it actually drove me off into vapor, and I vanished. You, standing
on your earth many miles below, never saw me—never could have seen
me—until this supreme moment, when, glowing with an instantaneous
fervor, I for a brief second became visible.
“Nature knows no annihilation, and though I had been driven off
into vapor and the trial by fire had scattered and dispersed me,
yet in the lofty heights of the atmosphere those vapors cooled and
condensed. They did not, they never could again reunite and reproduce
my pristine structure. Here and there in wide diffusion I repassed
from the vaporous to the solid form, and in this state I wore the
appearance of a streak of minute granules distributed all along the
highway I had followed. These granules gradually subsided through
the air to the earth. On Alpine snows, far removed from the haunts of
men and from contamination of chimneys, minute particles have been
gathered, many of which have unquestionably been derived from the
scattered remains of shooting stars. Into the sea similar particles
are forever falling, and they have been subsequently dredged up from
profound depths, having subsided through an ocean of water after
sinking through an ocean of air.”
Those splendid shooting stars which are often called fire-balls move
in every direction. They come from the east, and from the west,
from the north, and from the south. There is no hour of the night
at which they have not occasionally been seen. Even in daylight it
has happened not once or twice, but on several occasions, that a
brilliant meteor has forced itself upon our astonished notice. They
generally first make their appearance at a height which is between
fifty and one hundred miles above the ground. They hurry down their
inclined path, but generally become extinguished while still at least
twenty miles aloft. In their more ambitious flights meteors have been
known to span a kingdom. Nor are even greater strides unrecorded.
The length of a continent may be compared with the track of that
terrific meteor of 5th September, 1868, which broke into visibility
at an appalling height above the Black Sea, and had not expended
its stupendous energy until it passed over the smiling vineyards of
France.
Great fire-balls are much more numerous than any one would suppose
who had not paid attention to the subject. Nor need this be a
matter for surprise if it be remembered that when a fire-ball does
arrive it is only by a favorable combination of circumstances that
any particular individual is privileged to witness the exhibition.
As a random example of the yearly crop of fire-balls, I take from
the middle of 1877 to the middle of 1878. A list of the fire-balls
noticed during this period will be found in that store-house of
valuable information, the Reports of the British Association. In
the year referred to I see that eighty-six great fire-balls have
been recorded. They have appeared in various localities, both in the
old hemisphere and in the new. The most arduous observer may think
himself fortunate if he has even seen one of them.
As to the brilliant light from some of these great fire-balls, there
are numerous statements. We are not infrequently told that even the
beams of the full moon are ineffectual in comparison with the blaze
of the meteor; and we find a high authority asserting that one of
these bodies displayed a flash as “blinding as the sun.” On the
29th July, 1878, a fire-ball was seen which created so splendid an
illumination that “the smallest objects were visible at Manchester.”
Fortunate, indeed, would the astronomer have been who, guided by
some miraculous prescience, had gone to the ancient city of York on
the evening of the 23d of February, 1879, and on the tower of the
glorious minster spent the night in observation of the heavens. It
would have been his privilege to witness a majestic meteor under
circumstances of almost unique magnificence.
It was at seven minutes before three that such few stragglers as
the streets of York still contained saw a pear-shaped ball of fire
traveling across the sky. It drenched the ancient city with a flood
of light. The superb front of the minster never before glowed with a
more romantic illumination. The unwonted brilliancy streamed through
every aperture in every window in the city; every wakeful eye was
instantly on the alert; every light sleeper started up suddenly to
know what was the matter. Even those whom the blaze of midnight
light had failed to awaken were only permitted to protract their
slumbers for another minute and a half—only until an awful crash,
like a mighty peal of thunder, burst over the town, shaking the
doors, the windows, and even the houses themselves. The whole city
was thus alarmed. Every one started at the noise. But that noise was
not a clap of thunder. Nor was it produced by an earthquake. It was
merely the explosion of the fire-ball which flung itself against the
atmosphere after its immeasurable voyage through space.
Perhaps the most remarkable instance of the _explosion_ of a meteor
is recorded in the case of the great fire-ball so widely observed in
America on the 21st of December, 1876. The movements of this superb
object have been carefully studied by Professor H. A. Newton and
Professor D. Kirkwood. For the prodigious span of a thousand miles
this meteor tore over the American continent with a speed of some
ten or fifteen miles a second. It first appeared over Kansas at a
height of seventy-five miles. Thence it glided over the Mississippi,
over the Missouri; it passed to the south of Lake Michigan; it made a
short voyage over Lake Erie, and it can not have been very far from
the Falls of Niagara, when by becoming invisible all further traces
of its movements were lost. While passing a point midway between
Chicago and St. Louis a frightful explosion shivered the meteor into
a cluster of brilliant balls of fire, which seemed to chase each
other across the sky. This cluster must have been about forty miles
long and five miles wide. The detonation by which the explosion was
accompanied was a specially notable incident of this meteor. It was
not only heard with terrific intensity in the neighborhood, but the
volume of sound was borne to great distances.
The glory of a meteor is often so evanescent that we just get
a glimpse and it is gone. The sky resumes its ordinary aspect;
the familiar stars are there, and even the very situation of the
brilliant streak has become unrecognizable. But this is not always
so; it sometimes happens that the brief career of the meteor leaves
a notable trace behind it, so that for seconds and for minutes the
sky is diversified by an unwonted spectacle. The path of the meteor
leaves a stain of pearly light on the sky to mark the highway pursued
by our celestial visitor.
In its fearful career the meteor is often rent to fragments, reduced
to dust, dissolved into vapor. The glowing atoms of the wreck lie
strewn along the path, just as the ghastly remnants of Napoleon’s
mighty army limned out the awful retreat from Moscow.
A pencil-shaped cloud of meteoric débris, perhaps eighty or a hundred
miles in length, and four or five miles in diameter, thus hangs
poised in air. It is at night. The sun has sunk so far below the
horizon that there is no trace of the feeblest twilight glow. An
ordinary cloud would, of course, be invisible except as concealing
the stars; no beams of light fall upon it; there is nothing to render
it luminous. So, too, the meteoric streak will often pass instantly
into invisibility, but, as I have said, this is not always the case.
There is a well-authenticated instance in which the trail of a superb
meteor remained visible for nearly an hour. I have endeavored up to
the present to explain the various phenomena presented to us in the
fall of a meteor, but here, for the first time, we have to note a
circumstance for which it is not easy to account. We can explain why
it is that the long meteoric cloud should be there, but we can not
so easily explain why we should be able to see it. Whence comes this
beautiful pearly luminosity? It seems that the meteoric dust must
glow with some intrinsic luminosity.
We have spoken of dazzling fire-balls which generate for a brief
moment a light which eye-witnesses, with possibly a pardonable
exaggeration, have ventured to compare with the beams of the sun
himself. Other meteors are described as being as bright as the full
moon. Descending still lower in the scale of splendor, we read of
fire-balls as bright as Venus or Jupiter, as bright as Sirius, or as
a star of the first magnitude. With each step downward in brilliancy
we find the meteors to increase in numerical abundance. Shooting
stars as bright as the stars of the second or third magnitude are
comparatively frequent; they are still more numerous of the fourth
and fifth magnitudes. Every night brings its tale of shooting stars
whose brightness is just sufficient to impress the unaided eye. Nor
do the shooting stars which even the most attentive eye can detect
represent a fraction of their entire number. As there are telescopic
stars which the unaided eye can not see, so it might fairly be
conjectured that, as we can trace meteors of successive stages of
brightness down to the limit of unaided eye visibility, so there may
be meteors still and still smaller which would be detected could we
only direct a telescope toward them.
If we reflect that for every one that is seen there must be thousands
which dart in unseen, we obtain an imposing idea of the myriads of
shooting stars that daily rain in upon our globe.
The world is thus pelted on all sides day and night, year after year,
century after century, by troops and battalions of shooting stars
of every size, from objects not much larger than grains of sand up
to mighty masses which can only be expressed in tons. In the lapse
of ages our globe must thus be gradually growing by the everlasting
deposit of meteoric débris. Looking back through the vistas of time
past, it becomes impossible to estimate how much of the solid earth
may not owe its origin to this celestial source.
The first and most important truth with regard to the recurrence
of the meteors is their occasional appearance in what are known as
“meteoric showers.” During such displays it sometimes happens that
shooting stars in shoals break forth simultaneously, so as to produce
a spectacle which we now regard as of the utmost beauty and interest,
but which in earlier times has often been the source of the direst
terror and dismay.
Let me, for the sake of illustration, give some account of one of
these great showers of shooting stars.
In the year 1866 I occupied the position of astronomer to the late
Earl of Rosse. The memorable night between November 13th and 14th,
1866, was a very fine one; the moon was absent—a very important
consideration in regard to the effectiveness of the display. The
stars shone out clearly, and I was diligently examining some faint
nebulæ in the eye-piece of the great telescope, when a sudden
exclamation from the attendant caused me to look up from the
eye-piece just in time to catch a glimpse of a fine shooting star,
which, like a great sky-rocket, but without its accompanying noise,
shot across the sky over our heads. The great shooting star which had
already appeared was merely the herald announcing the advent of a
mighty host. At first the meteors came singly, and then, as the hours
wore on, they arrived in twos and in threes, in dozens, in scores,
in hundreds. Our work at the telescope was forsaken; we went to the
top of the castellated walls of the great telescope and abandoned
ourselves to the enjoyment of the gorgeous spectacle.
To number the meteors baffled all our arithmetic; while we strove
to count on the one side many of them hurried by on the other. The
vivid brilliance of the meteors was sharply contrasted with the
silence of their flight. We heard on that marvelous night no sounds
save those with which we were familiar. The flights of the celestial
rockets were attended with no noises that we could hear. The meteors
were no doubt somewhat various as to size, but the characteristic
feature of this shower, as contrasted with another great shower I
have also seen, was the remarkable brilliance of the shooting stars.
It was their exceptional splendor even more than their innumerable
profusion that gave to the shower its peculiarity. As to the actual
brilliancy of the meteors, I am enabled to give the accurate
estimate made by Mr. Baxendell at Manchester, where the shower was
well seen. Out of every hundred of these meteors ten were brighter
than a first magnitude star, and two or three of them were brighter
than Sirius. Fifteen out of each hundred were between the first and
second magnitudes, and twenty-five were between the second and third
magnitudes, while the remainder were smaller.
Some important facts with regard to ancient shooting-star showers
have survived the thousand and one casualties to which historical
records are exposed. A careful discussion of those which are
sufficiently accurate to be intelligible discloses to us the
startling fact that in general every thirty-three years a grand
shooting-star shower has rained down on our earth. It sometimes
happens that two consecutive years are rendered memorable by great
showers. At present the day of the year on which this particular
shower is wont to appear is about the 13th November; but in earlier
ages we find the date to shift slowly toward the commencement of the
year. Thus the display which took place in A. D. 1698 was on the 9th
of November; while, looking back still further to one of the very
earliest records, viz., that of the year 934, we find the date has
receded to October 14th. This change of the day on which the shower
occurs is of profound theoretical importance in connection with the
discovery of the orbit which these meteors pursue. The advance of the
date is, however, so slow that for the past few generations, as well
as for the next, we may sufficiently define this particular shower by
the meteors which enliven the skies between the 12th and the 14th of
November. In fact, the poetaster has parodied the well-known lines
for the days of the month by a similar effort, which will serve to
remind us also of another periodic shower of shooting stars which
occurs in August. He writes:
“If you November’s stars would see,
From twelfth to fourteenth watching be.
In August too stars shine from heaven,
On nights between nine and eleven.”
These lines are intended to imply that the days named will usually
bring, in every November, a few meteors at all events belonging to
the grand shower. These are stragglers, as it were, from the mighty
host which visits us three times in the century.
Astronomers have a special name for this group of November meteors.
They are called the “Leonids.” To explain why this name has been
given, and why it is appropriate, we must dwell on an important part
of the phenomena of the shower.
[Illustration: Fig. 23.—Position of Leo, Source of the Leonids]
Among the constellations there is a fine sickle-shaped group, forming
a part of Leo, one of the signs of the Zodiac. That part of the sky
defined by Leo is curiously related to the meteors of the 12th to
the 14th of November. Every shooting star truly belonging to that
great shower pursued a track across the heavens, the direction of
which, if carried back far enough, was always found to pierce through
the sickle of Leo. Indeed, the paths of all the meteors formed a set
of rays spreading away from that one point in the constellation. An
invariable characteristic of this particular shower is its connection
with the constellation of Leo, hence the appropriateness of the name
of Leonids.
It must be borne in mind that we can never see the meteors until the
fatal moment when they dive into our atmosphere. We could, indeed,
at any time point our telescope to the spot in the heavens where we
know the great shoal must certainly be located. But the mightiest
telescope in the world does not disclose the shoal to us. In fact,
we would never have seen these Leonids at all, we would never have
become conscious that such a shoal of meteors existed, had it
not been for a certain circumstance, which, for want of a better
expression, I must speak of as accidental.
Our globe pursues a certain definite track around the sun. Year after
year, with undeviating regularity, the earth performs the stages of
its journey. If it reaches certain points on the 1st of January and
the 12th of October in one year, then it reaches the same points on
the 1st of January and the 12th of October respectively on next year,
or any other year.
The Leonids and the earth have each a certain track. It might of
course have happened that one of these tracks lay quite outside or
quite inside the other. In the case of the Leonids, it has chanced
that their orbit does intersect the orbit of the earth, and to
this circumstance we are indebted for the glorious displays every
thirty-three years.
There are many other periodic showers of shooting stars besides those
notable Leonids. None of the other showers, however, possess the
same importance as the Leonids, nor do they ever manifest celestial
splendor comparable with that of those of the 13th of November.
The Perseids, for example, which appear from the 9th to the 11th
of August, are tolerably constant in their appearance, but have
little spectacular interest. There is also another shower called the
Andromedes, which occurs on the 27th of November. It has produced
certain displays, one of the most remarkable of which took place
in 1872. The meteors were excessively numerous on that occasion,
but they were so short in their paths, and so insignificant as to
brilliance, that the spectacle, though of great scientific interest,
could not be compared as to splendor with that of the Leonids in 1866.
There are also several other showers which appear with greater or
less regularity. Each of these possesses two distinct characteristics
by which its meteors can be identified. One of these characters is
the date on which the shower appears. The other is the constellation
or point on the heavens from which all the meteors appear to radiate.
Thus when we speak of the Andromedes on the 27th of November, we
express that the shower on the 27th November comes from the part of
the heavens marked by the constellation of Andromeda.
A striking discovery has been made which points to a curious
connection between comets and shooting stars. It has been found that
the track followed by a great shower of meteors is often identical
with the track pursued by a comet. It is wholly beyond the province
of mere chance that an orbit such as that of the Leonids should,
both as to its size and its position in space, be likewise that of a
comet, unless the comet and the meteor swarm were objects related to
each other.
The great sun guides our world through its long annual journey. The
mighty mass of the earth yields compliance to the potent sway of the
ruler of our system. But the sun does not merely exercise a control
over the vast planets which circulate around him. The supreme law
of gravitation constrains the veriest mote that ever floated in a
sunbeam, with the same unremitting care that it does the mightiest
of planets. Thus it is that each little meteor is guided in its
journeys for untold ages. Each of these little objects hurries along,
deflected at every moment, to follow its beautifully curved path by
the incessant attraction of the sun. At last, however, the fatal
plunge is taken. The long wanderings of the meteor have come to an
end and it vanishes in a streak of splendor.
COMETS.—SIR JOHN HERSCHEL
The extraordinary aspect of comets, their rapid and seemingly
irregular motions, the unexpected manner in which they often burst
upon us, and the imposing magnitudes which they occasionally assume,
have in all ages rendered them objects of astonishment, not unmixed
with superstitious dread to the uninstructed, and an enigma to those
most conversant with the wonders of creation and the operations
of natural causes. Even now, that we have ceased to regard their
movements as irregular, or as governed by other laws than those
which retain the planets in their orbits, their intimate nature,
and the offices they perform in the economy of our system, are as
much unknown as ever. No distinct and satisfactory account has yet
been rendered of those immensely voluminous appendages which they
bear about with them, and which are known by the name of their tails
(though improperly, since they often precede them in their motions),
any more than of several other singularities which they present.
The number of comets which have been astronomically observed, or of
which notices have been recorded in history, is very great, amounting
to several hundreds, and when we consider that in the earlier ages
of astronomy, and indeed in more recent times, before the invention
of the telescope, only large and conspicuous ones were noticed; and
that, since due attention has been paid to the subject, scarcely
a year has passed without the observation of one or two of these
bodies, and that sometimes two and even three have appeared at once;
it will be easily supposed that their actual number must be at least
many thousands. Multitudes, indeed, must escape all observation, by
reason of their paths traversing only that part of the heavens which
is above the horizon in the daytime. Comets so circumstanced can
only become visible by the rare coincidence of a total eclipse of the
sun—a coincidence which happened, as related by Seneca, sixty-two
years before Christ, when a large comet was actually observed very
near the sun. Several, however, stand on record as having been bright
enough to be seen with the naked eye in the daytime, even at noon
and in bright sunshine. Such were the comets of 1402, 1532 and 1843,
and that of 43 B. C. which appeared during the games celebrated by
Augustus in honor of Venus shortly after the death of Cæsar, and
which the flattery of poets declared to be the soul of that hero
taking its place among the divinities.
Comets consist for the most part of a large and more or less splendid
but ill-defined nebulous mass of light called the head, which is
usually much brighter toward its centre, and offers the appearance
of a vivid _nucleus_, like a star or planet. From the head, and in a
direction _opposite to that in which the sun is situated_ from the
comet, appear to diverge two streams of light, which grow broader and
more diffused at a distance from the head, and which most commonly
close in and unite at a little distance behind it, but sometimes
continue distinct for a great part of their course; producing an
effect like that of the trains left by some bright meteors, or
like the diverging fire of a sky-rocket (only without sparks or
perceptible motion). This is the tail. This magnificent appendage
attains occasionally an immense apparent length. Aristotle relates
of the tail of the comet of 371 B. C., that it occupied a third of
the hemisphere, or 60°; that of A. D. 1618 is stated to have been
attended by a train no less than 104° in length. The comet of 1680,
the most celebrated of modern times, and on many accounts the most
remarkable of all, with a head not exceeding in brightness a star
of the second magnitude, covered with its tail an extent of more
than 70° of the heavens, or, as some accounts state, 90°; that of
the comet of 1769 extended 97°, and that of the comet of 1843 was
estimated at about 65° when longest.
The tail is, however, by no means an invariable appendage of comets.
Many of the brightest have been observed to have short and feeble
tails, and a few great comets have been entirely without them. Those
of 1585 and 1763 offered no vestige of a tail; and Cassini describes
the comets of 1665 and 1682 as being as round and as well defined
as Jupiter. On the other hand, instances are not wanting of comets
furnished with many tails or streams of diverging light. That of 1744
had no less than six, spread out like an immense fan, extending to a
distance of nearly 30° in length. The small comet of 1823 had two,
making an angle of about 160°, the brighter turned as usual from the
sun, the fainter toward it, or nearly so. The tails of comets, too,
are often somewhat curved, bending, in general, toward the region
which the comet has left, as if moving somewhat more slowly, or as if
resisted in their course.
The smaller comets, such as are visible only in telescopes, or with
difficulty by the naked eye, and which are by far the most numerous,
offer very frequently no appearance of a tail, and appear only
as round or somewhat oval vaporous masses, more dense toward the
centre, where, however, they appear to have no distinct nucleus,
or anything which seems entitled to be considered as a solid body.
This was shown in a very remarkable manner in the case of the comet
discovered by Miss Mitchell in 1847, which on the 5th of October in
that year passed _centrally_ over a star of the fifth magnitude:
_so_ centrally that with a magnifying power of 100 it was impossible
to determine in which direction the extent of the nebulosity was
greatest. The star’s light seemed in no degree enfeebled; yet such
a star would be completely obliterated by a moderate fog, extending
only a few yards from the surface of the earth. And since it is an
observed fact that even those larger comets which have presented the
appearance of a nucleus have yet exhibited _no phases_, though we can
not doubt that they shine by the reflected solar light, it follows
that even these can only be regarded as great masses of thin vapor,
susceptible of being penetrated through their whole substance by
the sun-beams, and reflecting them alike from their interior parts
and from their surfaces. Nor will any one regard this explanation
as forced, or feel disposed to resort to a phosphorescent quality
in the comet itself, to account for the phenomena in question, when
we consider the enormous magnitude of the space thus illuminated,
and the extremely small _mass_ which there is ground to attribute to
these bodies. It will then be evident that the most unsubstantial
clouds which float in the highest regions of our atmosphere, and seem
at sunset to be drenched in light, and to glow throughout their
whole depth as if in actual ignition, without any shadow or dark
side, must be looked upon as dense and massive bodies compared with
the filmy and all but spiritual texture of a comet. Accordingly,
whenever powerful telescopes have been turned on these bodies, they
have not failed to dispel the illusion which attributes _solidity_ to
that more condensed part of the head which appears to the naked eye
as a nucleus; though it is true that in some a very minute stellar
point _has_ been seen, indicating the existence of something more
substantial.
[Illustration: Fig. 24.—Head of Comet]
That the luminous part of a comet is something in the nature of a
smoke, fog, or cloud, suspended in a transparent atmosphere, is
evident from a fact which has been often noticed, viz., that the
portion of the tail where it comes closest to and surrounds the
head is yet separated from it by an interval less luminous, as if
sustained and kept off from contact by a transparent stratum, as we
often see one layer of clouds over another with a considerable clear
space between. These, and most of the other facts observed in the
history of comets, appear to indicate that the structure of a comet,
as seen in section in the direction of its length, must be that of a
hollow envelope, of a parabolic form, inclosing near its vertex the
nucleus and head, something as represented in the preceding figure.
This would account for the apparent division of the tail into two
principal lateral branches, the envelope being oblique to the line of
sight at its borders, and therefore a greater depth of illuminated
matter being there exposed to the eye. In all probability, however,
they admit great varieties of structure, and among them may very
possibly be bodies of widely different physical constitution, and
there is no doubt that one and the same comet at different epochs
undergoes great changes, both in the disposition of its materials and
in their physical state.
We come now to speak of the motions of comets. These are apparently
most irregular and capricious. Sometimes they remain in sight only
for a few days, at others for many months; some move with extreme
slowness, others with extraordinary velocity; while not infrequently
the two extremes of apparent speed are exhibited by the same comet in
different parts of its course. The comet of 1472 described an arc of
the heavens of 40° of a great circle in a single day. Some pursue a
direct, some a retrograde, and others a tortuous and very irregular
course; nor do they confine themselves, like the planets, within any
certain region of the heavens, but traverse indifferently every part.
Their variations in apparent size, during the time they continue
visible, are no less remarkable than those of their velocity;
sometimes they make their first appearance as faint and slow-moving
objects, with little or no tail; but by degrees accelerate, enlarge,
and throw out from them this appendage, which increases in length
and brightness till (as always happens in such cases) they approach
the sun, and are lost in his beams. After a time they again emerge on
the other side, receding from the sun with a velocity at first rapid,
but gradually decaying. It is for the most part after thus passing
the sun that they shine forth in all their splendor, and that their
tails acquire their greatest length and development; thus indicating
plainly the action of the sun’s rays as the exciting cause of that
extraordinary emanation. As they continue to recede from the sun,
their motion diminishes and the tail dies away, or is absorbed into
the head, which itself grows continually feebler, and is at length
altogether lost sight of, in by far the greater number of cases never
to be seen more.
Without the clew furnished by the theory of gravitation, the enigma
of these seemingly irregular and capricious movements might have
remained forever unresolved. But Newton, having demonstrated the
possibility of any conic section whatever being described about the
sun, by a body revolving under the dominion of that law, immediately
perceived the applicability of the general proposition to the case
of cometary orbits; and the great comet of 1680, one of the most
remarkable on record, both for the immense length of its tail and for
the excessive closeness of its approach to the sun (within one-sixth
of the diameter of that luminary), afforded him an excellent
opportunity for the trial of his theory. The success of the attempt
was complete. From that time it became a received truth, that the
motions of comets are regulated by the same general laws as those of
the planets.
[Illustration: Fig. 25.—Orbit of Newton’s Comet (1680)]
Now calculations lead to the surprising fact, that the comets are by
far the most voluminous bodies in our system. The following are the
dimensions of some of those which have been made the subjects of such
inquiry.
The tail of the great comet of 1680, immediately after its perihelion
passage, was found by Newton to have been no less than 20,000,000 of
leagues in length, and to have occupied only two days in its emission
from the comet’s body! a decisive proof this of its being darted
forth by some active force, the origin of which, to judge from the
direction of the tail, must be sought in the sun itself. Its greatest
length amounted to 41,000,000 leagues, a length much exceeding the
whole interval between the sun and earth. The tail of the comet of
1769 extended 16,000,000 leagues, and that of the great comet of
1811, 36,000,000. The portion of the head of this last, comprised
within the transparent atmospheric envelope which separated it from
the tail, was 180,000 leagues in diameter. It is hardly conceivable
that matter once projected to such enormous distances should ever be
collected again by the feeble attraction of such a body as a comet—a
consideration which accounts for the surmised progressive diminution
of the tails of such as have been frequently observed.
The most remarkable of those comets which have been ascertained
to move in elliptic orbits is that of Halley, so called from the
celebrated Edmund Halley, who, on calculating its elements from its
perihelion passage in 1682, when it appeared in great splendor, with
a tail 30° in length, was led to conclude its identity with the great
comets of 1531 and 1607, whose elements he had also ascertained. The
intervals of these successive apparitions being 75 and 76 years,
Halley was encouraged to _predict_ its reappearance about the year
1759. So remarkable a prediction could not fail to attract the
attention of all astronomers, and, as the time approached, it became
extremely interesting to know whether the attractions of the larger
planets might not materially interfere with its orbital motion. The
computation of their influence from the Newtonian law of gravity, a
most difficult and intricate piece of calculation, was undertaken
and accomplished by Clairaut, who found that the action of Saturn
would retard its return by 100 days, and that of Jupiter by no less
than 518, making in all 618 days, by which the expected return would
happen later than on the supposition of its retaining an unaltered
period—and that, in short, the time of the expected perihelion
passage would take place within a month, one way or other, of the
middle of April, 1759. It actually happened on the 12th of March
in that year. Its next return was calculated by several eminent
geometers, and fixed successively for the 4th, the 7th, the 11th, and
the 26th of November, 1835; the two latter determinations appearing
entitled to the higher degree of confidence, owing partly to the more
complete discussion bestowed on the observations of 1682 and 1759,
and partly to the continually improving state of our knowledge of the
methods of estimating the disturbing effect of the several planets.
The last of these predictions, that of M. Lehmann, was published on
the 25th of July. On the 5th of August the comet first became visible
in the clear atmosphere of Rome as an exceedingly faint telescopic
nebula, within a degree of its place as predicted by M. Rosenberger
for that day. On or about the 20th of August it became generally
visible, and, pursuing very nearly its calculated path among the
stars, passed its perihelion on the 16th of November; after which,
its course carrying it south, it ceased to be visible in Europe,
though it continued to be conspicuously so in the Southern Hemisphere
throughout February, March, and April, 1836, disappearing finally on
the 5th of May.
[Illustration: Fig. 26.—Forms of Cometary Orbits]
Its first appearance, while yet very remote from the sun, was that
of a small round or somewhat oval nebula, quite destitute of tail,
and having a minute point of more concentrated light eccentrically
situated within it. It was not before the 2d of October that the tail
began to be developed, and thenceforward increased pretty rapidly,
being already 4° or 5° long on the 5th. It attained its greatest
apparent length (about 20°) on the 15th of October. From that time,
though not yet arrived at its perihelion, it decreased with such
rapidity that already on the 29th it was only 3°, and on November
the 5th 2½° in length. There is every reason to believe that before
the perihelion, the tail had altogether disappeared, as, though
it continued to be observed at Pulkowa up to the very day of its
perihelion passage, no mention whatever is made of any tail being
then seen.
Reflecting on these phenomena, and carefully considering the evidence
afforded by the numerous and elaborately executed drawings which have
been placed on record by observers, it seems impossible to avoid the
following conclusions: 1st. That the matter of the nucleus of a comet
is powerfully excited and dilated into a vaporous state by the action
of the sun’s rays, escaping in streams and jets at those points of
its surface which oppose the least resistance, and in all probability
throwing that surface or the nucleus itself into irregular motions
by its reaction in the act of so escaping, and thus altering its
direction.
2. That this process chiefly takes place in that portion of the
nucleus which is turned toward the sun; the vapor escaping chiefly in
that direction.
3. That when so emitted, it is prevented from proceeding in the
direction originally impressed upon it by some force directed _from_
the sun, drifting it back and carrying it out to vast distances
behind the nucleus, forming the tail or so much of the tail as can
be considered as consisting of material substance.
4th. That this force, whatever its nature, acts unequally on the
materials of the comet, the greater portion remaining unvaporized,
and a considerable part of the vapor actually produced remaining in
its neighborhood, forming the head and coma.
5th. That the force thus acting on the materials of the tail can not
possibly be identical with the ordinary gravitation of matter, being
centrifugal or repulsive, as respects the sun, and of an energy very
far exceeding the gravitating force toward that luminary. This will
be evident if we consider the enormous velocity with which the matter
of the tail is carried backward, in opposition both to the motion
which it had as part of the nucleus and to that which it acquired in
the act of its emission, both which motions have to be destroyed in
the first instance, before any movement in the contrary direction can
be impressed.
6th. That unless the matter of the tail thus repelled from the sun
be retained by a peculiar and highly energetic attraction to the
nucleus, differing from and exceptional to the ordinary power of
gravitation, it must leave the nucleus altogether; being in effect
carried far beyond the coercive power of so feeble a gravitating
force as would correspond to the minute mass of the nucleus; and
it is therefore very conceivable that a comet may lose, at every
approach to the sun, a portion of that peculiar matter, whatever it
be, on which the production of its tail depends, the remainder being
of course less excitable by the solar action, and more impassive to
his rays, and therefore, _pro tanto_, more nearly approximating to
the nature of the planetary bodies.
7th. That, considering the immense distances to which at least some
portion of the matter of the tail is carried from the comet, and
the way in which it is dispersed through the system, it is quite
inconceivable that the whole of that matter should be reabsorbed—that
therefore it must lose during its perihelion passage some portion
of its matter, and if, as would seem far from improbable, that
matter should be of a nature to be repelled from, not attracted by,
the sun, the remainder will, by consequence, be, _pro quantitate
inertiæ_, more energetically attracted to the sun than the mean of
both. If then the orbit be elliptic, it will perform each successive
revolution in a shorter time than the preceding, until, at length,
the whole of the repulsive matter is got rid of.
[Illustration: Fig. 27.—Halley’s Comet]
Besides the comet of Halley, several other of the great comets
recorded in history have been surmised with more or less probability
to return periodically, and therefore to move in elongated ellipses
around the sun. Such is the great comet of 1680, whose period is
estimated at 575 years, and which has been considered, with at least
a high _prima facie_ probability, to be identical with a magnificent
comet observed at Constantinople and in Palestine, and referred by
contemporary historians, both European and Chinese, to the year A.
D. 1106; with that of A. D. 531, which was seen at noonday close to
the sun; with the comet of 43 B. C., already spoken of as having
appeared after the death of Cæsar, and which was also observed in
the daytime; and finally with two other comets, mention of which
occurs in the Sibylline Oracles, and in a passage of Homer, and
which are referred, as well as the obscurity of chronology and
the indications themselves will allow, to the years 618 and 1194
B. C. It is to the assumed near approach of this comet to the
earth, about the time of the Deluge, that Whiston ascribed that
overwhelming tide-wave to whose agency his wild fancy ascribed that
great catastrophe—a speculation, it is needless to remark, purely
visionary. These coincidences of time are certainly remarkable,
especially when it is considered how very rare are the appearances of
comets of this class. Professor Encke, however, has discussed, with
all possible care, the observations recorded of the comet of 1680,
taking into consideration the perturbations of the planets (which are
of trifling importance, by reason of the great inclination of its
orbit to the ecliptic), and his calculations show that no elliptic
orbit, with such a period as 575 years, is competent to represent
them within any probable or even possible limits of error, the most
probable period assigned by them being 8814 Julian years. Independent
of this consideration, there are circumstances recorded of the comet
of A. D. 1106 incompatible with its motion in any orbit identical
with that of the comet of 1680, so that the idea of referring all
these phenomena to one and the same comet, however seducing, must be
relinquished.
Another great comet, whose return about the year 1848 had been
considered by more than one eminent authority in this department of
astronomy highly probable, is that of 1556, to the terror of whose
aspect some historians have attributed the abdication of the Emperor
Charles V. This comet is supposed to be identical with that of 1264,
mentioned by many historians as a great comet, and observed also in
China.
In 1661, 1532, 1402, 1145, 891, and 243 great comets appeared—that
of 1402 being bright enough to be seen at noonday. A period of 129
years would conciliate all these appearances, and should have brought
back the comet in 1789 or 1790 (other circumstances agreeing).
That no such comet was observed about that time is no proof that
it did not return, since, owing to the situation of its orbit, had
the perihelion passage taken place in July it might have escaped
observation.
We come now, however, to a class of comets of short period,
respecting whose return there is no doubt, inasmuch as two at
least of them have been identified as having performed successive
revolutions round the sun; have had their return predicted already
several times; and have on each occasion scrupulously kept to their
appointments. The first of these is the comet of Encke, so called
from Professor Encke of Berlin, who first ascertained its periodical
return. It revolves in an ellipse of great eccentricity (though not
comparable to that of Halley’s), the plane of which is inclined at an
angle of about 13° 22′ to the plane of the ecliptic, and in the short
period of 1,211 days, or about 3⅓ years. This remarkable discovery
was made on the occasion of its fourth recorded appearance, in 1819.
From the ellipse then calculated by Encke, its return in 1822 was
predicted by him, and observed at Paramata, in New South Wales,
by M. Rümker, being invisible in Europe: since which it has been
repredicted and reobserved in all the principal observatories, both
in the Northern and Southern Hemispheres, as a phenomenon of regular
occurrence.
Another comet of short period is that of _Biela_, so called from
M. Biela of Josephstadt, who first arrived at this interesting
conclusion on the occasion of its appearance in 1826. It is
considered to be identical with comets which appeared in 1772, 1805,
etc., and describes its very eccentric ellipse about the sun in 2,410
days, or about 6¾ years; and in a plane inclined 12° 34′ to the
ecliptic. It appeared again, according to the prediction, in 1832 and
in 1846.
This comet is small and hardly visible to the naked eye, even
when brightest. Nevertheless, as if to make up for its seeming
insignificance by the interest attaching to it in a physical point
of view, it exhibited, at its appearance in 1846, a phenomenon
which struck every astronomer with amazement, as a thing without
previous example in the history of our system. It was actually seen
to separate itself into two distinct comets, which, after thus
parting company, continued to journey along amicably through an arc
of upward of 70° of their apparent orbit, keeping all the while
within the same field of view of the telescope pointed toward them.
The first indication of something unusual being about to take place
might be, perhaps, referred to the 19th of December, 1845, when the
comet appeared to Mr. Hind pear-shaped, the nebulosity being unduly
elongated in a direction inclining northward. But on the 13th of
January, at Washington, in America, and on the 15th and subsequently
in every part of Europe, it was distinctly seen to have become
double; a very small and faint cometic body, having a nucleus of its
own, being observed appended to it, at a distance of about 2′ (in
arc) from its centre, and in a direction forming an angle of about
328° with the meridian, running northward from the principal or
original comet. From this time the separation of the two comets went
on progressively, though slowly. On the 30th of January the apparent
distance of the nucleus had increased to 3′, on the 7th of February
to 4′, and on the 13th to 5′, and so on, until on the 5th of March
the two comets were separated by an interval of 9′ 19″, the apparent
direction of the line of junction all the while varying but little
with respect to the parallel.
During this separation, very remarkable changes were observed to
be going on, both in the original comet and its companion. Both
had nuclei, both had short tails, parallel in direction and nearly
perpendicular to the line of junction; but whereas at its first
observation, on January 13th, the new comet was extremely small and
faint in comparison with the old, the difference both in point of
light and apparent magnitude diminished. On the 10th of February
they were nearly equal, although the day before the moonlight had
effaced the new one, leaving the other bright enough to be well
observed. On the 14th and 16th, however, the new comet had gained a
decided superiority of light over the old, presenting at the same
time a sharp and star-like nucleus, compared by Lieutenant Maury to a
diamond spark. But this state of things was not to continue. Already,
on the 18th, the old comet had regained its superiority, being nearly
twice as bright as its companion, and offering an unusually bright
and star-like nucleus. From this period the new companion began to
fade away, but continued visible up to the 15th of March. On the
24th the comet appeared again single, and on the 22d of April both
had disappeared.
While this singular interchange of light was going forward,
indications of some sort of communication between the comets were
exhibited. The new or companion comet, besides its tail, extending
in a direction parallel to that of the other, threw out a faint
arc light which extended as a kind of bridge from the one to the
other; and after the restoration of the original comet to its former
pre-eminence, it, on its part, threw forth additional rays, so as
to present the appearance of a comet with three faint tails forming
angles of about 120° with each other, one of which extended toward
its companion.
On the 22d of August, 1844, Signor de Vico, director of the
observatory of the Collegio Romano, discovered a comet, the motions
of which, a very few observations sufficed to show, deviated
remarkably from a parabolic orbit. It passed its perihelion on the
2d of September, and continued to be observed until the 7th of
December. Elliptic elements of this comet, agreeing remarkably well
with each other, were accordingly calculated by several astronomers,
from which it appears that the period of revolution is about 1,990
days, or 5½ (5.4357) years, which (supposing its orbit undisturbed in
the interim) would bring it back to the perihelion on or about the
13th of January, 1850, on which occasion, however, by reason of its
unfavorable situation with respect to the sun and earth, it could not
be observed.
This comet, when brightest, was visible to the naked eye, and had
a small tail. It is especially interesting to astronomers from the
circumstance of its having been rendered exceedingly probable by
the researches of M. Leverrier, that it is identical with one which
appeared in 1678, with some of its elements considerably changed
by perturbation. This comet is further remarkable from having been
concluded, by Messrs. Laugier and Mauvais, to be identical with the
comet of 1585 observed by Tycho Brahe, and possibly also with those
of 1743, 1766, and 1819.
By far the most remarkable comet, however, which has been seen during
the present century, is that which appeared in the spring of 1843,
and whose tail became visible in the twilight of the 17th of March
in England as a great beam of nebulous light, extending from a point
above the western horizon, through the stars of Eridanus and Lepus,
under the belt of Orion. This situation was low and unfavorable; and
it was not till the 19th that the head was seen, and then only as
a faint and ill-defined nebula, very rapidly fading on subsequent
nights. In more southern latitudes, however, not only the tail was
seen, as a magnificent train of light extending 50° or 60° in length;
but the head and nucleus appeared with extraordinary splendor,
exciting in every country where it was seen the greatest astonishment
and admiration. Indeed, all descriptions agree in representing it as
a stupendous spectacle, such as in superstitious ages would not fail
to have carried terror into every bosom. In tropical latitudes in the
Northern Hemisphere, the tail appeared on the 3d of March, and in
Van Diemen’s Land so early as the 1st, the comet having passed its
perihelion on the 27th of February.
There is abundant evidence of the comet in question having been seen
in full daylight, and in the sun’s immediate vicinity. It was so
seen on the 28th of February, the day after its perihelion passage,
by every person on board the H.E.I.C.S. “Owen Glenndower,” then off
the Cape, as a short dagger-like object close to the sun a little
before sunset. On the same day at 3^h 6^m P. M., and consequently in
full sunshine, the distance of the nucleus from the sun was actually
measured with a sextant by Mr. Clarke of Portland, United States, the
distance centre from centre being then only 3° 50′ 43″.
[Illustration: Fig. 28.—Orbits of the Nine Comets Captured by Jupiter
_Scale: 5 millimetres = 1 radius of the Earth’s orbit_]
It is by no means merely as a subject of antiquarian interest, or
on account of the brilliant spectacle which comets occasionally
afford, that astronomers attach a high degree of importance to all
that regards them. Apart even from the singularity and mystery which
appertains to their physical constitution, they have become, through
the medium of exact calculation, unexpected instruments of inquiry
into points connected with the planetary system itself, of no small
importance. We have seen that the movements of the comet Encke, thus
minutely and perseveringly traced by the eminent astronomer whose
name is used to distinguish it, have afforded ground for believing in
the presence of a resisting medium filling the whole of our system.
Similar inquiries, prosecuted in the cases of other periodical
comets, will extend, confirm, or modify our conclusions on this
head. The perturbations, too, which comets experience in passing
near any of the planets, may afford, and have afforded, information
as to the magnitude of the disturbing masses, which could not well
be otherwise obtained. Thus the approach of this comet to the planet
Mercury in 1838 afforded an estimation of the mass of that planet
the more precious, by reason of the great uncertainty under which
all previous determinations of that element labored. Its approach
to the same planet in the year 1848 was still nearer. On the 22d
of November their mutual distance was only fifteen times the moon’s
distance from the earth.
It is, however, in a physical point of view that these bodies offer
the greatest stimulus to our curiosity. There is, beyond question,
some profound secret and mystery of nature concerned in the
phenomenon of their tails. Perhaps it is not too much to hope that
future observation, borrowing every aid from rational speculation,
grounded on the progress of physical science generally (especially
those branches of it which relate to the ethereal or imponderable
elements), may ere long enable us to penetrate this mystery, and to
declare whether it is really _matter_ in the ordinary acceptation of
the term which is projected from their heads with such extravagant
velocity, and if not impelled, at least _directed_, in its course by
a reference to the sun, as its point of avoidance. In no respect is
the question as to the materiality of the tail more forcibly pressed
on us for consideration than in that of the enormous sweep which it
makes round the sun in perihelio, in the manner of a straight and
rigid rod, in defiance of the law of gravitation, nay, even of the
received laws of motion, extending (as we have seen in the comets
of 1680 and 1843) from near the sun’s surface to the earth’s orbit,
yet whirled round unbroken: in the latter case through an angle of
180° in little more than two hours. It seems utterly incredible that
in such a case it is one and the same material object which is thus
brandished. If there could be conceived such a thing as a _negative
shadow_, a momentary impression made upon the luminiferous ether
behind the comet, this would represent in some degree the conception
such a phenomenon irresistibly calls up. But this is not all. Even
such an extraordinary excitement of the ether, conceive it as we
will, will afford no account of the projection of lateral streamers;
of the effusion of light from the nucleus of a comet toward the sun;
and its subsequent _re_jection; of the irregular and capricious mode
in which that effusion has been seen to take place; none of the clear
indications of alternate evaporation and condensation going on in
the immense regions of space occupied by the tail and coma—none, in
short, of innumerable other facts which link themselves with almost
equally irresistible cogency to our ordinary notions of matter and
force.
LIFE IN OTHER WORLDS.—J. E. GORE
The question is often asked, Are the stars inhabited? To this we
can confidently answer, No. The stars themselves are certainly not
habitable by any forms of life with which we are familiar. That the
stars are luminous incandescent bodies, similar to the sun, seems
almost self-evident. That they shine by their own inherent light, and
not by light reflected from another body, like the planets of the
Solar System, is a fact which scarcely needs demonstration. There are
no bright objects near them from which they could derive their light,
and they are too far from the sun to obtain any illumination from
that source. But if any proofs were necessary, we have the evidence
of the spectroscope, which shows unmistakably that their light
emanates from incandescent bodies. Many of the stars show spectra
very similar to that of the sun. The light of others, although
differing somewhat in quality when analyzed by the prism, indicates
clearly that they are at a very high temperature—in many cases,
indeed, suggesting that they are actually hotter than the sun. It may
be objected, however, that in the case of binary or revolving double
stars, the smaller component may possibly shine by light reflected
from the brighter star. Indeed, this has been suggested in the case
of Sirius and its faint companion. But, if the companion of Sirius
shone merely by reflected light from its primary, it would be much
fainter than it is, and, indeed, would be utterly invisible in our
largest telescopes. Further, in some double stars, spectroscopic
observations suggest that the component stars have different spectra.
This is, of course, conclusive evidence against the hypothesis of
borrowed light; for were the smaller star to shine by reflected
light from the larger, the spectra of both would be identical, as
in the case of the sun and moon. We may therefore conclude that all
the visible stars are suns, and totally unfit for the habitation of
living creatures.
But may not the stars have planets revolving round them, forming
solar systems similar to our own? As they are evidently suns
shining by inherent light, may they not form centres of planetary
systems? In the case of those stars having spectra differing from
the solar spectrum, we can not speak with any confidence; but for
those which show spectra similar to that of our sun, and having,
therefore, probably a similar chemical constitution, the existence
of planets revolving round them seems from analogy very probable.
I refer to _single_ stars, that is stars which have no telescopic
close companion; for the double stars may, perhaps, form systems
differently constituted. In any case these binary systems would not
be strictly comparable with ours, for the sun is certainly a single
star.
Whether systems of planets really revolve round the stars referred
to, is a question which, unfortunately, can not be decided by
observation. If a planet equal in size to the “giant planet,”
Jupiter, were revolving round the nearest star—Alpha Centauri—at the
same distance from that star that Jupiter is from the sun, it would
be utterly invisible in our largest telescopes. The invisibility of
planets circling round the stars is therefore no proof whatever of
their non-existence. Each star of the solar type may possibly be
attended by a retinue of planets which may, perhaps, remain forever
invisible in the largest telescopes which man can construct. We can,
therefore, draw our conclusions only from analogy. If other suns
exist resembling our own sun in chemical constitution, which we know
to be a fact, is it not reasonable to suppose that they also form
centres of planetary systems similar to the Solar System?
“Consult with reason, reason will reply,
Each lucid point which glows in yonder sky,
Informs a system in the boundless space,
And fills with glory its appointed place;
With beams unborrowed brighten other skies,
And worlds to the unknown with heat and light supplies.”
The suns, which we call stars, were clearly not created for our
benefit. They are of very little practical use to the earth’s
inhabitants. They give us very little light; an additional small
satellite—one considerably smaller than the moon—would have been much
more useful in this respect than the millions of suns revealed by
the telescope. They must, therefore, have been formed for some other
purpose.
On Laplace’s Nebular Hypothesis, the condensation of an original
nebulous mass endowed with a motion of rotation would result not only
in the formation of a sun, similar to ours, but also in a system of
planets revolving round the central body. If, indeed, the primitive
nebula had no rotation or motions of any kind, the result would be
a sun without planets or satellites; but the motions with which all
the stars seem to be animated lead us to suppose that this would be a
case of very rare occurrence. We may therefore conclude, with a high
degree of probability, that the stars—at least those with spectra of
the solar type—form centres of planetary systems somewhat similar to
our own.
This being surmised, let us consider the conditions necessary for
the existence of life on these planets. There are various conditions
which must be complied with before we can imagine life, as we know
it, to be possible on any planet. Perhaps the most important of
these is the question of temperature. We know that in the universe
a great range of temperature exists, from the cold of interstellar
space—estimated at about 460° below the freezing-point of water—to
the intense heat which rages in the solar photosphere. In this long
thermal scale life is, at least on the earth, restricted within
rather narrow limits. Below a certain low temperature life can not
exist. The point is, however, far above the temperature of space. On
the other hand, above a certain high temperature—a low one, however,
compared with the intense heat of the solar surface—life is also
impossible, at least for highly organized beings like man and the
larger animals. For minute microscopic organisms the scale may,
perhaps, be somewhat extended; but even in its widest limits, the
range of temperature within which life is possible is, so far as we
know, certainly a narrow one.
For the support of life and vegetation, light is also necessary,
for without it no flowers would bloom, nor corn grow and ripen to
maturity. To obtain this supply of light and heat it is necessary
that a life-bearing planet should revolve at a suitable distance
from, and in a nearly circular orbit round, a central sun. These
conditions, it is hardly necessary to say, are fulfilled in the case
of the earth. Were we much nearer to the sun than we are, we should
suffer from excessive heat, and were we much further away, we should
probably perish from the cold. For this reason the existence of life
on the other planets of the Solar System seems very doubtful. Mercury
is probably too hot, and the other planets are certainly too cold,
so far as heat from the sun is concerned, unless, indeed, their
internal heat is sufficient to raise the temperature of their surface
to a point sufficient for the maintenance of life. Indeed, there is
good reason to suppose that in the planets Jupiter, Saturn, Uranus,
and Neptune, this internal heat is still so great that life would be
quite impossible on their surface. Venus, inside the earth’s orbit,
and Mars, outside, are the two planets which seem to approach nearest
to the required conditions. We know that both these planets possess
atmospheres somewhat similar to ours, and, in Mars at least, land and
water most probably exist on its surface. Venus is, of course, much
hotter than the earth, and Mars much colder, but possibly the polar
regions of Venus and the equatorial regions of Mars may form suitable
abodes for some forms, at least, of animal and vegetable life.
Let us proceed, however, to consider some other conditions necessary
for the existence of life on a planet. A suitable temperature is,
of course, indispensable, but this is not all. There are other
conditions which must be complied with. The planet must have a
rotation on its axis, so that every portion shall in turn receive its
due share of light and heat. Each point on its surface must have its
day and night, the day for work and the night for rest. The axis of
rotation must not lie in the plane of the planet’s orbit, but must
have a suitable inclination, so that each hemisphere may enjoy its
seasons, summer and winter, “seed-time and harvest,” in due course.
Further the velocity of rotation on its axis must not be too rapid.
If the earth rotated in a period of one and a quarter hours, bodies
at the equator would have no weight, and life would be impossible in
those regions.
The planet must also possess a mass sufficient to retain bodies
on its surface by the force of gravity. In the case of very small
bodies, such as the moons of Mars and some of the minor planets
between Mars and Jupiter, objects thrown into the air would pass away
into space never to return.
The planet should also have a mean density greater than that of
water, otherwise the seas would possess no stability, and destructive
waves would quickly destroy all life on its surface. All these
conditions are fulfilled in the case of Mars as well as on the earth.
In the planet Saturn, however, the density is less than that of
water, and in Uranus and Neptune only slightly greater.
The planet must also possess a suitable atmosphere. This is an
all-important condition for the support of animal life—at least
for the existence of man and the higher orders of animals. This
atmosphere must consist—so far as we know—of oxygen and nitrogen
gases mechanically mixed in proper proportions, and with a small
quantity of carbonic acid gas. Were the oxygen in smaller quantity
than it exists in the earth’s atmosphere, life could not be
supported. On the other hand, were it much in excess of its present
amount, a fever would be produced in the blood which would very soon
put an end to animal life. The presence of other gases in excessive
quantities would also render the air unfit for breathing. We see,
therefore, that a comparatively slight change in the composition of
a planet’s atmosphere would—so far as our experience goes—render the
planet uninhabitable by any of the higher forms of life with which we
are familiar.
For the support of life on a planet, water is also absolutely
necessary. Without this useful fluid the world would soon become
a desert, and life and vegetation would speedily vanish from its
surface.
Geological conditions must also be considered. It is clearly
necessary for the welfare of human beings at least that the surface
soil and rocks should contain coal, iron, lime, and other minerals,
substances almost indispensable for the ordinary wants of civilized
existence.
[Illustration: Nine Views of the Hour-Glass Sea on Mars
1, Nov. 26, 1864; 2, June 29, 1873; 3, Oct. 28, 1879; 4, June 2,
1888; 5, June 20, 1890; 6, Aug. 6, 1892; 7, Oct., 1894; 8, Dec. 3,
1896; 9, Dec. 7, 1896]
That all or any of the conditions considered would be complied with
in the case of a planet revolving round a star it is, of course,
impossible to say. But when we find stars showing by their spectra
that they contain chemical elements identical with those which exist
in the sun and the earth, analogy would lead us to suppose that very
possibly a planet resembling our earth may revolve round each of
these distant suns. I say _a_ planet, for evidently there would be
only _one_ distance from the central luminary—a distance depending
on its size—at which the temperature necessary for the support of
life would exist, as in the case of the earth, over the whole of
the planet’s surface. For other planets of the stellar system, life
would be, if it existed at all, most probably confined to restricted
regions of the planet’s surface. There would, therefore, be in each
system one planet, and only one, _especially_ suitable for the
support of animal life as we know it. This is with reference to light
and heat. If the other conditions were not complied with, then life
would probably not exist even on this one planet. In the case of a
star larger than the sun, the planet should be placed at a greater
distance than the earth is from the sun, but in this case the length
of the year and the seasons would be longer than ours.
The star which more nearly resembles the sun in the character of
the light which it emits is the bright star Capella. Arcturus has a
somewhat similar spectrum. But these are probably suns of enormous
size, if any reliance can be placed on the measures of their distance
from the earth. Other bright stars with spectra of the solar type are
Pollux, Aldebaran, Beta Andromedæ, Alpha Arietis, Alpha Cassiopeiæ,
Alpha Cygni, and Alpha Ursæ Majoris. Another star is Eta Herculis.
The magnitude of this star as measured with the photometer is about
3½. A parallax found by Bélopolsky and Wagner places it at a distance
of 515,660 times the sun’s distance from the earth. If the sun were
placed at this distance, I find that it would be reduced to a star of
the third magnitude. This result would imply that Eta Herculis is a
slightly smaller sun than ours; and a planet placed a little nearer
to the star than the earth is to the sun might, perhaps, fulfil the
conditions of a life-bearing world.
The number of stars visible in our largest telescopes is usually
estimated at 100,000,000. Of these we may perhaps assume that
10,000,000 have a spectrum of the solar type, and therefore closely
resemble our sun in their chemical constitution. If we suppose that
only one in ten of these is similar in size to the sun, and has a
habitable planet revolving round it, we have a total of 1,000,000
worlds in the visible universe fitted for the support of animal life.
We may therefore conclude, with a high degree of probability, that
among the “multitudinous” stellar hosts there are probably many stars
having life-bearing planets revolving round them.
THE SUN—WHAT WE LEARN FROM IT.—RICHARD A. PROCTOR
The Sun, the central and ruling body of the planetary system, and
the source of light and heat to our earth and all the members of
that system, is a globe about 852,900 miles in diameter. So far as
observation extends, his figure is perfectly spherical, no difference
having been observed between his polar and spherical diameters. It
has been well remarked, indeed, by Sir G. Airy, that if any observer
could by ordinary modes of measurement satisfy himself that a real
difference existed between the diameters, that observer would have
proved the inexactness of his own work; for the absence of any
measurable compression comes out as the result of comparisons between
thousands of observations of the sun’s limbs made at Greenwich
and other leading observatories. The volume of the sun exceeds
the earth’s 1,252,700 times. His mean density is almost exactly
one-fourth of the earth’s, and his mass exceeds hers about 316,000
times. Gravity at the surface of the sun exceeds terrestrial gravity
about 27.1 times, so that a body dropped from rest near the sun’s
surface would fall through 436 feet in the first second, and have
acquired a velocity of 872 feet per second.
Let the reader consider a terrestrial globe three inches in diameter,
and search out on that globe the tiny triangular speck which
represents Great Britain. Then let him endeavor to picture the town
in which he lives as represented by the minutest pin-mark that could
possibly be made upon this speck. He will then have formed some
conception, though but an inadequate one, of the enormous dimensions
of the earth’s globe, compared with the scene in which his daily life
is cast. Now, on the same scale, the sun would be represented by a
globe about twice the height of an ordinary sitting-room. A room
about twenty-six feet in length, and height, and breadth, would be
required to contain the representation of the sun’s globe on this
scale, while the globe representing the earth could be placed in a
moderately large goblet.
Such is the body which sways the motions of the Solar System. The
largest of his family, the giant Jupiter, though of dimensions which
dwarf those of the earth or Venus almost to nothingness, would yet
only be represented by a thirty-two inch globe, on the scale which
gives to the sun the enormous volume I have spoken of. Saturn would
have a diameter of about twenty-eight inches, his ring measuring
about five feet in its extreme span. Uranus and Neptune would be
little more than a foot in diameter, and all the minor planets would
be less than the three-inch earth. It will thus be seen that the sun
is a worthy centre of the great scheme he sways, even when we merely
regard his dimensions.
[Illustration: Fig. 29.—Sun Spot seen in 1870]
The sun outweighs fully seven hundred and forty times the combined
mass of all the planets which circle around him, so that, when we
regard the energy of his attraction, we still find him a worthy ruler
of the planetary scheme.
Viewed with the naked eye, the sun appears only as a luminous mass of
intense and uniform brightness; but when examined with the telescope,
his surface is frequently observed to be mottled over with a number
of dark spots, of irregular and ill-defined forms, constantly
varying in appearance, situation, and magnitude. These spots are
occasionally of immense size, so as to be visible even without the
aid of the telescope; and their number is frequently so great that
they occupy a considerable portion of the sun’s surface. Sir W.
Herschel observed one in 1779 the diameter of which exceeded 50,000
miles, more than six times the diameter of the earth; and Scheiner
affirms that he has seen no less than fifty on the sun’s disk at
once. Most of them have a deep black nucleus, surrounded by a fainter
shade, or _umbra_, of which the inner part, nearest to the nucleus,
is brighter than the exterior portion. The boundary between the
nucleus and umbra is in general tolerably well defined; and beyond
the umbra a stripe of light appears more vivid than the rest of the
sun.
[Illustration: Fig. 30.—Phase of Spot]
The discovery of the sun’s spots has been attributed to Fabricius,
Galileo, and Scheiner, and has been claimed for the English
astronomer Harriot. Amid these conflicting pretensions it is perhaps
impossible to arrive at the truth; but the matter is of little
importance; the discovery is one which followed inevitably that of
the telescope, and an accidental priority of observation can hardly
be considered as establishing any claim to merit.
The study of solar physics may be said to have commenced with the
discovery of the sun spots, about two hundred and sixty years ago.
These spots were presently found to traverse the solar disk in such
a way as to indicate that the sun turns upon an axis once in about
twenty-six days. Nor will this rotation appear slow, when we remember
that it implies a motion of the equatorial parts of the sun’s surface
at a rate exceeding some seventy times the motion of our swiftest
express train.
Next came the discovery that the solar spots are not surface stains,
but deep cavities in the solar substance. The changes of appearance
presented by the spots as they traverse the solar disk led Dr. Wilson
to form this theory so far back as 1779; but, strangely enough, it
is only in comparatively recent times that the hypothesis has been
finally established, since even within the last ten years a theory
was put forward which accounted satisfactorily for most of the
changes of appearance observed in the spots, by supposing them to be
due to solar clouds hanging suspended at a considerable elevation
above the true photosphere.
Sir William Herschel, reasoning from terrestrial analogies, was led
to look on the spot-cavities as apertures through a double layer
of clouds. He argued that, were the solar photosphere of any other
nature, it would be past comprehension that vast openings should
form in it, to remain open for months before they close up again.
Whether we consider the enormous rapidity with which the spots form
and with which their figure changes, or the length of time that many
of them remain visible, we find ourselves alike perplexed, unless we
assume that the solar photosphere resembles a bed of clouds. Through
a stratum of terrestrial clouds openings may be formed by atmospheric
disturbances, but while undisturbed the clouds will retain any form
once impressed upon them, for a length of time corresponding to the
weeks and months during which the solar spots endure.
And because the solar spots present two distinct varieties of light,
the faint penumbra and the dark umbra or nucleus, Herschel saw the
necessity of assuming that there are two beds of clouds, the outer
self-luminous and constituting the true solar photosphere, the inner
reflecting the light received from the outer layer, and so shielding
the real surface of the sun from the intense light and heat which it
would otherwise receive.
But while recent discoveries have confirmed Sir William Herschel’s
theory about the solar cloud-envelopes, they have by no means given
countenance to his view that the body of the sun may possibly
be cool. The darkness of the nucleus of a spot is found, on the
contrary, to give proof that in that neighborhood the sun is hotter,
because it parts less readily with its heat. We shall see presently
how this is. Meantime let it be noticed, in passing, that a close
scrutiny of large solar spots has revealed the existence of an
intensely black spot in the midst of the umbra. This black spot must
be regarded as the true nucleus.
The circumstance that the spots appear only on two bands of the sun’s
globe, corresponding to the sub-tropical zones on our own earth, led
the younger Herschel to conclusions as important as those which his
father had formed. He reasoned, like his father, from terrestrial
analogies. On our own earth the sub-tropical zones are the regions
where the great cyclonic storms have their birth, and rage with their
chief fury. Here, therefore, we have the analogue of the solar spots,
if only we can show reason for believing that any causes resembling
those which generate the terrestrial cyclone operate upon those
regions of the sun where the solar spots make their appearance.
We know that the cyclone is due to the excess of heat at the earth’s
equator. It is true that this excess of heat is always in operation,
whereas cyclones are not perpetually raging in sub-tropical climates.
Ordinarily, therefore, the excess of heat does not cause tornadoes.
Certain aerial currents are generated whose uniform motion suffices,
as a rule, to adjust the conditions which the excess of heat at the
equator would otherwise tend to disturb. But when through any cause
the uniform action of the aerial currents is either interfered with
or is insufficient to maintain equilibrium, then cyclonic or whirling
motions are generated in the disturbed atmosphere, and propagated
over a wide area of the earth’s surface.
Now we recognize the reason of the excess of heat at the earth’s
equator in the fact that the sun shines more directly upon that part
of the earth than on the zones which lie in higher latitudes. Can
we find any reason for suspecting that the sun, which is not heated
from without as the earth is, should exhibit a similar peculiarity?
Sir John Herschel considers that we can. If the sun has an atmosphere
extending to a considerable distance from his surface, then there
can be little doubt that, owing to his rotation upon his axis, this
atmosphere would assume the figure of an oblate spheroid, and would
be deepest over the solar equator. Here, then, more of the sun’s
heat would be retained than at the poles, where the atmosphere is
shallowest. Thus, that excess of heat at the solar equator which
is necessary to complete the analogy between the sun spots and
terrestrial cyclones seems satisfactorily established.
It must be remarked, however, that this reasoning, so far as the
excess of heat at the sun’s equator is concerned, only removes
the difficulty a step. If there were indeed an increased depth of
atmosphere over the sun’s equator sufficing to retain the requisite
excess of heat, then the amount of heat we receive from the sun’s
equatorial regions ought to be appreciably less than the amount
emitted from the remaining portions of the solar surface. This is
not found to be the case, so that either there is no such excess of
absorption, or else the solar equator gives out more heat, in other
words, is essentially hotter, than the rest of the sun. But this is
just the peculiarity of which we want the interpretation.
It may be taken for granted, however, that there is an analogy
between the sun spots and terrestrial cyclonic storms, though as yet
we are not very well able to understand its nature.
Then next we come to one of the most interesting discoveries ever
made respecting the sun—the discovery that the spots increase and
diminish in frequency in a periodic manner. We owe this discovery to
the laborious and systematic observations made by Herr Schwabe of
Dessau.
Schwabe found, in the course of about ten and a half years, the solar
spots pass through a complete cycle of changes. They become gradually
more and more numerous up to a certain maximum, and then as gradually
diminish. At length the sun’s face becomes not only clear of spots,
but a certain well-marked darkening around the border of his disk
disappears altogether for a brief season. At this time the sun
presents a perfectly uniform disk. Then gradually the spots return,
become more and more numerous, and so the cycle of changes is run
through again.
The astronomers who have watched the sun from the Kew Observatory
have found that the process of change by which the spots sweep
in a sort of “wave of increase” over the solar disk is marked by
several minor variations. As the surface of a great sea wave will be
traversed by small ripples, so the gradual increase and diminution in
the number of the solar spots are characterized by minor gradations
of change, which are sufficiently well marked to be distinctly
cognizable.
[Illustration: Fig. 31.—Ptolemaic System]
There seems every reason for believing that the periodic changes
thus noticed are due to the influence of the planets upon the solar
photosphere, though in what way that influence is exerted is not at
present perfectly clear. Some have thought that the mere attraction
of the planets tends to produce tides of some sort in the solar
envelopes. Then, since the height of a tide so produced varies as
the cube or third power of the distance, it has been thought that
a planet when in perihelion would generate a much larger solar tide
than when in aphelion. So that, as Jupiter has a period nearly equal
to the sun-spot period, it has been supposed that the attractions
of this planet are sufficient to account for the great spot period.
Venus, Mercury, the Earth, and Saturn have, in a similar manner,
been rendered accountable for the shorter and less distinctly marked
periods.
Without denying that the planets may be, and probably are, the
bodies to whose influence the solar-spot periods are to be ascribed,
I yet venture to express very strong doubts whether the attraction
of Jupiter is so much greater in perihelion than in aphelion as to
account for the fact that, whereas at one season the face of the sun
shows many spots, at another it is wholly free from them.[23]
However, we are not at present concerned so much with the explanation
of facts as with the facts themselves. We have to consider rather
what the sun is and what he does for the Solar System than why these
things are so.
Let us note, before passing to other circumstances of interest
connected with the sun, that the variable condition of his
photosphere must cause him to change in brilliancy as seen from
vast distances. If Herr Schwabe, for instance, instead of observing
the sun’s spots from his watch-tower at Dessau, could have removed
himself to a distance so enormous that the sun’s disk would have
been reduced, even in the most powerful telescope, to a mere point
of light, there can be no doubt that the only effect which he would
have been able to perceive would have been a gradual increase and
diminution of brightness, having a period of about ten and a half
years.
Our sun, therefore, viewed from the neighborhood of any of the stars,
whence undoubtedly he would simply appear as one among many fixed
stars, would be a “variable,” having a period of ten and a half
years. And further, if an observer, viewing the sun from so enormous
a distance, had the means of very accurately measuring its light, he
would undoubtedly discover that, while the chief variation of the
sun takes place in a period of ten and a half years, its light is
subjected to minor variations having shorter periods.
The discovery that the periodic changes of the sun’s appearance are
associated with the periodic changes in the character of the earth’s
magnetism is the next that we have to consider.
It had long been noticed that, during the course of a single day, the
magnetic needle exhibits a minute change of direction, taking place
in an oscillatory manner. And, when the character of this vibration
came to be carefully examined, it was found to correspond to a sort
of effort on the needle’s part to turn toward the sun. For example,
when the sun is on the magnetic meridian, the needle has its mean
position. This happens twice in a day, once when the sun is above
the horizon and once when he is below it. Again, when the sun is
midway between these two positions—which also happens twice in the
day—the needle has its mean position, because the northern and the
southern ends make equal efforts (so to speak) to direct themselves
toward the sun. Four times in the day, then, the needle has its mean
position, or is directed toward the magnetic meridian. But, when the
sun is not in one of the four positions considered, that end of the
needle which is nearest to him is slightly turned away from its mean
position toward him. The change of position is very minute, and only
the exact modes of observation made use of in the present age would
have sufficed to reveal it. There it is, however, and this minute
and seemingly unimportant peculiarity has been found to be full of
meaning.
The minute vibrations of the magnetic needle, thus carefully
watched—day after day, month after month, year after year—were found
to exhibit a yet more minute oscillatory change. They waxed and waned
within narrow limits of variation, but yet in a manner there was
no mistaking. The period of this oscillatory change was not to be
determined, however, by the observations of a few years. Between the
time when the diurnal vibration was least until it had reached its
greatest extent, and thence returned to its first value, no less than
ten and a half years elapsed, and a much longer time passed before
the periodic character of the change was satisfactorily determined.
The reader will at once see what these observations tend to. The
sun spots vary in frequency within a period of ten and a half years,
and the magnetic diurnal vibrations vary within a period of the same
duration. It might seem fanciful to associate the two periodic series
of changes together, and doubtless when the idea first occurred to
Lamont, it was not with any great expectation of finding it confirmed
that he examined the evidence bearing on the point. Judging from
known facts, we may see reasons for such an expectation in the
correspondence of the needle’s diurnal vibration with the sun’s
apparent motion, and the law which has been found to associate the
annual variations of the magnet’s power with the sun’s distance. But
undoubtedly when the idea occurred to Lamont it was an exceedingly
bold one, and the ridicule with which the first announcement of the
supposed law was received, even in scientific circles, suffices to
show how unexpected that relation was which is now so thoroughly
established. For a careful comparison between the two periods has
demonstrated that they agree most perfectly, not merely in length,
but maximum for maximum, and minimum for minimum. When the sun spots
are most numerous, then the daily vibration of the magnet is most
extensive, while, when the sun’s face is clear of spots, the needle
vibrates over its smallest diurnal arc.
Then the intensity of the magnetic action has been found to depend
upon solar influences. The vibrations by which the needle indicates
the progress of those strange disturbances of the terrestrial
magnetism which are known as magnetic storms have been found not
merely to be most frequent when the sun’s face is most spotted, but
to occur simultaneously with the appearance of signs of disturbance
in the solar photosphere. For instance, during the autumn of 1859,
the eminent solar observer, Carrington, noticed the apparition of
a bright spot upon the sun’s surface. The light of this spot was
so intense that he imagined the dark glass which protected his eye
had been broken. By a fortunate coincidence, another observer, Mr.
Hodgson, happened to be watching the sun at the same instant, and
witnessed the same remarkable appearance. Now it was found that
the self-registering magnetic instruments of the Kew Observatory
had been sharply disturbed at the instant when the bright spot was
seen. And afterward it was learned that the phenomena which indicate
the progress of a magnetic storm had been observed in many places.
Telegraphic communication was interrupted, and in some cases,
telegraphic offices were set on fire; auroras appeared both in the
Northern and Southern Hemisphere during the night which followed;
and the whole frame of the earth seemed to thrill responsively to
the disturbance which had affected the great central luminary of the
Solar System.
[Illustration: Fig. 32.—Copernican System: Facsimile of the Drawing
in the Volume by Copernicus Published in 1543]
The reader will now see why I have discussed relations which hitherto
he may perhaps have thought very little connected with my subject.
He sees that there is a bond of sympathy between our earth and the
sun; that no disturbance can affect the solar photosphere without
affecting our earth to a greater or less degree. But if our earth,
then also the other planets. Mercury and Venus, so much nearer the
sun than we are, surely respond even more swiftly and more distinctly
to the solar magnetic influences. But beyond our earth, and beyond
the orbit of moonless Mars, the magnetic impulses speed with the
velocity of light. The vast globe of Jupiter is thrilled from pole
to pole as the magnetic wave rolls in upon it; then Saturn feels
the shock, and then the vast distances beyond which lie Uranus and
Neptune are swept by the ever-lessening yet ever-widening disturbance
wave. Who shall say what outer planets it then seeks? or who, looking
back upon the course over which it has traveled, shall say that
planets alone have felt its effects? Meteoric and cometic systems
have been visited by the great magnetic wave, and upon the dispersed
members of the one and the subtle structure of the other effects even
more important may have been produced than those striking phenomena
which characterize the progress of the terrestrial or planetary
magnetic storms.
When we remember that what is true of a relatively great solar
disturbance, such as the one witnessed by Messrs. Carrington and
Hodgson, is true also (however different in degree) of the magnetic
influences which the sun is at every instant exerting, we see that a
new and most important bond of union exists between the members of
the solar family. The sun not only sways them by the vast attraction
of his gravity, not only illumines them, not only warms them, but he
pours forth on all his subtle yet powerful magnetic influences. A new
analogy between the members of the Solar System is thus introduced to
reinforce those other analogies which have been held so strikingly to
indicate that the ends for which our earth has been created are not
different from those which the Creator had in view when He planned
the other members of the Solar System.
The real end and aim of the telescope, as applied by the astronomer
to the examination of the celestial objects, is to gather together
the light which streams from each luminous point throughout
space. We may regard the space which surrounds us on every side
as an ocean without bounds or limits, an ocean across which there
are ever sweeping waves of light, either emitted directly from
the various bodies subsisting throughout space, or else reflected
from their surfaces. Other forms of waves also speed across those
limitless depths in all directions, but the light-waves are those
which at present concern us. Our earth is as a minute island placed
within the ocean of space, and to the shores of this tiny isle the
light-waves bear their message from the orbs which lie like other
isles amid the fathomless depths around us. With the telescope the
astronomer gathers together portions of light-waves which else
would have traveled in diverging directions. By thus intensifying
their action, he enables the eye to become cognizant of their true
nature. Precisely as the narrow channels around our shores cause the
tidal wave, which sweeps across the open ocean in almost insensible
undulations, to rise and fall through a wide range of variation, so
the telescope renders sensible the existence of light-waves which
would escape the notice of the unaided eye.
The telescope, then, is essentially a _light-gatherer_.
The spectroscope is used for another purpose. It might be called the
_light-sifter_. It is applied by the astronomer to analyze the light
which comes to him from beyond the ocean of space, and so to enable
him to learn the character of the orbs from which that light proceeds.
The principle of the instrument is simple, though the appliances by
which its full powers can alone be deduced are somewhat complicated.
A ray of sunlight falling on a prism of glass or crystal does not
emerge unchanged in character. Different portions of the ray are
differently bent, so that when they emerge from the prism they no
longer travel side by side as before. The violet part of the light
is bent most, the red least; the various colors from violet through
blue, green, and yellow, to red being bent gradually less and less.
The prism then _sorts_, or _sifts_, the light-waves.
But we want the means of sifting the light-waves more thoroughly. The
reader must bear with me while I describe, as exactly as possible in
the brief space available to me, the way in which the first rough
work of the prism has been modified into the delicate and significant
work of the spectroscope. It is well worth while to form clear views
on this point, because so many of the wonders of modern science are
associated with spectroscopic analysis.
If, through a small round hole in a shutter, light is admitted into
a darkened room, and a prism be placed with its refracting angle
downward and horizontal, a vertical spectrum, having its violet end
uppermost, will be formed on a screen suitably placed to receive it.
But now let us consider what this spectrum really is. If we take the
light-waves corresponding to any particular color, we know, from
optical considerations, that these waves emerge from the prism in a
pencil exactly resembling in shape the pencil of white light which
falls on the prism. They therefore form a small circular or oval
image on their own proper part of the spectrum. Hence the spectrum is
in reality formed of a multitude of overlapping images, varying in
color from violet to red. It thus appears as a rainbow-tinted streak,
presenting every gradation of color between the utmost limits of
visibility at the violet and red extremities.
If we had a square aperture to admit the light, we should get a
similar result. If the aperture were oblong, there would still be
overlapping images; but if the length of the oblong were horizontal,
then, since each image would also be a horizontally placed oblong,
the overlapping would be less than when the images were square.
Suppose we diminish the overlapping as much as possible? in other
words, suppose we make the oblong slit as narrow as possible? Then,
unless there were in reality an infinite number of images distributed
all along the spectrum from top to bottom, the images might be so
narrowed as not to overlap; in which case, of course, there would
be horizontal dark spaces or gaps in our spectrum. Or, again, if we
failed in finding gaps of this sort by simply narrowing the aperture,
we might lengthen the spectrum by increasing the refracting angle of
the prism, or by using several prisms, and so on.
The first great discovery in solar physics, by means of the analysis
of the prism (though the discovery had little meaning at the time),
consisted in the recognition of the fact that, by means of such
devices as the above, dark gaps or cross-lines _can_ be seen in the
solar spectrum. In other words, light-waves of the various gradations
corresponding to all the tints of the spectrum from violet to red
do _not_ travel to us from the great central luminary of our system.
Remembering that the effect we call color is due to the length of
the light-waves, the effect of red corresponding to light-waves of
the greatest length, while the effect of violet corresponds to the
shortest light-waves, we see that in effect the sun sends forth to
the worlds which circle around him light-waves of many different
lengths, but not of all. Of so complex and interesting a nature is
ordinary daylight.
But spectroscopists sought to interpret these dark lines in the solar
spectrum, and it was in carrying out this inquiry—which even to
themselves seemed almost hopeless, and to many would appear an utter
waste of time—that they lighted upon the noblest method of research
yet revealed to man.
They examined the spectra of the light from incandescent substances
(white-hot metals and the like), and found that in these spectra
there are no dark lines.
They examined the spectra of the light from the stars, and found that
these spectra are crossed by dark lines resembling those in the solar
spectrum, but differently arranged.
They tried the spectra of glowing vapors, and they obtained a
perplexing result. Instead of a number of dark lines across a
rainbow-tinted streak, they found bright lines of various colors.
Some gases would give a few such lines, others many, some only one or
two.
Then they tried the spectrum of the electric spark, and they found
here also a series of bright lines, but not always the same series.
The spectrum varied according to the substances between which the
spark was taken and the medium through which it passed.
Lastly, they found that the light from an incandescent solid or
liquid, when shining through various vapors, no longer gives a
spectrum without dark lines, but that the dark lines which then
appear vary in position, according to the nature of the vapor through
which the light has passed.
Here were a number of strange facts, seemingly too discordant and too
perplexing to admit of being interpreted. Yet one discovery only was
wanting to bring them all into unison.
In 1859, Kirchhoff, while engaged in observing the solar spectrum,
lighted on the discovery that a certain double dark line, which had
already been found to correspond exactly in position with the double
bright line forming the spectrum of the glowing vapor of sodium, was
intensified when the light of the sun was allowed to pass through
that vapor. This at once suggested the idea that the presence of
this dark line (or, rather, pair of dark lines) in the spectrum of
the sun is due to the existence of the vapor of sodium in the solar
atmosphere, and that this vapor has the power of absorbing the same
order of light-waves as it emits. It would of course follow from
this that the other dark lines in the solar spectrum are due to the
presence of other absorbent vapors in its atmosphere, and that the
identity of these would admit of being established in the same way,
supposing this general law to hold, that a vapor emits the same
light-waves that it is capable of absorbing.
Kirchhoff was soon able to confirm his views by a variety of
experiments. The general principles to which his researches led—in
other words, the principles which form the basis of spectrum
analysis—are as follows:
1. An incandescent solid or liquid gives a continuous spectrum.
2. A glowing vapor gives a spectrum of white lines, each vapor having
its own set of bright lines, so that, from the appearance of a
bright-line spectrum, one can tell the nature of the vapor or vapors
whose light forms the spectrum.
3. An incandescent solid or liquid shining through absorbent vapors
gives a rainbow-tinted spectrum crossed by dark lines, these dark
lines having the same position as the bright lines belonging to the
spectra of the vapors; so that, from the arrangement of the dark
lines in such a spectrum, one can tell the nature of the vapor or
vapors which surround the source of light.[24]
The application of the new method of research to the study of
the solar spectrum quickly led to a number of most interesting
discoveries. It was found that, besides sodium, the sun’s atmosphere
contains the vapors of iron, calcium, magnesium, chromium, and
other metals. The dark lines corresponding to these elements appear
unmistakably in the solar spectrum. There are other metals, such as
copper and zinc, which seem to exist in the sun, though some of the
corresponding dark lines have not yet been recognized. As yet it
has not been proved that gold, silver, mercury, tin, lead, arsenic,
antimony, or aluminium exist in the sun—though we can by no means
conclude, nor indeed is it at all probable, that they are absent from
his substance. The dark lines belonging to hydrogen are very well
marked indeed in solar spectrum, and, as we shall see presently,
the study of these lines has afforded most interesting information
respecting the physical constitution of the sun.
Now we notice at once how importantly these researches into the
sun’s structure bear upon the subject of this treatise. It would be
indeed interesting to consider the actual condition of the central
orb of the planetary scheme, to picture in imagination the metallic
oceans which exist upon his surface, the continual evaporation from
those oceans, the formation of metallic clouds, and the downpour of
metallic showers upon the surface of the sun. But apart from such
considerations, and viewing Kirchhoff’s discoveries simply in their
relation to the subject of other worlds, we have enough to occupy our
attention.
If it could have been shown that, in all probability, the substance
of the sun consists of materials wholly different from those which
exist in this earth, the conclusion obviously to be drawn from such
a discovery would be that the other planets also are differently
constituted. We could not find any just reason for believing that
in Jupiter or Mars there exist the elements with which we are
acquainted, when we found that even the central orb of the planetary
system exhibits no such feature of resemblance to the earth. But now
that we know, quite certainly, that the familiar elements, iron,
sodium, and calcium, exist in the sun’s substance, while we are led
to believe, with almost perfect assurance, that all the elements we
are acquainted with also exist there, we see at once that, in all
probability, the other planets are constituted in the same way. There
may of course be special differences: in one planet the proportionate
distribution of the elements may differ, and even differ very
markedly, from that which prevails in some other planet. But the
general conclusion remains, that the planets are formed of the
elements which have so long been known as terrestrial; for we can not
recognize any reason for believing that our earth alone, of all the
orbs which circle around the sun, resembles that great central orb in
general constitution.
Now, we have in this general law a means of passing beyond the
bounds of the Solar System, and forming no indistinct conceptions
as to the existence and character of worlds circling around other
suns. For these orbs, like our sun, contain in their substance many
of the so-called terrestrial elements, while it may not unsafely
be asserted that all, or nearly all, those elements, and few or no
elements unknown to us, exist in the substance of every single star
that shines upon us from the celestial concave. Hence we conclude
that round those suns also there circle orbs constituted like
themselves, and therefore containing the elements with which we are
familiar. And the mind is immediately led to speculate on the uses
which those elements are intended to subserve. If iron, for example,
is present in some noble orb circling around Sirius, we speculate
not unreasonably respecting the existence on that orb—either now or
in the past, or at some future time—of beings capable of applying
that metal to the useful purposes which man makes it subserve.
The imagination suggests immediately the existence of arts and
sciences, trades and manufactures, on that distant world. We know
how intimately the use of iron has been associated with the progress
of human civilization, and though we must ever remain in ignorance
of the actual condition of intelligent beings in other worlds, we
are yet led, by the mere presence of an element which is so closely
related to the wants of man, to believe, with a new confidence, that
for such beings those worlds must in truth have been fashioned.
I would fain dwell longer on the thoughts suggested by the researches
of Kirchhoff. Gladly too would I enter at length on an account of
those interesting discoveries which have been made in connection with
the total eclipses of the sun. One point, however, remains which is
too intimately connected with my subject to be passed over.
I refer to the sun’s corona.
It has been proved that the solar prominences consist of glowing
vapors, hydrogen being their chief constituent. It has been
found also, by comparing Mr. Lockyer’s observations of the
prominence-spectra with Dr. Frankland’s elaborate researches
into the peculiarities presented by the spectrum of hydrogen at
different pressures, that even in the very neighborhood of the solar
photosphere these vapors probably exist at a pressure so moderate as
to indicate that the limits of the sun’s vaporous envelope can not
lie very far (relatively) from the outer solar cloud-layer.
Now, the solar corona has been seen, during total eclipses of the
sun, to extend to a distance at least equal to the sun’s diameter
from the eclipsed orb. So that, assuming the corona to be a solar
atmosphere, it would have a depth of about eight hundred and fifty
thousand miles, and being also drawn toward the sun by his enormous
attractive energy (exceeding more than twenty-seven times that of
the earth), it could not fail to exert a pressure on his surface
exceeding many thousand-fold that of our air upon the earth. In
fact, such an atmosphere, let its outermost layers be as rare as we
can conceive, would yet have its lower layers absolutely liquefied,
if not solidified, by the enormous pressure to which they would
be subjected. We can not, then, believe this corona to be a solar
atmosphere.
[Illustration: Fig. 33.—Tychonic System]
Yet it is quite impossible to dissociate the corona, either wholly
or in part, from the sun. I am aware that physicists of eminence
have attempted to do this, and not only so, but to make of the
zodiacal light a terrestrial phenomenon. But they have overlooked
considerations which oppose themselves irresistibly to such a
conclusion.
In the first place, the mere fact that, during a total eclipse, the
moon looks black, in the very heart of the corona, affords, when
properly understood, the most conclusive evidence that the light of
the corona comes from behind the moon. If the glare of our atmosphere
could by any possibility account for the corona (which is not the
case), then that glare should appear over the moon’s disk also.
That this is so is proved by the fact that, when the glare really
does cover the moon, as while the sun is but slightly eclipsed,
the moon is not projected as a black disk on the background of the
_sky_, though, where her outline crosses the sun, it appears black,
by contrast with the intensity of his light.[25] The point seems,
however, too obvious to need discussion.
And, secondly, as Mr. Baxendell has pointed out, during totality the
part of the earth’s atmosphere between the eye and the corona is not
illuminated by the sun. Over a wide space all round the sun we are
looking through an atmosphere which is completely dark. In fact, if
the earth’s atmosphere alone were in question, we ought to see a dark
or negative corona around the sun, the illuminated atmosphere only
beginning to be faintly visible at a considerable angular distance
from the sun. This argument, rightly understood, is altogether
decisive of the question.[26]
But the spectroscope has given certain very perplexing evidence
respecting the light of the corona, and it remains that we should
endeavor to see how that evidence bears on the interesting problem
which the corona presents to our consideration.
During the total eclipse of 1868 the American observers found that
the spectrum of the corona is continuous, but crossed by certain
bright lines. If we accept the absence of dark lines as established
by the evidence (which is doubtful), this result seems at first
sight very difficult to explain. Referring to the principles of
spectroscopic analysis stated on pp. 338-339, it will be seen that
we should be led to infer that the corona consists of incandescent
matter surrounded by certain glowing gases. It is difficult to
suppose that this is the real explanation of the phenomenon.
Mr. Lockyer suggests that, if the corona shone by reflecting the
solar light, the continuous spectrum might be accounted for by
supposing the light from the glowing vapors around the sun to supply
the part wanting where the solar dark lines are, and that some of
these vapors shining yet more brightly would exhibit their bright
lines upon the continuous background of the spectrum. This view, as
applied by Mr. Lockyer to the theory that the corona is a terrestrial
phenomenon, is untenable, for the reasons already adduced. But,
independently of those reasons, there are others which render such a
solution of the difficulty unavailable.
Now, remembering that we have two established facts for our
guidance—(1) the fact that the corona can not be a solar atmosphere,
and (2) the fact that it must be a solar appendage—I think a way may
be found toward a satisfactory explanation.
Let it be premised that the bright lines of the coronal spectrum
correspond in position to those seen in the spectrum of the aurora,
and that the same lines are seen in the spectrum of the Zodiacal
Light, and in that of the phosphorescent light occasionally seen over
the heavens at night.
Since we have every reason to believe that the light of the aurora is
due to electrical discharges taking place in the upper regions of the
air, we are invited to the belief that the coronal light may be due
to similar discharges taking place between the particles (of whatever
nature) constituting the corona.
Now, though the appearance of an aurora is due to some special
terrestrial action (however excited), yet the material substances
between which the discharges take place must be assumed to be at all
times present in the upper regions of air. In all probability, they
are the particles of those meteors which the earth is continually
encountering. And since we know that meteor-systems must be
aggregated in far greater numbers near the sun than near the earth,
we may regard the coronal light as due to electrical discharges
excited by the sun’s action, and taking place between the members of
such systems. Besides this light, however, there must necessarily be
a large proportion of light reflected from these meteoric bodies.
In this way the peculiar character of the coronal spectrum may be
readily accounted for. We know, from the auroral spectrum, that the
principal bright lines due to the electrical discharges would be
precisely where we see bright lines in the coronal spectrum. But,
besides these, there would be fainter bright lines corresponding
to the various elements which exist in the meteoric masses. These
elements, we know, are the same as those in the substance of the sun.
Thus the bright lines would correspond in position with the dark
lines of the solar spectrum. Hence, as light reflected by the meteors
would give the ordinary solar spectrum, there would result from the
combination a continuous spectrum, on which the bright lines first
mentioned would be seen, as during the American eclipse.
What the polariscope has told us respecting the corona is in
accordance with this view.
In the same way the quality of the Zodiacal Light admits of being
perfectly accounted for, without resorting to the hypothesis that
this phenomenon is a terrestrial one.
The explanation thus put forward has at least the advantage of being
founded on well-established relations. We know that the auroral light
is associated with the earth’s magnetism, and that meteoric bodies
are continually falling upon the earth’s atmosphere. We know, also,
that the sun exerts magnetic influences a thousand-fold more intense
than those of the earth, and that in his neighborhood there must be
many million times more meteoric systems.
But we have other and independent reasons, which must not be
overlooked, for considering the corona to be of some such nature as I
have suggested. Leverrier has shown that there probably exists in the
neighborhood of the sun a family of bodies whose united mass suffices
appreciably to affect the motions of the planet Mercury. It would not
be safe to neglect considerations thus vouched for.
Mr. Baxendell also has shown that certain periodic variations in the
earth’s magnetism point to the existence of such a family of bodies;
and he has been able to assign to them a position according well with
that determined by Leverrier.
Now, whatever opinion we form as to the exact character of the
system of bodies pointed to by the researches of Leverrier and
Baxendell—whether we suppose that system to form a zone around
the sun, or that (as I believe) the system is merely due to the
aggregation of meteoric perihelia in the sun’s neighborhood—we may
be quite certain of this, that during a total solar eclipse the
system could not fail to become visible. Hence there is a double
objection to the view put forward by Mr. Lockyer and others. In the
first place, it fails to account for the appearance presented by
the corona; in the second place, it fails to render an account of
the implied non-appearance of the system which, according to the
researches of Leverrier and Baxendell, circles around the sun.
[Illustration: Fig. 34.—Scale of Planets
_Jupiter and Saturn are shown in their true axial positions, Uranus
and Neptune in the axial positions inferred from the motions of their
satellites_]
We know that the sun is the sole source whence light and heat are
plentifully supplied to the worlds which circle around him. The
question immediately suggests itself—Whence does the sun derive
those amazing stores of force from whence he is continually supplying
his dependent worlds? We know that, were the sun a mass of burning
matter, he would be consumed in a few thousand years. We know that,
were he simply a heated body, radiating light and heat continually
into space, he would in like manner have exhausted all his energies
in a few thousand years—a mere day in the history of his system.
Whence, then, comes the enormous supply of force which he has
afforded for millions on millions of years, and which also our reason
tells us he will continue to afford while the worlds which circle
around him have need of it—in other words, for countless ages to come?
Now, there are two ways in which the solar energies might be
maintained. The mere contraction of the solar substance, Helmholtz
tells us, would suffice to supply such enormous quantities of heat
that, if the heat actually given out by the sun were due to this
cause alone, there would not, in many thousands of years, be any
perceptible diminution of the sun’s diameter. But, secondly, the
continual downfall of meteors upon the sun would cause an emission
of heat in quantities vast enough for the wants of all the worlds
circling round him; while his increase of mass from this cause would
not be rendered perceptible in thousands of years, either by any
change in his apparent size or by changes in the motions of his
family of worlds.
It seems far from unlikely that both these processes are in operation
at the same time. Certainly the latter is, for we know, from the
motions of the meteoric bodies which reach the earth, that myriads
of these bodies must continually fall upon the sun. And if the corona
and Zodiacal Light really be due to the existence of flights of
meteoric systems circling around the sun, or to the existence in his
neighborhood of the perihelia of many meteoric systems, then there
must be a supply of light and heat from this source very nearly if
not quite sufficient to account for the whole solar emission.
It is well worthy of notice, too, that the association between
meteors and comets has an important bearing on this question. We
know that the most remarkable characteristic of comets is the
enormous diffusion of their substance. Now, in this diffusion there
resides an enormous fund of force. The contraction of a large comet
to dimensions corresponding to a very moderate mean density would
be accompanied by the emission of a vast supply of heat. And the
question is worth inquiring into, whether we can indeed assume that
the meteors which reach our atmosphere are solid bodies, and not
rather of cometic diffusion; since it is difficult otherwise to
account for the light and heat which they emit. Friction through the
rarer upper strata of our atmosphere will certainly not account for
these phenomena; nor, I think, will the compression of the atmosphere
in front of the meteors; on the other hand, the sudden contraction
of a diffused vapor would be accompanied by precisely such results.
But, be this as it may, it is certain that a large portion of the
substance of every comet is in a singularly diffused state. And since
the meteoric systems circling in countless millions round the sun
are, in all probability, associated in the most intimate manner with
comets, we may recognize in this diffusion, as well as in the mere
downfall of meteors, the source of an enormous supply of light and
heat.
And lastly, turning from our sun to the other suns which shine in
uncounted myriads throughout space, we see the same processes at
work upon them all. Each star-sun has its coronal and its zodiacal
disks, formed by meteoric and cometic systems; for otherwise each
would quickly cease to be a sun. Each star-sun emits, no doubt, the
same magnetic influences which give to the Zodiacal Light and to the
solar corona their peculiar characteristics. And thus the worlds
which circle round those orbs may resemble our own in all those
relations which we refer to terrestrial magnetism, as well as in the
circumstance that on them also there must be, as on our own earth, a
continual downfall of minute meteors. In those worlds, perchance, the
magnetic compass directs the traveler over desert wastes or trackless
oceans; in their skies, the aurora displays its brilliant streamers;
while, amid the constellations which deck their heavens, meteors
sweep suddenly into view, and comets extend their vast length athwart
the celestial vault, a terror to millions, but a subject of study and
research to the thoughtful.
FOOTNOTES:
[23] Professor Kirkwood has published a most interesting series of
inquiries, going far to prove that the real secret or the planetary
influences lies in the fact that the sun’s surface is not uniform,
and that on a certain solar longitude the planetary influences are
more effective than elsewhere.
[24] To these may be added the following law:
4. Light reflected from any opaque body gives the same spectrum as it
would have given before reflection.
5. But if the opaque body be surrounded by vapors, the dark lines
corresponding to these vapors make their appearance in the spectrum
with a distinctness proportioned to the extent to which the light has
penetrated those vapors before being reflected to us.
6. If the reflecting body be itself luminous, the spectrum belonging
to it is superadded to the spectrum belonging to the reflected light.
7. Glowing vapors surrounding an incandescent source of light may
cause bright lines or dark lines to appear in the spectrum, according
as they are more or less heated; or, they may emit just so much light
as to make up for what they absorb, in which case there will remain
no trace of their presence.
8. The electric spark presents a bright-line spectrum, compounded
of the spectra belonging to the vapors of those substances between
which, and of those through which, the discharge takes place.
According to the nature of these vapors and of the discharge itself,
the relative intensity of the component parts of the spectrum will be
variable.
Lastly, the appearance of the spectrum belonging to any element will
vary according to the circumstances of pressure and temperature under
which the element may emit light.
[25] It is also shown most conclusively, by a photograph of the
eclipse of August, 1868, taken an instant before the totality. Here
we see the glare trenching upon the moon’s disk (elsewhere black),
as it should theoretically. So soon as totality commenced, the glare
had reached the moon’s limb, whence it must immediately have passed
quickly away.
[26] In fact, if we take the mode of reasoning by which Mr. Lockyer
has endeavored to get over certain physical difficulties presently
to be mentioned, we shall be able to point definitely to the place
where his argument fails. He says, conceive a tiny moon placed so as
to appear coincident with the centre of the sun’s disk. There will
be atmospheric glare as well as direct sunlight. Now, conceive this
small moon to expand until it all but covers the sun. Still there
will be glare and a certain small proportion of direct sunlight. So
far his reasoning is most just. But when he allows his expanding moon
to cover the sun, and to extend beyond the solar disk as in total
eclipse, the atmospheric glare can no longer be assumed to exist all
round the expanding moon: at the moment when the moon just hides the
sun, the glare begins to leave the moon, a gradually expanding black
ring being formed round that body. It is only necessary to consider
where the glare comes from to see that this must be so.
I have taken no account of diffraction here, because it has been
abundantly proved that no corona of appreciable width could be formed
around the moon during total eclipse by the diffraction of the rays
of light as they pass near the moon’s limb.
MERCURY.—WILLIAM F. DENNING
Mercury is the nearest known planet to the sun. It is true that a
body, provisionally named Vulcan, has been presumed to exist in
the space inferior to the orbit of Mercury; but absolute proof is
lacking, and every year the idea is losing strength in the absence
of any confirmation of a reliable kind. Not one of the regular and
best observers of the sun has recently detected any such body during
its transits (which would be likely to occur pretty frequently), and
there is other evidence of a negative character; so that the ghost of
Vulcan may be said to have been laid, and we may regard it as proven
that no major planet revolves in the interval of 36,000,000 miles
separating Mercury from the sun.
Copernicus, amid the fogs of the Vistula, looked for Mercury in
vain, and complained in his last hours that he had never seen it.
Tycho Brahe, in the Island of Hueen, appears to have been far more
successful. The planet is extremely fugitive in his appearances, but
is not nearly so difficult to find as many suppose. Whenever the
horizon is very clear, and the planet well placed, a small sparkling
object, looking more like a scintillating star than a planetary body,
will be detected at a low altitude and may be followed to the horizon.
Mercury revolves round the sun in 87 days, 23 hours, 15 minutes,
and 44 seconds in an eccentric orbit, so that his distance from
that luminary varies from 43,350,000 to 28,570,000 miles. When in
superior conjunction the apparent diameter of the planet is 4″.5;
at inferior conjunction it is 12″.9, and at elongation 7″. His real
diameter is 3,000 miles.
Being situated so near to the sun, it is obvious that to an observer
on the earth he must always remain in the same general region of
the firmament as that body. His orbital motion enables him to
successively assume positions to the east and west of the sun, and
these are known as his elongations, which vary in distance from 18°
to 28°. He becomes visible at these periods either in the morning or
evening twilight, and under the best circumstances may remain above
the horizon two hours in the absence of the sun. The best times to
observe the planet are at his E. elongations during the first half of
the year, or at his W. elongations in the last half; for his position
at such times being N. of the sun’s place, he remains a long while in
view.
Occasionally he presents quite a conspicuous aspect on the horizon,
as in February, 1868, when I thought his lustre vied with that of
Jupiter, and in November, 1882, when he shone brighter than Sirius.
The planet is generally most conspicuous _a few mornings after his W.
elongations and a few evenings before his E. elongations_.
In the course of his orbital round, Mercury exhibits all the phases
of the moon. Near his elongations the disk is about half illuminated,
and similar in form to that of our satellite when in the first or
third quarter. But the phase is not to be distinctly made out unless
circumstances are propitious. Galileo’s telescope failed to reveal
it, and Hevelius, many years afterward, found it difficult. This
is explained by the small diameter of the planet and the rarity
with which his disk appears sharply defined. The phase is sometimes
noted to be less than theory indicates; for the planet has been seen
crescented when he should have presented the form of a semicircle.
Several observers have also remarked that his surface displays a rosy
tint, and that the terminator is more deeply shaded and indefinite
than that of Venus.
The atmosphere of Mercury is probably far less dense than that of
Venus. The latter being furthest from the sun might be expected to
shine relatively more faintly than the former, but the reverse is the
case. Mercury has a dingy aspect in comparison with the bright white
lustre of Venus. On May 12, 1890, when the two planets were visible
as evening stars, and separated from each other by a distance of
only 2°, I examined them in a 10-inch reflector, power 145. The disk
of Venus looked like newly polished silver, while that of Mercury
appeared of a dull leaden hue. A similar observation was made by Mr.
Nasmyth on September 28, 1878. The explanation appears to be that the
atmosphere of Mercury is of great rarity, and incapable of reflection
in the same high degree as the dense atmosphere of Venus.
As a naked-eye object, Mercury must necessarily be looked for when
near the horizon; but there is no such need in regard to telescopic
observation, which ought to be only attempted when the planet
surmounts the dense lower vapors and is placed at a sufficient
elevation to give the instrument a fair chance of producing a steady
image. The presence of sunshine need not seriously impair the
definition, or make the disk too faint for detail.
I have occasionally seen Mercury, about two or three hours after
his rising, with outlines of extreme sharpness and quite comparable
with the excellent views obtained of Venus at the time of sunrise
or sunset. Those who possess equatorials should pick up the planet
in the afternoon and follow him until after sunset, when the
horizontal vapors will interfere. Others who work with ordinary
altazimuth stands will find it best to examine the planet at his
western elongations during the last half of the year, when he may
be found soon after rising by the naked eye or with an opera-glass,
and retained in the telescope for several hours after sunrise if
necessary.
Mercury was displayed under several advantages in the morning
twilight of November, 1882, and I made a series of observations with
a 10-inch reflector, power 212. Several dark markings were perceived,
and a conspicuous white spot. The general appearance of the disk was
similar to that of Mars, and I forwarded a summary of my results to
Professor Schiaparelli of Milan, who favored me with the following
interesting reply:
“I have myself been occupied with this planet during the past year
(1882). You are right in saying that Mercury is much easier to
observe than Venus, and that his aspect resembles Mars more than any
other of the planets of the Solar System. It has some spots which
become partially obscured and sometimes completely so; it has also
some brilliant white spots in a variable position.”
Professor Schiaparelli used an 8½-inch refractor in this work, and
was able under some favorable conditions to apply a power of 400.
The outcome of his researches, encouraged since 1882 by the addition
of an 18-inch refractor to the appliances of his observatory, was
announced in the curious fact that the rotation of Mercury is
performed in the same time that the planet revolves round the sun! If
this conclusion is just, Mercury constantly presents one and the same
hemisphere to the sun, and the behavior of the moon relatively to the
earth has found an analogy.
Spots or markings of any kind have rarely been distinguished on
Mercury. On June 11, 1867, Prince recorded a bright spot, with
faint lines diverging from it northeast and south. The spot was a
little south of the centre. Birmingham on March 13, 1870, glimpsed
a large white spot near the planet’s east limb, and Vögel, at
Bothkamp, observed spots on April 14 and 22, 1871. These instances
are quoted by Webb, and they, in combination with the markings seen
by Schiaparelli at Milan and by the author at Bristol in 1882,
sufficiently attest that this object deserves more attentive study.
One of the most interesting phenomena, albeit a somewhat rare event,
in connection with Mercury, is that of a transit across the sun.
The planet then appears as a black circular spot. Observers have
noticed one or two very small luminous points on the black disk, and
an annulus has been visible round it. These features are probably
optical effects.
THE PLANET VENUS.—CAMILLE FLAMMARION
Revolving round the sun in 224 days, Venus has its motion combined
with ours in such a manner that it passes its inferior conjunction,
between the sun and us, every 584 days; but the plane in which it
revolves is inclined 3° 23′ to that in which the earth itself moves.
When Venus attains its greatest elongations from the sun it shines
in the west in the evening, then in the morning in the east, with
a splendid brightness which eclipses that of all the stars. It is,
without comparison, the most magnificent star of our sky. Its light
is so vivid that it casts a shadow. Sometimes, even, it pierces the
azure of the sky, in spite of the presence of the sun above the
horizon, and _shines in full daylight_.
The maximum visibility of Venus is produced by its greatest phase,
by its greatest elongation from the sun, and by the clearness of our
atmosphere.
The brilliant Venus was certainly the first planet noticed by the
ancients, as much on account of its brightness as its rapid motion.
Hardly is the sun set than it sparkles in the twilight; from
evening to evening it removes further from the west and increases
in brightness; during several months it reigns sovereign of the
skies, then plunges into the solar fires and disappears. It was
pre-eminently the star of the evening, the shepherd’s star, the star
of sweet confidences. It was the first of celestial beauties, and
the names conferred upon it correspond to the direct impression which
it produced on contemplative minds. Homer called it “Callistos,”
the _Beautiful_; Cicero named it _Vesper_, the evening star, and
_Lucifer_, the morning star, a name likewise given in the Bible and
the ancient mythologies to the chief of the celestial army.
The most ancient astronomical _observation_ we have of Venus is a
Babylonian record of the year 685 B. C. It is written on a brick and
preserved in the British Museum.
The best hours for examining Venus in a telescope are those of
daylight. In the night the irradiation produced by the brilliant
light of this beautiful planet prevents us from distinguishing
clearly the outlines of its phases.
When Venus occupies the region of its orbit behind the sun, with
reference to us—which is called the point of superior conjunction—it
is at its greatest distance, and is reduced to a disk of 9½ seconds
in diameter. It comes imperceptibly toward us, and when it passes
its quadrature, at its mean distance, it presents the aspect of a
half-moon. It soon attains its most brilliant light, at the epoch
when it shines at a distance of 39° from the sun, and shows the
third phase 69 days before its inferior conjunction. Its apparent
diameter is then 40 seconds, and the width of its illuminated part
is scarcely 10 seconds. In this position we see the fourth of the
disk illuminated; but this quarter emits more light than the more
complete phases. Finally, when it reaches the region of its orbit
nearest to the earth, it shows us nothing more than an excessively
thin crescent, since it is then between the sun and us, and presents
to us, so to say, its dark hemisphere. This is the position where
its apparent size is greatest, and it then measures 62 seconds in
diameter. After passing its inferior conjunction the phases are
reproduced, in inverse order, as a morning star.
Venus is constantly visible in full daylight in astronomical
instruments, even at the moment of its superior conjunction. It is
then round and quite small. At the epochs of its inferior conjunction
it presents itself under the form of a very thin crescent.
We sometimes notice that the interior of the crescent of Venus, the
remainder of the disk, is less black than the background of the sky.
This has been called the ashy light (_lumière cendrée_) of Venus,
although it has no satellite to produce it. It seems to me that this
visibility, rather subjective than objective, arises from clouds on
the planet, which whiten its disk and vaguely reflect the stellar
light scattered through space. The eye instinctively continues the
outline of the crescent, and imagines, rather than sees, the rest.
The revolution of Venus round the sun is performed in an orbit almost
exactly circular, and without perceptible eccentricity (0.0068), in a
period of 224 days, 16 hours, 49 minutes, 8 seconds.
The days of Venus, also, are a little more rapid than ours, but not
much. Since the year 1666 attentive observation of the planet led
Cassini to conclude that it turns on itself in 23 hours, 15 minutes.
This observation is extremely difficult, on account of the brightness
of the planet and the faintness of the irregularities visible on its
disk.
The year of Venus, composed of 224 terrestrial days, consequently
contains 231 of its own, since the day is a little shorter there than
here.
These same observations show that the axis of rotation of this planet
is much more inclined than ours, and that this inclination is 55
degrees. It follows that the seasons, although each lasting but 56
terrestrial days, or 58 Venusian days, are much more intense on this
world than on ours. They pass, without transition, from summer to
winter.
The inclination of the world of Venus being more than twice as great
as ours, we have only to take a terrestrial globe and incline it by
the same quantity to understand the climates and seasons which will
result. We may easily see that the torrid zone extends, in this case,
up to the frigid zone, and even beyond it; and, reciprocally, the
frigid zone extends to the torrid zone, and even encroaches on it; so
that no place remains for a temperate zone. There is not, then, on
Venus any temperate climate, but all latitudes are both tropical and
arctic.
It follows, then, from all these circumstances, that the seasons
and climates are much more violent and more varied than ours. This
neighboring world shows nearly the same dimensions as ours. Thus this
planet is truly the twin sister of ours.
The resemblance will be still more complete if we add that this world
is certainly surrounded by an atmosphere.
When we examine with the spectroscope the light reflected by this
planet we first find the lines of the solar spectrum (and this is
natural, since the planets have no light of their own, and merely
reflect that of the sun); but we notice besides several absorption
lines similar to those which the terrestrial atmosphere gives, and
particularly those of clouds and water vapor.
We may also add that attentive observation of the indentations
visible on the crescent of Venus has shown that the surface of this
planet is quite as uneven as that of the earth, and even more so;
that there are there Andes, Cordilleras, Alps, and Pyrenees, and
that the most elevated summits attain a height of 44,000 metres (27
miles). It has even been ascertained that the Northern Hemisphere is
more mountainous than the Southern.
Even the study of the geography of Venus has already been commenced.
But it is extremely difficult to draw, and the hours of sufficiently
pure atmosphere and possible observation are very rare. This
difficulty will be easily understood if we reflect that it is exactly
when Venus arrives at its nearest to us that it is least visible,
since, its illuminated hemisphere being always turned toward the
sun, it is its dark hemisphere which is presented to us. The nearer
it approaches us, the narrower the crescent becomes. Add to this
its vivid light and its clouds, and you may imagine what difficulty
astronomers have in dealing with it.
[Illustration: Twelve Views of Jupiter
Taken at Intervals within Six Consecutive Weeks]
However, by observing it in the daytime to avoid the glare, and
not waiting till the crescent becomes too thin, by choosing the
quadratures, and making use of moments of great atmospherical purity,
observers succeed, from time to time, in perceiving grayish spots,
which may indicate the place of its seas.
Of what nature are the inhabitants of Venus? Do they resemble us in
physical form? Are they endowed with an intelligence analogous to
ours? Do they pass their life in pleasure, as Bernardin de St. Pierre
said, or, rather, are they so tormented by the inclemency of their
seasons that they have no delicate perception, and are incapable
of any scientific or artistic attention? These are interesting
questions, to which we have no reply. All that we can say is, that
organized life on Venus must be little different from terrestrial
life, and that this world is one of those which resembles ours most.
The imaginary travelers to these worlds of the sky have always
carried with them their terrestrial ideas. The only scientific
conclusion which we can draw from astronomical observation is that
this world differs little from ours in volume, in weight, in density,
and in the duration of its days and nights; that it differs a little
more in the rapidity of its years, the intensity of its climates and
seasons, the extent of its atmosphere, and its greater proximity to
the sun. It should, then, be inhabited by vegetable, animal, and
human races but little different from those which people our planet.
As to imagining it desert or sterile, this is a hypothesis which
could not arise in the brain of any naturalist. The action of the
divine sun must be there, as in Mercury, still more fertile than his
terrestrial work, already so wonderful. We may add that Venus and
Mercury, having been formed after the earth, are relatively younger
than our planet.
The inhabitants of Venus see us shining in their sky like a
magnificent star of the first magnitude, soaring in the zodiac, and
showing motions similar to those which the planet Mars presents to
us; but instead of showing a reddish brightness, the earth shines
in the sky as a bluish light. It is from Venus that we are most
luminous. The inhabitants of Venus with the naked eye see our moon
shining beside the earth and revolving round it in twenty-seven days.
They form a magnificent couple. Our planet seen from there measures
65″, and the moon nearly 18″; the moon seen from Venus shows the same
diameter as the earth seen from the sun. Mercury is brilliant, and
comes immediately after the earth in brightness. Mars, Jupiter, and
Saturn are also visible as from here, but a little less luminous. The
constellations of the whole sky show exactly the same aspect as seen
from the earth.
THE EARTH AS A PLANET.—ÉLISÉE RECLUS
The earth on which we dwell is one of the lowest in rank among the
heavenly bodies. If an astronomer in some other planet were exploring
the immensity of space, our earth, owing to its small size, might
readily elude his intelligent view. A mere satellite of the sun,
the volume of which is 1,255,000 times greater, the earth is but a
point as compared with the immense tract of ether traversed by the
planets in their courses round their central globe. The sun itself is
only a spark, which seems lost amid the eighteen millions of stars
which Herschel’s telescope discerned in the Milky Way; the latter, an
immense agglomeration of suns and planets, which looks to us like a
broad streak of light round the whole universe, is in reality nothing
but a nebula. Beyond our own sky, other skies stretch far away into
infinity, and others beyond these, so that light notwithstanding its
prodigious rapidity, takes eternities to cross them. How small the
earth seems in this fathomless abyss of stars!
In the form of its orbit, in its movements round the sun and on its
own axis, in the succession of days and seasons, and in all the
phenomena governed by the great law of attraction, the earth becomes
the representative of all the other planets; in studying it, we study
all the heavenly bodies.
Our planet is a spheroid; that is, a sphere flattened at the
two poles and enlarged at the equator, so that all the circles
passing through the extremity of the polar axis form ellipses. The
presumed depression of each pole is about thirteen miles, nearly
a three-hundredth part of the radius of the earth; but it is not
altogether certain that the two poles are equally flattened. Perhaps
a contrast exists between the two hemispheres, not only in the
features of their continents and the distribution of seas, but also
in their geometrical shape. Be this as it may, it appears to be
proved that the curvature is not exactly the same at all points of
the earth at an equal distance from the poles; the meridians appear
without exception to be irregular ellipses.
The dimensions of the earth, as we have already seen, are almost as
nothing compared with the larger celestial bodies, and especially
with the extent of space which can be explored by the telescope. If
light, the speed of which has been adopted in astronomy as a term
of comparison, could be diffused in a curved line, it would travel
seven times round the globe in a second of time; this standard of
measurement, therefore, the only one suited to the stellary field, is
completely inapplicable to the surface of our globe.
The isolated globule in the immensity of space which we call the
earth is not motionless, as the ancients necessarily supposed,
looking upon it, as they did, as the immovable base of the firmament
of heaven. Hurried on in the vortex of universal vitality, our globe
is ever actuated by ceaseless motion, describing in ether a series
of elliptic spirals so complicated that astronomers have not yet
been able to calculate their various curves. Besides rotating on
its own axis, the earth describes an ellipse round the sun, and,
under the influence of this body, is drawn along from one heaven to
another toward distant constellations. It also oscillates and rocks
on its axis, and deviates more or less from its path, to salute, as
it were, every heavenly body which meets it. It is probable that
it never passes a second time through the same regions of the air;
yet, if it has again to traverse the spiral line of ellipses it has
already described, it would be after a cycle of so many thousands
of millions of years, that the earth itself, completely transformed,
would be no longer the same planet.
The motion of the earth, the immediate effects of which are the
most obvious to the notice of men, is the daily rotation which
takes place round an ideal axis passing through the two poles. The
globe turns from right to left, or from west to east—that is, in
a contrary direction to the apparent motion of the sun and stars,
which seem to rise in the east and to set in the west. As the earth’s
axis terminates at each pole, there is least surface-motion at
those points, and the motion is the more rapid in any part of the
surface of the globe the further it is from the central axis. At St.
Petersburg, in 60° latitude, the speed of rotation is about nine
miles a minute; in Paris, it exceeds eleven and a half miles during
the same brief time; on the equatorial line, which may be looked upon
as the ring of an immense wheel, the speed of the earth is twice as
great as it is at 60° of latitude—that is, about eighteen miles a
minute, or 528 yards a second—a rapidity equal to the flight of a
26-pound cannon-ball impelled by thirteen pounds of powder. By means
of this rotatory motion, the earth presents toward the sun each of
its faces alternately, and each also in turn toward the comparatively
darker regions of space; the succession of day and night is thus
constituted. In addition to this, the rotation of the earth is an
important fact which must always be taken into account in determining
the direction of fluids in motion on the surface of the globe, such
as streams and rivers, also marine and atmospheric currents.
The annual revolution which the earth performs round the sun follows
the line of an ellipse, one of the _foci_ of which is occupied by
the central star; the eccentricity of the ellipse is nearly equal
to 17/1000th of the great axis. The distance between the sun and
the earth always varies according to the particular point of its
orbit which the latter is traveling over. At its _aphelion_, that
is, at its greatest remoteness, this distance is about 93¾ millions
of miles; at the period of its _perihelion_, when the two heavenly
bodies are nearest to each other, it is approximately 90,259,000
miles. The mean distance, as estimated by astronomers since the
corrections of Encke, Hansen, Foucault, and Hind, is 91,839,000
miles. This extent of space is traversed by the solar rays in 8
minutes, 16 seconds; sound would take fifteen years in passing
through the same distance.
As Kepler has laid down in his celebrated laws, our planet moves with
an increased rapidity as it approaches nearer to the sun and travels
more slowly in proportion to its distance from that luminary; but
its mean speed may be estimated at nearly nineteen miles a second,
or sixty times the rapidity of a ball from the cannon’s mouth. This
speed, which makes one dizzy to think of, is to be added, as regards
each point in the surface of the earth, to the rotatory motion which
impels it round the polar axis.
After having turned round 366 times on its axis, our planet has
terminated its orbicular course, and is in the same position
relatively to the sun as at its starting-point; it has then
accomplished its _year_.
This daily rotation of the earth round its axis produces the
succession of days and nights, and, in the same way, its annual
revolution round the sun causes the alternations of the seasons. If
the axis of the earth, that is the ideal line which passes through
its two poles, were perpendicular to the plane of its annual orbit,
it is evident that the portion of the globe lighted by the sun would
invariably extend from one pole to the other, and that in both
hemispheres the days and nights would always consist of twelve hours
each. But this is not the case. The earth performs its revolutionary
movements in an inclined position; its ideal polar axis is sloped
about 23° 28′ from a perpendicular to its plane, and this position is
so far maintained that as regards the comparatively rapid succession
of days and seasons it may be looked upon as invariable. This
obliquity of axis causes continued changes in the phase presented
to the sun. The portion of the earth illumined by the rays of the
sun varies every day; for, although the planetary axis may appear
to maintain its extremity in a fixed position as regards some point
in infinite space, in respect to the sun it presents a constantly
varying degree of inclination, in consequence of the continual motion
of the earth. Twice during the course of the year it so happens that
the solar rays fall perpendicularly upon the equator of the earth; at
every other period in the annual revolution, sometimes the Northern
and sometimes the Southern Hemisphere receives the greatest amount of
light.
The astronomical year commences on the 20th of March, at the exact
moment when the sun illumines the equator in a vertical direction,
and the line of separation between light and shade passes through
the two poles. The period of darkness is then equal to that of light,
and admits of exactly twelve hours at all points of the earth. Hence
the name of “equinox” (equality of nights). But after this day, which
in the Northern Hemisphere serves as the starting-point of spring,
the earth continues its translatory movement. In consequence of the
inclination of its axis, the Northern Hemisphere, being turned toward
the sun, receives a greater quantity of light, while the southern
half of the globe is less vividly lighted. The vertical rays of the
sun now fall more and more to the north of the equator, and the
circle of light, far from arresting its progress at the poles, where
the day of six months’ duration is commencing to dawn, extends far
beyond it over the regions of the north. On the 21st of June, the day
of the first solstice, the axis of the earth being deeply inclined
toward the sun, this luminary shines on the zenith of the tropic
of Cancer at 23½° north of the equator, and its light illumines
the whole of the arctic zone, that is, the portion of the earth’s
surface extending to 23½° round the North Pole. Then spring ceases
and summer begins as regards the Northern Hemisphere. In the Southern
Hemisphere, on the contrary, autumn is giving place to winter. Above
the equator long days are prevailing, interrupted by short nights;
while in the south it is the nights which last the longest. In the
arctic zone the sun performs its apparent course of diurnal rotation
entirely above the horizon. The six months’ day, which spring
inaugurated at the North Pole, attains its high noon on the first
day of summer. At the same moment midnight arrives in the darkness
which is oppressing its antipodes.
Immediately after the 21st of June all the phenomena which took place
during the preceding season are directly reversed. The sun appears
to retrograde toward the southern horizon; its vertical rays cease
to fall on the line of the northern tropic, and constantly approach
the equator. The zone of light in the northern pole and of shade in
the southern equally diminish, and the days shorten in the Northern
Hemisphere in the same proportion as they lengthen in the Southern;
an equilibrium is gradually being re-established between the two
halves of the earth. On the 22d of September the position of the
sun is again exactly above the equator, and its light just reaches
both poles. The equinox, or the absolute equality of day and night
in every part of the globe, occurs for the second time in the year;
but this moment of equilibrium is, so to speak, but a mathematical
point between the two seasons. The axis of the earth which, during
the six months past, turned the North Pole toward the sun, now
presents to him the South Pole; the vertical rays of the central
luminary fall to the south of the earth’s equator, and the Southern
Hemisphere, in its turn, is the best endowed of the two halves of
the globe in the amount of light it receives and in the length of
its days. In the Southern Hemisphere spring is commencing; in the
Northern, autumn. Three months afterward, on the 21st of December,
the sun comes directly over the southern tropic, or the tropic of
Capricorn, 23½° south of the equator, and the whole of the antarctic
zone is presented to the solar rays. Summer has begun in the Southern
Hemisphere, and at the same time winter commences in that of the
north. Then, as the globe moves on, these two seasons follow each
other in their course, until at length the earth attains a position
similar to that from which it started; the March equinox, the first
day of spring in Europe, and the first day of autumn in Australia,
commences anew the astronomical year.
The elliptical form of the earth’s orbit and the unequal pace of the
globe in the various points of its course cause some considerable
variations in the duration of the seasons. In fact, from the 20th of
March to the 22d of September, that is, during the spring and summer
of the Northern Hemisphere, the earth takes 186 days to travel over
the first and largest half of its orbit, while during the winter
period, from the 22d of September to the 20th of March, only 179
days are required to accomplish the second half of its journey. The
summer period of the Northern Hemisphere actually exceeds by seven
or eight days, or about 187 hours, the corresponding period in the
southern half of the globe; added to this, in consequence of the
longer space of time during which the Arctic Pole remains inclined
toward the sun in the regions north of the equator, the hours of
daylight exceed the hours of night, while in the south the hours of
darkness predominate. This is, however, to some extent compensated
for; as, although in the southern regions of the earth the summer
lasts a shorter time, our planet is then closer to the sun; it is
at its perihelion, and consequently receives a larger proportion of
heat. There is, however, no doubt about the fact—as it is proved by a
direct observation, both of the winds and currents, and also of their
various temperatures—that, taking an equal distance from the equator,
the southern regions are colder than those of the north.
If an equality of seasons between the two halves of the world does
not at present exist, it will not fail to be established after a long
series of centuries by means of a slow terrestrial movement, which
has been known by the name of the _precession of the equinoxes_.
Just as a top (if we may be allowed to avail ourselves of so old an
illustration) turns round on the ground and bends over successively
in every direction, thus describing with its axis an ideal cone, so
the earth revolves in space, and slowly sways the line of its poles.
This line, which is always sloped at an angle of 66° 32′ to the
plane of the terrestrial orbit, turns round with a slight lateral
motion, so as always to point to a new region of the sky; if it were
prolonged indefinitely it would describe a circle amid the distant
stars. As the axis of the earth is constantly changing its direction
in this way, the plane of the equator must vary exactly to the same
extent in its position as regards the sun. In fact, every year
the exact moment of the March equinox anticipates by about twenty
minutes the time at which the corresponding equinox fell in the
year preceding. Each revolution of the earth round the sun brings a
fresh advance of twenty minutes in the determination of the equinox;
and as, during the long course of ages, the axis of the earth does
not intermit in this swaying motion, the time must come, after a
period of 12,900 years, that the conditions of the seasons will be
altogether changed. The hemisphere which hitherto received the larger
proportion of heat will receive the lesser share, and that half of
the globe which has endured the larger number of wintry days will
now, in its turn, enjoy the more lengthened period of summer. Then,
after a second period of 12,900 years, during which the relation
between the seasons of the two hemispheres is being gradually
modified, the axis of the earth completes its round of swaying,
which has lasted for 258 centuries, and the position of the globe in
respect to the sun being nearly the same as at its starting-point, a
second cycle of seasons will then commence.
We might call this period _the earth’s great year_, if, at the end of
it, the earth were in an identical position to that which it occupied
at the commencement; but this is not the case. The attraction of the
moon, and the disturbances caused by the vicinity of certain planets,
are incessantly modifying the curve described in the starry fields
of space by the earth’s axis, and complicate it with a multitude of
spirals, the various periods of which do not coincide with the great
period of the swaying of the axis. The successive undulations form a
continuous system of interwoven spirals. “It is a manifestation of
the infinite.”
But even this is not all. In addition to all the motions of the
globe which we have already pointed out—its diurnal rotation, its
annual revolution round the sun, the rhythmical swaying of its axis,
proved by the precession of the equinoxes, the nutation or more rapid
swaying which is caused by the attraction of the moon—we must now
notice the enormous translatory movement which is dragging it through
endless tracks of space in the train of the sun. Not many years ago,
this motion was entirely unknown to astronomers, and yet it is going
on with inconceivable rapidity—a rapidity more than double that of
the course of the planet round its central luminary. In one second
of time the earth moves about forty-four miles toward the point of
the heavens where we find the constellation of Hercules. During one
year only she travels 1,382 millions of miles in this direction.
Our own little earth itself is carried on from space to space, and
never closes the cycle of its revolutions. Ever since the time when
its particles were first grouped together, it has been describing in
space the infinite spiral of its ellipses, and thus will it go on
turning and oscillating in ether until the moment when it will exist
no longer as an independent planet. For the earth, too, must have an
end; like every other body in the universe, it comes into existence,
and lives only to die when its turn comes. Already its annual motion
of rotation is diminishing in speed; certainly this slackening of
pace is not very observable, since no astronomer from Hipparchus to
Laplace has yet exactly defined it. But, unless some cosmical force
acting in a contrary direction compensates for the loss of speed
caused by the friction of the tides against the bed and the shores of
the ocean, the impetus of our planet will every century diminish.
After various catastrophes which it is impossible to foresee, the
earth will eventually completely change its course of action, and
lose its independent existence, either uniting itself with other
planetary bodies or breaking up into fragments; or it will perhaps
terminate its course by falling like a mere aerolite upon the surface
of the sun.
THE MOON.—THOMAS GWYN ELGER
We know, both by tradition and published records, that from the
earliest times the faint gray and light spots which diversify the
face of our satellite excited the wonder and stimulated the curiosity
of mankind, giving rise to superstitions more or less crude and
erroneous as to their actual nature and significance. It is true
that Anaxagoras, five centuries before our era, and probably other
philosophers preceding him—certainly Plutarch at a much later
date—taught that these delicate markings and differences of tint,
obvious to every one with normal vision, point to the existence of
hills and valleys on her surface; the latter maintaining that the
irregularities of outline presented by the “terminator,” or line
of demarcation between the illumined and unillumined portion of
her spherical superficies, are due to mountains and their shadows;
but more than fifteen centuries elapsed before the truth of this
sagacious conjecture was unquestionably demonstrated. Selenography,
as a branch of observational astronomy, dates from the spring of
1609, when Galileo directed his “optic tube” to the moon, and in the
following year, in the _Sidereus Nuncius_, or the “Intelligencer of
the Stars,” gave to an astonished and incredulous world an account of
the unsuspected marvels it revealed.
The bright and dusky areas, so obvious to the unaided sight, were
found by Galileo to be due to a very manifest difference in the
character of the lunar surface, a large portion of the Northern
Hemisphere, and no inconsiderable part of the southeastern quadrant,
being seen to consist of large gray monotonous tracts, often
bordered by lofty mountains, while the remainder of the superficies
was much more conspicuously brilliant, and, moreover, included
by far the greater number of those curious ring-mountains and
other extraordinary features whose remarkable aspect and peculiar
arrangement first attracted his attention.
Before the close of the century when selenography first became
possible, Hevel of Dantzig, Scheiner, Langrenus (cosmographer to
the King of Spain), Riccioli, the Jesuit astronomer of Bologna, and
Dominic Cassini, the celebrated French astronomer, greatly extended
the knowledge of the moon’s surface, and published drawings of
various phases and charts, which, though very rude and incomplete,
were a clear advance upon what Galileo, with his inferior optical
means, had been able to accomplish. Langrenus, and after him Hevel,
gave distinctive names to the various formations, mainly derived
from terrestrial physical features, for which Riccioli subsequently
substituted those of philosophers, mathematicians, and other
celebrities; and Cassini determined by actual measurement the
relative position of many of the principal objects on the disk, thus
laying the foundation of an accurate system of lunar topography;
while the labors of T. Mayer and Schröter in the Eighteenth Century,
and of Lohrmann, Mädler, Neison (Nevill), Schmidt, and other
observers in the Nineteenth, have been mainly devoted to the study of
the minuter detail of the moon and its physical characteristics.
As was manifest to the earliest telescopic observers, its visible
surface is clearly divisible into strongly contrasted areas,
differing both in color and structural character. Somewhat less
than half of what we see of it consists of comparatively level dark
tracts, some of them many thousands of square miles in extent, the
monotony of whose dusky superficies is often unrelieved for great
distances by any prominent object; while the remainder, everywhere
manifestly brighter, is not only more rugged and uneven, but is
covered to a much greater extent with numbers of quasi-circular
formations differing widely in size, classed as walled-plains,
ring-plains, craters, craterlets, crater-cones, etc. (the latter
bearing a great outward resemblance to some terrestrial volcanoes),
and mountain ranges of vast proportions, isolated hills and other
features.
Though nothing resembling sheets of water, either of small or large
extent, has ever been detected on the surface of the moon, the
superficial resemblance, in small telescopes, of the large gray
tracts to the appearance which we may suppose our terrestrial lakes
and oceans would present to an observer on the moon, naturally
induced the early selenographers to term them Maria, or “seas”—a
convenient name, which is still maintained, without, however,
implying that these areas, as we now see them, are, or ever were,
covered with water.
There are twenty-three of these dusky areas which have received
distinctive names; seventeen of them are wholly, or in great part,
confined to the northern and to the southeastern quarter of the
Southern Hemisphere—the southwestern quadrant being to a great extent
devoid of them. By far the largest is the vast Oceanus Procellarum,
extending from a high northern latitude to beyond latitude 10° in the
southeastern quadrant, and, according to Schmidt, with its bays and
inflections, occupying an area of nearly two million square miles, or
more than that of all the remaining Maria put together. Next in order
of size come the Mare Nubium, or about one-fifth the superficies,
covering a large portion of the southeastern quadrant, and extending
considerably north of the equator, and the Mare Imbrium, wholly
confined to the northeastern quadrant, and including an area of about
340,000 square miles. These are by far the largest lunar “seas”. The
Mare Fœcunditatis, in the Western Hemisphere, the greater part of
it lying in the southwestern quadrant, is scarcely half so big as
the Mare Imbrium; while the Maria Serenitatis and Tranquilitatis,
about equal in area (the former situated wholly north of the equator
and the latter only partially extending south of it), are still
smaller. The arctic Mare Frigoris, some 100,000 square miles in
extent, is the only remaining large sea; the rest, such as the Mare
Vaporum, the Sinus Medii, the Mare Crisium, the Mare Humorum, and
the Mare Humboldtianum, are of comparatively small dimensions, the
Mare Crisium not greatly exceeding 70,000 square miles, the Mare
Humorum (about the size of England) 50,000 square miles, while the
Mare Humboldtianum, according to Schmidt, includes only about 42,000
square miles, an area which is approached by some formations not
classed with the Maria.
Among the Maria which exhibit the most remarkable arrangement of
ridges is the Mare Humorum, in the southeastern quadrant. Here, if
it be observed under a rising sun, a number of these objects will be
seen extending from the region north of the ring-mountain Vitello in
long undulating lines, roughly concentric with the western border
of the “sea,” and gradually diminishing in altitude as they spread
out, with many ramifications, to a distance of 200 miles or more
toward the north. At this stage of illumination they are strikingly
beautiful in a good telescope, reminding one of the ripple-marks left
by the tide on a soft, sandy beach. Like most other objects of their
class, they are very evanescent, gradually disappearing as the sun
rises higher in the lunar firmament, and ultimately leaving nothing
to indicate their presence beyond here and there a ghostly streak or
vein of a somewhat lighter hue than that of the neighboring surface.
The Maria, like almost every other part of the visible surface,
abound in craters of a minute type, which are scattered here
and there without any apparent law or ascertained principle of
arrangement.
Walled-plains, approximating more or less to the circular form,
though frequently deviating considerably from it, are among the
largest inclosures on the moon. They vary from upward of 150 to 160
miles or under in diameter, and are often encircled by a complex
rampart of considerable breadth, rising in some instances to a height
of 12,000 feet or more above the inclosed plain. This rampart is
rarely continuous, but is generally interrupted by gaps, crossed by
transverse valleys and passes and broken by more recent craters and
depressions. As a rule, the area within the circumvallation (usually
termed “the floor”) is only slightly, if at all, lower than the
region outside: it is very generally of a dusky hue, similar to that
of the gray plains of Maria, and, like them, is usually variegated
by the presence of hills, ridges, and craters, and is sometimes
traversed by delicate furrows, termed clefts or rills.
Ptolemæus, in the third quadrant and not far removed from the
centre of the disk, may be taken as a typical example of the class.
Here we have a vast plain, 115 miles from side to side, encircled
by a massive but much broken wall, which at one peak towers more
than 9,000 feet above a level floor, which includes details of a
very remarkable character. The adjoining Alphonsus is another, but
somewhat smaller object of the same type, as are also Albategnius and
Arzachel; and Plato, in a high northern latitude, with its noble,
many-peaked rampart and its variable steel-gray interior, Grimaldi,
near the eastern limb (perhaps the darkest area on the moon),
Schickard, nearly as big on the southeastern limb, and Bailly, larger
than either (still further south in the same quadrant), although they
approach some of the smaller “seas” in size, are placed in the same
category. The conspicuous central mountain, so frequently associated
with other types of ringed inclosures, is by no means invariably
found within the walled-plains; though, as in the case of Petavius,
Langrenus, Gassendi, and several other noteworthy examples, it is
very prominently displayed. The progress of sunrise on all these
objects affords a magnificent spectacle. Very often when the rays
infringe on their apparently level floor at an angle of from 1° to
2°, it is seen to be coarse, rough grained, and covered with minute
elevations, although an hour or so afterward it appears as smooth as
glass.
The more massive and extended mountain ranges of the moon are found
in the Northern Hemisphere, and (what is significant) in that
portion of it which exhibits few indications of other superficial
disturbances. The most prominently developed systems, the Alps, the
Caucasus, and the Apennines, forming a mighty western rampart to the
Mare Imbrium and giving it all the appearance of a vast walled-plain,
present few points of resemblance to any terrestrial chain. The
former include many hundred peaks, among which Mont Blanc rises to a
height of 12,000 feet, and a second, some distance west of Plato, to
nearly as great an altitude; while others ranging from 5,000 to 8,000
feet are common. They extend in a southwest direction from Plato to
the Caucasus, terminating somewhat abruptly, a little west of the
central meridian in about N. lat. 42°. One of the most interesting
features associated with this range is the so-called great Alpine
valley, which cuts through it west of Plato.
The Caucasus consist of a massive wedge-shaped mountain land,
projecting southward, and partially dividing the Mare Imbrium from
the Mare Serenitatis, both of which they flank. Though without peaks
so lofty as those pertaining to the Alps, there is one, immediately
east of the ring-plain Calippus, which, towering to 19,000 feet,
surpasses any of which the latter system can boast. The Apennines,
however, are by far the most magnificent range on the visible
surface, including as they do some 3,000 peaks, and extending in an
almost continuous curve of more than 400 miles in length from Mount
Hadley, on the north, to the fine ring-plain Eratosthenes, which
forms a fitting termination, on the south. The great headland Mount
Hadley rises more than 15,000 feet, while a neighboring promontory on
the southeast of it is fully 14,000 feet, and another, close by, is
still higher above the Mare. Mount Huyghens, again in N. lat. 20°,
and the square-shaped mass Mount Wolf, near the southern end of the
chain, include peaks standing 18,000 and 12,000 feet respectively
above the plain to which their flanks descend with a steep declivity.
The counterscarp of the Apennines, in places 160 miles in width from
east to west, runs down to the Mare Vaporum, with a comparatively
gentle inclination. It is everywhere traversed by winding valleys
of a very intricate type, all trending toward the southwest, and
includes some very bright craters and mountain-rings.
Whether variations in the visibility of lunar details, when observed
under apparently similar conditions, actually occur from time to
time from some unknown cause, is one of those vexed questions which
will only be determined when the moon is systematically studied by
experienced observers using the finest instruments at exceptionally
good stations; but no one who examines existing records of rills by
Gruithuisen, Lohrmann, Mädler, Schmidt, and other observers, can well
avoid the conclusion that the anomalies brought to light therein
point strongly to the probability of the existence of some agency
which occasionally modifies their appearance or entirely conceals
them from view. In short, the more direct telescopic observations
accumulate, and the more the study of minute detail is extended, the
stronger becomes the conviction that, in spite of the absence of an
appreciable atmosphere, there may be something resembling low-lying
exhalations from some parts of the surface which from time to time
are sufficiently dense to obscure, or even obliterate, the region
beneath them.
Sir John Herschel maintained that “the actual illumination of the
lunar surface is not much superior to that of weathered sandstone
rock in full sunshine. I have,” he says, “frequently compared the
moon setting behind the gray perpendicular façade of the Table
Mountain, illumined by the sun just risen in the opposite quarter of
the horizon when it has been scarcely distinguishable in brightness
from the rock in contact with it. The sun and moon being at nearly
equal altitudes, and the atmosphere perfectly free from cloud or
vapor, its effect is alike on both luminaries.” Zöllner’s elaborate
researches on this question are closely in accord with the above
observational result. Though he considers that the brightest parts
of the surface are as white as the whitest objects with which we are
acquainted, yet, taking the reflected light as a whole, he finds that
the moon is more nearly black than white. The most brilliant object
on the surface is the central peak of the ring-plain Aristarchus, the
darkest the floor of Grimaldi, or perhaps a portion of that of the
neighboring Riccioli. Between these extremes there is every gradation
of tone. Proctor, discussing this question on the basis of Zöllner’s
experiments respecting the light reflected by various substances,
concludes that the dark area just mentioned must be notably darker
than the dark gray syenite which figures in his tables, while the
floor of Aristarchus is as white as newly fallen snow.
MARS.—AGNES M. CLERKE
The furthest terrestrial planet from the sun is Mars, the “star of
strength.” No other heavenly body, except the moon, is so well placed
for observation from our position in space.
The diameter of Mars is 4,200 miles; its surface is equal to
two-sevenths, its volume to one-seventh those of the earth. But, in
consequence of its inferior mean density, nine such spheres would go
to make up the mass of our world. The superficial force of gravity
on Mars, compared with its terrestrial value, is as thirty-eight to
a hundred. A man could leap there a wall eight feet four inches in
height with no more effort than it would cost him here to spring over
a two-foot fence.
The planet’s rotation is performed in 24 hours, 37 minutes, on an
axis deviating from the vertical by 24° 50′. Hence its seasons
resemble our own, except in being nearly twice as long, for the
Martian year is of 687 days.
The disk of Mars is diversified with three shades of color—reddish,
or dull orange, dark grayish-green, and pure white. The last shows
mainly in two diametrically opposite patches. Each pole is surrounded
by a brilliant cap, suggesting the deposition of ice or snow over
the chilly spaces corresponding to our arctic and antarctic regions.
Nor is this all. Each of the polar hoods shrinks to a mere remnant
as the local summer advances, but regains its original size when
wintry influences are again in the ascendant. Here, and nowhere
else in the planetary system, we meet evidence of seasonal change;
and seasonal change is associated with vital possibilities. Again,
a globe upon which snow visibly melts must contain water; hence
the green markings can not but image to our minds seas and inlets
subdividing continents, the blond complexion of which may be caused
by some native peculiarity of the soil. It is in no way connected
with vegetation, since it neither fades nor flushes with the advent
of spring; and an atmospheric origin is excluded by the circumstance
that it becomes effaced by a whitish haze near the limb, just where
the densest atmospheric strata are traversed by the line of sight.
The spots on Mars are by no means so sharply defined as lunar craters
and _maria_; yet they are fundamentally permanent. Some can be
recognized from drawings made over two hundred years ago; and these
antique records have served modern astronomers to determine with
minute accuracy the rotation-period of the planet. Continents are
somewhat vaguely outlined. Great tracts of them are of an uncertain
and variable hue, as if subject to inundations. This peculiarity,
thoroughly certified during the favorable opposition of 1892, makes
a strong distinction between Mars and the Earth. Terrestrial oceans
keep within the limits assigned to them. On the neighboring planet—as
M. Faye observed in 1892—“water seems to march about at its ease,”
flooding from time to time regions as wide as France. The imperfect
separation of the two elements recalls the conditions prevailing
during the terrestrial carboniferous era.
The main part of the land of Mars is situated in the Northern
Hemisphere. It covers two-thirds of the entire globular surface.
Rather than land, indeed, it should be called a network of land and
water. The great continental block—so its orange tint declares it to
be—is cut up in all possible directions by an intricate system of
what appear to be waterways, running in perfectly straight lines—that
is, along great circles of the globe—for distances varying from 350
to upward of 4,000 miles. They are frequently seen in duplicate,
strictly parallel companions developing thirty to three hundred miles
apart from the original formations. This mysterious phenomenon is
evanescent, or rather periodical.
The canals invariably connect two bodies of water; hence they need
no locks or hydraulic machinery; their course is on a dead level.
The broadest of them are comparable with the Adriatic; those at the
limit of visibility, stretching like the finest spider-threads across
the disk, have a width of eighteen miles. “The canals,” Schiaparelli
says, “may intersect among themselves at all possible angles, but
by preference they converge toward the small spots to which we have
given the name of lakes. For example, seven are seen to converge in
Lacus Phœnicis, eight in Trivium Charontis, six in Lunæ Lacus, and
six in Ismenius Lacus.”
These “lakes” evidently form an integral part of the canal system.
They resemble huge railway junctions; and the largest of them—the
“Eye of Mars” (Schiaparelli’s Lacus Solis)—seems, in Mr. Lowell’s
phrase, like the hub of a five-spoked wheel. Mr. W. H. Pickering
in 1892, and Mr. Percival Lowell in 1894, were amazed at their
extraordinary abundance.
“Scattered over the orange-ochre groundwork of the continental
regions of the planet,” the latter wrote, “are any number of dark,
round spots. How many there may be it is not possible to state, as
the better the seeing, the more of them there seem to be. In spite,
however, of their great number, there is no instance of one occurring
unconnected with a canal. What is more, there is apparently none
which does not lie at the junction of several canals. Reversely, all
the junctions appear to be provided with spots.”
Most of these foci are about 120 miles in diameter, and appear most
precisely circular when most clearly seen. “Plotted upon a globe,”
Mr. Lowell continues, “they and their connecting canals make a
most curious network over all the orange-ochre equatorial parts of
the planet, a mass of lines and knots, the one marking being as
omnipresent as the other. Indeed, the spots are as peculiar and
distinctive a feature of Mars as the canals themselves.”
Like the canals, too, they emerge periodically, and in the same but
a retarded succession. They “are, therefore, in the first place,
seasonal phenomena, and, in the second place, phenomena that depend
for their existence upon the prior existence of the canals.”
Mr. Lowell terms them “oases,” and does not shrink from the full
implication of the term.
The most important result of the numerous observations of Mars,
made during the oppositions of 1892 and 1894, was the recognition
of a regular course of change dependent upon the succession of
its seasons. Schiaparelli had long anticipated this result; he is
commonly in advance of his time. These changes, moreover, when
closely watched, are really self-explanatory. The alternate melting
of the northern and southern snow-caps initiates and to some extent
determines them. As summer advances in either hemisphere, the wasting
of the corresponding white calotte can be followed in every minute
particular. “The snowy regions are then seen to be successively
notched at their edges; black holes and huge fissures are formed
in their interiors; great isolated fragments many miles in extent
stand out from the principal mass, dissolve, and disappear a little
later. In short, the same divisions and movements of these icy fields
present themselves to us at a glance that occur during the summer of
our own arctic regions.”
Indeed, glaciation on Mars is much less durable than on the earth. In
1894 the southern snow-cap vanished to the last speck 59 days after
the solstice and the remnant usually left looks scarcely enough to
make a comfortable cap for Ben Nevis. An immense quantity of water
is thus set free. The polar seas overflow; gigantic inundations
reinforced, doubtless, from other sources, spread to the tropics;
Syrtis regions of marsh or bog deepen in hue, and become distinctly
aqueous; canals dawn on the sight, and grow into undeniable
realities. We seem driven to believe that they discharge the function
of flood-emissaries.
Mr. Lowell does not hesitate to pronounce them of artificial
formation, and, on that large assumption, the purpose of their
connection with his “oases” becomes transparently clear. They bring
to these Tadmors in the wilderness the water supply by which they
are made to “blossom as the rose.” The junction-spots, we are told,
do not enlarge when the vernal freshet reaches them; they only
darken through the sudden development of vegetation. These circular
“districts, artificially fertilized by the canal system,” are strewn
broadcast over vast desert areas, the orange-ochreous sections of
Mars, covering the greater part of its surface, but deep buried in
the millennial dust of disintegrated red sandstone strata.
“Here, then,” Mr. Lowell remarks, “we have an end and reason for the
existence of canals, and the most natural conceivable—namely, that
the canals are constructed for the express purpose of fertilizing the
oases. When we consider the amazing system of the canal lines, we are
carried to this conclusion as forthright as is the water itself; what
we see being not the canal itself, indeed, but the vegetation along
its banks.”
The proportion of water to land is much smaller on Mars than on
the earth. Only two-sevenths of the disk are covered by the dusky
areas, and of late the aqueous nature of some, if not all, of
these has been seriously called in question. Professor Pickering
was convinced by his observations, in 1892 and 1894, “that the
permanent water area upon Mars, if it exist at all, is extremely
limited in its dimensions.” He estimated it at about half the size
of the Mediterranean. Professor Schaeberle is similarly incredulous.
If the dark markings are seas, he asks, how explain the irregular
gradations of shade in them? How, above all, explain their apparent
intersection by well-marked canals? Professor Barnard, observing
with the Lick thirty-six inch in 1894, discerned on the Martian
surface an astonishing wealth of detail, “so intricate, small, and
abundant, that it baffled all attempts to properly delineate it.”
It was embarrassing to find these minute features belonging more
characteristically to the “seas” than to the “continents.” Under the
best conditions, the dark regions lost all trace of uniformity.
Their appearance resembled that of a mountainous country, broken by
cañon, rift, and ridge, seen from a great elevation. These effects
were especially marked in the “ocean” area of the Hour-Glass Sea.
Evidently the relations of solid and liquid in that remote orb
are abnormal; they can not be completely explained by terrestrial
analogies. Yet a series of well-attested phenomena are intelligible
only on the supposition that Mars is, in some real sense, a
terraqueous globe. Where snows melt there must be water; and the
origin of the Rhone from a great glacier is scarcely more evident to
our senses than the dissolution of Martian ice-caps into pools and
streams.
The testimony of the spectroscope is to the same effect. Dr. Huggins
found, in 1867, the spectrum of Mars impressed with the distinct
traces of aqueous absorption, and the fact, although called in
question by Professor Campbell of Lick, in 1894, has been reaffirmed
both at Tulse Hill and at Potsdam. That clouds form and mists rise
in the thin Martian air, admits of doubt. During the latter half
of October, 1894, an area much larger than Europe remained densely
obscured. Whether or no actual rain was at that time falling over
the Maraldi Sea and the adjacent continent it would be useless to
conjecture. We only know that with the low barometric pressure at the
surface of Mars, the boiling point of water must be proportionately
depressed (Flammarion puts it at 115° Fahrenheit), which implies that
it evaporates rapidly, and can be transported easily.
If the Martian atmosphere be of the same proportionate mass as that
of our earth, it can possess no more than one-seventh its superficial
density. That is to say, it is more than twice as tenuous as the
air at the summits of the Himalayas. The corresponding height of
a terrestrial barometer would be four and a half inches. Owing,
however, to the reduced strength of gravity on Mars, this slender
envelope is exceedingly extensive. In the pure sky scarcely veiled by
it, the sun, diminished to less than half his size at our horizons,
probably exhibits his coronal streamers and prominences as a regular
part of his noontide glory; atmospheric circulation proceeds so
tranquilly as not to trouble the repose of a land “in which it
seemeth always afternoon”; no cyclones traverse its surface, only
mild trade-winds flow toward the equator, to supply for the volumes
of air gently lifted by the power of the sun, to carry reinforcements
of water-vapor north and south. Aerial movements are, in fact, by a
very strong presumption, of the terrestrial type, but executed with
greatly abated vigor.
Brilliant projections above the terminator of Mars were first
distinctly perceived at the Lick Observatory in 1890. They have
been reobserved at Nice, Arequipa, and Flagstaff (Mr. Lowell’s
observatory), coming into view, as a rule, when circumstances concur
to favor their visibility. They strictly resemble lunar peaks and
craters, catching the first rays of the sun, while the ground about
them is still immersed in darkness; and Professor Campbell connects
them with “mountain chains lying _across_ the terminator of the
planet,” and in some cases possibly snow-covered. He calculates their
height at about ten thousand feet. Their presence was unlooked for,
since a flat expanse is a condition _sine quâ non_ for the minute
intersection of land by water, which seems to prevail on Mars.
Although the sun is less than half as powerful on Mars as it is here,
the Martian climate, to outward appearance, compares favorably with
our own. Polar glaciation is less extensive and more evanescent, and
little snow falls outside the arctic and antarctic regions. Yet the
theoretical mean temperature is minus 4° C., or 61° of Fahrenheit
below freezing. This means a tremendous ice-grip. The coldest spot on
the earth’s surface is considerably warmer than this cruel average.
Fortunately, it exists only on paper. Some compensatory store of
warmth must then be possessed by Mars, and it can scarcely be
provided by its attenuated air. Possibly, internal heat may still be
effective, and we see exemplified in Mars the geological period when
vines and magnolias flourished in Greenland, and date-palms ripened
their fruit on the coast of Hampshire.
The climate of Mars, according to Schiaparelli, “must resemble that
of a clear day upon a high mountain. By day a very strong solar
radiation hardly at all mitigated by mist or vapor; by night a
copious radiation from the soil toward celestial space, and hence a
very marked refrigeration; consequently, a climate of extremes, and
great changes of temperature from day to night, and from one season
to another. And as on the earth, at altitudes of from 17,000 to
20,000 feet, the vapor of the atmosphere is condensed only into the
solid form, producing those whitish masses of suspended crystals
which we call cirrus-clouds, so in the atmosphere of Mars it would
be rarely possible to find collections of cloud capable of producing
rain of any consequence. The variation of temperature from one season
to another would be notably increased by their long duration, and
thus we can understand the great freezing and melting of the snow,
renewed in turn at the poles at each complete revolution of the
planet round the sun.”
The German astronomer Mädler searched in 1830 for a Martian
satellite, and although his telescope was of less than four inches
aperture, he satisfied himself that none with a diameter of as much
as twenty-three miles could be in existence. As it happened, he
was right. The pair of moons detected by Professor Asaph Hall with
the Washington twenty-six refractor, August 11 and 17, 1877, are
unquestionably below that limit of size. Neither of them can well
be more than ten miles across. Their names, “Deimos” and “Phobos,”
are taken from the _Iliad_, where Fear and Panic are introduced as
attendants upon the God of War. Deimos revolves in 30 hours and 18
minutes at a distance of 14,600 miles from the centre of Mars. And
since the planet rotates in 24 hours, 37 minutes, the diurnal motion
of the sphere from east to west is so nearly neutralized by the
orbital circulation of the satellite from west to east that nearly
132 hours elapse between its rising and its setting. During the
interval, it changes four times from new to full, and _vice versâ_.
Phobos is more effective in illumination, both because it is
larger and because it is less distant. At the Martian equator, its
brightness is equal to 1/60th that of our moon, but beyond 69° of
latitude it is permanently shut out from view by the curvature of the
globe.
THE PLANETOIDS.—CAMILLE FLAMMARION
On the first day of the last century (January 1, 1801), Piazzi, an
astronomer devoted to the sky, was observing at Palermo the small
stars of the constellation Taurus, and noting their exact positions,
when he remarked one which he had never seen before. The following
evening (January 2) he directed his telescope again toward the
same region of the sky, and remarked that the star was no longer
at the point where he had seen it the day before, and that it had
retrograded by 4′. It continued to retrograde up to the 12th,
stopped, and then moved in the direct way—that is to say, from west
to east. What was this moving star? The idea that it might be a
planet did not immediately occur to the mind of the observer, and he
took it for a comet, as William Herschel had done in 1781, when he
discovered Uranus.
However, the skilful Sicilian observer was a member of an association
which had for its special object the search for an unknown planet
between Mars and Jupiter. From the earliest times of modern astronomy
Kepler had described the disproportion, the void which exists between
the orbit of Mars and that of Jupiter. If we omit, in fact, the
orbit of the small planets or asteroids, we notice that the four
planets, Mercury, Venus, the earth, and Mars, are in some measure
crowded quite close to the sun, while Jupiter, Saturn, Uranus, and
Neptune extend far into immensity. The law of Titius indicates a
number, the number 28, as not being represented by any planet. It
was in 1772 that this _savant_ published this relation in a German
translation which he had made of the _Contemplation de la Nature_
of Charles Bonnet. Bode, Director of the Berlin Observatory, was so
astonished at the coincidence that he announced this arithmetical
relation as being a real law of nature, and spoke of it in such a
way that it is generally known only by his name. He even organized
an association of twenty-four astronomers to explore each hour of
the Zodiac and search for the unknown. This systematic exploration
had not yet produced any result when, by the merest chance, Piazzi
saw his moving star, and at first believed it to be a comet. But on
receipt of the news, Bode was convinced that this was the looked-for
planet.
The new planet was found to be at the distance 2.77, and to revolve
within a few days of the predicted period. Piazzi gave to the new
body the name of _Ceres_, the protecting divinity of Sicily in the
“good old times” of mythology.
The gap being thus filled up at the distance 28 by the discovery of
Ceres, no one thought that other planets might exist there; and if
Piazzi had supposed so, he might have at once discovered a dozen of
the small bodies which revolve in this region. An astronomer of
Bremen, Olbers, observed this planet on the evening of March 28,
1802, when he perceived in the constellation of the Virgin a star of
the seventh magnitude which was not marked on Bode’s chart, which
he used. The following day he found it had changed its place, and
recognized by this fact that it was a second planet. But it was much
more difficult to give citizenship to it than to its elder sister,
because, the gap being filled up, it was not required, and it was
more inconvenient than agreeable. They looked upon it, then, as a
comet until its motion proved that it revolved in the same region as
Ceres at the distance 2.77, and in 1,685 days (the period of Ceres is
1,681 days). They gave it the name of _Pallas_.
The unexpected discoveries of Ceres and Pallas led astronomers to
revise the catalogues of stars and celestial charts. Harding was of
the number of the zealous revisers. He was soon rewarded for his
trouble. On September 1, 1804, at ten o’clock in the evening, he saw
in the constellation of Pisces a star of the eighth magnitude which
was not noted in the _Histoire Céleste_ of Lalande. On September
4, he found it had perceptibly changed its place: it was a new
planet. It received the name of _Juno_. Its distance from the sun
is expressed by the number 2.67, and its revolution is performed in
1,592 days.
After these three discoveries, Olbers, noticing that the orbits of
these planets crossed each other in the constellation of the Virgin,
advanced the hypothesis that they might be nothing else but fragments
of a large shattered planet. Mechanics show that, in this case, the
fragments would again pass every year—that is to say, at each of
their revolutions—through the spot where the catastrophe took place.
Olbers then set himself to explore the constellation Virgo carefully,
and found on March 29, 1807, a fourth small planet, to which he gave
the name of _Vesta_. Its distance is but 2.36, and its revolution
only 1.326 days. This is the brightest of the small planets, and it
is sometimes seen with the naked eye (when we know where it is), like
a star of the sixth magnitude.
It seems surprising that after these brilliant beginnings
thirty-eight years should then have passed without the discovery of a
single planet, for it was only in 1845 that the fifth, _Astræa_, was
discovered by Hencke (who should not be confused with the astronomer
Encke), a simple amateur astronomer, postmaster at Berlin, who amused
himself by constructing charts of the stars. The principal reason
for this must be attributed to the want of good star-charts, for to
find these little moving points the first thing necessary is to have
a very precise chart of the region of the Zodiac which we observe,
in order to see whether one of the stars observed is in motion. The
earliest good Zodiacal charts are those which the Academy of Berlin
commenced to publish in 1830, taking as a basis the zones of Bessel
continued by Argelander. Those of the Paris Observatory, which are
more perfect, were only begun in 1854.
These small planets are all telescopic, invisible to the naked eye,
with the exception of Vesta, and sometimes Ceres, which good sight
can occasionally succeed in distinguishing; they are of the seventh,
eighth, ninth, tenth, and eleventh magnitudes, and even still
smaller, and it was for this reason also that so long an interval
of time elapsed between the fourth and fifth discoveries. It is
probable that all the small planets of any importance are now known,
but that a great number—several hundreds, perhaps—still remain to be
discovered of which the average brightness does not exceed that of
stars of the twelfth magnitude, and of which the diameter is but a
few miles. The diameter of the largest, Vesta, may be estimated at
400 kilometres (248 miles).
Hencke found successively the 5th and the 6th in 1845 and 1847;
Hind, the English astronomer, the 7th and 8th in 1847; Graham, an
English observer, the 9th in 1848; Gasparis, an Italian astronomer,
the 10th and 11th in 1849 and 1850, and afterward seven others. Hind
has further discovered eight others; Goldschmidt, a German painter
(a naturalized Frenchman), discovered fourteen between 1852 and
1861.[27] They are now discovered by swarms; Paliser alone has found
sixty-eight since 1874.
The names given to these small bodies commenced with the mythological
army of divinities of the earth and ancient heaven; but even before
the list had been exhausted certain scientific, or even national
or political, circumstances caused the preference to be given to
more modern names. It was thus that the 11th, discovered at Naples,
received the name of Parthenope; the 12th, discovered in England,
that of Victoria; the 20th, that of Massilia; the 21st, that of
Lutetia; the 25th, that of Phocæa, before even Urania had been
restored to the skies; the 45th was named in honor of the Empress
of the French; the 54th, in honor of the illustrious Alexander von
Humboldt; etc. The 87th, 107th, 141st, 154th, and 169th have been
named in honor of a young astronomer who has devoted his best years
to the culture of astronomy.
A rather curious fact is that they have put Wisdom (_Sapientia_)
in the sky only at the 275th, discovered in 1888; Bellona has been
placed there since the 28th (1854).
Of all this number of planets, the nearest to the sun is No. 149,
Medusa, of which the distance is 2.17—that is to say, about twice
as far from the sun as the earth; and the most distant is No. 279,
Thule, of which the distance is 4.26, about 4¼ times our distance.
A large number of these small bodies are remarkable for their great
eccentricity and for their high inclination to the ecliptic, an
inclination so great that some of them leave the Zodiac; thus, Pallas
(2) goes 34 degrees from the ecliptic; Euphrosyne (31) and Anna
(265) and Istria (183), to 26 degrees. They are sometimes northern
circumpolar stars, always above the horizon, sometimes southern
stars, not arising above the horizon of Paris. All these orbits are
so interlaced with each other that, if they were material hoops, we
could by means of one or two taken by chance raise all the others.
Are they globes? Yes, doubtless, for the most part. But several among
the smaller ones may be polyhedral, and may have proceeded from
subsequent explosions; the variations of brightness which have been
sometimes observed seem to imply surfaces irregularly broken.
Are they _worlds_? Why not? Is not a drop of water shown in the
microscope peopled with a multitude of various beings? Does not a
stone in a meadow hide a world of swarming insects? Is not the leaf
of a plant a world for the species which inhabit and prey upon it?
Doubtless among the multitude of small planets there are those which
must remain desert and sterile because the conditions of life (of
any kind) are not found united. But we can not doubt that on the
majority the ever-active forces of nature have produced, as in our
world, creations appropriate to these minute planets. Let us repeat,
moreover, that for nature there is neither great nor little. And
there is no necessity to flatter ourselves with a supreme disdain for
these little worlds, for in reality the inhabitants of Jupiter would
have more right to despise us than we have to despise Vesta, Ceres,
Pallas, or Juno: the disparity is greater between Jupiter and the
earth than between the earth and these planets.
FOOTNOTES:
[27] Goldschmidt passionately loved astronomy, and I have found
among his papers, which his family left me, numerous observations
and remarks which show how he loved the study of the sky. His
greatest ambition had been, at first, to possess a small telescope,
in order to make some observations, and the best day of his life
was that on which he found one in the possession of a dealer in old
stores. He hastened to direct it to the sky from his modest studio,
situated in one of the most frequented streets of Paris (Rue de
l’Ancienne-Comédie), above the Café Procope, formerly used as a
rendezvous by the stars of literature. There, _from his window_, he
discovered, in 1852, the 21st small planet, which received from Arago
the name of Lutetia; then, in 1854, the 32d (Pomona); then, in 1855,
the 36th (Atlanta); and afterward eleven others, all from his window.
Having often removed in search of a pure atmosphere, he finally
retired to Fontainebleau, where the forest offered him on all sides
admirable subjects for painting; and here he died in 1866.
JUPITER.—AGNES M. CLERKE
Jupiter is by far the most important member of the solar family.
The aggregate mass of all the other planets is only two-fifths of
his, which 316 earths would be needed to counterbalance. His size is
on a still more colossal scale than his weight, since in volume he
exceeds our globe 1,380 times. His polar and equatorial diameters
measure respectively 84,570 and 90,190 miles, giving a mean diameter
of 88,250 miles, and a polar compression of 1/16th. The corresponding
equatorial protuberance rises to 2,000 miles, so that the elliptical
figure of the planet strikes an observer at the first glance. This
at once indicates rapid axial movement; and Jupiter’s rotation is
accordingly performed in nine hours and fifty-five minutes, with an
uncertainty of a couple of minutes.
The numbers just given imply that this great planet is of somewhat
slight consistence, and its mean density is, in fact, a little less
than that of the sun. The sun is heavier than an equal bulk of water
in the proportion of 1.4 to 1, Jupiter in the proportion of 1.33 to
1. The earth is thus more than four times specifically heavier than
the latter globe. Three Jupiters would keep in equipoise four equal
globes of water, while the earth would turn the scale against five
and a half aqueous models of itself. This low density, an unfailing
characteristic of all the giant planets, is charged with meaning.
It at once gives us to understand that, in crossing the zone of
asteroids, we enter upon a different planetary region from that left
behind. The bodies revolving there are on an immensely larger scale
of magnitude than those on the hither side; they are of solar, rather
than terrestrial, density; they rotate much more rapidly, and are
in consequence of a more elliptical shape; they display, and most
likely possess, no solid surface; they are attended by retinues of
satellites.
Jupiter circulates round the sun in 11.86 years, in an orbit
deviating by less than one and a half degrees from the plane of the
ecliptic, but of thrice the eccentricity of the ellipse traced out
by the earth. With a mean distance from the sun of 483 millions
of miles, it accordingly approaches within 462 at perihelion, and
withdraws to 504 millions of miles at aphelion. Seasons it has none
worth mentioning; nor could they be of much effect even if they were
better marked.
Under propitious circumstances Jupiter comes within 369 million miles
of the earth. These occur when he is in opposition nearly at the
epoch of his perihelion passage. His maximum opposition distance, on
the other hand, is 411 million miles. He is then at aphelion. Thus,
at the most favorable opposition, he is 42 million miles nearer to us
than at the least favorable. The effect on his brightness is evident
to the eye. When his midnight culmination takes place in October, he
in fact sends us one and a half times more light than when the event
comes round to April. We need only recall the unusual splendor of his
appearance in September and October, 1892, when his lustre was double
that of Sirius. His opposition period, as we may call it, is 399 days.
The intrinsic brilliancy of his surfaces is surprising, especially
when we consider that it is somewhat deeply tinged with color.
The minimum diameter of the visible disk considerably exceeds the
maximum of that of Mars. Even with a low power it thus makes a
beautiful and interesting telescopic object. Its distinctive aspect
is that of a belted planet, the belts varying greatly in number and
arrangement. As many as thirty have, on occasions, been counted,
delicately ruling the disk from pole to pole. They are always
parallel to the equator, but are otherwise highly changeable, and
can not be too closely studied as an index to the planet’s physical
constitution. Two in particular are remarkable. They are called the
north and south equatorial belts, and inclose a lustrous equatorial
zone. The poles are shaded by dusky hoods.
This general scheme of markings, however, when viewed with one of the
great telescopes of the world, is so overlaid with minor particulars
as sometimes to be scarcely recognizable. One can not see the wood
for the trees. Lovely color-effects, too, come out under the best
circumstances of definition and aerial transparency. The tropical
belts may be summarily described as red; but they are of complex
structure, and their subordinate features and formations are marked
out, under the sway of alternating and tumultuous activities, by
strips and patches of vermilion, pink, purple, drab, and brown. The
intermediate space is divided into two bands by a line, or narrow
ribbon, pretty nearly coinciding with the equator, and rosy or
vivid scarlet in hue. The polar caps are sometimes of a delicate
wine-color, sometimes pale gray.
Professor Keeler made an elaborate study of the planet with the
Lick 36-inch in 1889, and executed a series of valuable drawings.
With a power of 320, the disk, he tells us, “was a most beautiful
object, covered with a wealth of detail which could not possibly be
accurately represented in a drawing.” Most of the surface was then
“mottled with flocculent and irregular cloud-masses. The edges of the
equatorial zone were brilliantly white, and were formed of rounded,
cloud-like masses, which, at certain places, extended into the red
belt as long streamers. These formed the most remarkable and curious
feature of the equatorial regions. They are the cause of the double
or triple aspect which the red belts present in small telescopes.”
Near their starting-points the streamers were white and sharply
defined, but became gradually diffused over the ruddy surface of
the belts. When at all elongated, they invariably flowed backward
_against_ the rotational drift, and were inferred to be cloud-like
masses expelled from the equatorial region, and progressively left
behind by its advance. This hypothesis was confirmed by the motion
of some bright points, or knots, on the streamers. “The portions of
the equatorial zone surrounding the roots of well-marked streamers
were somewhat brighter,” Professor Keeler continues, “than at other
places, and it is a curious circumstance that they were almost
invariably suffused with a pale olive-green color, which seemed to be
associated with great disturbance, and was rarely seen elsewhere.”
Now, if the material of the streamers had been simply a superficial
overflow, it should have carried with it into higher latitudes an
excess of linear rotational speed, and should hence have pushed its
way onward as it proceeded north and south. But, instead, it fell
behind; its velocity was less, not greater, than that of the belts
with which it eventually became incorporated. What are we to gather
from this fact? Evidently that the currents issuing north and south
were of eruptive origin. Their motion, in miles per second, was slow,
because they belonged to profound strata of the planet’s interior.
Their backward drift measured the depth from which they had been
flung upward.
The spots, red, white, and black, constantly visible on the Jovian
surface, excite the highest curiosity. They are of all kinds and
qualities, and their histories and adventures are as diverse as they
are in themselves. Some are quite evanescent; others last for years.
At times they come in undistinguished crowds, like flocks of sheep,
then a solitary spot will acquire notoriety on its own account. White
spots appear in both ways; black spots more often in communities; and
it is remarkable that the former frequent distinctively, though not
exclusively, the Southern, the latter the Northern Hemisphere. Red
spots, too, develop pretty freely; but the attention due to them has
been mainly observed by one striking specimen.
The Great Red Spot has been present with us for at least nineteen
years; and it is a moot point whether its beginnings were not watched
by Cassini more than two centuries ago. Its modern conspicuousness,
however, dates from 1878. Then of a full brick-red hue, and strongly
marked contour, it measured 30,000 by nearly 7,000 miles, and might
easily have inclosed three such bodies as the earth. It has since
faded several times to the verge of extinction, and partially
recovered; but there has never been a time when it ceased to dominate
the planet’s surface-configuration. More than once it has been
replaced by a bare elliptical outline, as if through an effusion
of white matter into a mold previously filled with red matter; and
just such a sketch was observed by Gledhill in 1870. The red spot
is attached, on the polar side, to the southern equatorial belt.
It might almost be described as jammed down upon it; for a huge
gulf, bounded at one end by a jutting promontory, appears as if
scooped out of the chocolate-colored material of the belt to make
room for it. Absolute contact, nevertheless, seems impossible. The
spot is surrounded by a shining aureola, which seemingly defends it
against encroachments, and acts as a _chevaux-de-frise_ to preserve
its integrity. The formation thus constituted behaves like an
irremovable obstacle in a strong current. The belt-stuff encounters
its resistance, and rears itself up into a promontory or “shoulder,”
testifying to the solid presence of the spot, even though it be
temporarily submerged. The great red spot, the white aureola, and the
brownish shoulder are indissolubly connected.
The spot is then no mere cloudy condensation. Yet it has no real
fixity. Its period of rotation is inconstant. In 1870-80, it was
of 9 hours, 55 minutes, 34 seconds; in 1885-86, it was longer by 7
seconds. The object had retrograded at a rate corresponding to one
complete circuit of Jupiter in six years, or of the earth in seven
months. It is not then fast moored, but floats at the mercy of the
currents and breezes predominant in the strange region it navigates.
A quiescent condition is implied by the approximate constancy of its
rotation-period during the last ten years. With the paling of its
color, its “proper motion” slackens or ceases. This must mean that,
at its maxima of agitation, it is the scene of uprushes from great
depths, which, bringing with them a slower linear velocity, occasion
the observed laggings. It is not self-luminous, and shows no symptom
of being depressed below the general level of the Jovian surface.
Jupiter has no certain and single period of rotation. Nearly all
the spots that from time to time come into view on its disk are in
relative motion, and thus give only individual results. The great
red spot has the slowest drift of all (with the rarest exceptions),
while the black cohorts of the Northern Hemisphere outmarch all
competitors. Mr. Stanley Williams, as the upshot of long study, has
delimitated nine atmospheric surfaces with definite periods. They
are well marked, and evidently have some degree of permanence, yet
the velocities severally belonging to them are distributed with
extreme irregularity. Thus, two narrow, adjacent zones differ in
movement by 400 miles an hour. This state of things must obviously
be maintained by some constantly acting force, since friction, if
unchecked, would very quickly abolish such enormous discrepancies.
The rotational zones are unsymmetrically placed; there is no
correspondence between those north and south of the Jovian equator;
and, although the equatorial drift is quicker than that of either
tropic, it is outdone in 20° to 24° north latitude.
Jupiter’s equatorial rotation, as indicated by observations of
spots, is accomplished in 9 hours 50 minutes; but Bélopolsky and
Deslandres’s spectrographic determinations gave rates of approach and
recession falling somewhat short of the corresponding velocity.
[Illustration: Three Views of Saturn
Showing Varying Aspects of the Ring taken at Different Intervals: 1,
Feb. 2, 1862; 2, Nov. 3, 1858; 3, March 23, 1856]
However this be, the rotation of the great planet, albeit
ill-regulated (if the expression be permissible), is distinctly
of the solar type. It is itself a “semi-sun,” showing no trace of
a solid surface, but a continual succession of cloud-like masses
belched forth from within. Jupiter’s low mean density, considered
apart from every other circumstance, suffices to demonstrate the
primitive nature of his state. In a sun-like body, the circulation is
bodily and vertical. That the processes going on in Jupiter are of
this kind is beyond question. Exchanges of hot and colder substances
are effected, not by surface-flows, but by up and down rushes. The
parallelism of his belts to his equator makes this visible to the
eye. An occasional oblique streak betokens a current in latitude, but
it is exceptional, and might be called out of character.
Jupiter’s true atmosphere encompasses the disturbed shell of vapors
observed telescopically. Its general absorptive action upon light
is betrayed by the darkening of the planet’s limb—another point of
resemblance to the sun; while its special, or selective, absorption
can only be detected with the spectroscope.
The actinic power of Jupiter’s light is very remarkable. It surpasses
that of moonlight nine times, and that of Mars twenty-four times. Dr.
Lohse further ascertained that the Southern Hemisphere is twice as
chemically effective as the Northern. This superiority is doubtless
connected with the greater physical agitation of the same region. A
series of photographs of Jupiter, taken in 1891 with the great Lick
refractor, were the first of any value for purposes of investigation.
Jupiter’s satellites were the first trophies of telescopic
observation. They are, indeed, bright enough for naked-eye
perception, could they be removed from the disk which obscures
them with its excessive splendor; and the first and third have
actually been seen, in despite of the glare, by a few persons with
phenomenally good eyesight. The mythological titles of the Galilean
group—Io, Europa, Ganymede, and Calypso (proceeding from within
outward)—have been superseded by prosaic numbers.
The Jovian family presents an animated and attractive spectacle. The
smallest of its original members (No. II) is almost exactly the size
of our moon; the largest (No. III), with its diameter of 3,550 miles,
considerably exceeds the modest proportions of Mercury. Satellite I
revolves in 42½ hours at the same average distance from Jupiter’s
surface that our moon does from that of the earth. No. II has a
period of 3 days 13 hours, and its distance from Jupiter’s centre is
415,000 miles. Both these orbits are sensibly circular; and Nos. III
and IV travel in ellipses of very small eccentricity, the one at a
mean distance of 664,000, the other at 1,167,000 miles, in periods
respectively of 7 days 4 hours, and 16 days 16½ hours. All four
revolve strictly in the plane of Jupiter’s equator.
They constitute a system bound together by peculiar dynamical
relations, in consequence of which they can never be all either
eclipsed or seen aligned at one side of their primary at the same
time. They can all, however, be simultaneously hidden behind it, or
in its shadow; although this moonless condition is looked out for as
a telescopic rarity.
The transits of the satellites across the Jovian disk present many
curious appearances, due to complicated and changeable effects of
light and shade both upon the planetary background and upon the
little circular objects self-compared with it. These, in the ordinary
course, show bright while near the dusky limb, then vanish during the
central passage, and re-emerge again bright at the opposite side.
But instead of duly vanishing, they now and then darken even to the
point of becoming indistinguishable from their own shadows, by which
they are preceded or followed. This difference of behavior can not
be attributed wholly to varieties of lustre in the sections of the
disk transited; otherwise it could be predicted. But this has never
been attempted; “black transits” come when least expected. The third
and fourth satellites are those chiefly subject to these phases; the
second has never been known to exhibit them; and they but slightly
affect the first. Indeed, all the satellites, except, perhaps, No.
II, are striped or spotted; and this leads to seeming deformations
in their shape, as well as fluctuations in their brightness, the
markings being evidently of atmospheric origin, and hence changeable.
Their distinct and accurate perception has been made possible by the
excellence of the Lick 36-inch refractor.
Jupiter’s moons seem to resemble him in constitution. The first three
possess the same high reflective power. No. II is as bright as the
planet’s brightest parts, so that its albedo can not fall short of
0.70. And even No. IV (formerly designated “Calypso” in reference
to its frequent obscurations) exactly matches, during its darkest
phases, the blue-gray polar hoods of its primary. On an average, too,
the satellites seem to be of about the same mean density as Jupiter,
No. I being considerably the lightest for its bulk; and their
spectra, according to Vogel’s observations in 1873, are composed of
solar rays modified in precisely the same way as those reflected by
the planet.
The discovery, September 9, 1892, of Jupiter’s “fifth satellite”
was one of the keenest astronomical surprises on record. Professor
Barnard seized the opportunity, lent by the specially favorable
opposition of 1892, to rummage the system for novelties. Keeping
the telescopic field dark by means of a metallic bar placed so as
to occult the gorgeous planetary round, he sought, night after
night, for what might appear. At length, on September 9, he caught
the glimmer he wanted, and made sure, September 10, that it truly
intimated the presence of a new satellite.
This small body revolves in a period of 11 hours, 57 minutes, 23
seconds at a mean distance of 112,160 miles from Jupiter’s centre, or
67,000 from his bulged equatorial surface. Hence, it should by right
be called “No. I” instead of “No. V.” The major axis of the ellipse
in which it circulates advances so rapidly, owing to the disturbance
caused by Jupiter’s spheroidal figure, as to complete a revolution in
five months. The implied eccentricity of its orbit, as M. Tisserand
has shown, very slightly exceeds that of the orbit of Venus, yet it
has been made obvious by Barnard’s observations of the differences
between its east and west elongations. Its orbital velocity of 16½
miles a second far surpasses that of any other satellite in the
solar system. Close vicinity to a mass so vast as Jupiter’s demands
counterbalancing swiftness. Its period of revolution being, however,
longer by one hour than Jupiter’s period of rotation, it so far
conducts itself normally as to rise in the east and set in the west.
On the other hand, since its progress over the sphere is measured by
the difference between the two periods, it spends five Jovian days in
journeying from one horizon to the other, running, in the meantime,
four times through all its phases. Yet it never appears full.
Jupiter’s voluminous shadow cuts off sunlight from it during nearly
one-fifth of each circuit.
SATURN.—AGNES M. CLERKE
Nearly twice as far from the sun as Jupiter revolves a planet,
the spacious orbit of which was, until 1781, supposed to mark the
uttermost boundary of the Solar System. The mean radius of that orbit
is 886 millions of miles; but in consequence of its eccentricity,
the sun is displaced from its middle point to the extent of 50
million miles, and Saturn is accordingly 100 million miles nearer
to him at perihelion than at aphelion. The immense round assigned
to the “saturnine” planet is traversed in 29½ years, at the tardy
pace of six miles a second. His seasons are thus twenty-nine times
more protracted than ours, and are nominally more accentuated,
since his axis of rotation deviates from the vertical by 27°. But
solar heat, however distributed, plays an insignificant part in his
internal economy. In the first place, its amount is only 1/91st its
amount on the earth; in the second, Saturn, like Jupiter—even more
than Jupiter—is thermally self-supporting. The bulk of his globe
comparatively to its mass suffices in itself to make this certain.
The mean diameter of Saturn is 71,000 miles, or nine times (very
nearly) that of the earth; if of equal density, its mass should then
be nine cubed, or 729 times the same unit. The actual proportion,
however, is 95; hence the planet has a mean density of only
95/729th, or between 1/7th and 1/8th the terrestrial, and being thus
composed of matter as light as cork, would float in water. Professor
G. H. Darwin has, moreover, demonstrated, from the movements of its
largest satellite, that its density gains markedly with descent into
the interior, so that its surface-materials must be lighter than any
known solid or liquid.
When at its nearest to the earth, Saturn is as large as a sixpence
held up at a distance of 210 yards. But instead of being round like
a sixpence, it is strongly compressed—more compressed even than
Jupiter. The spectra of the two planets are almost identical. Both
are impressed with traces of aqueous absorption, and include the “red
star line.”
Saturn resembles to the eye a large, dull star; its rays are entirely
devoid of the sparkling quality which distinguishes those of Jupiter.
But it shows telescopically an analogous surface-structure. Its most
conspicuous markings are tropical dark belts of a grayish or greenish
hue; the equatorial region is light yellow, diversified by vague
white spots; while the poles carry extensive pale blue canopies. The
apparent tranquillity of the disk may be attributed in part to the
vast distance from which it is viewed; yet not wholly.
From measures executed by Barnard in 1895, it appears that the
equatorial diameter of Saturn is 76,470, its polar diameter 69,770
miles, giving a mean diameter of 74,240, and a compression of about
1/12th. Gravity, at its surface, is only 1/5th more powerful than on
the earth.
Thus, Saturn not only belongs to the same celestial species as
Jupiter, but is a closely related individual of that species. There
is no probability that either is to any extent solid. Both exhibit
the same type of markings; both betray internal tumults by eruptions
of spots which, by their varying movements, supply a measure for the
profundity of their origin; both possess identically constituted
atmospheres, and are darkened marginally by atmospheric absorption.
Saturn is, however, distinguished by the possession of a unique set
of appendages. Nothing like them is to be seen elsewhere in the
heavens; and when well opened they form, with the globe they inclose,
and the retinue of satellites in waiting outside, a strange and
wonderful telescopic object. The rings, since they lie in the plane
of Saturn’s equator, are inclined 27° to the Saturnian orbit, and
28° to the ecliptic. The earth is, however, comparatively to Saturn,
so near the sun, that their variations in aspect, as viewed from it,
may in a rough way be considered the same as if seen from the sun.
They correspond exactly with the Saturnian seasons. At the Saturnian
equinoxes, the rings are illuminated edgewise, and disappear, totally
or approximately; at the Saturnian solstices, sunlight strikes them
nearly at the full angle of 27°, first from _below_, then from
_above_. At these epochs, we perceive the appendage expanded into
an ellipse about half as wide as it is long. Two concentric rings
(generally called A and B) are then very plainly distinguishable, the
inner being the brighter. The black fissure which separates them is
called “Cassini’s division,” because that eminent observer was, in
1675, the first to perceive it. A chasm known as “Encke’s division,”
in the outer ring (A), is a thinning-out rather than an empty space;
and temporary gaps frequently appear in A, while B is entirely
exempt from them. There are then two definite and permanent bright
rings, and no more; but with them is associated the dusky formation
discovered by W. C. Bond, November 15, 1850, and described by Lassell
as “something like a crape veil covering a part of the sky within
the inner ring.” It is semi-transparent, the limb of Saturn showing
distinctly through it.
The exterior diameter of the ring-system is 172,800, while its
breadth is 42,300 miles. The rings A and C are each 11,000 miles
wide; while B measures 18,000, Cassini’s division 2,270, and the
clear interval between C and the planetary surface somewhat less than
6,000 miles. Each ring, C included, is brightest at its outer edge;
but there is no gap between the shining and the dusky structures,
B shading by insensible gradations up to C, yet maintaining
distinctness from it. The earliest exact determinations of the former
were made by Bradley in 1719, since when they have been affected by
no appreciable change. The theoretically inevitable subversion of the
system is progressing with extreme slowness.
The thickness of the rings is quite inconsiderable. They are flat
sheets, without (so to speak) a third dimension. For this reason,
they disappear utterly in most telescopes, when their plane passes
through the earth, as it does twice in each Saturnian year. Only
under exceptional conditions, a narrow, knotted, often nebulous,
streak survives as an index to their whereabouts. On October 26,
1891, Professor Barnard, armed with the Lick refractor, found it
impossible to see them projected upon the sky, notwithstanding that
their shadow lay heavily on the planet. It was not until three
days later that “slender threads of light” came into view. The
corresponding thickness of the formation was estimated at less than
fifty miles. The phenomenon of ring disappearance will not recur
until July 29, 1907.
The constitution of this marvelous structure is no longer doubtful.
It represents what might be called the fixed form of a revolving
multitude of diminutive bodies. This was demonstrated by Clerk
Maxwell in the Adams Prize Essay of 1857. His conclusion proved
irreversible. The pulverulent composition of Saturn’s rings is one
of the acquired truths of science. An incalculable number of tiny
satellites revolving independently in distinct orbits, in the precise
periods prescribed by their several distances from the planet, are
aggregated into the unmatched appendages of Galileo’s _tergeminus
planeta_. The local differences in their brightness depend upon the
distribution of the component satelloids. Where they are closely
packed, as in the outer margins of rings A and B, sunlight is
copiously reflected; where the interspaces are wide, the blackness
of the sky is barely veiled by the scanty rays thrown back from the
thinly scattered cosmic dust. The appearance of the crape ring as
a _dark_ stripe on the planet results—as M. Seeliger has pointed
out—not from the transits of the objects themselves, but from the
flitting of their shadows in continual procession across the disk.
The albedo of these particles is so high as to render it improbable
that they are of an earthy or rocky nature, such as the meteorites
which penetrate our atmosphere. The rings they form are, on the
whole, more lustrous than Saturn’s globe; but this superiority is
held to be due to the absence of atmospheric absorption. Their
spectrum is that of unmodified sunlight.
An eclipse of Japetus, the eighth Saturnian moon, by the globe and
rings, November 1, 1889, was highly instructive as to the nature of
the dusky appendage. The satellite was never lost sight of during
its passage behind it; but became more and more deeply obscured as
it traveled outward; then, at the moment of ingress into the shadow
of ring B, suddenly disappeared. Certainty was thus acquired that
the particles forming the crape ring are most sparsely strewn at its
inner edge—which is, nevertheless, perfectly definite—and gradually
reach a maximum of density at its outer edge. Yet, while there is not
the smallest clear interval, a sharp line of demarcation separates
it from the contiguous bright ring. Professor Barnard was the only
observer of these curious appearances. The distribution of the
ring-constituents, like that of the asteroids, was governed by the
law of commensurable periods, Saturn’s moons replacing Jupiter as the
perturbing and regulating power.
The “satellite-theory” of Saturn’s rings has received confirmation
from apparently the least promising quarters. Professor Seeliger
of Munich showed, from photometric experiments in 1888, that their
constant lustre under angles of illumination ranging from 0° to
30° was proof positive of their composition out of discrete small
bodies. And Professor Keeler of Alleghany, by a beautiful and refined
application of the spectroscopic method, arrived at the same result
in April, 1895. “Under the two different hypotheses,” he remarked,
“that the ring is a rigid body, and that it is a swarm of satellites,
the relative motion of its parts would be essentially different.”
The former would necessarily involve increasing velocity _outward_,
the latter, increase of velocity _inward_, just for the same reason
that Mercury moves more swiftly than the earth, and the earth than
Saturn; while the sections of a solid body, which could have but one
period of rotation, should move faster, _in miles per second_, the
further they were from the centre of attraction. The line of sight
test is then theoretically available; but it was an arduous task to
render it practically so. The difficulties were, however, one by one
overcome; and a successful photograph of the spectra of Saturn and
its rings gave the required information in unmistakable shape. From
measurements of the inclinations of five dusky rays contained in it
with reference to a standard horizontal line, rates of movement were
derived of 12½ miles per second for the inner edge of ring B, and
of 10 miles for the outer edge of ring A. The agreement with theory
was, as nearly as possible, exact; the components of the rings were
experimentally demonstrated to be moving, each independently of every
other, under the dominion of Kepler’s laws.
For the globe of Saturn, Professor Keeler obtained, by the same
exquisite method, a rotational period of 10 hours, 14 minutes, 24
seconds, in precise accordance with that indicated by the white
spot of 1876, which thus seems to have had no proper motion, but to
have floated on the ochreous equatorial surface as tranquilly as a
water-lily upon a stagnant pool. The result, so far as it goes, hints
that Saturn may be really, as well as apparently, less ebullient than
Jupiter.
Seers into the future of the heavenly bodies consider that the rings
of Saturn, like the gills of a tadpole, are symptomatic of an early
stage of development; and will be disposed of before he arrives
at maturity. They can not be regarded otherwise than as abnormal
excrescences. No other planet retains matter circulating round it in
such close relative vicinity. It was proved by Roche of Montpellier
that no secondary body of importance can exist within less than 2.44
mean radii of its primary; inside of that limit it would be rent
asunder by tidal strain. But the entire ring-system lies within the
assigned boundary; hence, being _where_ it is, it can only exist _as_
it is—in flights of discrete particles. Will it, however, always
remain where it is?
“Clerk Maxwell,” wrote Mr. Cowper Ranyard, “used to describe the
matter of the rings as a shower of brickbats, among which there would
inevitably be continual collisions. The theoretical results of such
impacts would be a spreading of the ring both inward and outward.
The outward spreading will in time carry the meteorites beyond
Roche’s limit, where, in all probability, they will, as Professor
Darwin suggests, slowly aggregate, and a minute satellite will be
formed. The inward spreading will in time carry the meteorites at
the inner edge of the ring into the atmosphere of the planet, where
they will become incandescent, and disappear as meteorites do in our
atmosphere.”
Yet it may be that collisions are infrequent in this conglomeration
of “brickbats.” There is the strongest presumption that they
all circulate in the same direction, in orbits nearly circular,
and scarcely deviating from the plane of the Saturnian equator.
Those pursuing markedly eccentric tracks must long ago have been
eliminated. Thus, encounters can only occur through gravitational
disturbances by Saturn’s moons, and they must be of a mild character,
depending upon very small differences of velocity. The first sign of
a “spreading outward” should be the formation of an exterior “crape
ring,” of which no faintest trace has yet been perceived.
Saturn’s rings are entirely invisible from its polar regions, but
occasion prolonged and complex eclipse-effects in its temperate
and equatorial zones. They have been fully treated of from the
geometrical point of view by Mr. Proctor in _Saturn and its System_.
Of this planet’s eight satellites,[28] the largest, Titan (No.
VI), was discovered first (by Huygens in 1655), and the smallest,
Hyperion (No. VII), last (by Lassell and Bond in 1848). The five
others were detected by J. D. Cassini and William Herschel. Titan,
alone of the entire group, equals our moon in size. It measures,
according to Professor Barnard, 2,720 miles across. Its period of
revolution is nearly sixteen days, its distance from Saturn’s centre,
771,000 miles. The orbit of Japetus (No. VIII) is the largest, and
its period the longest of any secondary body in the Solar System.
It circulates in 79⅓ days at a distance of 2,225,000 miles, equal
to 59½ of Saturn’s equatorial radii. Hence its path is of about
the same _proportional_ dimensions as that of our moon. Japetus is
remarkable for its variability in light. It is capable of tripling
or quadrupling its minimum lustre. Sir William Herschel noticed that
these maxima coincided with a position on the western side of the
planet, and inferred rotation of the lunar kind. “From the changes
in this body,” he argued in 1792, “we may conclude that some part of
its surface, and this by far the largest, reflects much less light
than the rest; and that neither the darkest nor the brightest side is
turned toward the planet, but partly one and partly the other, though
probably less of the bright side.”
This explanation, however, he admitted to be incomplete. There
was, and is, outstanding variability, which seems to intimate the
presence of an atmosphere and the formation of clouds. But no
positive knowledge has yet been gained regarding the physical state
of Saturn’s moons. We may, nevertheless, conjecture that, since
tidal friction has destroyed the rotation (as regards Saturn) of the
remotest member of the family, it has not spared those more exposed
to its grinding-down action. All presumably rotate in the same time
that they revolve.
The five inner satellites move in approximately circular orbits; the
three outer in ellipses about twice as eccentric as the terrestrial
path. All, Japetus only excepted, keep strictly to the plane of the
rings. And since this makes an angle of 27° with the planet’s orbit,
eclipses are much less frequent here than in the Jovian system. They
can only occur when Saturn is within a certain distance (different
for each) from the node of the satellite-orbit. Even Mimas (No. I),
although it wheels round the ring at an interval of only 34,000
miles, often slips outside the obliquely projected shadow-cone. Its
distance from Saturn’s centre is 118,000 miles, and it completes
a circuit in 22½ hours. Perpetually wrapped in the glare of its
magnificent primary, it is a very shy object, only to be caught
sight of in its timid excursions by the very finest telescopes. Like
all the Saturnian moons, except Titan, and, by a rare conjunction,
Japetus, it is far too much contracted to be visible in transit
across the disk.
The movements of these bodies have been carefully studied, and their
mutual perturbations to some extent unraveled. They have proved
exceedingly interesting to students of celestial mechanics. Titan
has, in this department, chiefly to be reckoned with. He exercises in
the Saturnian system a similar overpowering influence to that wielded
by Jupiter in the Solar System.
FOOTNOTES:
[28] A ninth satellite, Phœbe, was discovered in 1904. Its
existence had been suspected for many years, and it was discovered
at the Arequipa Observatory, Peru, on March 14, 1899, by means of
photography. Since that date, it has been several times lost and
rediscovered.—E. S.
URANUS AND NEPTUNE.—WILLIAM F. DENNING
While Sir W. Herschel was a musician at Bath he formed the design of
making a telescopic survey of the heavens. While engaged in this, he
accidentally effected a discovery of great importance, for on the
night of March 13, 1781, an object entered the field of his 6.3-inch
reflector which ultimately proved to be a new major planet of our
system.
The acute eye of Herschel, directly it alighted upon the strange
body, recognized it as one of unusual character, for it had a
perceptible disk, and could be neither fixed star nor nebula. He
afterward found the object to be in motion, and its appearance being
“hazy and ill-defined,” with very high powers, he was led to regard
it as a comet, and communicated his discovery to the Royal Society at
its meeting on April 26, 1781.
The supposed comet soon came under the observation of others,
including Maskelyne, the Astronomer Royal, and Messier, the “Comet
Ferret,” of Paris. The latter, in a letter to Herschel, said:
“Nothing was more difficult than to catch it, and I can not conceive
how you could have hit this star or comet several times, for it
was absolutely necessary for me to observe it for several days in
succession before I could perceive that it was in motion.”
As observations began to accumulate, it was seen that a parabolic
orbit failed to accommodate them. Ultimately the secret was revealed.
The only orbit to represent the motion of the new body was found
to be an approximately circular one situated far outside the path
of Saturn, and the inference became irresistible that the supposed
“comet” must in reality be a new primary planet revolving on the
outskirts of the Solar System. This conclusion was justified by
facts of a convincing nature, and its announcement created no small
excitement in the scientific world. Every telescope was directed to
that part of the firmament which contained the new orb, and its pale
blue disk, wrapped in tiny proportions, was viewed again and again
with all the delight that so great a novelty could inspire. From the
earliest period of ancient history, no discovery of the same kind had
been effected. The Chaldeans were acquainted with five major planets,
in addition to the earth, and the number had remained constant until
the vigilant eye of Herschel enlarged our knowledge, and Saturn was
relieved as the sentinel planet going his rounds on the distant
frontiers of our system.
When the elements of the new body had been computed, a search was
instituted among the records of previous observers, and it was found
that Herschel’s planet had been seen on many occasions, but it had
invariably been mistaken for a fixed star. Flamsteed observed it
on six occasions between 1690 and 1715, while Le Monnier saw it on
twelve nights in the years 1750 to 1771, and it seems to have been
pure carelessness on the part of the latter which prevented him from
anticipating Herschel in one of the greatest discoveries of modern
times.
The name Uranus was applied to the new planet, though the discoverer
himself called it Georgium Sidus, and there were others who termed
it Herschel in honor of the man through whose sagacity it had been
revealed.
Uranus revolves around the sun in 30,687 days, which very slightly
exceeds 84 terrestrial years. His mean distance from the sun is
1,782,000,000 miles, but the interval varies between 1,699 and 1,865
millions of miles. The apparent diameter of the planet undergoes
little variation; the mean is 3″.6, but observers differ. His real
diameter is approximately 31,000 miles, and the polar compression
about 1/13, though this value is not that found by all authorities.
The planet near opposition shines like a star of the sixth magnitude,
and is observable with the naked eye. He emits a bluish light.
While engaged in meteoric observations, I have sometimes followed
the planet with the naked eye during several months, and noted the
changes in his position relatively to the stars near. It is clear
from this that Uranus admitted of detection before the invention of
the telescope.
A luminous ring, similar to that of Saturn, was at first supposed
to surround Uranus, and Herschel suspected the existence of such a
feature on several occasions; but it scarcely survived his later
researches, and modern observations have finally disposed of it.
In May and June, 1883, Professor Young, having the advantage of the
fine 23-inch refractor at the Princeton Observatory, observed two
faint belts, one on each side of the equator, and much like the belts
of Saturn. On March 18, 1884, Messrs. Thollon and Perrotin, with the
14-inch equatorial at Nice, remarked dark spots similar to those on
Mars, toward the centre of the disk, and a white spot was seen on the
limb. Two different tints were perceived, the color of the Northwest
Hemisphere being dark and that of the Southeast a bluish-white color.
In April observations were continued, and the white spot was seen
“rather as a luminous band than a simple spot,” but it was most
conspicuous near the limb. The observers thought the appearances
indicated a rotation-period of about ten hours. The brothers Henry
at Paris, in 1884, invariably noticed two belts lying parallel to
each other, and including between them the brighter equatorial zone
of the planet. Their results apparently show that the angle between
the plane of the Uranian equator and that of the satellite-orbits is
about 41°.
M. Perrotin, with the great 30-inch equatorial at Nice, reobserved
the belts in May and June, 1889. He wrote that dark parallel bands
were noticed several times, and they were very similar to the belts
of Jupiter. M. Perrotin notes that the bands of Uranus do not always
present the same aspect. They vary in size and number in different
parts of their circumference.
For many years it was supposed that Uranus possessed six satellites,
all of which were discovered by Sir W. Herschel, but later
observations proved that four of these had no existence. They were
small stars near the planet. But two of Herschel’s satellites were
fully corroborated, and two new ones were discovered by Lassell and
Struve. The number of satellites attending Uranus is four, and it is
probable that many others exist, though they are too minute to be
distinguished in the most powerful instruments hitherto constructed.
The following are the known satellites: 3d Ariel, discovered in 1847;
4th Umbriel, discovered in 1847; 1st Titania, discovered in 1787, and
2d Oberon, discovered in 1787.
Titania and Oberon are the two brightest satellites, but none of them
can be seen except in large instruments. From observations with large
modern instruments it appears highly probable that the four known
satellites must be considerably larger than any others which may be
revolving round the planet. A curious fact in connection with these
satellites is that their motions are retrograde.
The leading incidents in the narrative of the discovery of Uranus
and Neptune present a great dissimilarity—Uranus was discovered by
accident, Neptune by design. Telescopic power revealed the former,
while theory disclosed the latter. In one case optical appliance
afforded the direct means of success, while in the other the unerring
precision of mathematical analysis attained it. The telescope played
but a secondary part in the discovery of Neptune, for this instrument
was employed simply to realize or confirm what theory had proven.
Certain irregularities in the motion of Uranus could not be
explained but on the assumption of an undetected planet situated
outside the known boundaries of the system. Two able geometers
applied themselves to study the problem of these irregularities,
and to deduce from them the place of the disturbing body. This was
effected independently by Messrs. Le Verrier and Adams; and Dr. Galle
of Berlin, having received from Le Verrier the leading results of his
computations, and the intimation that the longitude of the suspected
planet was then 326°, found it with his telescope on the night of
September 23, 1846, in longitude 326° 52′. The calculated place by
Professor Adams was 329° 19′ for the same date.
The name given to the new planet was Neptune. When the elements were
computed it was found that they presented rather large differences
with those theoretically computed by Le Verrier and Adams. It was
also found that the planet had been previously observed by Lalande on
May 8 and 10, 1795, but its true character escaped detection. This
astronomer had observed a star of the eighth magnitude on May 8; but
on May 10, not finding the same star in the exact place noted on the
former evening, he rejected the first observation as inaccurate and
adopted the second, marking it doubtful. Lalande, like Le Monnier,
the unsuspecting discoverer of Uranus, let a valuable discovery slip
through his hands.
Neptune revolves round the sun in 60,126 days, which is equal to
rather more than 164½ of our years. His mean distance from the sun is
2,792,000,000 miles, and his usual diameter 2″.7. He exceeds Uranus
in dimensions, his real diameter being 37,000 miles.
Our knowledge of this distant orb is extremely limited, owing to his
apparently diminutive size and feebleness. No markings have ever been
sighted on his miniature disk, and we can expect to learn nothing
until one of the large telescopes is employed in the work. No doubt
this planet exhibits the same belted appearance as that of Uranus,
and there is every probability that he possesses numerous satellites.
Directly the new planet was discovered, Mr. Lassell turned his large
reflector upon it and sought to learn something of its appearance,
and possibly detect one or more of its satellites. On October 3 and
10, 1846, he was struck with the appearance of the disk, which was
obviously not spherical. He subsequently confirmed this impression,
and concluded that a ring, inclined about 70°, surrounded the planet.
Professor Challis supported this view, but later observations in
a purer sky led Mr. Lassell to abandon the idea. Thus the ring of
Neptune, like the ring of Uranus, though apparently obvious at first,
vanished in the light of more modern researches.
But if Mr. Lassell quite failed to demonstrate the existence of a
ring, he nevertheless succeeded in discovering a satellite belonging
to the planet. This was on October 10, 1846. The new satellite was
found to have a period of 5 days, 21 hours, and 3 minutes, and to be
situated about 220,000 miles distant from the planet.
END OF VOLUME ONE
TRANSCRIBER’S NOTE
Footnote [13] is referenced twice from page 102.
Obvious typographical errors and punctuation errors have been
corrected after careful comparison with other occurrences within
the text and consultation of external sources.
Some hyphens in words have been silently removed, some added,
when a predominant preference was found in the original book.
Except for those changes noted below, all misspellings in the text,
and inconsistent or archaic usage, have been retained.
Pg 3: ‘Caliph Al-Mamum’ replaced by ‘Caliph Al-Mamun’.
Pg 20: ‘Ninteenth Century’ replaced by ‘Nineteenth Century’.
Pg 21: ‘Map of the’ replaced by ‘Chart of the’.
Pg 21: ‘Hourglass Sea’ replaced by ‘Nine Views of the Hour-Glass Sea’.
Pg 74: ‘cose che redire’ replaced by ‘cose che ridire’.
Pg 74: ‘Nè sa, nè’ replaced by ‘Né sa, né’.
Pg 100: ‘Hesoid’ replaced by ‘Hesiod’.
Pg 122: ‘familar to most’ replaced by ‘familiar to most’.
Pg 150: ‘formed of myraids’ replaced by ‘formed of myriads’.
Pg 223: ‘may be interred’ replaced by ‘may be inferred’.
Pg 238: ‘Will some motral’ replaced by ‘Will some mortal’.
Pg 292: ‘its orbitual motion’ replaced by ‘its orbital motion’.
Pg 380: ‘the Mare Humorom’ replaced by ‘the Mare Humorum’.
Pg 390: ‘present themelves’ replaced by ‘present themselves’.
Pg 391: ‘Mr. Lowell remarks,*’ replaced by ‘Mr. Lowell remarks,’;
(the * anchor had no footnote and has been removed).
Pg 396: ‘permamently shut’ replaced by ‘permanently shut’.
Pg 418: ‘is a thining-out’ replaced by ‘is a thinning-out’.
*** END OF THE PROJECT GUTENBERG EBOOK THE STORY OF THE UNIVERSE, VOLUME I (OF 4) ***
Updated editions will replace the previous one—the old editions will
be renamed.
Creating the works from print editions not protected by U.S. copyright
law means that no one owns a United States copyright in these works,
so the Foundation (and you!) can copy and distribute it in the United
States without permission and without paying copyright
royalties. Special rules, set forth in the General Terms of Use part
of this license, apply to copying and distributing Project
Gutenberg™ electronic works to protect the PROJECT GUTENBERG™
concept and trademark. Project Gutenberg is a registered trademark,
and may not be used if you charge for an eBook, except by following
the terms of the trademark license, including paying royalties for use
of the Project Gutenberg trademark. If you do not charge anything for
copies of this eBook, complying with the trademark license is very
easy. You may use this eBook for nearly any purpose such as creation
of derivative works, reports, performances and research. Project
Gutenberg eBooks may be modified and printed and given away—you may
do practically ANYTHING in the United States with eBooks not protected
by U.S. copyright law. Redistribution is subject to the trademark
license, especially commercial redistribution.
START: FULL LICENSE
THE FULL PROJECT GUTENBERG LICENSE
PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK
To protect the Project Gutenberg™ mission of promoting the free
distribution of electronic works, by using or distributing this work
(or any other work associated in any way with the phrase “Project
Gutenberg”), you agree to comply with all the terms of the Full
Project Gutenberg™ License available with this file or online at
www.gutenberg.org/license.
Section 1. General Terms of Use and Redistributing Project Gutenberg™
electronic works
1.A. By reading or using any part of this Project Gutenberg™
electronic work, you indicate that you have read, understand, agree to
and accept all the terms of this license and intellectual property
(trademark/copyright) agreement. If you do not agree to abide by all
the terms of this agreement, you must cease using and return or
destroy all copies of Project Gutenberg™ electronic works in your
possession. If you paid a fee for obtaining a copy of or access to a
Project Gutenberg™ electronic work and you do not agree to be bound
by the terms of this agreement, you may obtain a refund from the person
or entity to whom you paid the fee as set forth in paragraph 1.E.8.
1.B. “Project Gutenberg” is a registered trademark. It may only be
used on or associated in any way with an electronic work by people who
agree to be bound by the terms of this agreement. There are a few
things that you can do with most Project Gutenberg™ electronic works
even without complying with the full terms of this agreement. See
paragraph 1.C below. There are a lot of things you can do with Project
Gutenberg™ electronic works if you follow the terms of this
agreement and help preserve free future access to Project Gutenberg™
electronic works. See paragraph 1.E below.
1.C. The Project Gutenberg Literary Archive Foundation (“the
Foundation” or PGLAF), owns a compilation copyright in the collection
of Project Gutenberg™ electronic works. Nearly all the individual
works in the collection are in the public domain in the United
States. If an individual work is unprotected by copyright law in the
United States and you are located in the United States, we do not
claim a right to prevent you from copying, distributing, performing,
displaying or creating derivative works based on the work as long as
all references to Project Gutenberg are removed. Of course, we hope
that you will support the Project Gutenberg™ mission of promoting
free access to electronic works by freely sharing Project Gutenberg™
works in compliance with the terms of this agreement for keeping the
Project Gutenberg™ name associated with the work. You can easily
comply with the terms of this agreement by keeping this work in the
same format with its attached full Project Gutenberg™ License when
you share it without charge with others.
1.D. The copyright laws of the place where you are located also govern
what you can do with this work. Copyright laws in most countries are
in a constant state of change. If you are outside the United States,
check the laws of your country in addition to the terms of this
agreement before downloading, copying, displaying, performing,
distributing or creating derivative works based on this work or any
other Project Gutenberg™ work. The Foundation makes no
representations concerning the copyright status of any work in any
country other than the United States.
1.E. Unless you have removed all references to Project Gutenberg:
1.E.1. The following sentence, with active links to, or other
immediate access to, the full Project Gutenberg™ License must appear
prominently whenever any copy of a Project Gutenberg™ work (any work
on which the phrase “Project Gutenberg” appears, or with which the
phrase “Project Gutenberg” is associated) is accessed, displayed,
performed, viewed, copied or distributed:
This eBook is for the use of anyone anywhere in the United States and most
other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online
at www.gutenberg.org. If you
are not located in the United States, you will have to check the laws
of the country where you are located before using this eBook.
1.E.2. If an individual Project Gutenberg™ electronic work is
derived from texts not protected by U.S. copyright law (does not
contain a notice indicating that it is posted with permission of the
copyright holder), the work can be copied and distributed to anyone in
the United States without paying any fees or charges. If you are
redistributing or providing access to a work with the phrase “Project
Gutenberg” associated with or appearing on the work, you must comply
either with the requirements of paragraphs 1.E.1 through 1.E.7 or
obtain permission for the use of the work and the Project Gutenberg™
trademark as set forth in paragraphs 1.E.8 or 1.E.9.
1.E.3. If an individual Project Gutenberg™ electronic work is posted
with the permission of the copyright holder, your use and distribution
must comply with both paragraphs 1.E.1 through 1.E.7 and any
additional terms imposed by the copyright holder. Additional terms
will be linked to the Project Gutenberg™ License for all works
posted with the permission of the copyright holder found at the
beginning of this work.
1.E.4. Do not unlink or detach or remove the full Project Gutenberg™
License terms from this work, or any files containing a part of this
work or any other work associated with Project Gutenberg™.
1.E.5. Do not copy, display, perform, distribute or redistribute this
electronic work, or any part of this electronic work, without
prominently displaying the sentence set forth in paragraph 1.E.1 with
active links or immediate access to the full terms of the Project
Gutenberg™ License.
1.E.6. You may convert to and distribute this work in any binary,
compressed, marked up, nonproprietary or proprietary form, including
any word processing or hypertext form. However, if you provide access
to or distribute copies of a Project Gutenberg™ work in a format
other than “Plain Vanilla ASCII” or other format used in the official
version posted on the official Project Gutenberg™ website
(www.gutenberg.org), you must, at no additional cost, fee or expense
to the user, provide a copy, a means of exporting a copy, or a means
of obtaining a copy upon request, of the work in its original “Plain
Vanilla ASCII” or other form. Any alternate format must include the
full Project Gutenberg™ License as specified in paragraph 1.E.1.
1.E.7. Do not charge a fee for access to, viewing, displaying,
performing, copying or distributing any Project Gutenberg™ works
unless you comply with paragraph 1.E.8 or 1.E.9.
1.E.8. You may charge a reasonable fee for copies of or providing
access to or distributing Project Gutenberg™ electronic works
provided that:
• You pay a royalty fee of 20% of the gross profits you derive from
the use of Project Gutenberg™ works calculated using the method
you already use to calculate your applicable taxes. The fee is owed
to the owner of the Project Gutenberg™ trademark, but he has
agreed to donate royalties under this paragraph to the Project
Gutenberg Literary Archive Foundation. Royalty payments must be paid
within 60 days following each date on which you prepare (or are
legally required to prepare) your periodic tax returns. Royalty
payments should be clearly marked as such and sent to the Project
Gutenberg Literary Archive Foundation at the address specified in
Section 4, “Information about donations to the Project Gutenberg
Literary Archive Foundation.”
• You provide a full refund of any money paid by a user who notifies
you in writing (or by e-mail) within 30 days of receipt that s/he
does not agree to the terms of the full Project Gutenberg™
License. You must require such a user to return or destroy all
copies of the works possessed in a physical medium and discontinue
all use of and all access to other copies of Project Gutenberg™
works.
• You provide, in accordance with paragraph 1.F.3, a full refund of
any money paid for a work or a replacement copy, if a defect in the
electronic work is discovered and reported to you within 90 days of
receipt of the work.
• You comply with all other terms of this agreement for free
distribution of Project Gutenberg™ works.
1.E.9. If you wish to charge a fee or distribute a Project
Gutenberg™ electronic work or group of works on different terms than
are set forth in this agreement, you must obtain permission in writing
from the Project Gutenberg Literary Archive Foundation, the manager of
the Project Gutenberg™ trademark. Contact the Foundation as set
forth in Section 3 below.
1.F.
1.F.1. Project Gutenberg volunteers and employees expend considerable
effort to identify, do copyright research on, transcribe and proofread
works not protected by U.S. copyright law in creating the Project
Gutenberg™ collection. Despite these efforts, Project Gutenberg™
electronic works, and the medium on which they may be stored, may
contain “Defects,” such as, but not limited to, incomplete, inaccurate
or corrupt data, transcription errors, a copyright or other
intellectual property infringement, a defective or damaged disk or
other medium, a computer virus, or computer codes that damage or
cannot be read by your equipment.
1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the “Right
of Replacement or Refund” described in paragraph 1.F.3, the Project
Gutenberg Literary Archive Foundation, the owner of the Project
Gutenberg™ trademark, and any other party distributing a Project
Gutenberg™ electronic work under this agreement, disclaim all
liability to you for damages, costs and expenses, including legal
fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT
LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE
PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE
TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE
LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR
INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH
DAMAGE.
1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a
defect in this electronic work within 90 days of receiving it, you can
receive a refund of the money (if any) you paid for it by sending a
written explanation to the person you received the work from. If you
received the work on a physical medium, you must return the medium
with your written explanation. The person or entity that provided you
with the defective work may elect to provide a replacement copy in
lieu of a refund. If you received the work electronically, the person
or entity providing it to you may choose to give you a second
opportunity to receive the work electronically in lieu of a refund. If
the second copy is also defective, you may demand a refund in writing
without further opportunities to fix the problem.
1.F.4. Except for the limited right of replacement or refund set forth
in paragraph 1.F.3, this work is provided to you ‘AS-IS’, WITH NO
OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE.
1.F.5. Some states do not allow disclaimers of certain implied
warranties or the exclusion or limitation of certain types of
damages. If any disclaimer or limitation set forth in this agreement
violates the law of the state applicable to this agreement, the
agreement shall be interpreted to make the maximum disclaimer or
limitation permitted by the applicable state law. The invalidity or
unenforceability of any provision of this agreement shall not void the
remaining provisions.
1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the
trademark owner, any agent or employee of the Foundation, anyone
providing copies of Project Gutenberg™ electronic works in
accordance with this agreement, and any volunteers associated with the
production, promotion and distribution of Project Gutenberg™
electronic works, harmless from all liability, costs and expenses,
including legal fees, that arise directly or indirectly from any of
the following which you do or cause to occur: (a) distribution of this
or any Project Gutenberg™ work, (b) alteration, modification, or
additions or deletions to any Project Gutenberg™ work, and (c) any
Defect you cause.
Section 2. Information about the Mission of Project Gutenberg™
Project Gutenberg™ is synonymous with the free distribution of
electronic works in formats readable by the widest variety of
computers including obsolete, old, middle-aged and new computers. It
exists because of the efforts of hundreds of volunteers and donations
from people in all walks of life.
Volunteers and financial support to provide volunteers with the
assistance they need are critical to reaching Project Gutenberg™’s
goals and ensuring that the Project Gutenberg™ collection will
remain freely available for generations to come. In 2001, the Project
Gutenberg Literary Archive Foundation was created to provide a secure
and permanent future for Project Gutenberg™ and future
generations. To learn more about the Project Gutenberg Literary
Archive Foundation and how your efforts and donations can help, see
Sections 3 and 4 and the Foundation information page at www.gutenberg.org.
Section 3. Information about the Project Gutenberg Literary Archive Foundation
The Project Gutenberg Literary Archive Foundation is a non-profit
501(c)(3) educational corporation organized under the laws of the
state of Mississippi and granted tax exempt status by the Internal
Revenue Service. The Foundation’s EIN or federal tax identification
number is 64-6221541. Contributions to the Project Gutenberg Literary
Archive Foundation are tax deductible to the full extent permitted by
U.S. federal laws and your state’s laws.
The Foundation’s business office is located at 809 North 1500 West,
Salt Lake City, UT 84116, (801) 596-1887. Email contact links and up
to date contact information can be found at the Foundation’s website
and official page at www.gutenberg.org/contact
Section 4. Information about Donations to the Project Gutenberg
Literary Archive Foundation
Project Gutenberg™ depends upon and cannot survive without widespread
public support and donations to carry out its mission of
increasing the number of public domain and licensed works that can be
freely distributed in machine-readable form accessible by the widest
array of equipment including outdated equipment. Many small donations
($1 to $5,000) are particularly important to maintaining tax exempt
status with the IRS.
The Foundation is committed to complying with the laws regulating
charities and charitable donations in all 50 states of the United
States. Compliance requirements are not uniform and it takes a
considerable effort, much paperwork and many fees to meet and keep up
with these requirements. We do not solicit donations in locations
where we have not received written confirmation of compliance. To SEND
DONATIONS or determine the status of compliance for any particular state
visit www.gutenberg.org/donate.
While we cannot and do not solicit contributions from states where we
have not met the solicitation requirements, we know of no prohibition
against accepting unsolicited donations from donors in such states who
approach us with offers to donate.
International donations are gratefully accepted, but we cannot make
any statements concerning tax treatment of donations received from
outside the United States. U.S. laws alone swamp our small staff.
Please check the Project Gutenberg web pages for current donation
methods and addresses. Donations are accepted in a number of other
ways including checks, online payments and credit card donations. To
donate, please visit: www.gutenberg.org/donate.
Section 5. General Information About Project Gutenberg™ electronic works
Professor Michael S. Hart was the originator of the Project
Gutenberg™ concept of a library of electronic works that could be
freely shared with anyone. For forty years, he produced and
distributed Project Gutenberg™ eBooks with only a loose network of
volunteer support.
Project Gutenberg™ eBooks are often created from several printed
editions, all of which are confirmed as not protected by copyright in
the U.S. unless a copyright notice is included. Thus, we do not
necessarily keep eBooks in compliance with any particular paper
edition.
Most people start at our website which has the main PG search
facility: www.gutenberg.org.
This website includes information about Project Gutenberg™,
including how to make donations to the Project Gutenberg Literary
Archive Foundation, how to help produce our new eBooks, and how to
subscribe to our email newsletter to hear about new eBooks.
