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Title: The story of the sun, moon, and stars
Author: Agnes Giberne
Release date: October 7, 2025 [eBook #77004]
Language: English
Original publication: Cincinnati: National Book Company, 1898
Credits: Alan, Tim Lindell and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive/American Libraries.)
*** START OF THE PROJECT GUTENBERG EBOOK THE STORY OF THE SUN, MOON, AND STARS ***
[Illustration: HIPPARCHUS IN THE OBSERVATORY OF ALEXANDRIA.]
THE STORY
OF THE
SUN, MOON, AND STARS
BY
AGNES GIBERNE
AUTHOR OF “THE STARRY SKIES,” “THE WORLD’S FOUNDATIONS,”
“RADIANT SUNS,” ETC.
[Illustration]
WITH COPIOUS ADDITIONS
Fully Illustrated
CINCINNATI
NATIONAL BOOK COMPANY
COPYRIGHT, 1898,
BY J. T. JONES.
INTRODUCTION.
BY CHARLES PRITCHARD, M. A., F. R. S.,
PROFESSOR OF ASTRONOMY IN THE UNIVERSITY OF OXFORD.
The pages of this volume on a great subject were submitted to my
criticism while passing through the press, with a request from a friend
that I would make any suggestions which might occur to me for its
improvement. Naturally, such a request was entertained, in the first
instance, with hesitation and misgiving. But after a rapid perusal
of the first sheet, I found my interest awakened, and then gradually
secured; for the book seemed to me to possess certain features of
no ordinary character, and, in my judgment, held out the promise of
supplying an undoubted want, thus enabling me to answer a question
which I have been often asked, and which had as often puzzled me,
to the effect, “Can you tell me of any book on astronomy suited to
beginners?” I think just such a book is here presented to the reader;
for the tale of the stellar universe is therein told with great
simplicity, and perhaps with sufficient completeness, in an earnest and
pleasant style, equally free, I think, from inaccuracy or unpardonable
exaggeration. We have here the outlines of elementary astronomy,
not merely detailed without mathematics, but to a very great extent
expressed in untechnical language. Success in such an attempt I regard
as a considerable feat, and one of much practical utility.
For the science of astronomy is essentially a science of great
magnitude and great difficulty. From the time of Hipparchus, some
century and a half before the Christian era, down to the present
day, the cultivation of astronomy has severely taxed the minds of a
succession of men endowed with the rarest genius. The facts and the
truths of the science thus secured have been of very slow accretion;
but like all other truths, when once secured and thoroughly understood,
they are found to admit of very simple verbal expression, and to lie
well within the general comprehension, and, I may safely add, within
the sympathies of all educated men and women.
Thus the great astronomers, the original discoverers of the last
twenty centuries, have labored, each in his separate field of the
vast universe of nature, and other men, endowed with other gifts,
have entered on their labors, and by systematizing, correlating, and
simplifying the expression of their results, have brought the whole
within the grasp of cultivated men engaged in other branches of the
varied pursuits of our complicated life. It is in this sort of order
that the amelioration and civilization of mankind have proceeded, and
at the present moment are, I hope and believe, rapidly proceeding.
It was, I suspect, under this point of view, though half unconsciously
so, that my attention was arrested by the book now presented to the
reader; for we have here many of the chief results of the laborious
researches of such men as Ptolemy, Kepler, Newton, Herschel,
Fraunhofer, Janssen, Lockyer, Schiaparelli, and others--no matter
where accumulated or by whom recorded--filtered through the mind of
a thoughtful and cultured lady, and here presented to other minds in
the very forms wherein they have been assimilated and pictured in her
own. And these forms and pictures are true. It is in this way that the
intellectual “protoplasm” of the human mind is fostered and practically
disseminated.
And, then, there is still another point of view from which this general
dissemination of great truths in a simple style assumes an aspect of
practical importance. I allude to the influences of this process on the
imaginative or poetic side of our complex nature.
Wordsworth, in one of his prefaces, has stated so clearly the truth
on this subject that I can not do better than give his words. “If the
time should ever come,” he says, “when what is now science becomes
familiarized to men, then the remotest discoveries of the chemist, the
botanist, the mineralogist, will be as proper objects of the poet’s
art as any upon which it can be employed. He will be ready to follow
the steps of the man of science; he will be at his side, carrying
sensation into the midst of the objects of science itself. The poet
will lend his divine spirit to aid the transfiguration, and will
welcome the being thus produced as a dear and genuine inmate of the
household of man.”
It is for reasons such as here stated that I heartily commend this
book to the attention of those who take an interest in the advancement
of the intellectual progress and culture of society. The story of the
Kosmos is told by the authoress in her own language and after her own
method. I believe, as I have said, the story is correct.
OXFORD UNIVERSITY.
CONTENTS.
CHAPTER. PAGE.
I. THE EARTH ONE OF A FAMILY, 13
II. THE HEAD OF OUR FAMILY, 24
III. WHAT BINDS THE FAMILY TOGETHER, 34
IV. THE LEADING MEMBERS OF OUR FAMILY.--FIRST
GROUP, 46
V. THE LEADING MEMBERS OF OUR FAMILY.--SECOND
GROUP, 55
VI. THE MOON, 62
VII. VISITORS, 73
VIII. LITTLE SERVANTS, 83
IX. NEIGHBORING FAMILIES, 95
X. OUR NEIGHBORS’ MOVEMENTS, 106
XI. MORE ABOUT THE SOLAR SYSTEM, 122
XII. MORE ABOUT THE SUN, 133
XIII. YET MORE ABOUT THE SUN, 148
XIV. MORE ABOUT THE MOON, 157
XV. YET MORE ABOUT THE MOON, 164
XVI. MERCURY, VENUS, AND MARS, 180
XVII. JUPITER, 199
XVIII. SATURN, 214
XIX. URANUS AND NEPTUNE, 225
XX. COMETS AND METEORITES, 240
XXI. MORE ABOUT COMETS AND METEORITES, 246
XXII. MANY SUNS, 267
XXIII. SOME PARTICULAR SUNS, 278
XXIV. DIFFERENT KINDS OF SUNS, 291
XXV. GROUPS AND CLUSTERS OF SUNS, 304
XXVI. THE PROBLEM OF SUN AND STAR DISTANCES, 310
XXVII. MEASUREMENT OF STAR DISTANCES, 320
XXVIII. THE MILKY WAY, 329
XXIX. A WHIRLING UNIVERSE, 337
XXX. READING THE LIGHT, 344
XXXI. STELLAR PHOTOGRAPHY, 355
XXXII. THE DAWN OF ASTRONOMY, 363
XXXIII. MODERN ASTRONOMY, 375
XXXIV. SIR ISAAC NEWTON, 382
XXXV. LATER ASTRONOMY, 393
LIST OF ILLUSTRATIONS.
PAGE.
Hipparchus in the Observatory at Alexandria _Frontispiece._
The Solar System 15
The Pleiades 19
Sun-spots 29
Phases of Mercury before Inferior Conjunction--Evening Star 50
Appearance of the Full Moon 63
One Form of Lunar Crater 69
The Earth as Seen from the Moon 71
Passage of the Earth and the Moon through the Tail of a Comet 76
Passage of the Comet of 1843 Close to the Sun 79
Meteor Emerging from Behind a Cloud 84
The Great Shower of Shooting-stars, November 27, 1872 90
Constellation of the Great Bear 108
The Great Bear 50,000 Years Ago 109
The Great Bear 50,000 Years Hence 109
Constellation of Orion as it Appears Now 110
Constellation of Orion 50,000 Years Hence 111
Position of Planets Inferior to Jupiter--Showing the Zone of the
Asteroids 123
Comparative Size of the Planetary Worlds 130
A Typical Sun-spot 135
A Solar Eruption 138
Sun-flames, May 3, 1892 143
Solar Corona and Prominence 152
Comparative Size of the Earth and Sun 155
The Moon--an Expired Planet 159
The Lunar Crater Copernicus 170
Lunar Eruption--Brisk Action 172
Lunar Eruption--Feeble Action 173
Nicolaus Copernicus 176
Phases of Venus 186
Mars and the Path of its Satellites 191
General Aspect of Jupiter--Satellite and its Shadow 200
Satellites of Jupiter Compared with the Earth and Moon 206
The System of Jupiter 208
Orbits of Nine Comets Captured by Jupiter 213
Saturn and the Earth--Comparative Size 215
Ideal View of Saturn’s Rings and Satellites from the Planet 222
Great Astronomers of Earlier Times 227
The Great Comet of 1811 247
Donati’s Comet, 1858 249
Great Comet of 1744 254
The First Comet of 1888--June 4 255
Great Comet of 1680 260
Great Comet of 1769 261
Baron Alexander Von Humboldt 264
A Telescopic Field in the Milky Way 270
Cygnus, Constellation of 279
Stars whose Distances are Best Known 281
Lyra, Constellation of 297
Constellations in the Northern Hemisphere 300
A Cluster of Stars in Centaurus 305
The Great Nebula in Andromeda, Compared with Size of the Solar
System 307
A Cluster of Stars in Perseus 332
Hercules, Constellation of 338
The Great Bear, Constellation of 341
The Prism and Spectrum 344
The Spectroscope 345
Spectra, Showing the Dark Lines 347
Great Astronomers of Later Times 377
Sir Isaac Newton 386
Eye-piece of Lick Telescope 407
Lick Observatory 409
Professor E. E. Barnard 410
THE STORY
OF THE
SUN, MOON, AND STARS.
CHAPTER I.
THE EARTH ONE OF A FAMILY.
What is this earth of ours?
Something very great--and yet something very little. Something very
great, compared with the things upon the earth; something very little,
compared with the things outside the earth.
And as our journeyings together are for a while to be away from earth,
we shall find ourselves obliged to count her as something quite small
in the great universe, where so many larger and mightier things are
to be found,--if indeed they are mightier. Not that we have to say
good-bye altogether to our old home. We must linger about her for a
while before starting, and afterwards it will be often needful to come
back, with speed swifter than the flight of light, that we may compare
notes on the sizes and conditions of other places visited by us.
But first of all: what _is_ this earth of ours?
It was rather a pleasant notion which men held in olden days, that
we--that great and important “We” which loves to perch itself upon
a height--stood firm and fixed at the very center of everything. The
earth was supposed to be a vast flat plain, reaching nobody could tell
how far. The sun rose and set for us alone; and the thousands of stars
twinkled in the sky at night for nobody’s good except ours; and the
blue sky overhead was a crystal covering for the men of our earth,
and nothing more. In fact, people seem to have counted themselves not
merely to have had a kind of kingship over the lower animals of our
earth, but to have been kings over the whole universe. Sun, stars, and
sky, as well as earth, were made for man, and for man only.
This was the common belief, though even in those olden days there were
_some_ who knew better. But the world in general knows better now.
Earth the center of the universe! Why, she is not that of even the
particular family in the heavens to which she belongs. For we do not
stand alone. The earth is one of a family of worlds, and that family is
called THE SOLAR SYSTEM. And so far is our earth from being the head of
the family, that she is not even one of the more important members. She
is merely one of the little sisters, as it were.
Men not only believed the earth, in past days, to be at the center
of the universe; but also they believed her to remain there without
change. Sun, moon, stars, planets, sky, might move; but never the
earth. The solid ground beneath their feet, _that_ at least was firm.
Every day the sun rose and set, and every night the stars, in like
manner, rose and set. But this was easily explained. We on our great
earth stood firm and still, while sun, moon, stars, went circling
round us once in every twenty-four hours, just for our sole and
particular convenience. What an important personage man must have felt
himself then in God’s great universe! Once again, we know better now!
[Illustration: THE SOLAR SYSTEM.]
For it is the earth that moves, and not the sun; it is the earth that
moves, and not the stars. The daily movements of sun and stars, rising
in the east, traveling over the sky, and setting in the west, are
no more real movements on their part than, when we travel in a rail
way-train, the seeming rush of hedges, telegraph posts, houses, and
fields is real. They are fixed and we are moving; yet the movement
appears to us to be not ours but theirs.
Still more strongly would this appear to be the case if there were no
noise, no shaking, no jarring and trembling, to make us feel that we
are not at rest. Sometimes when a train begins to move gently out of
a station, from among other trains, it is at the first moment quite
impossible to say whether the movement belongs to the train in which we
are seated or to a neighboring train. And in the motions of our earth
there is no noise, no shaking, no jarring--all are rapid, silent, and
even.
If you were rising through the air in a balloon, you would at first
only know your own movement by seeing the earth seem to drop away from
beneath you. And just so we can only know the earth’s movements by
seeing how worlds around us _seem_ to move in consequence.
When I speak of the Universe, I mean the whole of God’s mighty creation
as far as the stars reach. Sometimes the word is used in this sense,
and sometimes it is used only for a particular part of creation nearer
to us than other parts. At present, however, we will put aside all
thought of the second narrower meaning.
The wisest astronomer living can not tell us how far the stars reach.
We know now that there is no firm crystal covering over our heads,
dotted with bright points here and there; but only the wide open sky
or heaven, containing millions of stars, some nearer, some farther,
some bright enough to be seen by us all, some only visible through a
telescope.
People talk often of the stars being “set in space;” and the meaning of
“space” is simply “room.” Where you are must be space, or you could not
be there. But it is when we get away from earth, and travel in thought
through the wide fields of space, where God has placed his stars, that
we begin to feel how vast it is, and what specks we are ourselves--nay,
what a speck our very earth is, in this great and boundless creation.
For there is no getting to the borders of space. As one telescope after
another is made, each one stronger in power and able to reach farther
than the last, still more and more stars are seen, and yet more and
more behind and beyond, in countless millions.
It is the same all round the earth. The old notion about our world
being a flat plain has been long since given up. We know her now to be
a round globe, not fixed, but floating like the stars in space.
When you look _up_ into the sky, you are looking exactly in the
opposite direction from where you would be looking _up_ if you were
in Australia. For Australia’s “up” is our “down,” and our “down” is
Australia’s “up.” Or, to put it more truly, “up” is always in the
direction straight away from this earth, on whatever part of it you may
be standing; and “down” is always towards the center of the earth.
All round the globe, in north, south, east, west, whether you are in
Europe, Asia, Africa, or America, though you will see different stars
in certain different quarters of the earth, still overhead you will
find shining countless points of light.
And now, what are these stars? This is a matter on which people are
often confused, and on which it is well to be quite clear before going
one step farther.
Some of the stars you have most likely often noticed. The seven chief
stars of the Great Bear are known to a large number of people; and
there are few who have not admired the splendid constellation of Orion.
Perhaps you also know the W-shape of Cassiopeia, and the brilliant
shining of Sirius, and the soft glimmer of the Pleiades.
The different constellations or groups of principal stars have been
watched by men for long ages past. They are called the fixed stars, for
they do not change. How many thousands of years ago they were first
arranged by men into these groups, and who first gave them their names,
we can not tell.
True, night by night, through century after century, they rise, and
cross the sky, and set. But those are only seeming movements. Precisely
as the turning round of the earth upon her axis, once in every
twenty-four hours, makes the sun appear to rise and cross the sky and
set, in the day-time; so also the same turning of the earth makes the
stars appear to do the same in the night-time.
There is another seeming movement among the stars, which is only in
seeming. Some come into view in summer which can not be seen in
winter; and some come into view in winter which can not be seen in
summer. For the sun, moving on his pathway through the sky, hides those
stars which shine with him in the day-time. The zodiac is an imaginary
belt in the heavens, sixteen or eighteen degrees wide, containing the
twelve constellations through which the sun passes. And as he passes
from point to point of his pathway, he constantly conceals from us
fresh groups of stars by day, and allows fresh groups to appear by
night. Speaking generally, however, the stars remain the same year
after year, century after century. The groups may still be seen as of
old, fixed and unchanging.
[Illustration: THE PLEIADES.]
What are these stars? Stars and planets have both been spoken about.
There is a great difference between the two.
Perhaps if you were asked whether the sun is most like to a star or a
planet, you would be rather at a loss; and many who have admired the
brilliant evening star, Venus, often to be seen after sunset, would be
surprised to learn that the evening star is in reality no star at all.
A star is a sun. Our sun is nothing more nor less than a star. Each one
of the so-called “fixed stars,” that you see shining at night in the
sky, is a sun like our sun; only some of the stars are larger suns and
some are smaller suns than ours.
The main reason why our sun looks so much larger and brighter than the
stars is, that he is so very much nearer to us. The stars are one and
all at enormous distances from the earth. By and by we will go more
closely into the matter of their distance, compared with the distance
of the sun.
At present it is enough to say that if many of the stars were placed
just as near to us as our sun is placed, they would look just as large
and bright; while there are some that would look a great deal larger
and brighter. And if our sun were to travel away from us, to the
distance of the very nearest of the little twinkling stars, he would
dwindle down and down in size and brilliancy, till at last we should
not be able to tell him apart from the rest of the stars.
I have told you that the stars are called “fixed” because they keep
their places, and do not change from age to age. Though the movement
of the earth makes them seem all to sweep past every night in company,
yet they do not travel in and out among one another, or backwards
and forwards, or from side to side. At all events, if there be such
changes, they are so slow and so small as to be exceedingly difficult
to find out. Each group of stars keeps its own old shape, as for
hundreds of years back.
But among these fixed stars there are certain stars which do go to and
fro, and backwards and forwards. Now they are to be seen in the middle
of one constellation, and now in the middle of another. These restless
stars were long a great perplexity. Men named them Planets or Wanderers.
We know now that the planets are in reality not stars at all, and also
that they are not nearly so far away from us as the fixed stars. In
fact, they are simply members of our own family--the Solar System. They
are worlds, more or less like the world we live in; and they travel
round and round the sun as we do, each more or less near to him; and
they depend upon him for heat and light, in more or less the same
manner as ourselves.
Therefore, just as our sun is a star, and stars are suns, so our earth
or world is a planet, and planets are worlds. EARTH is the name we
give to that particular world or planet on which we live. Planets may
generally be known from stars by the fact that they do not twinkle.
But the great difference between the two lies in the fact that a star
shines by its own radiant, burning light, whereas a planet shines
merely by light reflected or borrowed from the sun.
But how were the earth and the other worlds made? Let us imagine an
immense gaseous mass placed in space. Attraction is a force inherent in
every atom of matter. The denser portion of this mass will insensibly
attract towards it the other parts, and in the slow fall of the more
distant molecules towards this more attractive region, a general motion
is produced, incompletely directed towards this center, and soon
involving the whole mass in the same motion of rotation. The simplest
form of all, even in virtue of this law of attraction, is the spherical
form. It is that which a drop of water takes, and a drop of mercury if
left to itself.
The laws of mechanics show that, as this gaseous mass condenses and
shrinks, the motion of rotation of the nebula is accelerated. In
turning, it becomes flattened at the poles, and gradually takes the
form of an immense lens-shaped mass of gas. It has begun to turn so
quickly as to develop at its exterior circumference a centrifugal force
superior to the general attraction of the mass, as when we whirl a
sling. The inevitable consequence of this excess is a rupture of the
equilibrium, which detaches an external ring. This gaseous ring will
continue to rotate in the same time and with the same velocity; but
the nebulous mother will be henceforth detached, and will continue to
undergo progressive condensation and acceleration of motion. The same
feat will be reproduced as often as the velocity of rotation surpasses
that by which the centrifugal force remains inferior to the attraction.
It may have happened also that secondary centers of condensation would
be formed even in the interior of the nebula.
In our system the rings of Saturn still subsist.
The successive formation of the planets, their situation near the
plane of the solar equator, and their motions of translation round the
same center, are explained by the theory which we are discussing. The
most distant known planet, Neptune, would be detached from the nebula
at the epoch when this nebula extended as far as the planet, out to
nearly three thousand millions of miles, and would turn in a slow
revolution requiring a period of 165 years for its accomplishment. The
original ring could not remain in the state of a ring unless it was
perfectly homogeneous and regular; but such a condition is, so to say,
unrealizable, and it did not delay in condensing itself into a sphere.
Successively, Uranus, Saturn, Jupiter, the army of small planets, Mars,
would thus be detached or formed in the interior of this same nebula.
Afterwards came the earth, of which the birth goes back to the epoch
when the sun had arrived at the earth’s present position. Venus and
Mercury would be born later. Will the sun give birth to a new world?
This is not probable.
CHAPTER II
THE HEAD OF OUR FAMILY.
People began very early in the history of the world to pay close
attention to the sun. And no wonder. We owe so much to his heat and
light that the marvel would be if men had not thought much about him.
Was the sun really any larger than he looked, and if so, how much
larger was he? And what was his distance from the earth? These were two
of the questions which puzzled our ancestors the longest. If once they
could have settled exactly how far off the sun really was, they could
easily have calculated his exact size; but this was just what they
could not do.
So one man supposed that the sun must be quite near, and very little
larger than he looked. Another thought he might be seventy-five miles
in diameter. A third ventured to believe that he was larger than the
country of Greece. A fourth was so bold as to imagine that he might
even outweigh the earth herself.
After a while many attempts were made to measure the distance of the
sun; and a great many different answers to this difficult question
were given by different men, most of them very wide of the mark. It
is only of late years that the matter has been clearly settled. And,
indeed, it was found quite lately that a mistake of no less than three
millions of miles had been made, notwithstanding all the care and all
the attention given. But though three millions of miles sounds a great
deal, yet it is really very little--only a tiny portion of the whole.
For the distance of the sun from the earth is no less than about
NINETY-THREE MILLIONS OF MILES. Ninety-three millions of miles! Can
you picture that to yourself? Try to think what is meant by a thousand
miles. Our earth is eight thousand miles in diameter. In other words,
if you were to thrust a gigantic knitting-needle through her body,
from the North Pole to the South Pole, it would have to be about eight
thousand miles long.
To reach the thought of one million, you must picture _one thousand
times one thousand_. Our earth is about twenty-five thousand miles
round. If you were to start from the mouth of the River Amazon, in
South America, and journey straight round the whole earth on the
equator, till you came back to the same point, you would have traveled
about twenty-five thousand miles. But that would be a long way off from
a million miles. You would have gone only once round the earth. Now a
cord one million miles in length could be wrapped, not once only, but
_forty times_, round and round the earth. And when you have managed
to reach up to the thought of one million miles, you have then to
remember that the sun’s distance is ninety-three times as much again.
So, to picture clearly to ourselves the actual meaning of “ninety-three
millions of miles” is not so easy.
Suppose it were possible to lay a railroad from here to the sun. If
you could journey thither in a perfectly straight line, at the rate
of thirty miles an hour, never pausing for one single minute, night or
day, you would reach the sun in about three hundred and forty-six years.
Thirty miles an hour is a slow train. Suppose we double the speed, and
make it an express train, rushing along at the pace of sixty miles
an hour. Then you might hope to reach the end of your journey in one
hundred and seventy-three years. If you had quitted this earth early in
the eighteenth century, never stopping on your way, you would be, just
about now, near the end of the nineteenth, arriving at the sun.
So much for the sun’s distance from us. Now as to his size.
I have already mentioned that our earth’s diameter--that is, her
_through measure_, as, for instance, the line drawn straight from
England through her center to New Zealand--is about eight thousand
miles. This sounds a good deal. But what do you think of the diameter
of the sun being no less than _eight hundred and fifty-eight thousand
miles_? The one is eight thousand miles, the other over eight hundred
thousand!
Suppose you had a long slender pole which would pass through the middle
of the earth, one end just showing at the North Pole and the other at
the South Pole. You would need more than a hundred and eight of such
poles, all joined together, to show the diameter of the sun.
The sun seems not to be made of nearly such heavy materials as the
earth. He is what astronomers call less “dense,” less close and compact
in his make, just as wood is less dense and heavy than iron. Still,
his size is so enormous, that if you could have a pair of gigantic
scales, and put the sun into one scale and the earth with every one of
her brother and sister planets into the other, the sun’s side would go
down like lightning. He would be found to weigh seven hundred and fifty
times as much as all the rest put together. And, it would take more
than twelve hundred thousand little earths like ours, rolled into one
huge ball, to make a globe as large as the sun.
In the beginning of the seventeenth century a man named Fabricius was
startled by the sight of a certain black spot upon the face of the
sun. He watched till too dazzled to look any longer, supposing it to
be a small cloud, yet anxious to learn more. Next day the spot was
there still, but it seemed to have moved on a little way. Morning after
morning this movement was found to continue, and soon a second spot,
and then a third spot, were observed creeping in like manner across
the sun. After a while they vanished, one at a time, round his edge,
as it were; but after some days of patient waiting on the part of the
lookers-on, they appeared again at the opposite edge, and once more
began their journey across.
Fabricius seems to have been the first, but he was not the last, to
watch sun-spots. Many astronomers have given close attention to them.
Modern telescopes, and the modern plan of looking at the sun through
darkened glass, have made this possible in a way that was not possible
two or three hundred years ago.
The first important discovery made through the spots on the sun, was
that the sun turns round upon his axis, just in the same manner that
the earth turns round upon hers. Instead of doing so once in the course
of each twenty-four hours, like the earth, he turns once in the course
of about twenty-five days.
It must not be supposed that the spots seen now upon the sun are the
same spots that Fabricius saw so long ago. There is perpetual change
going on; new spots forming, old spots vanishing; one spot breaking
into two, two spots joining into one, and so on. Even in a single hour
great alterations are sometimes seen to take place.
Still, many of the spots do remain long enough and keep their shapes
closely enough to be watched from day to day, and to be known again as
old friends when they reappear, after being about twelve days hidden on
the other side of the sun. So that the turning of the sun upon his axis
has become, after long and careful examination, a certain known fact.
For more than thirty years one astronomer kept close watch over the
spots on every day that it was possible to see the sun. Much has been
learned from his resolute perseverance.
Now what are these spots?
One thing seems pretty sure, and that is that they are caused by some
kind of tremendous storms or cyclones taking place on the surface of
that huge ball of fire.
[Illustration: SUN SPOTS.]
The first attentive observer of the sun, Scheiner, at first regarded
the spots as satellites--an indefensible opinion, which, however,
some have attempted to revive. Galileo attributed them to clouds or
vapors floating in the solar atmosphere; this was the best conclusion
which could be drawn from the observations of that epoch. This opinion
met for a long time with general approval; it has even been renewed
in our day. Some astronomers, and among them Lalande, believed, on
the contrary, that they were mountains, of which the flanks, more
or less steep, might produce the aspect of the penumbra--an opinion
irreconcilable with the proper motion which the spots sometimes possess
in a very marked manner. It is not usual, in fact, to see mountains
traveling. Derham attributed them to the smoke issuing from the
volcanic craters of the sun, an opinion revived and maintained in
recent times by Chacornac. Several _savants_, regarding the sun as a
liquid and incandescent mass, have also explained the spots by immense
cinders floating on this ocean of fire. But a century had scarcely
elapsed after the epoch when the spots were observed for the first
time, when an English astronomer, Wilson, showed with certainty that
the spots are hollow.
We do not know with any certainty whether the sun is through and
through one mass of glowing molten heat, or whether he may have a solid
and even cool body within the blazing covering. Some have thought the
one, and some the other. We only know that he is a mighty furnace of
heat and flame, beyond anything that we can possibly imagine on our
quiet little earth.
It seems very sure that no such thing as “quietness” is to be found on
the surface of the sun. The wildest and fiercest turmoil of rushing
wind and roaring flame there prevails. The cyclones or hurricanes which
take place are sometimes so rapid and so tremendous in extent, tearing
open the blazing envelope of the sun, and showing glimpses of fiery
though darker depths below, that we, on our far-distant earth, can
actually watch their progress.
Astronomers speak of the “quivering fringe of fire” all around the edge
of the sun, visible through telescopes. But the sun is perpetually
turning on his axis, so that each hour fresh portions of his surface
thus pass the edge, showing the same appearance. What conclusion
can we come to but that the whole enormous surface of the sun is one
restless, billowy sea of fire and flame?
It sounds to us both grand and startling to hear of the mighty
outbreaks from Mount Vesuvius, or to read of glowing lava from a
volcano in Hawaii pouring in one unbroken stream for miles.
But what shall be thought of rosy flames mounting to a height of fifty
or a hundred thousand miles above the edge of the sun? What shall be
thought of a tongue of fire long enough to fold three or four times
round our solid earth? What shall be thought of the awful rush of
burning gases, sometimes seen, borne along at the rate of one or two or
even three hundred miles in a single second across the sun’s surface?
What shall be thought of the huge dark rents in this raging fiery
ocean, rents commonly from fifty to one hundred thousand miles across,
and not seldom more?
Fifty thousand miles! A mere speck, scarcely visible without a
telescope; yet large enough to hold seven earths like ours flung in
together. The largest spot measured was so enormous that eighteen
earths might have been arranged in a row across the breadth of it, like
huge boulders of rock in a mountain cavern; and to have filled up the
entire hole about one hundred earths would have been needed.
It is well to grow familiar with certain names given by astronomers to
certain parts of the sun.
The round shining disk or flat surface, seen by all of us, is called
the _photosphere_, or “light-sphere.” It has a tolerably well-defined
edge or “limb,” and dazzles the eye with its intense brightness.
Across the photosphere the spots move, sometimes many, sometimes
few, in number. Besides the dark spots, there are spots of extreme
brilliancy, standing out on even that dazzling surface, which causes
a piece of white-hot iron to look black and cold by contrast. These
extra-radiant spots which come and go, and at times change with great
rapidity, are named _faculæ_--a Latin word meaning “torches.”
At the edge of the photosphere astronomers see what has been already
mentioned, that which is sometimes called the _chromotosphere_, which
one has named “the _sierra_,” and which another has described as “a
quivering fringe of fire.” The waves of the sea, on a stormy day, seen
in the distance rising and breaking the horizon-line, may serve as an
illustration; only in the sun the waves are of fire, not water. What
must their height be, to be thus visible at a distance of ninety-three
millions of miles?
Outside the _sierra_ are seen, at certain seasons, bright,
“rose-colored prominences” or flames of enormous height. During an
eclipse, when the dark body of the moon comes between the sun and us,
exactly covering the photosphere, these red tips stand out distinctly
beyond the edge of sun and moon. Their changes have been watched, and
their height repeatedly measured. Some are so lofty that ten little
earths such as ours might be heaped up, one upon another, without
reaching to their top. Whether they are in character at all like the
outbursts from our own volcanoes it is hard to say. To compare the
two would be rather like comparing a small kitchen fire with a mighty
iron-smelting furnace. The faculæ, the sierra, and the prominences are
visible only through a telescope.
Outside these red flames or “prominences” is the corona, commonly
divided into the inner and outer coronas.
Many different explanations have been offered of this beautiful crown
of light round the sun, plainly visible to the naked eye during an
eclipse. But here again we know little, and must be content to watch
and wait.
CHAPTER III.
WHAT BINDS THE FAMILY TOGETHER?
What is it which binds together all the members of the Solar System?
Ah, what? Why should not the sun at any moment rush away in one
direction, the earth in a second, the planets in half a dozen others?
What is there to hinder such a catastrophe? Nothing--except that
they are all held together by a certain close family tie; or, more
correctly, by the powerful influence of the head of the family.
This mysterious power which the sun has, and which all the planets
have also in their smaller degrees, is called Attraction. Sometimes
it is named Gravitation or Gravity. When we speak, as we often do,
of the _law_ of attraction or gravitation, we mean simply this--that
throughout the universe, in things little and great, is found a certain
wonderful _something_, in constant action, which we call a “law.” What
the “something” may be, man can not tell; for he knows it only by its
effects. But these effects are seen everywhere, on all sides, in the
earth and in the universe. It is well named in being called a “law;”
for we are compelled to obey it. None but the Divine Lawgiver who made
this law can for a single moment suspend its working.
What causes an apple to fall to the ground when it drops from the
branch? Why should it not, instead, rise upwards? Because, of course,
it is heavy, or has weight. But what _is_ weight? Simply this--that
the earth draws or drags everything downwards towards herself by the
power of attraction. Every substance, great or small, light or heavy,
is made up of tiny atoms. Each one of these atoms attracts or draws all
the other atoms towards itself; and the closer they are together, the
more strongly they pull one another.
The atoms in a piece of iron are much closer than the atoms in a piece
of wood; therefore the iron is called the “more _dense_” of the two,
and its weight or “mass” is greater. The more closely the atoms are
pressed together, the greater the number of them in a small space, and
the more strong the drawing towards the earth; for the earth draws each
one of these atoms equally. That is only another way of saying that
a thing is “heavier.” If you drop a stone from the top of a cliff,
will it rise upwards or float in the air? No, indeed. The pull of the
earth’s attraction, dragging and still dragging downward, makes it rush
through the air, with speed quickening each instant, till it strikes
the ground. Every single atom in every single body _pulls_ every other
atom, whether far or near. The nearer it is, the stronger always the
pulling.
We do not always feel this, because the very much greater attraction of
the earth hides--or smothers, as we may say--the lesser attraction of
each small thing for another. But though you and I might stand side by
side upon earth, and feel no mutual attraction, yet if we could mount
up a few thousands of miles, far away from earth, and float in distant
space, there we should find ourselves drawn together, and unable to
remain apart.
Now precisely as an apple falling from a tree and a stone dropping
from a cliff are dragged downward to the earth, just so our earth and
all the planets are dragged downwards towards the sun and towards each
other. The law of the earth’s attraction of all objects on its surface
to itself was indistinctly suspected a very long time ago; but it was
the great Newton who first discovered that this same law was to be
found working among the members of the whole Solar System. The sun
attracts the earth, and the earth attracts the sun. But the enormous
size of the sun compared with our earth--like a great nine-foot globe
beside a tiny one-inch ball--makes our power of attraction to be quite
lost sight of in his, which is so much greater.
The principle of gravitation is of far wider scope than we have yet
indicated. We have spoken merely of the attraction of the earth, and
we have stated that its attraction extends throughout space. But the
law of gravitation is not so limited. Not only does the earth attract
every other body, and every other body attract the earth, but each of
these bodies attracts each other; so that, in its more complete shape,
the law of gravitation announces that “every body in the universe
attracts every other body with a force which varies inversely as the
square of the distance.” It is impossible for us to overestimate the
importance of this law. It supplies the clue by which we can unravel
the complicated movements of the planets. It has led to marvelous
discoveries, in which the law of gravitation has enabled us to
anticipate the telescope, and, indeed, actually to feel the existence
of bodies before those bodies have even been seen.
We come now to another question. If the sun is pulling with such power
at the earth and all her sister planets, why do they not fall down upon
him? What is to prevent their rolling some day into one of those deep
rents in his fiery envelope? Did you ever tie a ball to a string, and
swing it rapidly round and round your head? If you did, you must have
noticed the steady outward pull of the ball. The heavier the ball, and
the more rapid its whirl, the stronger the pull will be. Let the string
slip, and the rush of the ball through the air to the side of the room
will make this yet more plain. Did you ever carry a glass of water
quickly along, and then, on suddenly turning a corner, find that the
water has not turned with you? It has gone on in its former direction,
leaving the glass, and spilling itself on the floor.
The cause in both cases is the same. Here is another “law of nature,”
so-called. Though we can neither explain nor understand why and how
it is so, we see it to be one of the fixed rules of God’s working in
every-day life throughout his universe. The law, as we see it, seems
to be this: Everything which is at rest must remain at rest, until
set moving by some cause outside of, or independent of itself; and
everything which is once set moving, must continue moving in a straight
line until checked.
According to this a cannon-ball lying on the ground ought to remain
there until it is set in motion; and, once set in motion by being fired
from a cannon, it ought to go on forever. Exactly so--if nothing
stops it. But the earth’s attraction draws the cannon-ball downward,
and every time it strikes the ground it is partly checked. Also each
particle of air that touches it helps to bring it to rest. If there
were no earth and no air in the question, the cannon-ball might rush on
in space for thousands of years.
Why did the water get spilled? Because it necessarily continued moving
in a straight line. Your sudden change of direction compelled the solid
glass to make the same change, but the liquid water was free to go
straight on in its former course; so it obeyed this law, and _did_ go
on. Why did the ball pull hard at the string as you swung it round?
Because at each instant it was striving to obey this same law, and to
rush onward in a straight line. The pull of the string was every moment
fighting against that inclination, and forcing the ball to move in a
circle.
Just such is the earth’s movement in her yearly journey round the sun.
The string holding in the ball pictures the sun’s attraction holding
in the earth. The pulling of the ball outward in order to continue
its course in a straight line, pictures the pulling of our earth each
moment to break loose from the sun’s attraction and to flee away into
distant space.
For the earth is not at rest. Each tick of the clock she has sped
onward over more than eighteen miles of her pathway through the sky.
Every instant the sun is dragging, with the tremendous force of his
attraction, to make her fall nearer to him. Every instant the earth is
dragging with the tremendous force of her rapid rush, to get away from
him. These two pullings so far balance each other, or, more strictly,
so far combine together, that between the two she journeys steadily
round and round in her nearly circular orbit.
If the sun pulled a little harder she would need to travel a little
faster, or she would gradually go nearer to him. If the earth went
faster, and the sun’s attraction remained the same as it is now, she
would gradually widen her distance. Indeed, it would only be needful
for the earth to quicken her pace to about five-and-twenty miles a
second, the sun’s power to draw her being unchanged, and she would then
wander away from him for ever. Day by day we on our earth should travel
farther and farther away, leaving behind us all light, all heat, all
life, and finding ourselves slowly lost in darkness, cold, and death.
For what should we do without the sun? All our light, all our warmth,
come from him. Without the sun, life could not exist on the earth.
Plants, herbage, trees, would wither; the waters of rivers, lakes,
oceans, would turn to masses of ice; animals and men would die. Our
earth would soon be one vast, cold, forsaken tomb of darkness and
desolation.
We may not think so, but everything which moves, circulates, and lives
on our planet is the child of the sun. The most nutritious foods come
from the sun. The wood which warms us in winter is, again, the sun in
fragments. Every cubic inch, every pound of wood, is formed by the
power of the sun. The mill which turns under the impulse of wind or
water, revolves only by the sun. And in the black night, under the rain
or snow, the blind and noisy train, which darts like a flying serpent
through the fields, rushes along above the valleys, is swallowed up
under the mountains, goes hissing past the stations, of which the pale
eyes strike silently through the mist,--in the midst of night and cold,
this modern animal, produced by human industry, is still a child of the
sun. The coal from the earth which feeds its stomach is solar work,
stored up during millions of years in the geological strata of the
globe.
As it is certain that the force which sets the watch in motion is
derived from the hand which has wound it, so it is certain that all
terrestrial power proceeds from the sun. It is its heat which maintains
the three states of bodies--solid, liquid, and gaseous. The last
two would vanish, there would be nothing but solids; water and air
itself would be in massive blocks,--if the solar heat did not maintain
them in the fluid state. It is the sun which blows in the air, which
flows in the water, which moans in the tempest, which sings in the
unwearied throat of the nightingale. It attaches to the sides of the
mountains the sources of the rivers and glaciers, and consequently the
cataracts and the avalanches are precipitated with an energy which
they draw directly from him. Thunder and lightning are in their turn
a manifestation of his power. Every fire which burns and every flame
which shines has received its life from the sun. And when two armies
are hurled together with a crash, each charge of cavalry, each shock
between two army corps, is nothing else but the misuse of mechanical
force from the same star. The sun comes to us in the form of heat,
he leaves us in the form of heat; but between his arrival and his
departure, he has given birth to the varied powers of our globe.
I have spoken before about the old-world notion that our earth was a
fixed plain, with the sun circling round her. When the truth dawned
slowly upon some great minds, anxious only to know what really was the
truth, others made a hard struggle for the older and pleasanter mode
of thinking. It went with many sorely against the grain to give up all
idea of the earth being the chief place in the universe. Also there was
something bewildering and dizzying in the notion that our solid world
is never for one moment still. But truth won the victory at last. Men
consented slowly to give up the past dream, and to learn the new lesson
put before them.
We still talk of the sun rising and setting, and of the stars doing the
same. This is, however, merely a common form of speech, which means
just the opposite. For instead of the sun and stars moving, it is the
earth which moves.
The earth has two distinct movements. Indeed, I ought to say that she
has three; but we will leave all thought of the third for the present.
First, she turns round upon her axis once in every twenty-four hours.
Secondly, she travels round the sun once in about every three hundred
and sixty-five days and a quarter. No wonder our ancestors were
startled to learn that the world, which they had counted so immovable,
was perpetually spinning like a humming-top, and rushing through space
like an arrow.
You may gain some clear notions as to the daily rising and setting of
our sun, with the help of an orange. Pass a slender knitting-needle
through the orange, from end to end, and hold it about a yard distant
from a single candle in a room otherwise darkened. Let the needle or
axis slant somewhat, and turn the orange slowly upon it.
The candle does not move; but as the orange turns, the candle-light
falls in succession upon each portion of the yellow rind. Half of the
orange is always in shade, and half is always in light; while at either
side, if a small fly were standing there, he would be passing round out
of shade into candle-light, or out of candle-light into shade.
Each spot on our earth moves round in turn into half-light, full-light,
half-light, and darkness; or, in other words, has morning-dawn,
midday-light, evening twilight, and night. Each spot on our earth would
undergo regularly these changes every twenty-four hours throughout the
year, were it not for another arrangement which so far affects this
that the North and South Poles are, by turns, cut off from the light
during many months together.
Thus the sun is in the center of the Solar System, turning slowly on
his axis; and the earth and the planets travel round him, each spinning
like a teetotum, so as to make the most of his bright warm rays. But
for this spinning movement of the earth, our day and night, instead of
being each a few hours long, would each last six months.
You may notice that, as you turn the orange steadily round, the outside
surface of the skin has to move much more slowly in those parts close
to the knitting-needle, than in those parts which bulge out farthest
from it. Near the North and South Poles the surface of our earth
travels slowly round a very small circle in the course of twenty-four
hours. But at the equator every piece of ground has to travel about
twenty-five thousand miles in the same time; so that it rushes along at
the rate of more than one thousand miles an hour. A man standing on the
earth at the equator is being carried along at this great speed, not
_through_ the air, for the whole atmosphere partakes of the same rapid
motion, but _with_ the air, round and round the earth’s axis.
Now about the other movement of the earth--her yearly journey round the
sun. While she moves, the sun, as seen from the earth, seems to change
his place. First he is observed against a background of one group of
stars, then against a second, then against a third. Not that the stars
are visible in the daytime when the sun is shining, but their places
are well known in the heavens; and also they can be noted very soon
after he sets, or before he rises, so that the constellations nearest
to him may each day be easily found out.
Of course, in old times the sun was thought to be really taking this
journey among the stars, and men talked of “the sun’s path” in the
heavens. This path was named “the Ecliptic,” and we use the word still,
though we know well that the movements are not really his, but ours.
As the earth’s daily movement causes day and night, so the earth’s
yearly movement causes spring, summer, autumn, and winter. A few pages
back I mentioned, in passing, one slight yet important fact which lies
at the root of this matter about the seasons. The earth, journeying
round the sun, travels with her axis _slanting_. Put your candle in the
middle of the table, and stand at one end, holding your orange. Now let
the knitting-needle, with the orange upon it, so slant that one end
shall point straight over the candle, towards the upper part of the
wall at the farther end of the room. Call the upper end so pointing the
North Pole of your orange. You will see that the candle-light falls
chiefly upon the upper half of the orange; and as you turn it slowly,
to picture day and night, you will find that the North Pole has no
night, and the South Pole has no day. That is summer in the northern
hemisphere, and winter in the southern.
Walk round next to one side of the table, towards the right hand,
taking care to let the knitting-needle point steadily still in exactly
the same direction, not towards the same _spot_, for that would alter
its direction as you move, but towards the same _wall_. Stop, and you
will find the candle lighting up one-half of your orange, from the
North to the South Poles. Turn it slowly, never altering the slope of
the axis, and you will see that every part of the orange comes by turns
under the light. This is the Autumnal Equinox, when days and nights all
over the world are equal in length.
Walk on to the other end of the table, still letting the needle slope
and point steadily as before. Now the candle-light will shine upon the
lower or South Pole, and the North Pole will be entirely in the shade.
This is summer in the southern hemisphere, and winter in the northern.
Pass on to the fourth side of the table, and once more you will find
it, as at the second side, equal light from North Pole to South Pole.
This will be the Spring Equinox.
It is an illustration that may be easily practiced. But everything
depends upon keeping the slant of the needle or axis unchanged
throughout. If it be allowed to point first to right, and then to left,
first towards the ceiling, and then towards the wall, the attempt will
prove a failure.
CHAPTER IV.
THE LEADING MEMBERS OF OUR FAMILY--FIRST GROUP.
The chief distinction between stars and planets is, as before said,
that the stars shine entirely by their own light, while the planets
shine chiefly, if not entirely, by reflected light. The stars are suns;
mighty globes of glowing flame. The planets simply receive the light of
the sun, and shine with a brightness not their own.
A lamp shines by its own light; but a looking-glass, set in the sun’s
rays and flashing beams in all directions, shines by reflected light.
In a dark room it would be dark. If there were no sun to shine upon
Mars or Venus, we should see no brightness in them. The moon is like
the planets in this. She has only borrowed light to give, and none of
her own.
Any one of the planets removed to the distance of the nearest fixed
star, would be invisible to us. Reflected light will not shine nearly
so far as the direct light of a burning body. There may be thousands or
millions of planets circling round the stars--those great and distant
suns--just as our brother-planets circle round our sun. But it is
impossible for us to see them. The planets which we can see are close
neighbors, compared with the stars. I do not mean that they are near in
the sense in which we speak of nearness upon earth. They are only near
in comparison with what is so very much farther away.
For a while we must now leave alone all thought of the distant stars,
and try to gain a clear idea of the chief members of our own Family
Circle--that family circle of which the sun is the head, the center,
the source of life and warmth and light. There are two ways in which
astronomers group the planets of the Solar System. One way is to divide
them into the Inferior Planets, and the Superior Planets.
As the earth travels in her pathway round the sun, two planets travel
on their pathways round the sun nearer to him than ourselves. If the
pathway or orbit of our earth were pictured by a hoop laid upon the
table, with a ball in the center for the sun; then those two planets
would have two smaller hoops of different sizes _within_ ours; and the
rest would have larger hoops of different sizes _outside_ ours. The two
within are called inferior planets, and the rest outside are called
superior planets.
A round hoop would not make a good picture of an orbit. For the yearly
pathway of our earth is not in shape perfectly round, but slightly
oval; and the sun is not exactly in the center, but a little to one
side of the center. This is more or less the case with the orbits of
all the planets.
But the laying of the hoops upon the table would give no bad idea of
the way in which the orbits really lie in the heavens. The orbits of
all the chief planets do not slope and slant round the sun in all
manner of directions. They are placed almost in the same _plane_ as it
is called--or, as we might say, in the same _flat_. In these orbits
the planets all travel round in the same direction. One may overtake a
second on a neighboring orbit, and get ahead of him, but one planet
never goes back to meet another.
In speaking of the orbits, I do not mean that the planets have visible
marked pathways through the heavens, any more than a swallow has a
visible pathway through the sky, or a ship a marked pathway through the
sea. Yet each planet has his own orbit, and each planet so distinctly
keeps to his own, that astronomers can tell us precisely whereabouts in
the heavens any particular planet will be, at any particular time, long
years beforehand.
There is also another mode of grouping the planets, besides dividing
them into superior and inferior planets. By this other mode we find two
principal groups or quartets of planets, separated by a zone or belt of
a great many very small planets.
{ Mercury.
First Group { Venus.
{ Earth.
{ Mars.
The Asteroids or Planetoids.
{ Jupiter.
Second Group { Saturn.
{ Uranus.
{ Neptune.
The first four are small compared with the last four, though much
larger than any in the belt of tiny Asteroids.
It was believed at one time that a planet had been discovered nearer to
the sun than Mercury, and the name Vulcan was given to it. But no more
has been seen of Vulcan, and his existence is so doubtful that we must
not count him as a member of the family without further information.
Mercury is very much smaller than our Earth. The diameter of the earth
is eight thousand miles, but the diameter of Mercury is only about
three thousand miles--not even half that of the earth. Being so much
nearer to the sun than ourselves, the pulling of his attraction is much
greater, and this has to be balanced by greater speed, or Mercury would
soon fall down upon the sun. Our distance from the sun is ninety-three
millions of miles. Mercury’s distance is only about one-third of ours;
and instead of traveling, like the earth, at the rate of eighteen miles
each second, Mercury dashes headlong through space at the mad pace
of twenty-nine miles each second. It is a good thing the earth does
not follow his example, or she would soon break loose from the sun’s
control altogether.
The earth takes more than three hundred and sixty-five days, or twelve
months, to journey round the sun in her orbit. That is what we call
“the length of our year.” But Mercury’s year is only eighty-eight days,
or not quite three of our months. No wonder!--when his pathway is so
much shorter, and his speed so much greater than ours. So Mercury
has four years to one year on earth; and a person who had lived on
Mercury as long as five earthly years, would then be twenty years old.
The increased number of birthdays would scarcely be welcome in large
families, supposing we could pay a long visit there.
The sun, as seen from Mercury, looks about four and a half times as
large as from here; the heat and glare being increased in proportion.
No moon has ever been found, belonging to Mercury.
The planet Mercury is, like the earth and the moon, a globe of dark
matter which only shines and is visible by the illumination of the
solar light. Its motion round the central star, which brings it
sometimes between the sun and us, sometimes in an oblique direction,
sometimes at right angles, and shows us a part incessantly variable,
of its illuminated hemisphere, produces in its aspect, as seen in a
telescope, a succession of phases similar to those which the moon
presents to us. The cut represent the apparent variations of size and
the succession of phases, visible in the evening after sunset; when the
planet attains its most slender crescent, it is in the region of its
orbit nearest to the earth, and passes between the sun and us; then,
some weeks afterwards, it emerges from the solar rays and passes again
through the same series of phases in inverse order, as we see them by
reversing the figure.
[Illustration: PHASES OF MERCURY BEFORE INFERIOR CONJUNCTION--EVENING
STAR.]
Venus, the second inferior planet, is nearly the same size as our
earth. Seen from the earth, she is one of the most brilliant and
beautiful of all the planets. Her speed is three miles a second faster
than ours, and her distance from the sun is about two-thirds that of
our own; so that the orbit of Venus lies half-way between the orbit of
Mercury and the orbit of the earth. The day of Venus is about half an
hour shorter than ours. Her year is nearly two hundred and twenty-five
days, or seven and a half of our months. One or two astronomers have
fancied that they caught glimpses of a moon near Venus; but this is
still quite doubtful, and indeed it is believed to have been a mistake.
Venus and Mercury are only visible as morning and evening planets.
Venus, being farther from the sun, does not go before and follow after
him quite so closely as Mercury, and she is therefore the longer within
sight.
When Venus, traveling on her orbit, comes just between the sun and us,
her dark side is turned towards the earth, and we can catch no glimpse
of her. When she reaches that part of her orbit which is farthest from
us, quite on the other side of the sun, her great distance from us
makes her light seem less. But about half-way round on either side, she
shows exceeding brilliancy, and that is the best view we can get of her.
Seen through a telescope, Venus undergoes phases like those of Mercury,
or of our moon. That is to say, we really have “new Venus,” “quarter
Venus,” “half Venus,” “full Venus,” and so on.
Of all the luminaries in the heavens, the sun and moon excepted, the
planet Venus is the most conspicuous and splendid. She appears like
a brilliant lamp amid the lesser orbs of night, and alternately
anticipates the morning dawn, and ushers in the evening twilight.
When she is to the westward of the sun, in winter, she cheers our
mornings with her vivid light, and is a prelude to the near approach
of the break of day and the rising sun. When she is eastward of that
luminary, her light bursts upon us after sunset, before any of the
other radiant orbs of heaven make their appearance; and she discharges,
in some measure, the functions of the absent moon. The brilliancy of
this planet has been noticed in all ages, and has been frequently
the subject of description and admiration both by shepherds and by
poets. The Greek poets distinguished it by the name of Phosphor,
“light-bringer,” when it rose before the sun, and Hesperus, “the west,”
when it appeared in the evening after the sun retired. It is now
generally distinguished by the name of the Morning and Evening Star.
Next to the orbit of Venus comes the orbit of our own Earth, the third
planet of the first group.
Mars, the fourth of the inner quartet, but the first of the superior
planets, is a good deal smaller than Venus or the earth. The name Mars,
from the heathen god of war, was given on account of his fiery reddish
color. Mars is better placed than Venus for being observed from earth.
When he is at the nearest point of his orbit to us, we see him full in
the blaze of sunlight; whereas Venus, at her nearest point, turns her
bright face away.
The length of the day of Mars--or, in other words, the time he takes
to turn upon his axis--is only forty minutes longer than that of
earth. Mars’ journey round the sun is completed in the course of six
hundred and eighty-seven days, not much less than two of our years.
His distance from the sun is about one hundred and forty millions of
miles, and his speed is fourteen miles a second. We shall find, with
the increasing distance of each planet, that the slower pace balances
the lessened amount of the sun’s attraction.
Passing on from Mars, the last of the first group of planets, we reach
the belt of Asteroids, sometimes called Planetoids, Minor Planets, or
Telescopic Planets. They are so tiny that Mercury is a giant compared
with the largest among them.
The zone of space containing all these little planets is more than a
hundred millions of miles broad. Their orbits do not lie flat in almost
the same plane, but slant about variously in a very entangled fashion.
If a neat model were made of this zone, with a slender piece of wire to
represent each orbit, it would be found impossible to lift up one wire
without pulling up all the rest with it. Those asteroids lying nearest
to the sun take about three of our years to travel round him, and those
lying farthest take about six of our years.
New members of the group are very often found. The number of asteroids
now known amounts to about three hundred and twenty. Ceres is estimated
by Professor Barnard, of the Yerkes Observatory, Chicago University, to
be six hundred miles in diameter. Vesta is about two hundred and fifty
miles in diameter. Meta, on the other hand, is less than seventy-five
miles. There are some smaller ones that do not measure twenty miles in
diameter, and it is probable that there are many which are so small as
to be absolutely invisible to the best telescopes, and which measure
only a few hundred rods in diameter. Twenty thousand Vestas would be
needed to make one globe equal to our earth in size.
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. A world of two, three, or four hundred miles
in diameter is still a continent worthy to satisfy the ambition of a
Xerxes or a Tamerlane.
CHAPTER V.
THE LEADING MEMBERS OF OUR FAMILY--SECOND GROUP.
Leaving behind us the busy zone of planetoids, hurrying round and round
the sun in company, we cross a wide gap, and come upon a very different
sight.
The distance from the sun which we have now reached is little less than
four hundred and fifty millions of miles, or about five times as much
as the earth’s distance; and the sun in the heavens shows a diameter
only one-fifth of that which we are accustomed to see. Slowly--yet not
slowly--floating onwards through space, in his far-off orbit, we find
the magnificent planet Jupiter.
Is eight miles each second slow progress? Compared with the wild whirl
of little Mercury, or even compared with the rate of our own earth’s
advance, we may count it so; but certainly not, compared with our
notions of speed upon earth. Eight miles each second is five hundred
times as fast as the swiftest express-train ever made by man. No mean
pace that for so enormous a body. For Jupiter is the very largest of
all the members of the Solar System except the sun himself--quite
the eldest brother of the family. His diameter is about eighty-five
thousand miles, his axis being nearly eleven times as long as that of
earth. Though in proportion to his great bulk not nearly so heavy as
our earth, yet his bulk is so vast that more than twelve hundred earths
would be needed to make one Jupiter.
It must not, however, be forgotten that there is a certain amount of
uncertainty about these measurements of Jupiter. He seems to be so
covered with a dense atmosphere and heavy clouds that it is quite
impossible for us to learn the exact size of the solid body within.
Jupiter does not travel alone. Borne onwards with him, and circling
steadily around him, are five moons; the smallest with a probable
diameter of about one hundred miles; one about the same size as our
own moon, and the others all larger. The nearest of the five, though
distant from the center of the planet over one hundred and twelve
thousand miles, revolves about him in twelve hours; the second, though
farther from Jupiter than our moon from the earth, speeds round him
in less than two of our days. The most distant, though over a million
miles away, takes scarcely seventeen days to accomplish its long
journey. Jupiter and his moons make a little system by themselves--a
family circle within a family circle.
Like the smaller planets, Jupiter spins upon his axis; and he does this
so rapidly that, notwithstanding his great size, his day lasts only ten
hours, instead of twenty-four hours like ours. But if Jupiter’s day
is short, his year is not. Nearly twelve of our years pass by before
Jupiter has traveled once completely round the sun. So a native of
earth who had just reached his thirty-seventh year, would, on Jupiter,
be only three years old.
Passing onward from Jupiter, ever farther and farther from the sun,
we leave behind us another vast and empty space--empty as we count
emptiness, though it may be that there is in reality no such thing as
emptiness throughout the length and breadth of the universe. The width
of the gap which divides the pathway of Jupiter from the pathway of his
giant brother-planet Saturn is nearly five times as much as the width
of the gap separating the earth from the sun. The distance of Saturn
from the sun is not much less than double the distance of Jupiter.
With this great space in our rear, we come upon another large and
radiant planet, the center, like Jupiter, of another little system;
though it can only be called “little” in comparison with the much
greater Solar System of which it forms a part.
Saturn’s diameter is less than that of Jupiter, but the two come near
enough to be naturally ranked together. Nearly seven hundred earths
would be needed to make one globe as large as Saturn. But here again
the dense and cloudy envelope makes us very uncertain about the
planet’s actual size. Saturn is like Jupiter in being made of lighter
materials than our earth; and also in his rapid whirl upon his axis,
the length of his “day” being only ten and a half of our hours.
From Jupiter’s speed of eight miles each second, we come down in the
case of Saturn to only five miles each second. And Jupiter’s long
annual journey looks almost short, seen beside Saturn’s long journey
of thirty earthly years. A man aged sixty, according to our fashion of
reckoning time, would on Saturn have just kept his second birthday.
The system or family of Saturn is yet more wonderful than that of
Jupiter. Not five only but eight moons travel ceaselessly round Saturn,
each in its own orbit; and in addition to the eight moons, he has
revolving round him three magnificent rings. These rings, as well as
the moons, shine, not by their own brilliancy, for they have none, but
by borrowed sunlight. The farthest of the moons wanders in his lonely
pathway about two millions of miles away from Saturn. The largest of
them is believed to be about the same size as the planet Mars. Of the
three rings circling round Saturn, almost exactly over his equator, the
inside one is dusky, purplish, and transparent; the one outside or over
that is very brilliant; and the third, outside the second, is rather
grayish in hue.
Another vast gap--more enormous than the last. It is a wearisome
journey. From the orbit of Jupiter to the orbit of Saturn at their
nearest points, was five times as much as from the sun to the earth.
But from the orbit of Saturn to the orbit of Uranus, the next member of
the sun’s family, we have double even that great space to cross.
Still, obedient to the pulling of the sun’s attractive power, Uranus
wanders onward in his wide pathway round the sun, at the rate of four
miles a second. Eighty-four of our years make one year of Uranus.
This planet has four moons, and thus forms a third smaller system
within the Solar System; but he may have other satellites also, as yet
undiscovered. In size he is seventy-four times as large as our earth.
One more mighty chasm of nine hundred millions of miles, for the same
distance which separates the pathway of Saturn from the pathway of
Uranus, separates also the pathway of Uranus from the pathway of
Neptune. Cold, and dark, and dreary indeed seems to us the orbit on
which this banished member of our family circle creeps round the sun,
in the course of one hundred and sixty-five years, at the sluggish rate
of three miles a second.
On the planet Saturn, the quantity of light and heat received from
the sun is not much more than a hundredth part of that which we are
accustomed to receive on earth. But by the time we reach Neptune, the
great sun has faded and shrunk in the distance until to our eyes he
looks only like an exceedingly brilliant and dazzling star.
We know little of this far-off brother, Neptune, except that he is
rather larger than Uranus, being one hundred and five times as big as
the earth; that he has at least one moon; and also that, like Uranus,
he is made of materials lighter than those of earth, but heavier than
those of Jupiter or Saturn.
After all, it is no easy matter to gain clear ideas as to sizes and
distances from mere statements of “so many miles in diameter,” and “so
many millions of miles away.” A “million miles” carries to the mind a
very dim notion of the actual reality.
Now if we can in imagination bring down all the members of the Solar
System to a small size, keeping always the same proportions, we may
find it a help. “Keeping the same proportions” means that all must be
lessened alike, all must be altered in the same degree. Whatever the
supposed size of the earth may be, Venus must be still about the same
size as the earth, Saturn seven hundred times as large, and so on.
Also, whatever the distance of the earth from the sun, in miles, or
yards, or inches, Mercury must still be one-third as far, Jupiter still
five times as far, and thus with the rest.
First, as to size alone. Suppose the earth is represented by a small
globe, exactly three inches in diameter. It will be a very small globe.
Not only men and houses, but mountains, valleys, seas, will all have to
be reduced to so minute a size, as to be quite invisible to the naked
eye.
Fairly to picture the other members of the Solar System, in due
proportion, you will have them as follows:
Mercury and Mars will be balls smaller than the earth, and Venus nearly
the same size of the earth. Uranus and Neptune will be each somewhere
about a foot in diameter. Saturn will be twenty-eight inches and
Jupiter thirty-two inches in diameter. The sun will be a huge dazzling
globe, _twenty-six feet_ in diameter. No wonder he weighs seven hundred
and fifty times as much as all his planets put together.
Next let us picture the system more exactly on another and smaller
scale. First, think of the sun as a brilliant globe, about nine feet,
or three yards, in diameter, floating in space.
About one hundred yards from the sun travels a tiny ball, not half
an inch in diameter, passing slowly round the sun--slowly, because
as sizes and distances are lessened, speed must in due proportion be
lessened also. This is Mercury. About two hundred yards from the sun
travels another tiny ball, one inch in diameter. This is Venus. Nearly
a quarter of a mile from the sun travels a third tiny ball, one inch
again in diameter; and at a distance of two feet and a half from it a
still smaller ball, one quarter of an inch in diameter, journeys round
it and with it. These are the earth and the moon. About half as far
again as the last-named ball, travels another, over half an inch in
diameter. This is Mars.
Then comes a wide blank space, followed by a large number of minute
objects, no bigger than grains of powder, floating round the sun in
company. These are the Asteroids. Another wide blank space succeeds the
outermost of them.
About one mile distant from the sun journeys a globe, ten inches in
diameter. Round him, as he journeys, there travel five smaller balls,
the largest of which is about the third of an inch in diameter. Their
distances from the bigger globe vary from one and a third to twelve and
a half feet. These are Jupiter and his moons.
Nearly two miles distant from the sun journeys another globe, about
eight and a half inches in diameter. Eight tiny balls and three
delicate rings circle round him as he moves. These are Saturn and his
belongings. About four miles distant from the sun journeys another
globe, four inches in diameter, with four tiny balls accompanying
him--Uranus and his moons. Lastly, at a distance of six miles from
the sun, one more globe, not much larger than the last, with one tiny
companion, pursues his far-off pathway.
These proportions as to size and distance will serve to give a clear
idea of the Solar System.
CHAPTER VI.
THE MOON.
Come, and let us pay a visit to the moon. We seem to feel a personal
interest in her, just because she is, in so peculiar a sense, our own
friend and close attendant. The sun shines for us; but, then, he shines
for all the members of the Solar System. And the stars--so many as we
can see of them--shine for us too; but no doubt they shine far more
brilliantly for other and nearer worlds. The moon alone seems to belong
especially to ourselves.
Indeed, we are quite in the habit of speaking about her as “our moon.”
Rather a cold and calm friend, some may think her, sailing always
serenely past, whatever may be going on beneath her beams; yet she has
certainly proved herself constant and faithful in her attachment.
We have not very far to travel before reaching her,--merely about
two hundred and forty thousand miles. That is nothing, compared with
the weary millions of miles which we have had to cross to visit some
members of our family. A rope two hundred and forty thousand miles
long would fold nearly ten times round the earth at the equator. You
know the earth’s diameter--about eight thousand miles. If you had
thirty poles, each eight thousand miles long, and could fasten them all
together, end to end, one beyond another, you would have a rod long
enough to reach from the earth to the moon.
Let us take a good look at her before starting. She is very beautiful.
That soft silvery light, so unlike sunlight or gaslight, or any other
kind of light seen upon earth, has made her the darling of poets and
the delight of all who love nature. Little children like to watch
her curious markings, and to make out the old man with his bundle of
sticks, or the eyes, nose, and mouth of the moon--not dreaming what
those markings really are. And in moods of sadness, how the pure calm
moonlight seems to soothe the feelings! Who would suppose that the
moon’s beauty is the beauty rather of death than of life?
[Illustration: APPEARANCE OF THE FULL MOON.]
The stars have not much chance of shining through her bright rays. It
is well for astronomers that she is not always at the full. But when
she is, how large she looks--quite as large as the sun, though in
reality her size, compared with his, is only as a very small pin’s
head compared with a school globe two feet in diameter. Her diameter is
little more than two thousand miles, or one quarter that of our earth;
and her whole surface, spread out flat, would scarcely equal North and
South America, without any of the surrounding islands.
The reason she looks the same size as the sun, is that she is so very
much nearer. The sun’s distance from us is more than one-third as many
_millions_ of miles as the moon’s distance is _thousands_ of miles.
This makes an enormous difference.
We call our friend a “moon,” and say that she journeys round the earth,
while the earth journeys round the sun. This is true, but it is only
part of the truth. Just as certainly as the earth travels round the
sun, so the moon also travels round the sun. And just as surely as
the earth is a planet, so the moon also is a planet. It is a common
mode of expression to talk about “the earth and her satellite.” A no
less correct, if not more correct, way would be to talk of ourselves
as “a pair of planets,” journeying round the same sun, each pulled
strongly towards him, and each pulling the other with a greater or less
attraction, according to her size and weight. For the sun actually does
draw the moon with more force than that with which the earth draws her.
Only as he draws the earth with the same sort of force, and nearly in
the same degree, he does not pull them apart.
The moon, like the other planets, turns upon her axis. She does this
very slowly, however; and most singularly she takes exactly the same
time to turn once upon her axis that she does to travel once round
the earth. The result of this is, that we only see one face of the
moon. If she turned upon her axis, and journeyed round the earth in two
different lengths of time, or if she journeyed round us and did not
turn upon her axis at all, we should have views of her on all sides,
as of other planets. But as her two movements so curiously agree, it
happens that we always have one side of the moon towards us, and never
catch a glimpse of the other side.
And now we are ready to start on our journey of two hundred and forty
thousand miles. An express train, moving ceaselessly onward night and
day, at the rate of sixty miles an hour, would take us there in about
five months and a half. But no line of rails has ever yet been laid
from the earth to the moon, and no “Flying Dutchman” has ever yet plied
its way to and fro on that path through the heavens. Not on the wings
of steam, but on the wings of imagination, we must rise aloft. Come--it
will not take us long. We shall pass no planets or stars on the road,
for the moon lies nearer to us than any other of the larger heavenly
bodies.
Far, far behind us lies the earth, and beneath our feet, as we descend,
stretch the broad tracts of moonland. For “downward” now means towards
the moon, and away from the distant earth.
What a strange place we have reached! The weird, ghastly stillness
of all around, and the glaring, dazzling, cloudless heat, strike us
first and most forcibly. Nothing like this heat have we ever felt on
earth. For the close of the moon’s long day--on this side of its
globe--is approaching, and during a whole fortnight past the sun’s
fierce rays have been beating down on these shelterless plains. Talk
of sweltering tropics on earth! What do you call _this_ heat? Look
at the thermometer: how the quicksilver is rising! Well, that is not
a common thermometer which we have brought with us. The mercury has
stopped--three hundred degrees above boiling water!
Not a cloud to be seen overhead; only a sky of inky blackness, with a
blazing sun, and thousands of brilliant stars, and the dark body of
our own earth, large and motionless, and rimmed with light. Seen from
earth, sun and moon look much the same size; but seen from the moon,
the earth looks thirteen times as large as our full moon. Not even a
little mistiness in the air to soften this fearful glare! Air! why,
there is no air; at least not enough for any human being to breathe
or feel. If there were air, the sky would be blue, not black, and the
stars would be invisible in the daytime. It looks strange to see them
now shining beside the sun.
And then, this deadly stillness! Not a sound, not a voice, not a murmur
of breeze or water. How could there be? Sound can not be carried
without air, and of air there is none. As for breeze--wind is moving
air, and where we have no air we can have no wind. As for water--if
there ever was any water on the moon, it has entirely disappeared. We
shall walk to and fro vainly in search of it now. No rivers, no rills,
no torrents in those stern mountain ramparts rising on every side. All
is craggy, motionless, desolate.
How very, very slowly the sun creeps over the black sky! And no
marvel, since a fortnight of earth-time is here but one day, answering
to twelve hours upon earth. Can not we find shelter somewhere from
this blazing heat? Yonder tall rock will do, casting a sharp shadow
of intense blackness. We never saw such shadows upon earth. There the
atmosphere so breaks and bends and scatters about the light, that
outlines of shadows are soft and hazy, even the clearest and darkest of
them, compared with this.
When will the sun go down? But he is well worth looking at meanwhile.
How magnificent he appears, with his pure radiant photosphere, fringed
by a sierra of dazzling pink and white, orange and gold, purple
and blue. For here no atmosphere lies between to blend all into
yellow-white brightness. And how plainly stand out those prominences
or tongues of tinted flame, not merely rose-colored, as seen from
earth, but matching the sierra in varying hues; while beyond spreads
a gorgeous belt of pink and green, bounded by lines and streams of
delicate white light, reaching far and dying slowly out against the jet
background. The black spots on the face of the sun are very distinct,
and so also are the brilliant faculæ.
We must take a look around us now at moonland, and not only sit gazing
at the sun, though such a sky may well enchain attention. How unlike
our earthly landscapes! No sea, no rivers, no lakes, no streams, no
brooks, no trees, bushes, plants, grass, or flowers; no wind or breeze;
no cloud or mist or thought of possible rain; no sound of bird or
insect, of rustling leaves or trickling water. Nothing but burning,
steadfast, changeless glare, contrasting with inky shadows; sun and
earth and stars in a black heaven above; silent, desolate mountains and
plains below.
For though we stand here upon a rough plain, this moon is a mountainous
world. Ranges of rugged hills stretch away in the distance, with
valleys lying between--not soft, green, sloping, earthly valleys, but
steep gorges and precipitous hollows, all white dazzle and deep shade.
But the mountains do not commonly lie in long ranges, as on earth. The
surface of the moon seems to be dented with strange round pits, or
craters, of every imaginable size. We had a bird’s-eye view of them as
we descended at the end of our long journey moonward. In many parts
the ground appears to be quite honeycombed with them. Here are small
ones near at hand, and larger ones in the distance. The smaller craters
are surrounded by steep ramparts of rock, the larger ones by circular
mountain-ranges. We have nothing quite like them on earth.
Are they volcanoes? So it would seem; only no life, no fire, no action,
remain now. All is dead, motionless, still. Is this verily a blasted
world? Has it fallen under the breath of Almighty wrath, coming out
scorched and seared? Is it simply passing through a certain burnt-out,
chilled phase of existence, through which other planets also pass, or
will pass, at some stage of their career? Who can tell?
We will move onward, and look more closely at that towering mass of
rugged rocks, beyond which the sun will by and by go down. Long jetty
shadows lie from them in this direction. No wonder astronomers on
earth can through their telescopes plainly see these black shadows
contrasting with the glaring brightness on the other side.
A “mass of rocks” I have said; but as, with our powers of rapid
movement, we draw near, we find a range of craggy mountains sweeping
round in a vast circle. Such a height in Switzerland would demand many
hours of hard climbing. But on this small globe attraction is a very
different matter from what it is on earth; our weight is so lessened
that we can leap the height of a tall house without the smallest
difficulty. No chamois ever sprang from peak to peak in his native
Switzerland with such amazing lightness as that with which we now
ascend these mighty rocks.
[Illustration: ONE FORM OF LUNAR CRATER.]
Ha! what a depth on the other side! We stand looking down into one of
the monster craters of the moon. A sheer descent of at least eleven
thousand feet would land us at the bottom. Why, Mont Blanc itself
is only about fifteen thousand feet in height. And what a crater!
Fifty-six miles across in a straight line, from here to the other side,
with these lofty rugged battlements circling round, while from the
center of the rough plain below a sharp, cone-shaped mountain rises to
about a quarter of the height of the surrounding range.
It is a grand sight; peak piled upon peak, crag upon crag, sharp rifts
or valleys breaking here and there the line of the narrow, uplifted
ledge; all wrapped in silent and desolate calm. There are many such
craters as this on the moon, and some much larger.
The sun slowly nears his setting, and sinks behind the opposite range.
How we shiver! The last ray of sunlight has gone and already the ground
is pouring out its heat into space, unchecked by the presence of air
or clouds. The change takes place with marvelous quickness. A deadly
chill creeps over all around. A whole fortnight of earth-time must pass
before the sun’s rays will again touch this spot. Verily the contrasts
of climate in the moon, during the twelve long days and nights which
make up her year, are startling to human notions.
But though the sun is gone we are not in darkness. The stars shine with
dazzling brightness, and the huge body of the earth, always seeming to
hang motionless at one fixed point in the sky, gives brilliant light,
though at present only half her face is lit up and half is in shadow.
Still her shape is plainly to be seen, for she has ever round her a
ring of light, caused by the gathered shining of stars as they pass
behind her thick atmosphere. She covers a space on the sky more than a
dozen times as large as that covered by the full moon in our sky.
[Illustration: THE EARTH AS SEEN FROM THE MOON.]
It would be worth while to stay here and watch the half-earth grow
into magnificent full-earth. But the cold is becoming fearful--too
intense for even the imagination to endure longer. What must be the
state of things on the other side of the moon, where there is no bright
earth-light to take the place of the sun’s shining, during the long two
weeks’ night of awful chill and darkness?
It seems probable that a building on the moon would remain for century
after century just as it was left by the builders. There need be no
glass in the windows, for there is no wind and no rain to keep out.
There need not be fireplaces in the rooms, for fuel can not burn
without air. Dwellers in a city in the moon would find that no dust
can rise, no odors be perceived, no sounds be heard. Man is a creature
adapted for life in circumstances which are very narrowly limited.
A few degrees of temperature, more or less; a slight variation in
the composition of air, the precise suitability of food, make all
the difference between health and sickness, between life and death.
Looking beyond the moon, into the length and breadth of the universe,
we find countless celestial globes, with every conceivable variety of
temperature and of constitution. Amid this vast number of worlds with
which space teems, are there any inhabited by living beings? To this
great question science can make no response save this: We can not tell.
Time for us to wend our way homewards from this desolate hundred-fold
arctic scene. We have more to learn by and by about our friend and
companion. For the present--enough.
CHAPTER VII.
VISITORS.
We come next to the very largest members of our Solar System.
From time to time in past days--and days not very long past
either--people were startled by the sight of a long-tailed star, moving
quickly across the sky, called a comet. We see such long-tailed stars
still, now and then; but their appearance no longer startles us.
It is hardly surprising, however, that fears were once felt. The great
size and brilliancy of some of these comets naturally caused large
ideas to be held as to their weight, and the general uncertainty about
their movements naturally added to the mysterious notions afloat with
respect to their power of doing harm.
A collision between the earth and a comet seemed no unlikely event; and
if it happened--what then? Why, then, of course, the earth would be
overpowered, crushed, burnt up, destroyed. So convinced were many on
this point that the sight of a comet and the dread of the coming “end
of the world” were fast bound together in their minds.
Even when astronomers began to understand the paths of some of the
comets, and to foretell their return at certain dates, the old fear
was not quickly laid to rest. So late as the beginning of the present
century, astronomers having told of an approaching comet, other people
added the tidings of an approaching collision. “If a collision, then
the end of the world,” was the cry; and one worthy family, living and
keeping a shop in a well-known town on the south coast of England,
packed up and fled to America--doubtless under full belief that the
destruction of the Old World would not include the destruction of the
New.
The nature of these singular bodies is somewhat better known in the
present day; yet even now, among all the members of the Solar System,
they are perhaps the ones about which we have most to learn. The
nucleus, or bright and star-like spot, which, with the surrounding
coma or “hair,” we sometimes call the “head” of the comet, is the
densest and heaviest part of the whole. The comets are of immense size,
sometimes actually filling more space than the sun himself, and their
tails stream often for millions of miles behind them; nevertheless,
they appear to be among the lightest of the members of the Solar System.
This excessive lightness greatly lessens the comet’s power of
harm-doing. In the rebound from all the old exaggerated fears, men
laughed at the notion of so light and delicate a substance working any
injury whatever, and even declared that a collision might take place
without people on earth being aware of the fact. It is now felt that
we really know too little about the nature of the said substance to
be able to say what might or might not be the result of a collision.
A certain amount of injury to the surface of the earth might possibly
take place. But of the “end of the world,” as likely to be brought
about by any comet in existence, we may safely banish all idea.
The word “comet” means “a hairy body,” the name having been given from
the hairy appearance of the light around the nucleus. About seventeen
hundred different comets have been seen at different times by men--some
large, some small; some visible to the naked eye, but most of them only
visible through telescopes. These hundreds are, there is no doubt, but
a very small number out of the myriads ranging through the heavens.
If you were seated in a little boat in mid-ocean, counting the number
of fishes which in one hour passed near enough in the clear water for
your sight to reach them, you might fairly conclude, even if you did
not know the fact, that for every single fish which you could see,
there were tens of thousands which you could not see.
Reasoning thus about the comets, as we watch them from our earth-boat
in the ocean of space, we feel little doubt that for each one which we
can see, millions pass to and fro beyond reach of our vision. Indeed,
so long ago as the days of Kepler, that great astronomer gave it as his
belief that the comets in the Solar System, large and small, were as
plentiful as the fishes in the sea. And all that modern astronomers can
discover only tends to strengthen this view.
Why should the comets be called “visitors?” I call them so simply
because many of them _are_ visitors. Some, it is true, belong to the
Solar System. But even in their case, strong doubts are felt whether
they were not once visitors from a distance, caught in the first
instance by the attraction of one of the larger planets, and retained
thenceforward, for a time at least, by the strong attraction of the sun.
[Illustration: PASSAGE OF THE EARTH AND THE MOON THROUGH THE TAIL OF A
COMET.]
Every comet, like every planet, has his own orbit or pathway in the
heavens, though the kind of orbit varies with different comets. There
are, first, those comets which travel round and round the sun in
“closed orbits”--that is, in a ring with joined ends. Only the ring is
always oval, not round. There are, secondly, those which travel in an
orbit which _may_ be closed; but if so, the oval is so long and narrow,
and the farther closed end is at so great a distance, that we can not
speak certainly. There are, thirdly, those which decidedly are mere
visitors. They come from the far-off star-depths, flash once with their
brilliant trains of light through our busy Solar System, causing some
little excitement by the way, and go off in another direction, never to
return.
Only a small part of the orbits of these comets can be seen from
earth; but by careful attention astronomers learn something of the
shape of the curve in which they travel. It is in that way possible to
calculate, sometimes certainly, and sometimes uncertainly, whether a
comet may be expected to return, or whether we have seen him for the
first and the last time. By looking at _part_ of a curve, the rest of
which is hidden from us, we are able to judge whether that part belongs
to a circle or an oval, or whether the two ends pass away in different
directions, and do not join.
The comets, whether members of our family circle or visitors from a
distance, are altogether very perplexing. They are often extremely
large, yet they are always extremely light. They reflect the sun’s
brightness like a planet, yet in some measure they seem to shine by
their own light, like a star. They obey the attraction of the sun, yet
he appears to have a singular power of driving the comets’ tails away
from himself.
For, however rapidly the comet may be rushing round the sun, and
however long the tail may be, it is almost always found to stream in an
opposite direction from the sun. An exception to this rule was seen in
the case of a certain comet with two tails, one of which did actually
point towards the sun; but the inner tail may have been only a “jet” of
unusual length, like in kind to the smaller jets often thus poured out
from the nucleus.
Very curious changes take place in comets as they journey, especially
as they come near the sun. One was seen in the course of a few days to
lose all his hair, and also his tail. Another was seen to break into
two pieces, both of which pieces at last disappeared. Sometimes the one
tail divides into two tails.
Traveling, as the comets do, from intense cold into burning heat,
they are very much affected by the violent change of climate. For the
paths of the comets are such long ovals, or ellipses, that, while they
approach the sun very closely in one part of their “year,” they travel
to enormous distances in the other part.
“Halley’s Comet,” which takes seventy-six of our years to journey round
the sun, comes nearer to him than Venus, and goes farther away from him
than Neptune. As this comet draws gradually closer, he has to make up
for the added pull of the sun’s increasing attraction by rushing onward
with greater and greater rapidity, till he whirls madly past the sun,
and then, with slowly slackening speed, journeys farther and farther
away, creeps at length lazily round the farther end of his orbit in the
chill, dark, neighborhood of Neptune, and once more travels towards the
sun with growing haste.
“Encke’s Comet” has a year of only three and a half of our years, so
he may be said to live quite in our midst. But many comets travel much
farther away than the one named after Halley. It is calculated of some
that, if they ever return at all, it can not be for many hundreds of
years.
[Illustration: PASSAGE OF THE COMET OF 1843 CLOSE TO THE SUN (FEBRUARY
27TH, 10 HOURS, 29 MINUTES.)]
“Newton’s Comet,” seen about two centuries ago, has a journey to
perform of such length that he is not expected again to appear for
several thousand years. Yet, at the nearest point in his orbit, he
approached the sun so closely, that the heat which he endured was
about two thousand times that of red-hot iron. Changes were seen to be
taking place in his shape, as he drew near to the sun, and disappeared.
Four days he was hidden in the sun’s rays. He vanished, with a tail
streaming millions of miles behind him. He made his appearance again
with a tail streaming millions of miles in front of him. But how this
wonderful movement took place is beyond man’s power to explain.
The comet of 1843 was one of the most attractive seen during the
present century. It was first observed in March, and it appeared with
a suddenness which had quite a startling effect. It was an imposing
object in the southern regions. The comet was seen in Italy on the 28th
of February; at Washington, on the 6th of March; at Oporto, on the
14th; but owing to unfavorable weather, it was not visible in England,
or any of the northern countries of Europe, previous to the 17th. A
little after sunset on that day the tail was observed in the western
sky, but the head had already sunk below the horizon. The whole of the
comet appeared on the following evenings for a short time, for it was
traveling away from the sun with great velocity, having doubled the
solar orb before it became visible; and about the beginning of April it
finally disappeared.
The appearance of this startling stranger, as observed at Washington,
is thus described by Lieutenant Maury, of the Hydrographical Office in
that city: “On Monday morning, March 6th, our attention was called to
a paragraph in the newspapers, stating that a comet was visible near
the sun at midday with the naked eye. The sky was clear; but not being
able to discover any thing with the unassisted eye, recourse was had to
the telescope, but with no better success. About sunset in the evening,
the examination was renewed with great diligence, but to no purpose.
The last faint streak of day gilded the west; beautiful and delicate
fleeces of cloud curtained the bed of the sun; the upper sky was
studded with stars, and all hopes of seeing the comet that evening had
vanished. Soon after we had retired, the officer of the watch announced
its appearance in the west. The phenomenon was sublime and beautiful.
The needle was greatly agitated, and a strongly marked pencil of light
was streaming up from the path of the sun in an oblique direction to
the southward and eastward; its edges were parallel. It was 30° long.
Stars could be seen twinkling through it, and no doubt was at first
entertained that this was the tail of the comet.”
The tail, as seen in northerly countries, spread over an arc of the
heavens of about 40°; but in southern latitudes it extended to from 60°
to 70°. It had an absolute length of two hundred millions of miles; so
that, had it been coiled around the earth like a serpent, it would have
girdled it eight thousand times at the equator. This comet approached
still nearer the sun than that of Newton, and must therefore have been
exposed to a heat of greater intensity. Its center is computed to
have been within a hundred thousand miles of the solar surface; and
according to Sir John Herschel’s calculations, it was then exposed to a
heat equal to that which would be received by an equal portion of the
earth’s surface, if it were subject to the influence of forty-seven
thousand suns, placed at the common distance of the actual sun. It is
difficult to conceive how a flimsy substance in such circumstances
could escape being entirely dissipated. But such was its velocity that
it wheeled round the sun in less than two hours.
The general question of the probability and the consequences of a
collision with a comet may be legitimately entertained. With reference
to the first point, it can not be denied that collision is possible;
but, at the same time, it is so extremely improbable that it may be
safely dismissed from apprehension. The fact of such an event not
having been experienced in the known course of terrestrial history is
surely some guarantee against its occurrence. Another may be found in
the small volume of the earth and of comets when compared with the
immensity of space in which they move. According to the well-understood
principles of probabilities, Arago has calculated that, upon the
appearance of a new comet, the odds are as 281,000,000 to 1, that it
will not strike against our globe. But even supposing collision to
occur, all that we know of the constitution of comets justifies the
conclusion that the encounter would involve no terrestrial convulsion,
nor any result incompatible with full security to life and happiness.
So much for the largest members of our circle--largest, though
lightest; members some, visitors others. Now we turn to the smallest.
CHAPTER VIII.
LITTLE SERVANTS.
If you walk out any night after dark, and watch the bright stars
shining in a clear sky--shining as they have done for ages past--you
will probably see, now and then, a bright point of light suddenly
appear, dart along a little distance, and as suddenly vanish.
That which you have seen was not the beginning of a story, but in
ninety-nine cases out of a hundred it was the end of a story. The
little shooting-star was in existence long before you saw him, whirling
through space with millions of little companions. But he has left them
all, and dropped to earth. He is a shooting-star no longer.
If such a journey to the moon as the one described two chapters back
were indeed possible, the voyage aloft would hardly be so easily and
safely performed as is there taken for granted. Putting aside the
thought of other difficulties, such as lack of conveyance and lack of
air, there would be the danger of passing through a very considerable
storm of missiles--a kind of “celestial cannonade”--which, to say the
least, would prove very far from agreeable.
These “starlets” and “meteor planets,” as they have been called, are
not visible in a normal condition, because of their minuteness. But on
entering our atmosphere they are rendered luminous, owing to the heat
evolved by the sudden and violent compression of the air in front of
the moving body. According to this view, shooting-stars, which simply
dart across the heavens, may be regarded as coming within the limits of
the atmosphere, and carried out of it again, by their immense velocity,
passing on in space. Meteoric showers may result from an encounter with
a group of these bodies, while aërolites are those which come so far
within the sphere of the earth’s attraction as to fall to its surface.
More than two thousand years ago the Greeks venerated a famous stone
which fell from the heavens on the river Ægos.
[Illustration: METEOR EMERGING FROM BEHIND A CLOUD (NOV. 23, 1877).]
It will scarcely be believed what numbers of these shooting-stars or
meteorites constantly fall to the earth. As she travels on her orbit,
hurrying along at the rate of nineteen miles each second, she meets
them by tens of thousands. They too, like the earth, are journeying
round the great center of our family. But they are so tiny, and
the earth by comparison is so immense, that her strong attraction
overpowers one after another, drags it from its pathway, and draws it
to herself.
And then it falls, flashing like a bright star across the sky, and
the little meteorite has come to his end. His myriads of companions,
hastening still along their heavenly track--for the meteorites seem to
travel commonly in vast flocks or companies--might, had they sense,
mourn in vain for the lost members of their family.
Any one taking the trouble to watch carefully some portion of the sky
after dark, may expect to see each hour about four to eight of these
shooting-stars--except in the months of August and November, when the
number is much larger. About six in an hour does not sound a great
deal. But that merely means that there have been six in one direction,
and near enough for you to see. Somebody else, watching, may have
seen six in another direction; and somebody else, a few miles away,
may have seen six more. It is calculated that, in the course of every
twenty-four hours, about four hundred millions of meteorites fall to
earth, including those visible only through telescopes.
This is rather startling. What if you or I should some day be struck
by one of these solid, hard little bodies, darting as they do towards
earth with speed swifter than that of a cannon-ball? True, they are not
really stars, neither are they really planets. But they are, to say
the least, often much larger than a cannon-ball, and a cannon-ball can
destroy life.
Four hundred millions every twenty-four hours! Does it not seem
singular that we do not see them constantly dropping to the ground?
The truth is, we _should_ see them, and feel them too, and dire would
be the danger to human life, but for a certain protecting something
folded round this earth of ours to ward off the peril. That “something”
is the earth’s atmosphere. But for the thick, soft, strong, elastic
air through which the meteorites have to pass, they would fall with
fearful violence, often doing terrible mischief. As it is, we are
guarded. The shooting-star, drawn by the earth’s attraction, drops into
her atmosphere, darting with tremendous speed. In consequence of this
speed and the resistance of the air, it catches fire. That is when we
first see it. The meteorites are believed to appear at a height of
about seventy miles, and to disappear at a height of about fifty miles.
So that, in one instant’s flash, the shooting-star has traveled some
twenty miles toward us. Then the light goes out. The little meteorite
is burnt. It falls to earth still; but only as fine dust, sinking
harmlessly downward.
The meteorites do not always vanish so quickly. Now and then a larger
one--too large to be rapidly burnt--does actually reach the ground. If
any man were struck by such a stone he would undoubtedly be killed.
When meteorites thus fall to earth they are usually called _aërolites_.
Some are found no bigger than a man’s fist, while others much exceed
this size. There is one, kept carefully in the British Museum, which
weighs three tons and a half; and we hear of another, lying in South
America, between seven and eight feet in length. Such a sky-visitant
would be very unwelcome in any of our towns.
We must remember that, whatever size an aërolite may be when it reaches
the earth, it must have been far greater when journeying round the sun,
since a good part of it has been burnt away during its rush downward
through the earth’s atmosphere.
Meteors, or bolides, or fireballs, are of much the same nature as
meteorites; but they are larger, longer to be seen, and slower in
movement. Also, it is not uncommon for them to burst with a loud
explosion. Early in the present century such a meteor visited Normandy.
It exploded with a noise like the roll of musketry, scattering
thousands of hot stones over a distance of several miles. A great
number of them were collected, still smoking. The largest of these
stones weighed no less than twenty pounds. Happily no one seems to have
been injured. Other such falls have taken place from time to time.
Sometimes bright, slowly-moving meteors have been seen, looking as
large as the moon.
A remarkable fireball appeared in England on November 6, 1869. This
fireball was extensively seen from different parts of the island, and
by combining and comparing these observations, we obtain accurate
information as to the height of the object and the velocity with which
it traveled. It appears that this meteor commenced to be visible at
a point ninety miles above Frome, in Somersetshire, and that it
disappeared at a point twenty-seven miles over the sea, near St. Ives,
in Cornwall. The whole length of its course was about 170 miles, which
was performed in a period of five seconds, thus giving an average
velocity of thirty-four miles a second. A remarkable feature in the
appearance which this fireball presented was the long, persistent
streak of luminous cloud, about fifty miles long and four miles wide,
which remained in sight for fully fifty minutes. We have in this
example an illustration of the chief features of the phenomena of a
shooting-star presented on a very grand scale. It is, however, to be
observed that the persistent luminous streak is not a universal, nor,
indeed, a very common characteristic of a shooting-star.
If we may liken comets to the _fishes_ of the Solar System--and in
their number, their speed, their varying sizes, their diverse motions,
they may be fairly so likened--we may perhaps speak of the meteorites
as the _animalcula_ of the Solar System. For, in comparison with the
planets, they are, in the matter of size, as the animalcula of our
ponds in comparison with human beings. In point of numbers they are
countless.
Take a single drop of water from some long-stagnant pond, and place
it under a powerful microscope. You will find it to be full of life,
teeming with tiny animals, darting briskly to and fro. The drop of
water is in itself a world of living creatures, though the naked eye
of man could never discover their existence. So with the meteorites.
There is good reason to believe that the Solar System fairly teems with
them. We talk of “wide gaps of empty space,” between the planets; but
how do we know that there is any such thing as empty space to be found
throughout all the sun’s domain?
Not only are the meteorites themselves countless, a matter easily
realized, but the families or systems of meteorites appear to be
countless also. They, like the systems of Jupiter and Saturn, are
each a family within a family--a part of the Solar System, and yet a
complete system by themselves. Each circles round the sun, and each
consists of millions of tiny meteorites. When I say “tiny,” I mean it
of course only by comparison with other heavenly bodies. Many among
them may possibly be hundreds of feet and even more in diameter, but
the greater proportion appear to be much smaller. It is not impossible
that multitudes beyond imagination exist, so small in size that it is
impossible we should ever see them, since their dying flash in the
upper regions of our atmosphere would be too faint to reach our sight.
The earth, traveling on her narrow orbit round the sun, crosses the
track of about one hundred of these systems, or rings. Sometimes she
merely touches the edge of a ring, and sometimes she goes into the very
thick of a dense shower of meteorites. Twice every year, for instance,
on the 10th of August and the 11th of November, the earth passes
through such a ring, and very many falling stars may be seen on those
nights. Numbers of little meteorites, dragged from their orbits and
entangled in the earth’s atmosphere, like a fly caught in a spider’s
web, give their dying flash, and vanish. It used to be supposed that
the August and November meteorites belonged to one single system; but
now they are believed to be two entirely distinct systems.
[Illustration: THE GREAT SHOWER OF SHOOTING-STARS, NOVEMBER 27, 1872.]
The comet discovered on February 27, 1827, by Biela, and ten days later
at Marseilles by Gambart, who recognized that it was the same as that
of 1772 and 1805, returned six and a half years later, in 1832. In
fact it crossed, as we have seen, the plane of the terrestrial orbit
at the respectable distance of fifty millions of miles from the earth;
but if there was any danger in this meeting, it was rather for it
than for us; for it was certainly strongly disturbed in its course.
It returned in 1839, but under conditions too unfavorable to enable
it to be observed--in the month of July, in the long days, and too
near the sun. It was seen again in 1845, on November 25th, near the
place assigned to it by calculation, and its course was duly followed.
Everything went on to the general satisfaction, when--unexpected
spectacle!--on January 13, 1846, _the comet split into two_! What had
passed in its bosom? Why this separation? What was the cause of such a
celestial cataclysm? We do not know; but the fact is, that instead of
one comet, two were henceforth seen, which continued to move in space
like two twin-sisters--two veritable comets, each having its nucleus,
its head, its coma, and its tail, slowly separating from each other. On
February 10th there was already a hundred and fifty thousand miles of
space between the two. They would seem, however, to have parted with
regret, and during several days a sort of bridge was seen thrown from
one to the other. The cometary couple, departing from the earth, soon
disappeared in the infinite night.
They returned within view of the earth in the month of September, 1852.
On the 26th of this month the twins reappeared, but much farther apart,
separated by an interval of twelve hundred and fifty thousand miles.
But this is not the strangest peculiarity which this curious body
presented to the attention of astronomers. The catastrophe which was
observed in 1846 was only a presage of the fate which awaited it; for
now its existence is merely imagined, the truth being that _this comet
is lost_. Since 1852 all attempts to find it again have been unavailing.
To be lost is interesting, especially for a comet. But this, doubtless,
was not enough; for it reserved for us a still more complete surprise.
Its orbit intersects the terrestrial orbit at a point which the earth
passes on November 27th. Well, nothing more was thought about it--it
was given up as hopeless, when, on the evening of November 27, 1872,
there fell from the sky a veritable _rain of shooting stars_. The
expression is not exaggerated. They fell in great flakes. Lines of fire
glided almost vertically in swarms and showers--here, with dazzling
globes of light; there, with silent explosions, recalling to mind those
of rockets; and this rain lasted from seven o’clock in the evening
till one o’clock next morning, the maximum being attained about nine
o’clock. At the observatory of the Roman College, 13,892 were counted;
at Montcalieri, 33,400; in England a single observer counted 10,579,
etc. The total number seen was estimated at _a hundred and sixty
thousand_. They all came from the same point of the sky, situated near
the beautiful star Gamma of Andromeda.
On that evening I happened to be at Rome, in the quarter of the Villa
Medicis, and was favored with a balcony looking towards the south. This
wonderful rain of stars fell almost before my eyes, so to say, and I
shall never cease regretting not having seen it. Convalescent from
a fever caught in the Pontine Marshes, I was obliged to go into the
house immediately after the setting of the sun, which on that evening
appeared from the top of the Coliseum to sleep in a bed of purple
and gold. My readers will understand what disappointment I felt next
morning when, on going to the observatory, Father Secchi informed me
of that event! How had he observed it himself? By the most fortunate
chance: A friend of his, seeing the stars fall, went to him to ask
an explanation of such a phenomenon. It was then half-past seven. The
spectacle had commenced; but it was far from being finished, and the
illustrious astronomer was enabled to view the marvelous shower of
nearly _fourteen thousand_ meteors.
This event made a considerable stir in Rome, and the pope himself did
not remain indifferent; for, some days afterwards, having had the
honor of being received at the Vatican, the first words that Pius IX
addressed to me were these: “Have you seen the shower of Danaë?” I had
admired, some days before in Rome, some admirable “Danaës,” painted by
the great masters of the Italian school in a manner which left nothing
to be desired; but I had not had the privilege of finding myself under
the cupola of the sky during this new celestial shower, more beautiful
even than that of Jupiter.
What was this shower of stars? Evidently--and this is not doubtful--the
encounter with the earth of myriads of small particles of matter moving
in space along the orbit of Biela’s comet. The comet itself, if it
still existed, would have passed twelve weeks before. It was not, then,
to speak correctly, the comet itself which we encountered, but perhaps
a fraction of its decomposed parts, which, since the breaking-up of the
comet in 1846, would be dispersed along its orbit behind the head of
the comet.[1]
[1] No doubt can remain of the identity of this swarm of shooting-stars
with the comet of Biela. On November 27, 1885, the same encounter
occurred. A magnificent shower of stars was observed all over Europe
just at the moment when the earth crossed the comet’s orbit.
Once in every thirty-three years we have a grand display of meteorites
in November; tens of thousands being visible in one single night. The
meteorites in that ring have their “year” of thirty-three earthly
years, and once in the course of that long year our earth’s orbit
carries her deep into their midst. In this single November ring there
are myriads upon myriads of meteorites, spreading through millions of
miles of space.
Yet this system is but one among many. There is no reason whatever
to suppose that the streams of meteorites cluster more thickly about
the orbit of the earth, than in other parts of the Solar System. No
doubt the rest of the planets come across quite as many. Indeed,
the wonderful rings of Saturn are probably formed entirely of
meteorites--millions upon millions of them whirling round the planet
in a regular orbit-belt, lit up by the rays of the sun. Also it is
believed that the meteorite families cluster more and more closely in
the near neighborhood of the sun, rushing wildly round him, and falling
by millions into the ocean of flame upon his surface. It has even been
guessed that they may serve in part as fuel to keep up his mighty
furnace heat.
There is a curious cone-shaped light seen sometimes in the west after
sunset. It is called the “Zodiacal Light,” and men have often been much
puzzled to account for it. The shining is soft and dim, only to be
seen when the sky is clear, and only to be seen in the neighborhood of
the sun. This, too, _may_ be caused by reflected light from countless
myriads of meteorites gathering thickly round the sun.
CHAPTER IX.
NEIGHBORING FAMILIES.
We have now to take flight in thought far, far beyond the outskirts of
our little Solar System. Yes, our _great_ Solar System, with its mighty
sun, its planets, its moons, its comets and meteorites, its ceaseless
motions, its vast distances,--even all this sinks to littleness beside
the wider reaches of space which now have to be pictured to our minds.
For our sun, in all his greatness, is only a single star--only one star
among other stars--and not by any means one of the largest of the stars.
How many stars are there in the sky? Look overhead some cloudless
night, and try to count the brilliant points of light. “Millions,” you
would most likely give as your idea of their number. Yet you would be
wrong; for you do not really perceive so many. The stars visible to
man’s naked eye have been mapped and numbered. It is found that from
two to three thousand are, as a rule, the utmost ever seen at once,
even on a favorable night, and with particularly good sight.
But what is actually the full number of the stars? Two or three
thousand overhead. Five or six thousand round the whole world. So
much visible to man’s unaided eyes. Ah, but take a telescope, and see
through it the opening fields of stars beyond stars. Take a stronger
telescope, and note how, as you pierce deeper into space, fresh stars
beyond fresh stars shine faintly in the measureless distance. Take the
most powerful telescope ever made, and again it will be the same story.
There has been a chart or map drawn of known stars in the northern
hemisphere--including those visible in telescopes down to a certain
magnitude--containing over three hundred thousand. But that is only a
part of even what man can see. Sir William Herschel calculated roughly
that the number of stars within reach of his powerful telescope, round
the whole earth, amounted probably to something like twenty millions.
Twenty millions of suns! For that is what it really means. Twenty
millions of radiant, burning, heavenly bodies--some the same size as
our sun; some larger, perhaps very much larger; some smaller, perhaps
very much smaller; but all SUNS. And any number of these suns may have,
just like our own, families of planets traveling round them, enjoying
their light and their heat.
We talk about stars of the first, second, and other magnitudes. Stars
can be seen without a telescope as low down as the sixth magnitude;
after that they become invisible to the naked eye. This word
“magnitude” is rather misleading. “Magnitude” means size, and whatever
the real size of the stars may be, they have to our sight no seeming
size at all. So when we speak of different _magnitudes_, we really mean
different _brightnesses_. The brightest stars are those of the first
magnitude, the next brightest those of the second magnitude, and so on.
No doubt many a star of the third or fourth magnitude is really much
larger than many a star of the first or second magnitude, only being
farther away it shines more dimly, or the higher-magnitude star may in
itself possess greater natural brilliancy.
Of first-magnitude stars there are altogether about twenty; of
second-magnitude stars about sixty-five; of third-magnitude stars about
two hundred; and so the numbers increase till of the sixth-magnitude
stars we find more than three thousand. These are all that can be
commonly seen with the naked eye, amounting to five or six thousand.
With telescopes the numbers rise rapidly to tens of thousands, hundreds
of thousands, and even millions.
For a long while it was found quite impossible to measure the distances
of the stars. To this day the distances of not over two dozen, among
all those tens of thousands, have been discovered. The difficulty of
finding out the distance of the sun was as nothing compared with the
difficulty of finding out the distances of the stars.
No base-line sufficient for the purpose could for years be obtained.
I must explain slightly what is meant by a “base-line.” Suppose you
were on the brink of a wide river, which you had no means of crossing,
though you wished to discover its breadth. Suppose there were on the
opposite brink a small tree, standing alone. As you stood, you would
see the tree seeming to lie against a certain part of the country
beyond. Then, if you moved along your bank some fifty paces, the tree
would seem to lie against quite a different part of the country beyond.
Now if you had a long piece of string to lay down along the fifty paces
you walked, and if two more pieces of string were tied, one from each
_end_ of the fifty paces, both meeting at the tree, then the three
pieces of string would make one large triangle, and the “fifty paces”
would be the “base” of your triangle.
If you could not cross the river, you could not of course tie strings
to the tree. But having found your _base-line_, and measured its exact
length, and having also found the shape of the two angles at its two
ends, by noting the seeming change of the tree’s position, it would
then be quite easy to find out the distance of the tree. The exact
manner in which this calculation is made can hardly be understood
without some slight knowledge of a science called trigonometry. The
tree’s distance being found, the breadth of the river would be known.
This mode of measuring distance was found comparatively easy in the
case of the moon. But in the case of the sun there was more difficulty,
on account of the sun’s greater distance. No base-line of ordinary
length would make the sun seem to change his position in the sky in the
slightest degree. Nor till the very longest base-line on the earth was
tried could the difficulty be overcome. That base-line is no less than
eight thousand miles long. One man standing in England looking at the
sun, and another man standing in Australia looking at the sun, have
such a base-line lying between them, straight through the center of the
earth.
In the case of the stars this plan was found useless. So closely has
the sky been mapped out, and so exactly is the place of each star
known, that the tiniest change would have been at once noticed. Not
a star showed the smallest movement. The eight thousand miles of the
earth’s diameter was a mere point with regard to them.
A bright idea came up. Here was our earth traveling round the sun, in
an orbit so wide that in the middle of summer she is over one hundred
and eighty millions of miles away from where she is in the middle of
winter. Would not that make a magnificent base-line? Why not observe
a star in summer and observe the same star again in winter, and then
calculate its distance.
This, too, was done. For a long while in vain! The stars showed no
signs of change, beyond those due to causes already known. Astronomers
persevered, however, and with close and earnest care and improved
instruments, success at last rewarded their efforts. A few--only a few,
but still a few--of those distant suns have submitted to the little
measuring-line of earth, and their distance has been roughly calculated.
Now, what is their distance? Alpha Centauri, the second star which was
attempted with success, is the nearest of all whose distance we know.
You have heard how far the sun is from the earth. The distance of Alpha
Centauri is _two hundred and seventy-five thousand times as much_. Can
you picture to yourself that vast reach of space--a line ninety-three
millions of miles long, repeated over and over again two hundred and
seventy-five thousand times?
But Alpha Centauri is one of the very nearest. The brilliant Sirius is
at least twice as far away. Others utterly refuse to show the smallest
change of position. It is with them, as had been said, much the same as
if a man were to look at a church-steeple, twenty miles distant, out
of one pane in a window, and then were to look at it out of the next
pane. With the utmost attention he would find no change of position in
the steeple. And like the base-line of two glass panes to that steeple,
so is the base-line formed by our whole yearly journey to thousands
of distant stars. We _might_ measure how far away they are, only the
longest base-line within our reach is too short for our purpose.
The planet Neptune has a wider orbit than ours. But even his orbit,
seen from the greater number of the stars, would shrink to a single
point. After all, how useless to talk of two hundred and seventy-five
thousand times ninety-three millions of miles! What does it mean? We
can not grasp the thought.
Let us look at the matter from another view. Do you know how fast light
travels--this bright light shining round us all day long? Light, so far
as we know, does not exist everywhere. It travels to and fro, from the
sun to his planets, from the planets to one another; from the sun to
the moon, from the moon to the earth, and from the earth to the moon
again.
Light takes time to travel. This sounds singular, but it is true. Light
can not pass from place to place in no time. Light, journeying through
space, is invisible. Only when it strikes upon something, whether a
solid body or water or air, does it become visible to our eyes. The
shining all round us in the day-time is caused by the sunlight being
reflected, not only from the ground, but from each separate particle
of air. If we had no atmosphere, we should see still the bright rays
falling on the ground, but the sky above would be black. Yet that black
sky would be full of millions of light-rays, journeying hither and
thither from sun and stars, invisible except where they alight upon
something.
The speed of light is far beyond that of an express train, far beyond
that of the swiftest planet. In one tick of the clock, Mercury has
rushed onward twenty-nine miles. In one tick of the clock, storm-flames
upon the surface of the sun will sweep over two or three hundred miles.
But in one tick of the clock a ray of light flashes through one hundred
and eighty-six thousand three hundred and thirty-seven miles.
One hundred and eighty-six thousand miles! That is the same as to say
that, during one single instant, a ray of light can journey a distance
equal to about eight times round and round our whole earth at the
Equator.
By using this wonderful light-speed as a measurement, we gain clearer
ideas about the distances of the stars. A ray of light takes more
than eight minutes to pass from the sun to the earth. Look at your
watch, and note the exact time. See the hand moving slowly through the
minutes, and imagine one single ray of light, which has left the sun
when first you looked, flashing onward and onward through space, one
hundred and eighty-six thousand miles each second. Eight minutes and a
half are over. The ray falls upon your hand. In those few minutes it
has journeyed ninety-three millions of miles.
So much for the sun’s distance. How about the stars? Alpha Centauri,
a bright star seen in the southern hemisphere, is one of our nearest
neighbors. Yet each light-gleam which reaches the eye of man from that
star, left Alpha Centauri four years and a third before. During four
years and a third, from the moment when first it quitted the surface
of the blazing sun, it has flashed ceaselessly onward, one hundred and
eighty-six thousand miles each second, dwindling down with its bright
companion-rays from a glare of brilliancy to a slender glimmer of light
till it reaches the eye of man. Four years and a third sounds much,
side by side with the eight minutes’ journey from the sun. Sound would
take more than three millions of years to cross the same abyss. At
the constant velocity of thirty-seven miles an hour, an express-train
starting from the sun Alpha Centauri would not reach the earth until
after an uninterrupted course of nearly seventy-five millions of years.
This is our _neighbor_ star. The second, the nearest after it, is
nearly double as far, and is found in quite another region of space, in
the constellation of Cygnus, the Swan, always visible in our northern
hemisphere. If we wish to understand the relative situation of our sun
and the nearest two, let us take a celestial globe, and draw a plane
through the center of the globe and through Alpha Centauri and 61
Cygni. We shall thus have before us the relation which exists between
our position in infinitude and those of these two suns. The angular
distance which separates them on the celestial sphere is 125°. Let
us make this drawing, and we shall discover certain rather curious
particulars. In the first place, these two nearest stars are in the
plane of the Milky Way, so that we can also represent the Milky Way on
our drawing; again, this celestial river is divided into two branches,
precisely in the positions occupied by these two nearest stars, the
division remaining marked along the whole interval which separates
them. This drawing shows us, further, that if we wish to trace the
curve of the Milky Way with reference to the distance of our two stars,
it will be nearer to us in the constellation of the Centaur than in
that of the Swan; and, in fact, it is probable that the stars of that
region of the sky are nearer than those of the opposite region. Another
very curious fact is, that both the nearest stars are double.
But look at Sirius, that beautiful star so familiar to us all. The
light which reaches you to-night, left his surface from nine to ten
years ago. Look at the Little Bear, with the Pole-star shining at the
end of his tail. That ray of soft light quitted the Pole-star some
forty years ago. Almost half a century it has been speeding onward and
ever onward, with ceaseless rapidity, till its vast journey is so far
accomplished that it has reached the earth. Look at Capella, another
fair star of the first magnitude. The light which reaches you from her
has taken over thirty years to perform its voyage. Ten, thirty, forty
years--at the rate of 186,000 miles per second!
These stars are among the few whose distance can be roughly measured.
Others lie at incalculable distances beyond. There are stars whose
light must, it is believed, have started hundreds or even thousands of
years before the soft, faint ray at length reaches the astronomer’s
upturned eye through the telescope.
Look at Capella, how gently and steadily she shines! You see Capella,
not as she is _now_, but as she _was_ thirty years ago. Capella may
have ceased to exist meantime. The fires of that mighty sun may have
gone out. If so, we shall by and by learn the fact--more than thirty
years after it happened. Look at the Pole-star. Is there any Pole-star
now? I can not tell you. I only know there _was_ one, forty years ago,
when that ray of light started on its long journey. Stars have ceased
to shine before now. What may have happened in those forty years, who
can say? Look at that dim star, shining through a powerful telescope
with faint and glimmering light. We are told that in all probability
the tiny ray left its home long before the time of Adam.
There is a strange solemnity in the thought. Hundreds of years
ago--thousands of years ago--some say, even tens or hundreds of
thousands of years ago! It carries us out of the little present into
the unknown ages of a past eternity.
If the neighboring stars are placed at tens and hundreds of trillions
of miles from us, it is at quadrillions, at quintillions of miles
that most of the stars lie which are visible in the sky in telescopic
fields. What suns! what splendors! Their light comes from such
distances! And it is these distant suns which human pride would like
to make revolve round our atom; and it was for our eyes that ancient
theology declared these lights, invisible without a telescope, were
created! No contemplation expands the thought, elevates the mind, and
spreads the wings of the soul like that of the sidereal immensities
illuminated by the suns of infinitude. We are already learning that
there is in the stellar world a diversity no less great than that
which we noticed in the planetary world. As in our own Solar System
the globes already studied range from 6 miles in diameter (satellites
of Mars) up to 88,000 miles (Jupiter)--that is to say, in the
proportion of 1 to 14,000--so in the sidereal system the suns present
the most enormous differences of volume and brightness: 61 Cygni, the
stars numbered 2,398 in the catalogue of Lalande, and 9,352 in the
catalogue of Lacaille, and others of the eighth or ninth magnitude, are
incomparably smaller or less luminous than Sirius, Arcturus, Capella,
Canopus, Rigel, and the other brilliants of the firmament.
CHAPTER X.
OUR NEIGHBORS’ MOVEMENTS.
How high! how distant! how mighty! How little we know about them, yet
how overwhelming the little we know, and how wonderful that we do know
it!
We have now to consider the movements of these distant
neighbors--first, their seeming movements; secondly, their real
movements.
I have already spoken about the seeming motions of the stars as a
whole, once believed to be real, and now known to be only caused by the
motions of our earth. For just as the turning of the earth upon her
axis makes the sun seem to rise every morning in the east, and to set
every evening in the west, so that same continued turning makes the
stars seem to rise every evening in the east and to set every morning
in the west.
When we speak of the stars as rising in the east, we do not mean that
they all rise at one point in the east, but that all rise, more or
less, in an easterly direction--northeast, east, and southeast. So also
with respect to the west. It is to the east and west of the earth as a
whole that they rise and set--not merely to the east and west of that
particular spot on earth where one man may be standing. All night long
fresh stars are rising, and others are setting, and if it were not for
the veil of light made by the sunshine in our atmosphere, we should see
the same going on all day long as well.
There are some constellations, or groups of stars, always visible at
night in our northern hemisphere; and there are some constellations
never visible to us, but only seen by people living in the southern
hemisphere--in Australia, for instance. There are other constellations
which appear in summer and disappear in winter, or which appear in
winter and disappear in summer. This change is caused by our earth’s
journey round the sun. It is not that the constellations have altered
their place in the heavens with respect to the other constellations,
it is merely that the earth has so altered its position in the heavens
that the groups of stars which a short time ago were above the horizon
with the sun by day are now above the horizon without him by night.
Mention has been a good many times made of the axis of the earth ending
in the North and South Poles. If this axis were carried straight onward
through space, a long, slender pole passing upwards into the sky
without any bend, from the North Pole in one direction and from the
South Pole in the other,--this would be the pole of the heavens. The
places of the stars in the sky are counted as “so many degrees” from
the North and South Celestial Poles, just as the places of towns on
earth are counted as “so many degrees” from the North and South Poles
of earth. There are atlases of the sky made as well as atlases of the
earth.
The constellation of the Great Bear is known to all who have ever used
their eyes at all to watch the heavens. Almost equally well known
are the two bright stars in this constellation named the Pointers,
because, taken together, they point in nearly a straight line to a
certain important star in the end of the Little Bear’s tail, not very
distant.
This star, important less from its brightness than from its position,
lies close to that very spot in the heavens where the celestial North
Pole passes. It is called the Pole-star. Night after night, through
the year, it there remains, all but motionless, never going below
the horizon for us in the northern hemisphere, or northern half of
the earth; never rising above the horizon for those in the southern
hemisphere. It shines ever softly and steadily in its fixed position.
If you travel further south, the Pole-star sinks downward towards the
horizon. If you travel further north, the Pole-star rises higher above
the horizon. If you were at the North Pole, you would see the Pole-star
exactly overhead.
[Illustration: CONSTELLATION OF THE GREAT BEAR.]
Very near the Pole-star is the constellation of the Great Bear,
with Cassiopeia nearly opposite on the other side of the Little
Bear, and other groups between the two, completing the circle. These
constellations do not, to us who live in the northern hemisphere,
rise or set; for they simply move in a circle round and round the
Pole-star, never going below the horizon. All day and all night long
this circling movement continues, though only visible at night. It is
caused entirely by the earth’s own motion on her axis.
[Illustration: THE GREAT BEAR 50,000 YEARS AGO.]
[Illustration: THE GREAT BEAR 50,000 YEARS HENCE.]
Lower down, or rather further off from the Pole-star, comes another
ring of constellations. These in just the same manner appear to
travel round and round the Pole-star. But being further away, each
dips in turn below the horizon--or, as we call it, each sets and
rises again. And by the time we come to yet another circle of leading
constellations, we reach those which are so far affected by the
earth’s yearly journey as to be only visible through certain months,
and to be hidden during other months.
[Illustration: CONSTELLATION OF ORION, AS IT APPEARS NOW.]
If we could stand exactly at the North Pole, during part of its six
months’ night, we should see the Pole-star just overhead, and all the
constellations circling round it once in every twenty-four hours. Those
nearest would move slowly, in a small ring. Those furthest, and lowest
down, would in the same length of time sweep round the whole horizon.
But the stars would not there seem to rise or set. If we were standing
at the South Pole, we should see exactly the same kind of seeming
movement, only with altogether a different set of stars. If we were
standing on the Equator at night, we should see the rising and setting
very plainly. The whole mass of stars would appear to rise regularly
and evenly in an easterly direction, to pass steadily across the sky,
each taking its own straightforward path, and to set in a westerly
direction.
We who are placed midway between the Pole and the Equator, see a
mixture of these two motions. Some stars seem to circle round and
round, as all would do if we stood at the North Pole. Some stars seem
to rise and set, as all would do if we stood at the Equator. So much
for the seeming movements of the stars.
But now, about their real movements. Are the stars fixed, or are they
not? These seeming daily and yearly motions do not affect the question,
being merely caused by our own motions. Trees and hedges may appear to
move as we rush past them in a train, yet they are really fixed.
[Illustration: CONSTELLATION OF ORION 50,000 YEARS FROM NOW.]
During a long while, after it was found out that the quick, daily
movements of all the stars in company were merely apparent, men
believed that they really had no “proper motions”--that is, no
movements of their own. For century after century the constellations
remain the same. Hundreds of years ago the seven chief stars of the
Great Bear shone in company as they shine now. Who could suppose that
each one of those seven stars is hurrying on its path through space
with a speed exceeding far that of the swiftest express-train? Yet so
it is. Hundreds of years ago the grand group of Orion, with belt and
sword, gleamed brilliantly night by night as it gleams in these days;
and Cassiopeia had her W form, and Hercules and Draco and Andromeda
were shaped as they are shaped still. Who would imagine that through
those hundreds of years each star of these different constellations was
hastening with more or less of speed along its heavenly road? Yet so it
is.
Cases have been decisively ascertained of stars changing their places
among the other stars by a slow and gradual motion. Three of the most
conspicuous of them--Sirius, Arcturus, and Aldebaran--have been proved,
by the comparison of modern with some ancient observations, to have
experienced a change of place to the southward, to the extent of more
than the breadth of the moon in all the three. And during the period of
accurate modern measurement, other instances have been ascertained of
steady change of place by the effect of proper motion.
But if the stars are thus rapidly moving in all directions, how is it
that we do not _see_ them move? How is it that, night after night, year
after year, century after century, even thousand years after thousand
years, the shapes of the constellations remain unaltered?
Suppose you and I were standing on the seashore together, watching
the movements of scores of sea-craft, little boats and large boats,
steamers, yachts, and ships. Suppose we stood through a full quarter
of an hour looking on. Some might move, it is true, very slowly; yet
their movements in every case would plainly be seen. There could be no
possibility of mistaking the fact, or of supposing them to be “fixed.”
Just so we see the nearer planets move. Little danger of our supposing
them to be “fixed stars.”
In the matter of the stars themselves, we must carry our illustration
further. Come with me up to the top of that lofty hill on the border
of the sea, and let us look from the cliff. We see still the movements
among boats and smacks, yachts and steamers, only the increased
distance makes the movements seem slower. But our view is widened. Look
on the far horizon and see three distant dots, which we know to be
ships--one and two close together, and a third a little way off, making
a small constellation of vessels. Watch them steadily for a quarter of
an hour. You will detect no movement, no increased distance or nearness
between any two of the three. The group remains unchanged.
Are they really moving? Of course they are, more or less rapidly,
probably with differing speed and in different directions. But at
so great a distance, one quarter of an hour is not long enough for
their motions to become visible to the naked eye. If we could watch
longer--say, for two or three hours--ah, that would make all the
difference! If only we could watch longer! But the hundreds, and even
thousands of years during which men have watched the stars, sink, at
our vast distance, into no more than one quarter of an hour spent in
watching the far-off ships from the high hilltop. The motions can not
be detected. In ten thousand years you might see something. In fifty
thousand years you might see much. But four or five thousand years are
not sufficient.
One other mode there is, by means of which the movements of the ships
on the horizon might be made plain. Suppose you had no more than the
quarter of an hour to spare, but suppose you had at your command a
powerful telescope. Then you may practically bring the ships nearer,
and by magnifying the small, slow, distant motions, you may make them,
as it were, larger, quicker, more easy to see.
Telescopes will do this for us, likewise, in the matter of the stars.
By means of telescopes, with the assistance of careful watching and of
close calculation it has been found that the stars are really moving
quickly, each one in his own pathway. The very speed of some of them
has been measured.
Arcturus is one of those stars, the motions of which are most plainly
to be seen. In the course of about one thousand years he changes
visibly his place in the sky by a space equal to the apparent diameter
of the moon. The seeming movement of Arcturus in one thousand years is
the same as the seeming width of the round moon that we see.
But the actual speed with which Arcturus rushes through space is said
to be no less than fifty-four miles each second, or not far from two
hundred thousand miles an hour. That is nearly three times as fast as
our own earth’s motion round the sun. How enormous the distance must be
which can shrink such speed to such seeming slowness!
Another beautiful star, Capella, is believed to travel at the rate of
thirty miles each second. The speed of Sirius is slower, being only
fourteen miles a second. The Pole-star creeps along at the rate of only
one mile and a half each second.
So also with the rest of the stars. There seems good reason to believe
that every star we see shining in the heavens, every star visible in
powerful telescopes, is perpetually hastening onward. But hastening
whither? God knows! We do not.
In all probability not one of the tens of millions of stars which
may be seen through telescopes is in repose. This is a matter of
conjecture, of reasoning from analogy, and of reasoning also from
the working of known laws. We _know_, as a consequence of direct
observation, apart from the new spectroscopic method, that at least
hundreds are upon the wing. We _assume_, as a matter of the greatest
possible likelihood, that all the millions besides, which can not be
actually seen to stir, are equally on the move. Knowing what we do know
of the laws by which the universe of stars is governed, it seems to us
an absolute impossibility that any single star, amid the whole vast
host, can be or could be permanently at rest.
If by any means a star were brought to repose--what then would happen?
It would inevitably start off again, drawn by the attraction of other
stars. From whatever direction the strongest pull came, the impulse
would be given. Lengthened repose would be out of the question. And
this, it seems to us, must be true, not of one star only, here or
there, but of every star in the enormous host of radiant suns which
make up the mighty Stellar System.
Our forefathers, one thousand years ago, could not measure the precise
positions of individual stars, as astronomers now are able to do.
They had no modern observatories, no telescopes, no spectroscopes, no
photographic appliances. These methods of observation we shall explain
in another chapter. Their measurements at best were rough, their
scientific knowledge was crude. Had we any such accurate observations
handed down from one thousand years ago as are made in these days, we
should no doubt see clearly many slight differences in the positions
of many stars which are not now apparent. But even then we should see
no changes sufficient in amount to affect the general outlines of the
leading constellations.
A star, which in the course of a century makes visible advance over a
space in the sky equal to only a small portion of the breadth of the
full moon, is looked upon as a fast voyager. One hundred times this
degree of movement, if it took place in a considerable number of stars
in our sky, would not in centuries very materially change the face of
our midnight heavens.
And all motions which would in the remotest degree affect the shapes of
constellations, must be sideway motions. Those line-of-sight motions,
of which the spectroscope alone tells us, could never have been
discovered by simple observation of the sky. Until the new method came
to light we had no means whatever of perceiving such movements among
the stars.
Suppose you are looking at two men in the distance, upon a wide, flat
plain. One of the two is walking very slowly _across_ your line of
vision. The second man is moving very slowly straight _towards_ you.
If you watch with care you may find out both the movements. The sideway
walking will be apparent first and most easily, because, as the man
moves, he has constantly a fresh part of the horizon behind him. But
in time the advance of the second man towards you will also become
apparent; for although he is seen still against precisely the same spot
on the horizon, he slowly occupies a larger spot on the retina of your
eye,--in other words, he seems to grow bigger. And that, as you know
from long experience, can only mean increasing nearness.
If a star seemed to grow larger as it drew nearer, we should then be
able to perceive that movement also. But no star in the sky ever does
seem to grow any larger. Every star is to us but one point of light.
It may be rushing towards us at an enormous rate of speed, yet still
as a single point it remains, always at the same point in our sky.
Therefore we have no chance of perceiving its movement; or rather, we
_had_ no chance until the discovery of this new method. In fact, the
very nearest known star is at so enormous a distance that, supposing it
to be coming towards us at the rate of one hundred miles each second,
it would still gain, in the course of a century, only one-fortieth part
more of brightness than it has now. It would not increase at all in
apparent size.
When we leave behind us the thought of starry motions, as we faintly
detect them at this great distance, and picture to ourselves the actual
far-off whirl of all those glorious suns, the effect upon the mind is
overwhelming. Stars are found to be rushing hither and thither, at
every degree of speed, in every imaginable direction: stars to right,
and stars to left; stars towards us, and stars away from us; stars
alone, and stars in company,--all this, and more, deciphered out of
the tiny gleam of quivering light, which streams through the vast
abyss of space from each distant orb to earth. So real star-motions
were known--first, through telescopic observation; secondly, through
spectroscopic observation; and now, lastly, photography has stepped in,
bringing with it much increase of exactitude.
But if all the stars are moving, what of our sun? Our sun is a star.
And our sun also is moving. He is pressing onward, in a wide sweep
through space, bearing along with him his whole vast family--planets,
satellites, comets, meteorites--round or towards some far distant
center. For aught we know, every star in the heavens may have a like
family traveling with him.
In infinite space the stars are strewn in immense clusters, like
archipelagoes of islands in the ocean of the heavens. To go from one
star to another in the same archipelago light takes years; to pass
from one archipelago to another it takes thousands of years. Each of
these stars is a sun similar to ours, surrounded, doubtless, at least
for the most part, by worlds gravitating in its light; each of these
planets possesses, sooner or later, a natural history adapted to its
constitution, and serves for many ages as the abode of a multitude of
living beings of different species. Attempt to count the number of
stars which people the universe, the number of living beings who are
born and die in all these worlds, the pleasures and pains, the smiles
and tears, the virtues and vices! Imagination, stop thy flight!
The sun is not one of the most quickly-moving stars. His rate of speed
has not been found out with any certainty, but it is believed to be
about four or five miles a second.
And where are we going? This has been in part discovered. If you and
I were driving through a forest of trees, we should see the trees on
each side of us seeming to move backward, while behind they would close
together, and in front they would open out.
Astronomers--and first among them, William Herschel--reasoned that
if our Solar System were really in motion, we ought to be able to
see these changes among the stars. And some such changes have become
visible through careful watching--not so much those ahead and behind as
those at the sides.
It is not actually so simple a matter as looking at the trees in a
forest, because the trees would be at rest, whereas each star has his
own particular real motion, as well as his seeming change of place
caused by our sun’s motion. It is more like moving in a small steamer
at sea, among hundreds of other craft, each of which is going on its
own way, at the same time that all on either side seem to move backward
because we are moving forward.
So each movement had to be noted, and the real motions had to be
separated from the seeming backward drift of stars to the right and
left of the sun’s pathway. The result of all this is that the sun,
with his planets, is found to be hastening towards a certain far-off
constellation named Hercules.
It is a strange and unexpected fact, but absolutely true, that each sun
of space is carried along with a velocity so rapid that a cannon-ball
represents rest in comparison; it is at neither a hundred, nor three
hundred, nor five hundred yards per second that the earth, the sun,
Sirius, Vega, Arcturus, and all the systems of infinitude travel: it
is at ten, twenty, thirty, a hundred thousand yards a second; all run,
fly, fall, roll, rush through the void--and still, seen as a whole, all
seems in repose.
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 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. “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 towards 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.
How do such contemplations enlarge and transfigure the vulgar idea
which is generally entertained of the world! Should not the knowledge
of these truths form the first basis of all instruction which aims
at being serious? Is it not strange to see the immense majority of
human beings living and dying without suspecting these grandeurs,
without thinking of learning something of the magnificent reality which
surrounds them?
Where the sun and his planets will journey in future ages no living man
can say. Indeed, though it is a question which does not lack interest
to a thoughtful mind, yet there are numberless other questions about
centuries near at hand which concern man far more nearly. The history
of the Universe, and the history of this Earth of ours, must have
advanced many broad stages before our sun and his attendant planets can
have traveled so far that any change will be apparent in the shape of
the star-constellations which spangle our sky.
CHAPTER XI.
MORE ABOUT THE SOLAR SYSTEM.
We have now reached a point where it ought not to be difficult for
us to picture to ourselves, with something of vividness, the general
outlines of the Solar System. Awhile ago this Solar System was a very
simple matter in the eyes of astronomers. There was the great sun
fixed in the center, with seven planets circling round him--seven of
course, it was said, since seven was the perfect number--and a few
moons keeping pace with some of the planets, and an occasional comet,
and a vast amount of black, empty space. But astronomers now begin to
understand better the wonderful richness of the system as a whole; the
immense variety of the bodies contained in it; the perpetual rush and
stir and whirl of life in every part. Certainly there is no such thing
as dull stagnation throughout the family.
First, we have the great, blazing, central sun; not a sun at rest, as
regards the stars, but practically at rest as regards his own system,
of which he is always head and center. Then come the four smaller
planets, rapidly whirling round him, all journeying in the same
direction, and all having their oval pathways lying on nearly the same
flat plane in space. Then the broad belt of busy little planetoids.
Then the four giant planets,--Jupiter nearly five times as far as our
earth from the sun; Saturn nearly twice as far as Jupiter; Uranus
nearly twice as far as Saturn; Neptune as far from Uranus as Uranus
from Saturn,--all keeping on very nearly the same level as the four
inner planets.
[Illustration: POSITION OF PLANETS INFERIOR TO JUPITER--SHOWING THE
ZONE OF THE ASTEROIDS.]
And between and about these principal members of the system, with their
accompanying moons, we have thousands of comets flashing hither and
thither, with long, radiant trains; and myriads of meteorites, gathered
often into dense, vast herds or families, but also scattered thickly
throughout every part of the system, each tiny ball reflecting the
sun’s rays with its little glimmer of light.
Broad reaches of black and empty space! Where are they? Perhaps
nowhere. We are very apt, in our ignorance, to imagine that where we
see nothing, there must of necessity be nothing. But, for aught we
know, the whole Solar System, not to speak of sky-depths lying beyond,
may be bright with reflecting bodies great and small, from the mighty
Jupiter down to the fine diamond-dust of countless meteorites. In this
earth of ours we find no emptiness. Closer and closer examination with
the microscope only shows tinier and yet tinier wonders of form and
life, each perfect in finish. Not of form only, but of _life_. How
about that matter as regards the Solar System? Is our little world
the one only spot in God’s great universe which teems with life? Are
all other worlds mere barren, empty wastes? Surely not. We may safely
conclude that life of one kind or another has been, is, or will be,
upon our brother and sister worlds.
The same reasoning may be used for the distant stars--those millions
of suns lying beyond reach of man’s unassisted eyes. Are they formed
in vain? Do their beams pour uselessly into space, carrying light,
warmth, and life-giving power to nothing? Surely around many of them,
as around our sun, must journey worlds; and not only worlds, but worlds
containing life.
We shall have to speak of this matter again as we go on. Whether men
and women like ourselves could live on the other planets is another
question. In some cases it looks doubtful, in some it would seem to
be impossible. But the endless variety of life on this earth--life
on land, life in the air, life in water, life underground, life in
tropical heat, life in arctic cold--forbids us in anywise to be
positive as to what may or may not be.
If no animals that we know could exist there, animals that we do not
know might be found instead. Any one of the planets may, and very
likely does, abound with life. Nay, the very meteorites themselves,
before they catch fire and burn in our air, _may_ be the homes of tiny
animalcula, altogether different from anything we see on earth. We can
not fancy life without air, but neither could we fancy life in water,
if we had not seen and known it to be possible.
I have spoken of the probable _brightness_ of the Solar System as a
whole. We are so apt to think of things merely as we see them with our
short sight, that it is well sometimes to try to realize them as they
actually are. Picture to yourself the great central sun pouring out in
every direction his burning rays of light. A goodly abundance of them
fall on our earth, yet the whole amount of light and heat received over
the whole surface of this world is only the two-thousand-millionth
part of the enormous amount which he lavishly pours into space. How
much of that whole is wasted? None, though God gives his gifts with a
kingly profusion which knows no bounds. Each ray has its own work to
do. Millions of rays are needed for the lighting and nourishing and
warming of our companion-planets, while others are caught up by passing
comets, and myriads flash upon swift, tiny meteorites. Of the rays not
so used, many pass onwards into the vast depths beyond our system, and
dwindle down into dim, starlike shining till they reach the far-off
brother-stars of our sun.
Have they work to do there? We can not tell. We do not know how far the
sun’s influence reaches. As head and center, he reigns only in his own
system. As a star among stars, a peer among his equals, he may, for
aught we can tell, have other work to do.
In an early chapter, mention was made of the earth’s three motions, two
only being explained. First, she spins ceaselessly upon her axis. So
does the sun, and so do the planets. Secondly, she travels ceaselessly
round and round the sun in her fixed orbit. So does each one of the
planets. Thirdly, she journeys ceaselessly onward through space with
the sun. So also do the rest of the planets. These last two movements,
thought of together, make the earth’s pathway rather perplexing at
first sight. We talk of her orbit being an ellipse or oval; but how can
it be an ellipse, if she is always advancing in one direction?
The truth is, the earth’s orbit is and is not an ellipse. As regards
her yearly journey round the sun, roughly speaking, we may call it
an ellipse. As regards her movement in space, it certainly is not an
ellipse.
Think of the Solar System, with the orbits of all the planets, as lying
_nearly flat_--in the manner that hoops might be laid upon a table,
one within another. The asteroids, comets, and meteorites do not keep
to the same level; but their light weight makes the matter of small
importance.
Having imagined the sun thus in the center of a large table--a small
ball, with several tiny balls traveling round him on the table at
different distances--suppose the sun to rise slowly upwards, not
directly up, but in a sharp slant, the whole body of planets continuing
to travel round, and at the same time rising steadily with him.
By carefully considering this double movement, you will see that the
real motion of the earth--as also of each of the planets--is not a
going round on a flat surface to the same point from which she started,
but is a corkscrew-like winding round and round upwards through space.
Yet as regards the central sun, the shape of the orbit comes very near
being an ellipse, if calculated simply by the earth’s distance from him
at each point in turn of her pathway through the year.
An illustration may help to explain this. On the deck of a moving
vessel, you see a little boy walking steadily round and round the mast.
Now is that child moving in a circle, or is he not? Yes, he is. No, he
is not. He walks in a circle as regards the position of the mast, which
remains always the center of his pathway. But his movement _in space_
is never a circle, since he constantly advances, and does not once
return to his starting-point. You see how the two facts are possible
side by side. Being carried forward by the ship, with no effort of his
own, the forward motion does not interfere with the circling motion.
Each is performed independently of the other.
It is the same with the earth and the planets. The sun, by force of
his mighty attraction, bears them along wherever he goes--no exertion
on their part, so to speak, being needed. That motion does not in the
least interfere with their steady circling round the sun.
Just as--to use another illustration--the earth, turning on her axis,
bears through space a man standing on the Equator at the rate of one
thousand miles an hour. But this uniform movement, unfelt by himself,
does not prevent his walking backwards, or forwards, or in circles, as
much as he will.
So, also, a bird in the air is unconsciously borne along with the
atmosphere, yet his freedom to wheel in circles for any length of time
is untouched.
A few words about the orbits of the planets. I have more than once
remarked that these pathways are, in shape, not circles, but ellipses.
A circle is a line drawn in the shape of a ring, every part of which
is at exactly the same distance from the center-point or focus. But an
ellipse, instead of being, like a circle, perfectly round, is oval in
shape; and instead of having only one focus, it has two foci, neither
being exactly in the center. Foci is the plural word for focus. If
an ellipse is only slightly oval--or slightly _elliptical_--the two
foci are near together. The more oval or _eccentric_ the ellipse, the
farther apart are the two foci.
You may draw a circle in this manner. Lay a sheet of white paper on a
board, and fix a nail through the paper into the board. Then pass a
loop of thread--say an inch or an inch and a half in length--round the
nail, and also round a pencil, which you hold. Trace a line with the
pencil, keeping the loop tight, so that the distance of your line from
the nail will be always equal, and when it joins you have a circle. The
nail in the center is the focus of the circle.
To draw an ellipse, you must fix _two_ nails. Let them be about half an
inch apart; pass a loop over both of them, and again placing a pencil
point within the loop, again trace a line carefully all round, keeping
the thread drawn tight. This time an oval instead of a circle will
appear. By putting the nails nearer together or farther apart, you may
vary as you will the shape of the ellipse.
In the orbits of the earth and the planets, all of which are ellipses
in shape, the sun is not placed in the exact center, but in one of
the two foci, the second being empty. So at one time of the year the
planet is nearer to the sun than at another time. Our earth is no
less than three millions of miles nearer in winter than she is in
summer--speaking of the winter and summer of the northern hemisphere.
Three millions of miles is so tiny a piece out of ninety-three millions
of miles, that it makes little or no difference in our feelings of heat
or cold.
The orbits of the comets are ellipses also, but ellipses often so
enormously lengthened out, that the two foci are almost--if one may so
speak--at the two _ends_ of the oval. To draw a good comet orbit, you
must fix the two nails on your paper some five or six inches apart,
with a loop of thread just large enough to slip over them both, and to
allow the pencil to pass round them. When your ellipse is drawn, you
must picture the sun in the place of one of the two nails, and you will
see how, in their pathways, the comets at one time pass very near the
sun, and at another time travel very far away from him.
[Illustration: COMPARATIVE SIZE OF THE PLANETARY WORLDS.]
It is generally found in families, not only that the parent or head
of the family has great influence over all the members, but that each
member has influence over each other member. Brother influences
brother, and sister influences sister.
This, too, we find in the Solar System. Not only does the sun, by his
powerful attraction, bind the whole family together, but each member
of the family attracts each other member. True, the force of the sun’s
attraction is overpowering in amount compared with others. The sun
attracts the planets, and the planets attract the sun; but their feeble
pulling is quite lost in the display of his tremendous strength.
Among themselves we see the power more plainly. The earth attracts the
moon, keeping her in constant close attendance; and the moon attracts
the earth, causing a slight movement on her part, and also causing the
tides of the sea. Each planet has more or less power to hinder or help
forward his nearest brother-planet. For instance, when Jupiter on his
orbit draws near the slower Saturn on his orbit, Saturn’s attraction
pulls him on, and makes him move faster than usual; but as soon as he
gets ahead of Saturn then the same attraction pulls him back, and makes
him go more slowly than usual. Jupiter has the same influence over
Saturn; and so also have Saturn and Uranus over one another, or Uranus
and Neptune.
In early days astronomers were often greatly puzzled by these quickened
and slackened movements, which could not be explained. Now the
“perturbations” of the planets, as they are called, are understood and
allowed for in all calculations. Indeed, it is by means of this very
attraction that Neptune was discovered, and the planets have actually
been weighed. What a wonderful difference we find in this picture of
the Solar System, as we now know it to be, from the old-world notion of
our earth as the center of the universe!
When we think of all the planets, and of the magnificent sun; when we
pass onward in imagination through space, and find our sun himself
merely one twinkling star amid the myriads of twinkling stars scattered
broadcast through the heavens, while planets and comets have sunk to
nothing in the far distance,--then indeed we begin to realize the
unutterable might of God’s power! Why, our earth and all that it
contains may be regarded as but one grain of dust in the wide universe.
CHAPTER XII.
MORE ABOUT THE SUN.
Not among the least of the wondrous things of creation are the
tremendous disturbances taking place upon the surface of the sun--that
raging, roaring sea of flame.
A good many explanations have been from time to time offered as to the
dark spots seen to move across the face of the sun. Some one or more of
these explanations may be true; but we know little about the matter.
A sun-spot does not commonly consist of merely one black patch. There
is the dark center, called the _umbra_--plural, _umbræ_. There is the
grayish part surrounding the umbra, called the _penumbra_. Also, in
the center of the umbra there is sometimes observable an intensely
black spot, called the _nucleus_. Sometimes a spot is made up of
nucleus and umbra alone, without any penumbra. Sometimes it is made
of penumbra alone, without any umbra. Sometimes in one spot there are
several umbræ, with the gray penumbra round the whole, and gray bridges
dividing the umbræ.
The enormous size of these spots has been already described in an
earlier chapter. Fifty thousand to one hundred thousand miles across
is nothing unusual. In the year 1839 a spot was seen which measured no
less than one hundred and eighty-six thousand miles in diameter.
One explanation proposed was, that the sun might be a cool body,
covered over with different envelopes or dense layers of cloudy form,
one above another. The inside envelope--or, as some say, the inside
atmosphere--would then be thick and dull-colored, protecting the solid
globe within and reflecting light, but having none of its own. The next
envelope would be one mass of raging, burning gases--the _photosphere_,
in fact. The outer envelope would be a transparent, surrounding
atmosphere, lighted up by the sea of fire within. A sun-spot would then
consist of the tearing open of one or more of these envelopes, so as to
give glimpses of the gray, inner atmosphere, or even of the dark, cool
globe at the center.
There may be some truth in this explanation; but the notion of a
cool and dark body within is now pretty well given up. The apparent
blackness of a spot-nucleus does not prove actual blackness or absence
of heat. A piece of white-hot iron, held up against the sun, looks
black; and it may be merely the contrast of the glowing photosphere
which makes the nucleus seem so dark. It is even believed that the
blackest parts may be the most intensely hot of all.
Another proposed explanation was of dark clouds floating in the sun’s
atmosphere. There, again, are found difficulties, particularly in
the fact that, as the spots move across the sun, the changes which
regularly take place in their appearance make it pretty clear that
their shape is not flat, but hollow and cave-like. The changes here
spoken of are seeming changes of shape, caused by change of position.
There are also real changes constantly taking place. Although the spots
often keep their general outlines long enough to be watched across the
face of the sun, and even to be known again after spending nearly a
fortnight hidden on the other side, still they are far from being fixed
in form.
[Illustration: A TYPICAL SUN-SPOT.]
The alterations are at times not only very great, but very rapid.
Sometimes in a single hour of watching, an astronomer can see marked
movement going on--as you or I might in an hour observe movements
slowly taking place in a high layer of clouds. For movement to show at
all in one hour, at so immense a distance, proves that the actual rate
of motion must be very great.
The first great fact which was got from the study of these spots was
this, that this great sun is very much like our own earth, in so far
as it rotates on an axis in exactly the same way that our earth does.
Not only do the spots change their position on the face of the sun, in
consequence of the sun’s rotation on its axis, but they change very
much from day to day, and even from hour to hour; so that we have
evidence not only that the sun is rotating like our earth, but that the
atmosphere of the sun is subjected to most tremendous storms--storms
so tremendous, in fact, that the fiercest cyclones on our own earth
are not for one moment to be compared with them. The fact that the
spots do really move with the sun, and are really indentations,
saucer-like hollows, in the photosphere, is shown by the appearance,
which is always presented by a spot when it is near the edge of the
sun. You know if you take a dinner-plate, and look it full in the face,
it is round; but if you look at it edgeways, it is not round. Take
two different views of the same spot: In one, you look the sun-spot
straight in the face, and you see into it and can learn all about it;
but in the other, when it has nearly gone round the corner, and is
disappearing on the sun’s edge, you see it in the same way that you
would see a plate looked at edgeways.
These sun-cyclones must indeed be of terrific force and extent,
compared with anything we see on earth. It was calculated that the
speed of movement perceived in one spot was about three hundred and
sixty-three miles each second.
And still all this is nothing, or almost nothing, in comparison with
the real power of the sun! The liquid state of the ocean; the gaseous
state of the atmosphere; the currents of the sea; the raising of the
clouds, the rains, storms, streams, rivers; the calorific value of
all the forests of the globe and all the coal-mines of the earth; the
motion of all living beings; the heat of all humanity; the stored-up
power in all human muscles, in all the manufactories, in all the
guns,--all that is almost nothing compared with that of which the
sun is capable. Do we think that we have measured the solar power
by enumerating the effects which it produces on the earth? Error!
profound, tremendous, foolish error! This would be to believe still
that this star has been created on purpose to illuminate terrestrial
humanity. In reality, what an infinitesimal fraction of the sun’s total
radiation the earth receives and utilizes! In order to appreciate
it, let us consider the distance of ninety-three millions of miles
which separates us from the central star, and at this distance let us
see what effect our little globe produces, what heat it intercepts.
Let us imagine an immense sphere _traced at this distance from the
sun_, and entirely surrounding it. Well, on this gigantic sphere,
the spot intercepted by our little earth is only equivalent to the
fraction 1/2,138,000,000; that is to say, that the dazzling solar
hearth radiates all round it through immensity a quantity of light
and heat two thousand one hundred and thirty-eight million times more
than that which we receive, and of which we have just now estimated
the stupendous effects. The earth only stops in its passage the _two
thousand millionth part of the total radiation_.
Sometimes the storms or outbursts come in the shape of a bright spot
instead of a dark one.
[Illustration: A SOLAR ERUPTION.]
Two astronomers were one day watching the sun from two different
observatories, when they saw such an event take place. An intense
and dazzling spot of light burst out upon the surface of the sun--so
intense, so dazzling, as to stand quite apart from the radiant
photosphere. To one astronomer it looked like a single spot, while the
other saw two spots close together. In about a minute the light grew
more dim, and in five minutes all was over. But in those five minutes
the spot or spots had traveled a distance of thirty-five thousand
miles.
It was a very remarkable thing that the _magnets_ on earth--those
delicate little needles which point so steadily yet perseveringly
towards the North Pole--seemed to be strongly agitated by the distant
solar outburst. This brings us to another interesting fact.
The spots on the sun are not always the same in number. Sometimes they
are many, sometimes they are few. Long and close watching has made it
clear that they pass through a regular _order_ of changes: some years
of many spots being followed by other years of less and less spots;
then some years of very few spots being followed by other years of more
and more spots,--decrease and increase being seemingly regular and
alternate.
This turn or _cycle_ of changes--from more to less, and then from less
to more again--is found to run its course about once in every eleven
years, with some variations.
Now, it has long been known that the magnetic-needle goes through
curious variations. Though we speak of it as pointing always north,
yet it does not always so point exactly. Every day the needle is found
to make certain tiny, delicate motions, as if faintly struggling to
follow the daily movements of the sun--just a little towards the east,
or just a little towards the west. These tiny motions, having been
long watched and measured, were found to go through a regular course
of changes--some years more, and some years less, waxing and waning by
turns. It was discovered that the course of changes from more to less,
and from less to more again, took place in about eleven years.
These two things, you see, were quite independent of one another.
Those who watched the sun-spots were not thinking of the magnets, and
those who watched the magnets were not thinking of the sun-spots. But
somebody did at last happen to think of both together. He was laughed
at, yet he took the trouble carefully to compare the two. And, strange
to say, he found that these two periods in the main agreed--the eleven
years of alternate changes in the number of sun-spots, and the eleven
years of alternate changes in the movements of the magnetic-needle.
When the spots are most, the needle moves most. When the spots are
least, the needle moves least. So much we know. But to explain the why
and the wherefore is beyond our power.
This study of the magnetism of our wandering planet is very
interesting, and one which is still very little known. Here is a weak
needle, a slip of magnetic iron, which, with its restless and agitated
finger, incessantly seeks a region near the north. Carry this needle
in a balloon up to the higher aerial regions, where human life begins
to be extinguished; shut it up in a tomb closely separated from the
light of day; take it down into the pit of a mine, to more than a
thousand yards in depth,--and incessantly, day and night, without
fatigue and without rest, it watches, trembles, throbs, seeks the
point which attracts it across the sky, through the earth, and through
the night. Now--and here is a coincidence truly filled with notes of
interrogation--the years when the oscillation of this innocent little
steel wire is strongest are the years when there are more spots, more
eruptions, more tempests in the sun; and the years when its daily
fluctuations are weakest are those when we see in the day-star neither
spots, eruptions, nor storms. Does there exist, then, a magnetic bond
between the immense solar globe and our wandering abode? Is the sun
magnetic? But the magnetic currents disappear at the temperature of
red-hot iron, and the incandescent focus of light is at a temperature
incomparably higher still. Is it an electrical influx which is
transmitted from the sun to the earth across a space of ninety-three
millions of miles? On September 1, 1859, two astronomers--Carrington
and Hodgson--were observing the sun, independently of each other;
the first on a screen which received the image; the second directly
through a telescope,--when, in a moment, a dazzling flash blazed
out in the midst of a group of spots. This light sparkled for five
minutes above the spots without modifying their form, as if it were
completely independent, and yet it must have been the effect of a
terrible conflagration occurring in the solar atmosphere. Each observer
ascertained the fact separately, and was for an instant dazzled. Now,
here is a surprising coincidence: at the very moment when the sun
appeared inflamed in this region the magnetic instruments of the Kew
Observatory, near London, where they were observing, manifested a
strange agitation; the magnetic needle jumped for more than an hour
as if infatuated. Moreover, a part of the world was on that day and
the following one enveloped in the fires of an aurora borealis, in
Europe as well as in America. It was seen almost everywhere,--at Rome,
at Calcutta, in Cuba, in Australia, and in South America. Violent
magnetic perturbations were manifested, and at several points the
telegraph-lines ceased to act. Why should these two curious events not
be associated with each other? A similar coincidence was observed on
August 3, 1872, by Professor Charles A. Young, of Princeton, N. J.: a
paroxysm in the solar chromosphere, magnetic disturbances everywhere.
There is a very singular appearance seen upon the sun which must not
be passed over without mention. Some astronomers speak of the whole
surface as being _mottled_ all over with a curious rough look when
examined through a powerful telescope. This “mottling” is described by
various observers in various ways. One speaks of “luminous spots shaped
like rice-grains.” Another, of “luminous spots resembling strokes
made with a camel’s-hair pencil.” Another, of “luminous objects or
granules.” Others, of “multitudes of leaves,” “nodules,” “crystalline
shapes,” “leaves or scales, crossing one another in all directions,
like what are called spills in the game of spillikens.” They have also
been pictured as “certain luminous objects of an exceedingly definite
shape and general uniformity of size, whose form is that of the oblong
leaves of a willow-tree.” These cover the whole disk of the sun,
excepting the space occupied by the spots, in countless millions, and
lie crossing each other in every imaginable direction.
In size they are said to be about one thousand miles long, by two
or three hundred broad, but they vary a good deal. Where there is a
spot, the willow-leaves at its edge are said to point pretty regularly
towards the center. Whether they are, as they seem to be, solid in
form; whether they are, as some suppose, the chief source of the
sun’s light and heat; whether they lie on his surface or float in
his atmosphere; what is their real nature, and what is their real
use,--about these questions we are at present quite in the dark.
We have next to think a little more about the edge or limb of the sun,
and the stormy flames and outburst there seen. Until quite lately
the only time for observing such appearances was during a total
eclipse of the sun. Lately, by means of the new instrument called the
“spectroscope,” it has been found possible to take observations when no
eclipse is going on.
[Illustration: SUN-FLAMES, MAY 3, 1892.]
A few words of explanation as to eclipses of the sun seem needful,
before going further. An eclipse of the sun is caused simply by the
round body of the moon passing exactly between the sun and the earth,
so as to hide the sun from us.
Let there be a candle on the table, while you stand near. The rays of
light from the candle fall upon your face. Now move slowly, to and
fro, a round ball between you and the candle. So long as it is not
precisely in the line between--so long as it is a little higher, or
a little lower, or a little to one side--then you can still see the
flame. Once you let the ball come just between the light and your
eyes, and you see it no more. In other words the candle-flame is
eclipsed--hidden, veiled, cut off--by the ball.
It may seem curious at first sight that the moon, which is very small
compared with the sun, should have power to cover the sun. But remember
the difference of the distance. The sun is very far, and the moon
is very near. Any small object very near will easily hide from your
sight a large object at a considerable distance. You may hold up a
shilling-piece at arm’s length, and make it cover from sight a man, or
even a house, if the latter be far enough away. The sun at a distance
of ninety-three millions of miles, and the moon at a distance of two
hundred and forty thousand miles, have to our vision the same seeming
size. So, when the moon glides between, her round face just about
covers the sun’s round face.
If the moon were traveling exactly in the same plane as the plane of
the earth’s orbit, an eclipse would be a very common affair indeed. But
the plane of the moon’s orbit being not quite the same as the plane of
the earth’s orbit, she passes sometimes a little above, and sometimes a
little below, the exact spot where she would hide the sun’s rays from
us. Now and then, at certain intervals, she goes just between. And so
well is the moon’s path in the heavens understood that astronomers can
tell us, long years beforehand, the day and the hour an eclipse will
take place.
An eclipse of the sun is sometimes partial, sometimes total, sometimes
annular. In a partial eclipse, the moon does indeed pass between, but
only so as to hide from us _part_ of the sun. She is a little too
low, or a little too high, to cover his face. In a total eclipse, the
moon covers the sun completely, so that for a few minutes the bright
photosphere seems blotted out from the heavens, a black round body,
surrounded by light, taking its place. In an annular eclipse, the moon
in like manner crosses the sun, but does not succeed in covering him
entirely, a rim of bright photosphere showing round the black moon. For
in an annular eclipse, the moon, being a little farther away from the
earth than at the time of a total eclipse, has too small a disk quite
to hide the sun’s disk.
The blackness of the moon during an eclipse is caused by the fact that
her bright side is turned towards the sun, and her dark side toward us.
An eclipse of the sun can take place only at new moon, never at full
moon. At her full, the moon is outside the earth’s orbit, away from the
sun, and can not by any possibility pass between.
Eclipses, like comets, have always been interpreted as the indication
of inevitable calamities. Human vanity sees the finger of God making
signs to us on the least pretext, as if we were the end and aim of
universal creation. Let us mention, for example, what passed even in
France, with reference to the announcement of an eclipse of the sun, on
August 21, 1560. For one, it presaged a great overthrow of States and
the ruin of Rome; for another, it implied another universal deluge;
for a third, nothing less would result than a conflagration of the
globe; finally, for the less excited, it would infect the air. The
belief in these terrible effects was so general that, by the express
order of the doctors, a multitude of frightened people shut themselves
up in very close cellars, well heated and perfumed, in order to shelter
themselves from these evil influences. Petit relates that the decisive
moment approached, that the consternation was at its height, and that
a parish priest of the country, being no longer able to confess his
parishioners, who believed their last hour had come, was obliged to
tell them in a sermon “not to be so much hurried, seeing that, on
account of the wealth of the penitents, the eclipse had been postponed
for a fortnight.” These good parishioners found no more difficulty in
believing in the postponement of the eclipse than they had in believing
in its unlucky influence.
History relates a crowd of memorable acts on which eclipses have
had the greatest influence. Alexander, before the battle of Arbela,
expected to see his army routed by the appearance of a phenomenon of
this kind. The death of the Athenian general Nicias and the ruin of
his army in Sicily, with which the decline of the Athenians commenced,
had for their cause an eclipse of the moon. We know how Christopher
Columbus, with his little army, threatened with death by famine at
Jamaica, found means of procuring provisions from the natives by
depriving them in the evening of the light of the moon. The eclipse
had scarcely commenced when they supplied him with food. This was the
eclipse of March 1, 1504. We need not relate other facts of this
nature, in which history abounds, and which are known to every one.
Eclipses no longer cause terror to any one, since we know that they
are a natural and inevitable consequence of the combined motions of
the three great celestial bodies--the sun, the earth, and the moon;
especially since we know that these motions are regular and permanent,
and that we can predict, by means of calculation, the eclipses which
will be produced in the future as well as recognize those which have
occurred in the past.
The following description of the total eclipse of 1860 will be found
interesting. Mr. Lowe, who observed it at Santander, Spain, thus writes:
“Before totality commenced, the colors in the sky and on the hills were
magnificent beyond all description. The clear sky in the north assumed
a deep indigo color, while in the west the horizon was first black
like night. In the east the clear sky was very pale blue, with orange
and red, like sunrise. On the shadow creeping across, the deep blue in
the north changed, like magic, to pale, sunrise tints of orange and
red, while the sunrise appearance in the east had changed to indigo.
The darkness was great; the countenances of men were of a livid pink.
The Spaniards lay down, and their children screamed with fear; fowls
hastened to roost, ducks clustered together, pigeons dashed against the
houses; flowers closed; many butterflies flew as if drunk, and at last
disappeared. The air became very humid, so much so that the grass felt
to one of the observers as if recently rained upon.”
CHAPTER XIII.
YET MORE ABOUT THE SUN.
Having seen something of storms taking place on the sun’s photosphere,
we must next give our attention to storms taking place at his edge. But
it should be remembered that the said edge, far from being a mere rim
to a flat surface, is a kind of horizon-line--is, in fact, just that
part of the photosphere which is passing out of or coming under our
sight. The surface there is, in kind, the same as the surface of the
broad disk facing us. In watching outbursts at the edge of the sun, we
have a side view instead of a bird’s-eye view.
In the year 1871, Professor Charles A. Young, of Princeton, was looking
at a large hydrogen cloud on the edge of the sun. When I speak of a
“cloud,” it must not be supposed that anything like a damp, foggy,
earthly cloud is meant. This solar cloud was a huge mass of red-hot
gas, about one hundred thousand miles long, rising to a height of fifty
thousand miles from the sun’s surface, and appearing to rest on glowing
pillars of fire.
The professor, while watching, was called away for half an hour. He
came back, expecting to find things much as he had left them. Instead
of this, a startling change had taken place. The whole mass of glowing
fire seemed to have been actually “blown to shreds” by some tremendous
outburst from below. In place of the motionless cloud were masses of
scattered fire, each from about four thousand to fourteen thousand
miles long, and a thousand miles wide.
As the professor gazed, these “bits” of broken cloud rose rapidly
upwards, away from the surface of the sun. When I say “rapidly,” I mean
that the real movement, which the professor could calculate, was rapid.
The seeming movements were of course slow, and over a small space. The
actual motions were not tardy, for in ten minutes these huge, fiery
cloud-pieces rushed upwards to a height of two hundred thousand miles
from the edge of the sun, moving at a rate of at least one hundred and
sixty-seven miles each second. Gradually they faded away.
But what caused this sudden change? Just before the professor was
interrupted, he had noticed a curious little brilliant lump--a sort of
suspicious thunder-cloud appearance--below the quiet, bright cloud. And
after this tremendous shattering, the little bright lump rose upwards
into a huge mass of rolling flame, reaching like a pyramid to a height
of fifty thousand miles. In the course of a few minutes these enormous
flames could be seen to move and bend, and to curl over their gigantic
tips. But they did not last long. At half-past twelve the professor
had been called away; by half-past two the rolling flames completely
vanished.
Now, whatever may be the full explanation of this sight, there is no
doubt that on that day was observed from earth a tremendous outburst,
compared with which our mightiest volcanoes are like the sputtering of
a farthing dip beside a roaring furnace. The awful force and greatness
of such a solar eruption are more than we can possibly picture to
ourselves. At our distance we may catch a faint glimpse of what is
going on, and calculate speed of movement. But vividly to realize the
actual terrific grandeur of what took place is past our power.
Possibly this was much the same kind of outburst as that seen by the
two English astronomers; only theirs was a bird’s-eye view, as it
were, looking down on the top of the sight, while the professor had a
side-view, certainly much the best for observation.
It does not follow from what he saw that the eruption must have taken
place exactly at the “edge” of the sun. Probably it happened near
the edge. All he could say, was that the flames rose fifty thousand
miles, and the pieces of cloud were carried two hundred thousand miles,
away from the edge. The eruption may have begun on the other side of
the sun, at any distance from the horizon-edge where it first became
visible to earthly eyes.
Also, while the professor found that the shattered cloudlets moved at
a rate of about one hundred and sixty-seven miles each second, it is
calculated that the first fearful outburst must have caused movement,
near the surface of the sun, at a rate of at least three hundred miles
each second. Probably the hydrogen cloud was borne upwards along with
a vast mass of fragments flung out from the sun. We are here upon
doubtful ground; but this tremendous power of eruption in the sun,
and of driving matter out of and away from his surface, should not be
forgotten.
Though such a sun-storm as that just described is not often to be seen,
yet there are at all times certain strange red prominences, or glowing
flames, rising up here and there from the sun’s “limb.” Doubtless they
rise also from other parts of the photosphere, though they are only
visible to us when near enough to the edge to stand out beyond it.
Seen during an eclipse, these prominences have clear, sharp outlines,
and are usually bright rose-red in color. They are described as
sometimes wide and low, sometimes tall and slender; sometimes jagged,
sometimes regular; sometimes keeping long the same shape, sometimes
changing quickly in a few minutes. They are said to be like flames,
like mountains, like the teeth of a saw, like icebergs, like floating
cloudlets.
As to their height, from fifty to eighty thousand miles is nothing
unusual. We must not speak of Mont Blanc or Mount Everest here. Jupiter
placed bodily on the surface of the sun, beside such a fire-mountain,
would not far overtop it. The earth, Venus, Mars, and Mercury, would
lie like little toy-balls at its foot. And these are common-sized
sun-flames. One was measured which reached to the enormous height
of two hundred thousand miles. The spectroscope shows these solar
prominences or jets to be made--at least in part--of burning hydrogen
gas.
Beyond the sierra or chromatosphere--that border of rippling, crimson
flame-billows round the edge of the sun, with red flame-mountains
rising out of it here and there--beyond these, stretches the corona.
The corona, as seen from earth, is a bright, far-reaching glory of
light, shining round the sun in a total eclipse. The moon then comes
between the sun and the earth, her dark, round body creeping over the
face of the sun till the bright photosphere is completely covered. But
the sierra and the tall, red flames stand out from behind the black
moon, and the beautiful, soft corona-light stretches far beyond.
[Illustration: SOLAR CORONA AND PROMINENCE.]
It was long doubted whether the corona really belonged to the sun or to
the moon. There seems now no doubt that it is a part of the sun.
Various descriptions of the corona have been given at different
times, as observed during different eclipses. It has been seen as a
steady, beamy, white cloud behind the moon, showing no flickering.
It has been seen marked with bright lines of light, and seeming to
move rapidly round and round. It has been seen silvery white, sending
off long streams of brightness. It has been seen in the form of
white light, with bluish rays running over it. It has been seen with
entangled jets of light, like “a hank of thread in disorder.” It has
been seen silvery-white again, with a faint tinge of greenish-violet
about the outer edge. It has been seen from a high mountain-top as a
mass of soft, bright light, “through which shot out, as if from the
circumference of the moon, straight, massive, silvery rays, seeming
distinct and separate from each other, to a distance of two or three
diameters of the lunar disk, the whole spectacle showing as upon a
background of diffused, rose-colored light.”
Majestic, indeed, are the proportions of some of those mighty flames
which leap from the surface of the sun, yet these flames flicker, as
do our terrestrial flames, when we allow them time comparable to their
gigantic dimensions. Drawings of the same prominence often show great
changes in a few hours, or even less. The magnitude of the changes
could not be less than many thousands of miles, and the actual velocity
with which such masses move is often not less than one hundred miles
a second. Still more violent are the solar convulsions, which some
observers have been so fortunate as to behold, when from the sun’s
surface, as from a mighty furnace, vast incandescent masses are
projected upwards. All indications point to the surface of the sun as
the seat of the most frightful storms and tempests, in which the winds
sweep along incandescent vapors.
The corona consists of two parts--the inner and brighter corona, the
outer and fainter corona. The shape of the whole seems to change much
at different times. The outer edge is usually blurred and indistinct,
fading gently away.
Many explanations have been suggested. At one time the corona was
supposed to be a solar atmosphere, reflecting light like our own
atmosphere. Some have thought the light might be caused by countless
myriads of meteorite systems, revolving in the close neighborhood of
the sun. Some suppose it may be owing, in part at least, to solar
eruptions, and the pouring outward of burning gas and matter. But our
knowledge of the true nature of the corona is yet in its infancy.
A few closing words as to the size and weight of the sun. In diameter,
eight hundred and fifty-eight thousand miles, and in bulk equal to
one million two hundred and eighty thousand earths, his weight is in
proportion less. Our earth is about four times as dense as the sun. If
her size were increased to the sun’s size, her density being the same
as now, she would be very much heavier than the sun, and would attract
much more strongly. Still, though the sun is of lighter materials than
the earth, his immense size gives him weight equal to seven hundred and
fifty times as much as all the planets put together.
The attraction on the surface of the sun is also very great--so great
that we can hardly picture it to ourselves. If life exists there at
all--supposing it possible that any kind of life can be in such a
fiery atmosphere--it must be life very different from any known in
this world. A man who on earth weighs one hundred and sixty pounds,
and walks lightly erect, would, on the sun, lie helplessly bound to
the ground, crushed by his own overpowering weight. It is said that a
cannon-ball, reposing on the sun, if lifted one inch and allowed to
fall, would dash against the ground with a speed three times greater
than that of our fastest express-trains. For weight on earth is merely
caused by the amount of force with which the earth draws downward a
body towards herself--a force greater or less according to the density
of that body. So weight on the sun would be immensely increased by his
immensely greater power of attraction.
[Illustration: COMPARATIVE SIZE OF THE EARTH AND SUN.]
It is an interesting question how far the sun’s attractive influence
reaches effectually through space. The nearer a body is to the sun,
the greater the attraction which he exercises over it. At the distance
of the planet Mercury, a speed of twenty-nine miles each second is
needful to overcome or balance it sufficiently for the planet to remain
in his orbit. At the distance of the planet Neptune, about three miles
each second is enough. If a planet were journeying at four times the
distance of Neptune, the speed would need to be not over two miles each
second, lest the planet should break loose and wander away. But even
two miles a second is no mean speed--more than seven thousand miles an
hour. If we come to speak of that which we on earth call rapid motion,
we shall gain a clearer idea as to the extent of the sun’s power.
Suppose a planet were traveling through space at the rate of one of our
fastest express-trains--sixty miles an hour. It has been calculated
that, unless the sun’s attraction were interfered with and overpowered
by some nearer sun, the said planet, though placed at a distance ten
or twelve times as great as that of the far-off star Alpha Centauri,
would still be forced by the sun’s attraction to journey round him in a
closed orbit. At such a speed it would not be free to wander off into
the depths of space.
CHAPTER XIV.
MORE ABOUT THE MOON.
From a globe all fire, all energy, all action, we come to a globe
silent, voiceless, changeless, lifeless. So, at least, the moon seems
to us. But it will not do to speak too confidently.
True, we can find no trace of an atmosphere in the moon. If there is
any atmosphere at all, it must be so thin as to be less than that which
we on earth count as actually none. We pump away the air from a glass
inclosure in an air-pump, and say the glass is empty. Only it is not
quite empty. There is always just a very little air remaining, though
so little that fire would not burn and animals could not live in it.
Some believe that air, up to that amount, may be found in the moon. But
this is much the same as to say there is none at all. For, of course,
with either no air or so very little air, life can not possibly exist
on the moon. We can not imagine such a thing for a moment. That is
just how the matter stands. We “can not imagine,” and therefore we
conclude it to be an impossibility. As if we knew a hundredth part of
the possibilities in any one corner of God’s great universe! As if our
being unable to picture a thing proves that thing not to exist!
Suppose we had always lived in tropical heat, and had never seen,
known, or heard of such a fact as life in Arctic snows. Should we
consider it a thing possible? Suppose we had always lived on dry land,
with never a sight of sea or river or pond, and never a proof that
animal life could exist under water--aye, and that some living animals
may be suffocated by air, just as other living animals are suffocated
by water. Should we not, in our wisdom, reason out such a state of
affairs to be utterly impossible?
There _may_ be no life on the moon. It _may_ be that she is now passing
through a dead, cold, blasted stage, either at the close of some past
history, or in preparation for some future history--or both. But, on
the other hand, it _may_ be that the moon is no less full of life than
the earth; only the life must be different in kind, must be something
which we do not know any thing at all about.
The moon is very much smaller than the earth. Her diameter is about
two-sevenths of the earth’s diameter; her entire surface is about
two twenty-sevenths of the earth’s surface; her size is about two
ninety-ninths of the earth’s size; and her whole weight is about
one-eightieth of the earth’s weight. Attraction or gravitation on the
surface of the moon is very different from what it is on the earth. Her
much smaller bulk greatly lessens her power of attraction. While a man
from earth would, on the surface of the sun--supposing he could exist
there at all--lie helpless, motionless, and crushed by his own weight,
he would on the moon find himself astonishingly light and active. A
leap over a tall house would be nothing to him.
[Illustration: THE MOON--AN EXPIRED PLANET.]
The moon, unlike the sun, has no light or heat of her own to give out.
She shines merely by reflected light. Rays of sunlight falling upon
her, rebound thence, and find their way earthward. This giving of
reflected light is not a matter all on one side. We yield to the moon a
great deal more than she yields to us. Full earth, seen from the moon,
covers a space thirteen times as large as full moon seen from earth.
Perhaps you may have noticed, soon after new moon, when a delicate
crescent of silver light shows in the sky, that within the said
crescent seems to lie the body of a round, dark moon, only not
perfectly dark. It shows a faint glimmer. That glimmer is called
earth-shine. The bright crescent shines with reflected sunlight. The
dim portion shines with reflected earth-light. What a journey those
rays have had! First, leaving the sun, flashing through ninety-three
millions of miles to earth, rebounding from earth and flashing over two
hundred and forty thousand miles to the dark shaded part of the moon,
then once more rebounding and coming back, much wasted and enfeebled,
across the same two hundred and forty thousand miles, to shine dimly in
your eyes and mine. The popular description of this particular view of
the moon is “the old moon in the arms of the new.”
Now about the _phases_ of the moon; that is, her changes from “new”
to “full,” and back again to “new.” If the moon were a starlike body,
shining by her own light, she would always appear to be round. But as
she shines by reflected sunlight, and as part of her bright side is
often turned away from us, the size and shape of the bright part seem
to vary. For, of course, only that half of the moon which is turned
directly towards the sun is bright. The other half turned away is dark,
and can give out no light at all, unless it has a little earth-shine to
reflect.
As the moon travels round the earth, she changes gradually from new to
full moon, and then back to new again. “New moon” is when the moon, in
her orbit, comes between the sun and the earth. The half of her upon
which the sun shines is turned away from us, and only her dark side is
towards us. So at new moon she is quite invisible. It is at new moon
that an eclipse of the sun takes place, when the moon’s orbit carries
her in a line precisely between sun and earth.
Passing onwards round the earth, the moon, as we get a little glimpse
of her shining side, first shows a slender sickle of light, which
widens more and more till she reaches her first quarter. She is then
neither between earth and sun, nor outside the earth away from the sun,
but just at one side of us, passing over the earth’s own orbit. Still,
as before, half her body is lighted up by the sun. By this time _half_
the bright part and _half_ the dark part are turned towards us; so
that, seeing the bright quarter, we name it the “first quarter.”
On and on round us moves the moon, showing more light at every step.
Now she passes quite outside the earth’s orbit, away from the sun. Not
the slightest chance here of an eclipse of the sun, though an eclipse
of the moon herself is quite possible. But more of that presently. As
she reaches a point in a line with earth and sun--only generally a
little higher or lower than the plane of the earth’s orbit--her round,
bright face, shining in the sun’s rays, is turned exactly towards us.
Then we have “full moon.”
Still she goes on. Once more her light narrows and wanes, as part of
her bright half turns away. Again at the “last quarter,” as at the
first, she occupies a “sideways” position, turning towards us half
her bright side and half her dark side. Then she journeys on, with
lessening rim of light, till it vanishes, and once more we have the
dark, invisible “new moon.”
It was these phases and aspects of the moon which formerly gave birth
to the custom of measuring time by months, and by weeks of seven days,
on account of the return of the moon’s phases in a month, and because
the moon appears about every seven days, so to say, under a new form.
Such was the first measure of time; there was not in the sky any signal
of which the differences, the alternations, and the epochs were more
remarkable. Families met together at a time fixed by some lunar phase.
The new moons served to regulate assemblies, sacrifices, and public
functions. The ancients counted the moon from the day they first
perceived it. In order to discover it easily, they assembled at evening
upon the heights. The first appearance of the lunar crescent was
watched with care, reported by the high priest, and announced to the
people by the sound of trumpets. The new moons which correspond with
the renewal of the four seasons were the most solemn; we find here the
origin of the “ember weeks” of the Church, as we find that of most of
our festivals in the ceremonies of the ancients. The Orientals, the
Chaldeans, Egyptians, and Jews religiously observed this custom.
An eclipse of the sun has already been described. An eclipse of the
moon is an equally simple matter. An eclipse of the sun is caused by
the dark, solid body of the moon passing just between earth and sun,
hiding the sun from us, and casting its shadow upon the earth. An
eclipse of the moon is also caused by a shadow--the shadow of our own
earth--falling upon the moon.
Here again, if the plane of the moon’s orbit were the same as ours,
eclipses of the moon would be very common. As it is, her orbit carries
her often just a little too high or too low to be eclipsed, and it is
only now and then, at regular intervals, that she passes through the
shadow of the earth.
If a large, solid ball is hung up in the air, with bright sunlight
shining on it, the sunlight will cast a _cone of shadow_ behind the
ball. It will throw, in a direction just away from the sun, a long,
round shadow, the same as the ball at first, but tapering gradually off
to a point. If the ball is near the ground, a round shadow will rest
there, almost as large as the ball. The higher the ball is placed, the
smaller will be the round shadow, till at length, if the ball be taken
far enough upwards, the shadow will not reach the ground at all. Our
earth and all the planets cast just such tapering cones of dark shadow
behind them into space. The cone always lies in a direction away from
the sun.
It is when the moon comes into this shadow that an “eclipse of the
moon” takes place. Sometimes she only dips half-way into it, or just
grazes along the edge of it, and that is called a “partial eclipse.”
Sometimes she goes in altogether, straight through the midst of the
shadow, so that the whole of her bright face for a short time grows
quite dark. Then we have a “total lunar eclipse.”
CHAPTER XV.
YET MORE ABOUT THE MOON.
There are two ways of thinking about the moon. One way is to consider
her as merely the earth’s attendant satellite. The other way is to
consider her as our sister-planet, traveling with us round the central
sun.
The first is the more common view; but the second is just as true as
the first.
For the sun does actually pull the moon towards himself, with a very
much stronger pulling than that of the earth. The attraction of the sun
for the moon is more than double the attraction of the earth for the
moon. If it were not that he pulls the earth quite as hard as he pulls
the moon, he would soon overpower the earth’s attraction, and drag the
moon away from us altogether.
People are often puzzled about the orbit or pathway of the moon through
the heavens. For in one sense they have to think of her as traveling
round and round in a fixed orbit, with the earth in the center. In
another sense they have to think of her as always journeying onwards
with the earth in her journey round the sun, and thus never returning
to the same point.
There are two ways of meeting this difficulty. First of all, remember
that the one movement does not interfere with the other. Just as in the
case of the earth traveling round the sun, and also traveling onward
with him through space; just as in the case of a boy walking round and
round a mast, and also being borne onwards by the moving vessel,--so it
is here. The two movements are quite separate and independent of each
other. As regards the earth alone, the moon journeys round and round
perpetually, not in a circle, but in a pathway which comes near being
an ellipse. As regards the actual _line_ which the moon’s movements may
be supposed to draw in space, it has nothing elliptical about it, since
no one point of it is ever reached a second time by the moon.
But according to this last view of the question, nobody ever can or
will walk in a circle or an oval. Take a walk round your grass-plot,
measuring your distance carefully at all points from the center. Is
that a circle? All the while you moved, the surface of the earth was
rushing along and bearing you with it, and the whole earth was hurrying
round the sun, and was being also carried by him in a third direction.
Whatever point in space you occupied when you started, you can _never
fill that particular part of space again_. The two ends of your
so-called circle can never be joined.
But then you may come back to the same point _on the grass_, as
that from which you started. And this is all that really signifies.
Practically you have walked in a circle. Though not a circle as regards
space generally, it is a circle as regards the earth. So also the moon
comes back to the same point _in her orbit round the earth_. Letting
alone the question of space, and considering only the earth, the moon
has--roughly speaking--journeyed in an ellipse. You may, however, look
at this matter in quite another light. Forget about the moon being the
earth’s satellite, and think of earth and moon as two sister-planets
going round the sun in company.
The earth, it is true, attracts the moon. So, also, the moon attracts
the earth; though the far greater weight of the earth makes her
attraction to be far greater. If earth and moon were of the same size,
they would pull each other with equal force.
But though the pull of the earth upon the moon is strong, the pull of
the sun upon the moon is more than twice as strong. And greatly as the
earth influences the moon, yet the actual center of the moon’s orbit is
the sun, and not the earth. Just as the earth travels round the sun, so
also the moon travels round the sun.
The earth travels steadily in her path, being only a little swayed and
disturbed by the attraction of the moon. The moon on the contrary,
while traveling in her orbit, is very much swayed and disturbed indeed
by the earth’s attraction. In fact, instead of being able to journey
straight onwards like the earth, her orbit is made up of a succession
of delicate curves or scallops, passing alternately backwards and
forwards over the orbit of the earth. Now she is behind the earth; now
in front of the earth; now between earth and sun; now outside the earth
away from the sun. The order of positions is not as here given, but
each is occupied by her in turn. Sometimes she moves quickly, sometimes
she moves slowly, just according to whether the earth is pulling her
on or holding her back. Two hundred and forty thousand miles sounds
a good deal. That is the distance between earth and moon. But it is,
after all, a mere nothing, compared with the ninety-three millions of
miles which separate the sun from the earth and moon.
If we made a small model, with the sun in the center, and the earth and
moon traveling a few inches off, only one slender piece of wire would
be needed to represent the path of earth and moon together. For not
only would the earth and the moon be so small as to be quite invisible,
but the whole of the moon’s orbit would have disappeared into the
thickness of the single wire. This question of the moon’s motions is
in its nature intricate, and in its details quite beyond the grasp of
any beginner in astronomy. But so much at least may be understood,
that though the earth’s attraction powerfully affects the moon, and
causes in her motions _perturbations_, such as have been already spoken
about as taking place among the planets, yet that in reality the great
controlling power over the moon is the attraction of the sun.
The tides of the ocean are chiefly brought about by the moon’s
attraction. The sun has something to do with the matter, but the moon
is the chief agent. This action of the moon can best be seen in the
southern hemisphere, where there is less land. As the moon travels
slowly round the earth, her attraction draws up the yielding waters
of the ocean in a vast wave which travels slowly along with her. The
same pulling which thus lifts a wave on the side of the earth towards
the moon, also pulls the earth gently _away from_ the water on the
opposite side, and causes a second wave there. The parts of the ocean
between these two huge waves are depressed, or lower in level. These
two waves on the opposite sides of the earth sweep steadily onwards,
following the moon’s movements,--not real, but seeming movements,
caused by the turning of the earth upon her axis.
Once in every twenty-four hours these wide waves sweep round the whole
earth in the southern ocean. They can not do the same in the north, on
account of the large continents, but offshoots from the south waves
travel northwards, bringing high-tide into every sea and ocean inlet.
If there were only one wave, there would be only one tide in each
twenty-four hours. As there are two waves, there are two tides, one
twelve hours after the other. In the space between these two high-tides
we have low-tide.
Twice every month we have very high and very low tides. Twice every
month we have tides not so high or so low. The highest are called
“spring-tides,” and the lowest “neap-tides.” When the moon is between
us and the sun, or when she is “new moon,” there are spring-tides;
for the pull or attraction of sun and moon upon the ocean act exactly
together. It is the same at full moon, when once more the moon is in a
straight line with earth and sun. But at the first and last quarters,
when the moon has her _sideways_ position, and when the sun pulls in
one direction and the moon pulls in another, each undoes a little of
the other’s work. Then we only have neap-tides; for the wave raised is
smaller, and the water does not flow so high upon our shores.
In speaking of the surface of the moon, we are able only to speak
about one side. The other is entirely hidden from us. This is caused
by the curious fact that the moon turns on her axis and travels round
the earth in exactly the same length of time. One-half of the moon is
thus always turned towards us, though of that half we can only see so
much as is receiving the light of the sun. But the half turned in our
direction is always the same half. One part of the moon--not quite so
much as half, though always the same portion--is turned away from us. A
small border on each side of that part becomes now and then visible to
us, owing to certain movements of the earth and the moon.
What sort of a landscape may lie in the unknown district, it is idle to
imagine. Many guesses have been made. Some have supposed it possible
that air _might_ be found there; that water _might_ exist there; that
something like earthly animals _might_ live there. It is difficult to
say what may not be, in a place about which we know nothing whatever.
But judging from our earthly experience, nothing seems more unlikely
than that air, water, clouds, should be entirely banished from over
one-half of a globe, and collected together in the space remaining.
We are on safer ground when we speak about that part of the moon
which is turned towards us. For we can say with confidence that if
any atmosphere exist there, it must be in thickness less than the
two-thousandth part of our earthly atmosphere. It seems equally clear
that water also must be entirely wanting. The tremendous heat of the
long lunar day would raise clouds of vapor, which could not fail to
be visible. But no such mistiness ever disturbs the sharply-defined
outline of the moon, and no signs of water action are seen in the
craggy mountains and deep craters.
[Illustration: THE LUNAR CRATER COPERNICUS.]
The craters which honeycomb the surface of the moon are various in
size. Many of the larger ones are from fifty to a hundred miles in
diameter. These huge craters--or, as we may call them, deep circular
plains--are surrounded by mighty mountain ramparts, rising to the
height of thousands of feet. Usually they have in their center a
sugar-loaf or cone-shape mountain, or even two or more such mountains,
somewhat lower in height than the surrounding range. The sunset-lights
upon certain of these distant mountain-peaks were first watched by
Galileo through his telescope, and have since been seen by many an
observer--intense brightness contrasting with intense blackness of
shadow.
In addition to her great craters, the moon seems to be thickly covered
with little ones, many of them being as small as can be seen at all
through a telescope. Whether these are all volcano-craters remains to
be discovered. It is not supposed that any of them are now active. From
time to time, signs of faint changes on the moon’s surface have been
noticed, which it was thought might be owing to volcanic outbursts.
Such an outburst as the worst eruptions of Mount Vesuvius would be
invisible at this distance. But the said changes may be quite as well
accounted for by the startling fortnightly variations of climate which
the moon has to endure. The general belief now inclines to the idea
that the moon-volcanoes are extinct, though no doubt there was in the
past great volcanic activity there.
A description has been given earlier of the rain of meteorites
constantly falling to our earth, and only prevented by the atmosphere
from becoming serious. But the moon has no such protecting atmosphere,
and the amount of cannonading which she has to endure must be by no
means small. Perhaps in past times, when her slowly-cooling crust
was yet soft, these celestial missiles showering upon her may have
occasionally made deep round holes in her surface.
This is another guess, which time may prove to be true. Guesses at
possible explanations of mysteries do no harm, so long as we do not
accept them for truth without ample reason.
[Illustration: LUNAR ERUPTION--BRISK ACTION.]
The origin of the lunar craters must be referred to some ancient epoch
in the moon’s history. How ancient that epoch is we have no means of
knowing; but in all probability the antiquity of the lunar craters is
enormously great. At the time when the moon was sufficiently heated
to have these vast volcanic eruptions, of which the mighty craters
are the survivals, the earth must have been very much hotter than it
is at present. It is not, indeed, at all unreasonable to believe that
when the moon was hot enough for its volcanoes to be active, the earth
was so hot that life was impossible on its surface. This supposition
would point to an antiquity for the moon’s craters far too great to
be estimated by the centuries and the thousands of years which are
adequate for the lapse of time as recognized by the history of human
events. It seems not unlikely that millions of years may have elapsed
since the mighty craters of Plato or of Copernicus consolidated into
their present form.
[Illustration: LUNAR ERUPTION--FEEBLE ACTION.]
It will now be possible for us to attempt to account for the formation
of the lunar craters. The most probable views on the subject are
certainly those adopted by Mr. Nasmyth, as represented in the cuts,
though it must be admitted that they are by no means free from
difficulty. We can explain the way in which the rampart around the
lunar crater is formed, and the great mountain which so often adorns
the center of the plain. The first of these cuts contains an imaginary
sketch of a volcanic vent on the moon in the days when the craters were
active. The eruption is here in the full flush of its energy, when the
internal forces are hurling forth a fountain of ashes or stones, which
fall at a considerable distance from the vent; and these accumulations
constitute the rampart surrounding the crater. The second cut depicts
the crater in a later stage of its history. The prodigious explosive
power has now been exhausted, and perhaps has been intermitted for some
time. A feeble jet issues from the vent, and deposits the materials
close around the orifice, and thus gradually raises a mountain in the
center.
Besides the craters and their surrounding barriers, there are ranges of
mountains on the moon, and flat plains which were once named “seas,”
before it was found that water did not exist there. Astronomers also
see bright ridges, or lines, or cracks of light, hard to explain.
One of the chief craters is called “Ptolemy,” and in size it is roughly
calculated to be no less than one hundred and fourteen miles across.
Another, “Copernicus,” is about fifty-six miles; and another, “Tycho,”
about fifty-four miles. The central cone-mountain of Tycho is five
thousand feet high. The crater of “Schickard” is supposed to be as much
as one hundred and thirty-three miles in diameter.
Astronomers have agreed to name these craters after the great
discoverers who enlarged our knowledge of the solar and sidereal
systems. It is fitting that these great names are suggested every time
the moon is seen through a telescope. To Ptolemy we are indebted for
what is known as “The Ptolemaic System of the Universe,” which makes
the earth the center around which the sun, moon, planets, and stars all
revolve, and explains the apparently erratic movements of the planets
by supposing their orbits to be epicycles; that is, curves returning
upon themselves and forming loops. Tycho Brahe was willing to allow,
with Copernicus, that the planets all revolved around the sun; but he
taught that both sun and planets turned around the earth.
The system known as “The Copernican” is now known to be the only true
one, and it is universally accepted. It is so called after Nicolaus
Copernicus, who was born in Thorn, Prussia, February 19, 1473. He was
educated for the Church, and studied medicine, but devoted himself
especially to astronomy.
Without being precisely a great genius, this quiet and thoughtful monk
seems to have been wise far beyond the age in which he lived, and
remarkable for his independence of mind. He had a “profound sagacity,”
and a wide general grasp of scientific subjects.
The extremely complicated and cumbrous nature of the Ptolemaic
system appeared to his judgment hardly compatible with the harmony
and simplicity elsewhere characteristic of nature. Moreover, he was
impressed and perplexed by the very marked changes in the brilliancy
of the planets at different seasons. These changes _now_ are no
difficulty at all. Venus, Mars, Jupiter, when on the same side of the
sun as ourselves, are comparatively near to us, and naturally look much
more bright in consequence of that nearness than when they are on
the opposite side of the sun from ourselves. But under the Ptolemaic
system, each planet was supposed to revolve round our earth, and to
be always at about the same distance from us; therefore, why such
variations in their brilliancy?
[Illustration: NICOLAUS COPERNICUS.]
During thirty-six long years he patiently worked out this theory, and
during part of those thirty-six years he wrote the one great book of
his lifetime, explaining the newer view of the Solar System which
had taken hold of his reason and imagination. This book, named _De
Revolutionibus Orbium Cœlestium_, or, “Concerning the Revolutions of
the Celestial Spheres,” which came out only a few hours before his
death, was dedicated to the Bishop of Rome--a little touch of worldly
wisdom which doubtless staved off for a while the opposition of the
Vatican.
Copernicus was not the first who thought of interpreting the
celestial motions by the theory of the earth’s motion. That immortal
astronomer has taken care to give, with rare sincerity, the passages
in the ancient writers from which he derived the first idea of the
probability of this motion--especially Cicero, who attributed this
opinion to Nicetas of Syracuse; Plutarch, who puts forward the names
of Philolaus, Heraclides of Pontus, and Ecphantus the Pythagorean;
Martianus Capella, who adopted, with the Egyptians, the motion of
Mercury and Venus around the sun, etc. Even a hundred years before the
publication of the work of Copernicus, Cardinal Nicolas of Cusa, in
1444, in his great theological and scientific encyclopedia, had also
spoken in favor of the idea of the earth’s motion and the plurality
of worlds. From ancient times to the age of Copernicus, the system of
the earth’s immobility had been doubted by clear-sighted minds, and
that of the earth’s motion was proposed under different forms. But all
these attempts still leave to Copernicus the glory of establishing it
definitely.
Not content with merely admitting the idea of the earth’s motion as a
simple arbitrary hypothesis, which several astronomers had done before
him, he wished--and this is his glory--to demonstrate it to himself
by acquiring the conviction by study, and wrote his book to prove it.
The true prophet of a creed, the apostle of a doctrine, the author of
a theory, is the man who, by his works, demonstrates the theory, makes
the creed believed in, and spreads the doctrine. He is not the creator.
“There is nothing new under the sun,” says an ancient proverb. We may
rather say, Nothing which succeeds is entirely new. The newborn is
unformed and incapable. The greatest things are born from a state of
germ, so to say, and increase unperceived. Ideas fertilize each other.
The sciences help each other; progress marches. Men often feel a truth,
sympathize with an opinion, touch a discovery, without knowing it. The
day arrives when a synthetical mind feels in some way an idea, almost
ripe, becoming incarnate in his brain. He becomes enamored of it, he
fondles it, he contemplates it. It grows as he regards it. He sees,
grouping round it, a multitude of elements which help to support it.
To him the idea becomes a doctrine. Then, like the apostles of good
tidings, he becomes an evangelist, announces the truth, proves it by
his works, and all recognize in him the author of the new contemplation
of nature, although all know perfectly well that he has not invented
the idea, and that many others before him have foreseen its grandeur.
Such is the position of Copernicus in the history of astronomy. The
hypothesis of the earth’s motion had been suggested long before his
birth on this planet. This theory counted partisans in his time.
But he--he did his work. He examined it with the patience of an
astronomer, the rigor of a mathematician, the sincerity of a sage, and
the mind of a philosopher. He demonstrated it in his works. Then he
died without seeing it understood, and it was not till a century after
his death that astronomy adopted it, and popularized it by teaching
it. However, Copernicus is really the author of the true system of the
world, and his name will remain respected to the end of time.
The so-called “seas” on the moon are those large dark spots to be seen
on its surface, in the shape of “eyes, nose, and mouth,” or of the
famous old man with his bundle of sticks. The brighter parts are the
more mountainous parts.
The chief ranges of lunar mountains have been named by astronomers
after mountains on earth, such as the Apennines, the Alps, the
Caucasian range, the Carpathian and the Altai Mountains.
CHAPTER XVI.
MERCURY, VENUS, AND MARS.
Once again we have to journey through the high-roads of the Solar
System, paying a brief visit to each in turn of our seven chief
brother-and-sister planets, and learning a few more leading facts about
them. Having gone the same way before, it will not now seem quite so
far.
Busy, hurrying Mercury! we must meet him first in his wild rush through
space. If he were to slacken speed for a single instant, he would begin
to fall with fearful rapidity towards the sun. And if Mercury were to
drop into one of those huge black chasms of rent furnace-flame on the
sun’s surface, there would be a speedy end to his life as a planet.
Mercury’s day is about the same length as our day, and his year is
about one quarter the length of our year. If Mercury has spring,
summer, autumn, and winter, each season must be extremely short; but
this depends upon whether Mercury’s axis slopes like the earth’s
axis--a matter difficult to find out. Mercury is always so near to the
sun, that it is by no means easy to observe him well.
We know more about his orbit than his axis. The earth’s orbit, as
before explained, is not a circle, but an ellipse or oval. Mercury’s
orbit is an ellipse also, and a much longer--or, as it is called, a
more eccentric--ellipse. The earth is three millions of miles nearer
to the sun at one time of the year, than six months before or after.
Mercury is no less than fifteen millions of miles nearer at one time
than another, which must make a marked difference in the amount of heat
received.
Even when the distance is greatest, the sun as seen from Mercury looks
four and a half times as large as the sun we see. What a blazing
splendor of light! It is not easy to imagine human beings living there,
in such heat and glare, and with either no changes of season at all, or
such very short seasons rapidly following one another. Mercury may, and
very likely does, abound with living creatures, as much as the earth
abounds with them; only one fancies they must be altogether a different
kind of living creatures from any ever seen on earth. And yet we do not
know. Man can so wonderfully adapt himself or be adapted to different
climates on earth, from extreme heat to extreme cold, that we can not
tell how far this adapting power may reach.
Both Mercury and Venus seem to be enfolded in dense, cloud-laden
atmospheres, rarely parting so as to allow us to get even a glimpse
of the real planets within the thick, light-reflecting covering. Some
have thought that a heavy, moist, protecting atmosphere may help to
ward off the intense heat, and to make Mercury a more habitable place.
Our earthly atmosphere is rather of a kind to store up heat, and to
make us warmer than we should be without it; but there might be vapors
differently constituted which might act in some other way. At all
events, we know how easily God can have adapted either the planet to
the creatures he meant to place there, or the creatures to the climate.
“All things are possible” to him. The how and the what are interesting
questions for us, but we must often be content to wait for an answer.
A thick, gray ring or belt has been noticed round the small, black disk
of Mercury, while it has passed between us and the sun. The edge of
Mercury, seen against the bright photosphere beyond, would, if there
were no atmosphere, be sharp and clear as the edge of the airless moon.
This surrounding haze seems to show that Mercury has an atmosphere. The
sunlight reflected from Mercury’s envelope of clouds shines at least as
brightly as if it were reflected from his solid body.
The small size of Mercury makes attraction on his surface much less
than on earth. A lump of iron weighing on earth one pound, would weigh
on Mercury only about seven ounces, or less than half as much. So a
man would be a very light leaper indeed there, and an elephant might
be quite a frolicsome animal. If there are star-gazers in Mercury, and
if the cloud-laden atmosphere allows many clear views of the sky, the
earth and Venus must both be beautiful to look upon. Each of the two
would shine far more brightly than Jupiter, as seen at his best from
earth.
Like Mercury, Venus, the next planet, has an orbit lying inside our
orbit. Mercury and Venus are always nearer to the sun than we are;
and if Mercury and Venus traveled round the sun in orbits, the planes
of which were exactly the same as the plane of the earth’s orbit, we
should very often see them creeping over the surface of the sun. Not
that they really “creep over” it; only, as they journey between the sun
and us, we can see them pass like little black dots across the sun’s
disk. This is the same thing as when the moon passes across the sun’s
disk and eclipses it. But Mercury and Venus are too far away from us to
cause any eclipse of the sun’s light.
Mercury has given more trouble to astronomers than any other member of
the system; for, owing to his proximity to the sun, he is usually lost
in the solar glory, and is never seen in a dark part of the heavens,
even at the time of his greatest distance. This circumstance, together
with a small mass and an immense velocity, renders it difficult to
catch and watch him. In high latitudes, where the twilight is strong
and lengthened, while mists often overhang the horizon, the planet can
seldom be seen with the naked eye. Hence an old astro-meteorologist
contemptuously describes him as “a squirting lacquey of the sun, who
seldom shows his head in these parts, as if he was in debt.” Copernicus
lamented that he had never been able to obtain a sight of Mercury; and
the French astronomer Delambre saw him not more than twice with the
naked eye.
The most favorable times for making observations are about an hour and
three-quarters before sunrise in autumn, and after sunset in spring;
but very clear weather and a good eye are required. At certain periods,
when between the earth and the sun, on a line joining the centers of
the two bodies, Mercury appears projected on the solar disk as a small,
round, dark spot. This is called a _transit_, and would occur during
every revolution if the plane of his orbit coincided with that of the
orbit of the earth. But as one-half of his orbit is a little above
that of the earth, and the other half a little below it, he passes
above or below the sun to the terrestrial spectator, except at those
intervals, when, being between us and the sun, he is also at one of the
two opposite points where the planes of the respective orbits intersect
each other. Then, stripped of all luster, the planet passes over
the face of the great luminary as a black circular speck, affording
evidence of his shining by reflected light, and of his spherical form.
The first transit of Mercury recorded in history was predicted by
Kepler, and witnessed by Gassendi, at Paris, on the morning of a
cloudy day, the 7th of November, 1631. It was toward nine o’clock
when he saw the planet; but owing to its extreme smallness, he was
at first inclined to think it was a minute solar spot. It was then
going off the sun; and made its final egress toward half-past ten. The
observed time of the transit was nearly five hours in advance of the
computed time. This was a very satisfactory accordance for that age
between theory and observation; but it was the effect of a fortuitous
combination of circumstances, rather than of computations founded
upon well-established data. “The crafty god,” wrote Gassendi, in the
peculiar style of his day, “had sought to deceive astronomers by
passing over the sun a little earlier than was expected, and had drawn
a veil of dark clouds over the earth in order to make his escape more
effectual. But Apollo, knowing his knavish tricks from his infancy,
would not allow him to pass altogether unnoticed. To be brief, I have
been more fortunate than those hunters after Mercury who have sought
the cunning god in the sun. I found him out, and saw him where no one
else had hitherto seen him.” Since Gassendi’s time a number of transits
have been observed.
These crossings of the sun’s face, or “transits,” as they are called,
have been important matters. The transit of Venus especially was once
eagerly looked for by astronomers, since, by close observations of
Venus’s movements and positions, the distance of the sun could at that
time be better calculated than in any other way. Other methods are now
coming into vogue.
The transits of Venus are rare. Two come near together, separated by
only eight years, and then for more than one hundred years the little
dark body of Venus is never seen from earth to glide over the sun’s
photosphere. There was a transit of Venus in the year 1761, and another
in the year 1769. There was a transit of Venus in 1874, and another in
1882. At the last transits it was found that the sun, instead of being
ninety-five millions of miles away, as astronomers thought, was only
ninety-three millions of miles away. The reason why these transits
happen so seldom, is that the orbits of Mercury and Venus lie in rather
a different plane or level from the earth’s orbit. So, like the moon,
though often passing between us and the sun, they generally go just a
little higher or just a little lower than his bright face.
Mercury and Venus show phases like the moon, although they do not
circle round the earth as the moon does. These “phases,” or changes of
shape, are probably never visible except through a telescope. It will
be easier to think about the phases of Venus alone, than to consider
both together. Her orbit lies within the earth’s orbit, and the earth
and Venus travel round the sun--as do all the planets--in the same
direction. But as Venus’s pathway is shorter than ours, and as her
speed is greater, she is much the quickest about her yearly journey,
and she overtakes us again and again at different points of our orbit
in turn.
[Illustration: PHASES OF VENUS.]
At one time she comes between us and the sun. That is her nearest
position to us, and she is then only about twenty-five millions of
miles distant. A beautiful sight she would be, but unfortunately her
bright side is entirely turned away, and only her dark side is turned
towards us. So then she is “new Venus,” and is invisible.
At another time she is completely beyond the sun, and at her farthest
position away from us. Her shining is quite lost in the sun’s rays
coming between. And though we get a good view of her as “full Venus,”
at a little to one side or the other, yet so great is her distance--as
much as one hundred and fifty-seven millions of miles--that her size
and brightness are very much lessened.
Between these two nearest and farthest points, she occupies two middle
distances, one on each side of the sun. Then, like the moon at her
“quarters,” she turns to us only half of her bright side. But this
is the best view of Venus that we have, as a brilliant, untwinkling,
starlike form,--the Evening Star of ancients and of poets. Between
these four leading positions Venus is always traveling gradually
from one to another--always either waxing or waning in size and in
brightness. Mercury passes through the same seeming changes.
Inhabitants of Venus must have a glorious view of the earth, with her
attendant moon. For just at the time when the two planets are nearest
together, and when she is only “new Venus” to us, a dark and invisible
body, the earth is “full earth” to Venus. The very best sight we ever
have of Venus can not come near that sight. But if Mercury and Venus
really are so often covered with heavy clouds as astronomers believe,
this must greatly interfere with any habits of star-gazing.
Venus and the earth have often been called twin-sister planets. There
are many points of likeness between them. In size they differ little,
and in length of day they are within an hour of being the same. Earth
certainly has a companion-moon, and Venus, it is believed, has not. At
one time several astronomers were pretty certain that they had caught
glimpses of a moon; but the supposed moon has of late quite vanished,
and nobody can say whether it ever really existed. Venus travels in
an ellipse which comes nearer to being a circle than the orbit of any
other planet.
Mountain-shadows have been watched through the telescope, in Venus,
as in the moon. Some astronomers have believed that they saw signs of
very lofty mountains--as much as twenty-eight miles, or four times the
height of our highest earthly mountains, but this requires confirmation.
There is a good deal of uncertainty about the climate of Venus. The
heat there must greatly surpass heat ever felt on earth--the sun being
about double the apparent size of our sun, and pouring out nearly
double the amount of light and heat that we receive.
This difference may be met, as already stated, by a sheltering, cloudy
atmosphere, or the inhabitants may have frames and eyesight suited to
the increased glare and warmth.
An atmosphere and water exist there as here. From what we have seen
above of the rapid and violent seasons of this planet, we might think
that the agitations of the winds, the rains, and the storms would
surpass everything which we see and experience here, and that its
atmosphere and its seas would be subject to a continual evaporation
and precipitation in torrential rains--an hypothesis confirmed by its
light, due, doubtless, to reflection from its upper clouds, and to the
multiplicity of the clouds themselves. To judge by our own impressions,
we should be much less pleased with this country than with our own, and
it is even very probable that our physical organization, accommodating
and complaisant as it is, could not become acclimatized to such
variations of temperature. But it is not necessary to conclude from
this that Venus is uninhabitable and uninhabited. We may even suppose,
without exaggeration, that its inhabitants, organized to live in the
midst of these conditions, find themselves at their ease, like a fish
in water, and think that our earth is too monotonous and too cold to
serve as an abode for active and intelligent beings.
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 resemble ours most. 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 an 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.
It is believed also that the axis of Venus, instead of being slanted
only as much as the earth’s axis, is tilted much more. Even if
the tilting is less than some have supposed, it is probably very
considerable. If Venus really does “lie over” in such a manner,
certain startling changes of climate on its surface--unpleasant
changes, according to our ideas--would take place.
Like earth, Venus would have her two arctic regions, where a burning
summer’s day would succeed a bitter winter’s night, each half a year in
length. She would have also her tropical region--only in that region
intense cold would alternate with intense heat, brief seasons of each
in turn. And between the tropics and the arctic regions would lie wide
belts, by turns entirely tropical and entirely arctic. The rapidity and
severity of these changes, following one another in a year about as
long as eight of our months, would seem to be too much for any human
frame to endure. But it all rests upon an _if_. And we may be quite
sure that _if_ there are any manner of human beings in Venus, their
frames are well suited to the climate of their world.
Though Mars is one of the inner group of four small planets, divided by
the zone of asteroids from the outer group of four great planets, yet
he belongs to the outside set of Superior Planets. His orbit surrounds
ours, being at all points farther off from the sun. Very slight
“phases” have been seen in Mars. He turns to us, from time to time,
just enough of his dark side to prove that he _has_ a dark side, and
that he does not shine like a star by his own light. But the phases on
that planet are by no means marked as they are with Venus.
Until lately it was believed that Mars possessed no moons. Two very
small ones have, however, been lately found circling round him.[2]
They have been named Deimos and Phobos, after the “sons of Mars” in
Greek mythology. Deimos travels round Mars in thirty-nine hours, while
Phobos performs the same journey in the astonishingly short period of
seven hours and a half!
[2] The discovery of the two moons is due to the agency of a woman.
Professor Asaph Hall, of Washington City, was searching by the aid of
the most powerful telescope which had yet been directed to Mars, and at
the very moment when the planet was in the most favorable condition for
observation. Having searched in vain during several evenings in August,
1877, he was about to give up, when Mrs. Hall begged him to search a
little more. He did so, and on the night of the 11th he discovered the
first of the satellites, and then on the 17th the second.
[Illustration: MARS AND THE PATH OF ITS SATELLITES.]
We have here, then, a system very different from that of the earth
and moon. But the most curious point is the rapidity with which the
inner satellite of Mars revolves round its planet. This revolution
is performed in seven hours, thirty-nine minutes, fifteen seconds,
although the world of Mars rotates on itself in twenty-four hours,
thirty-seven minutes--that is to say, this moon turns much more
quickly than the planet itself. This fact is inconsistent with all the
ideas we have had up to the present on the law of formation of the
celestial bodies. Thus, while the sun appears to revolve in the Martian
sky in a slow journey of more than twenty-four hours, the inner moon
performs its entire revolution in a third of a day. It follows that
_it rises in the west_ and _it sets in the east_! It passes the second
moon, eclipses it from time to time, and goes through all its phases
in eleven hours, each quarter not lasting even three hours. What a
singular world!
These satellites are quite small--they are the smallest celestial
bodies we know. The brightness of the planet prevents us from measuring
them exactly. It seems, however, that the nearer is the larger, and
shows the brightness of a star of the tenth magnitude, and that the
second shines as a star of the twelfth magnitude. According to the
most trustworthy photometric measures, the first satellite may have a
diameter of 7.45 miles, and the second a diameter of 6.2 miles. _The
larger of these two worlds is scarcely larger than Paris._ Should we
honor them with the title of worlds? They are not even terrestrial
continents, nor empires, nor kingdoms, nor provinces, nor departments.
Alexander, Cæsar, Charlemagne, or Napoleon, might care but little to
receive the scepter of such worlds. Gulliver might juggle with them.
Who knows, however? The vanity of men being generally in the direct
ratio of their mediocrity, the microscopical reasoning mites which
doubtless swarm on their surface have also, perhaps, permanent armies,
which mutilate each other for the possession of a grain of sand.
These two little moons received from their discoverer the names of
_Deimos_ (Terror) and _Phobos_ (Flight), suggested by the two verses of
Homer’s “Iliad” which represent Mars descending on the earth to avenge
the death of his son Ascalaphus:
“He ordered Terror and Flight to yoke his steeds,
And he himself put on his glittering arms.”
_Phobos_ is the name of the nearer satellite; _Deimos_, that of the
more distant.
The existence of these little globes had already been suspected from
analogy, and thinkers had frequently suggested that, since the earth
has one satellite, Mars should have two, Jupiter four, Saturn eight;
and this is indeed the fact--though as Jupiter is now found to have
_five_ satellites, this arithmetical progression is upset. But as we
experience too often in practice the insufficiency of these reasonings
of purely human logic, we can not give them more value than they
really possess. We might suppose in the same way now that Uranus has
sixteen satellites, and Neptune thirty-two. This is possible; but
we know nothing of them, and have not even the right to consider
this proportion as probable. It is not the less curious to read the
following passage, written by Voltaire in 1750 in his masterpiece, the
“Micromégas:”
“On leaving Jupiter, our travelers crossed a space of about a hundred
millions of leagues, and reached the planet Mars. They saw _two
moons_, which wait on this planet, and which have escaped the gaze
of astronomers. I know well that Father Castel wrote against the
existence of these two moons; but I agree with those who reason from
analogy. These good philosophers know how difficult it would be for
Mars, which is so far from the sun, to get on with less than two moons.
However this may be, our people found it so small that they feared they
might not find anything to lie upon, and went on their way.”
Here we have unquestionably a very clear prophecy, a rare quality
in this kind of writing. The astronomico-philosophical romance of
“Micromégas” has been considered as an imitation of Gulliver. Let us
open the masterpiece of Swift himself, composed about 1720, and we
read, word for word, in Chapter III of the “Voyage to Laputa:”
“Certain astronomers ... spend the greatest part of their lives in
observing the celestial bodies, which they do by the assistance of
glasses far excelling ours in goodness. For this advantage hath enabled
them to extend the discoveries much farther than our astronomers in
Europe; for they have made a catalogue of ten thousand fixed stars,
whereas the largest of ours do not contain above one-third part of that
number. They have likewise discovered two lesser stars, or satellites,
which revolve about Mars, whereof the innermost is distant from the
center of the primary planet exactly three of his diameters, and the
outermost five. The former revolves in the space of ten hours, and
the latter in twenty-one and a half; so that the squares of their
periodical times are very near in the same proportion with the cubes of
their distance from the center of Mars, which evidently shows them to
be governed by the same law of gravitation that influences the other
heavenly bodies.”
What are we to think of this double prediction of the two satellites of
Mars? Indeed, the prophecies which have been made so much of in certain
doctrinal arguments have not always been as clear, nor the coincidences
so striking. However, it is evident that no one had ever seen these
satellites before 1877, and that there was in this hit merely the
capricious work of chance. We may even remark that both the English and
French authors have only spoken ironically against the mathematicians,
and that in 1610, Kepler, on receiving the news of the discovery of the
satellites of Jupiter, wrote to his friend Wachenfals that “not only
the existence of these satellites appeared to him probable, but that
doubtless there might yet be found two to Mars, six or eight to Saturn,
and perhaps one to Venus and Mercury.” We can not, assuredly, help
noticing that reasoning from analogy is here found on the right road.
However this may be, this discovery truly constitutes one of the most
interesting facts of contemporary astronomy.
Mars is not only much smaller than the earth, but a good deal less
dense in his “make.” His material is only about three-quarters as heavy
as an equal amount of the earth’s material. A very heavy man on earth
would be a most light and active individual on Mars. Gold taken from
earth to Mars would weigh there no more than tin weighs upon earth.
Mars has, it seems, an atmosphere, even as earth has. Of all the
planets Mars, is the only one whose actual surface is discernible in
the telescope. Mercury and Venus are so hidden by dense envelopes of
clouds that the real planets within are only now and then to be dimly
caught sight of. Jupiter and Saturn are so completely enwrapped in
mighty masses of vapor, that we can not even be certain whether there
are any solid bodies at all inside.
But Mars can be studied. Here and there, it is true, clouds sweep over
the landscape, hiding from view for a little while one continent or
another, one sea or another, growing, changing, melting away, as do
the clouds of earth. Still, though these clouds come and go, there are
other markings on the surface of Mars which do not change. Or, rather,
they only change so much as the continents and oceans of earth would
seem to vary, if watched from another planet, as the daily movement of
earth carried them from west to east, or as they might be hidden for
a while by cloud-layers coming between. It has been curiously noted
that these clouds over Mars form often in the morning and evening, and
are afterwards dispersed by the heat of midday. Also there seems every
reason to believe that rainfalls take place in Mars as upon earth.
The red color of Mars is well known. This does not vanish in the
telescope, but it is found that parts only have the red or orange
hue, while other parts are dark and greenish. These are the markings
which remain always the same, and they have been so closely examined
that more is known about the geography of Mars than of any other world
outside our own.
Mars, at his nearest point, does not draw closer to us than forty
millions of miles. At such a distance one must not speak too
confidently. There are, however, many reasons for believing that the
red portions are continents and that the green portions are oceans.
The spectroscope has lately shown us that water does really exist in
the atmosphere of Mars--unlike the dreary, waterless moon. So we no
longer doubt that the cloudlike appearances are clouds, and that rain
sometimes falls on Mars. If there is rain, and if there are clouds and
vapor, there are probably oceans also.
Two singular white spots are to be seen at the north and south poles,
which we believe to be polar ice and snow. Somebody looking at our
earth in like manner from a distance, would doubtless perceive two
such white snow-spots. These two polar caps are seen to vary with the
seasons. When the north pole of Mars is turned towards the sun, the
white spot there grows smaller; and at the same time, the south pole of
Mars being turned away from the sun, the white spot there grows larger.
Again, when the south pole is towards the sun, and the north pole away
from the sun, the white spot at the south is seen to be the smallest,
and the white spot at the north is seen to be the largest. This is
exactly what takes place in the summers and winters of our north and
south poles.
The markings of Mars have been so carefully studied, that at last a map
has been made of the planet--a map of a world, never less than forty
millions of miles away! Names have been given to the continents and
oceans--such as Dawes Continent, Herschel Continent, De La Rue Ocean,
Airy Sea, Huggins Inlet, and so on.
Land and water seem to be very differently arranged on Mars from what
they are on earth. Here we have about three times as much water as
land, and to get from one continent to another without crossing the
sea is in some cases impossible. But a traveler there might go most
conveniently to and fro, hither and thither, to all parts of his world,
either on land or on water, without any change. If he preferred water,
he would never need to set foot on land; and if he preferred land, he
would never need to enter a boat. The two are so curiously mingled
together, narrow necks of land running side by side with long, narrow
sea-inlets, that Atlantic and Pacific Oceans are unknown.
Some have wondered whether the reddish color of the land may be caused
by grass and trees being red instead of green. Very strange if so it
were. But in that case, no doubt the inhabitants of Mars would find
green just as trying to their eyesight, as we should find red trying to
ours.
CHAPTER XVII.
JUPITER.
Passing at one leap over the belt of tiny asteroids, about which we
know little beyond their general movement, and the size and weight of
a few among them, we reach at once the giant planet Jupiter: Mighty
Jupiter, hurrying ever onward, with a speed, not indeed equal to that
of Mercury or of our earth, yet eighty times as rapid as the speed of
a cannon-ball! Think of a huge body, equal in bulk to twelve hundred
earths, equal in weight to three hundred earths, rushing ceaselessly
through space, at the rate of seven hundred thousand miles a day!
Jupiter’s shape is greatly flattened at the poles. He spins rapidly on
his axis, once in nearly ten hours, and has therefore a five hours’
day and a five hours’ night. As the slope of his axis is exceedingly
slight, he can boast little or no changes of season. The climate near
the poles has never much of the sun’s heat. In fact, all the year round
the sun must shine upon Jupiter much as he shines on the earth at the
equinoxes.
But the amount of light and heat received by Jupiter from the sun is
only about one twenty-fifth part of that which we receive on earth; and
the sun, as seen from Jupiter, can have but a small, round surface, not
even one-quarter the diameter of the sun we see in the sky.
When looked at with magnifying power, the bright, starlike Jupiter
grows into a broad, softly-shining disk or plate, with flattened top
and bottom, and five tiny, bright moons close at hand. Sometimes one
moon is on one side, and four are on the other; sometimes two are one
side and three on the other; sometimes one or more are either hidden
behind Jupiter or passing in front of him. Jupiter has also curious
markings on his surface, visible through a telescope. These markings
often undergo changes; for Jupiter is no chill, fixed, dead world, such
as the moon seems to be.
[Illustration: GENERAL ASPECT OF JUPITER--SATELLITE AND ITS SHADOW.]
There are dark belts and bright belts, usually running in a line with
the equator, from east to west. Across the regions of the equator lies
commonly a band of pearly white, with a dark band on either side of
“coppery, ruddy, or even purplish” hue. Light and dark belts follow one
after another, up to the north pole and down to the south pole.
When we talk of “north and south poles” in the other planets, we merely
mean those poles which point towards those portions of the starry
heavens which we have chosen to call “northern” and “southern.” You
know that all the chief planets travel round the sun in very nearly the
same _plane_ or flat surface that we do ourselves. That plane is called
the “plane of the ecliptic.” Suppose that you had an enormous sheet of
cardboard, and that in the middle of this cardboard the sun were fixed,
half his body being above and half below. At a little distance, fixed
in like manner in the card, would be the small body of the earth, half
above and half below, her axis being in a slanting position. The piece
of cardboard represents what is called in the heavens the _plane of
the ecliptic_--an imaginary flat surface, cutting exactly through the
middle of the sun and of the earth.
If the planets all traveled in the same precise plane, they would
all be fixed in the cardboard just like the earth, half the body of
each above and half below. As they do not so travel, some would have
to be placed a little higher, some a little lower, according to what
part of their orbits they were on. This supposed cardboard “plane of
the ecliptic” would divide the heavens into two halves. One half,
containing the constellations of the Great Bear, the Little Bear,
Cepheus, Draco, and others, would be called the Northern Heavens. One
end of the earth’s axis, pointing just now nearly to the Polar Star,
we name the North Pole; and all poles of planets pointing towards this
northern half of the heavens, are in like manner named by us their
north poles.
With regard to west and east, lay in imagination upon this cardboard
plane a watch, with its face upwards; remembering that all the planets
and nearly all the moons of the Solar System are said both to spin on
their axes, and to travel in their orbits round the sun, _from west to
east_. Note how the hands of your watch would move in such a position.
The “west to east” motions of planets and moons would be in exactly the
opposite direction from what the motions of the watch-hands would be.
To return to Jupiter. It is believed that these bands of color are
owing to a heavy, dense atmosphere, loaded with vast masses of cloudy
vapor. By the “size” of Jupiter, we really mean the size of this
outside envelope of clouds. How large the solid body within may be,
or whether there is any such solid body at all, we do not know. The
extreme lightness of Jupiter, as compared with his great size, has
caused strong doubts on this head.
The white belts are supposed to be the outer side of cloud-masses
shining in the sunlight. Travelers in the Alps have seen such
cloud-masses, spreading over the whole country beneath their feet,
white as driven snow, and shining in the sunbeams which they were
hiding from villages below; or looking like soft masses of cotton-wool,
from which the mountain-peaks rose sharply here and there.
The dark spaces between seem to be rifts or breaks in the clouds.
Whether, when we look at those dark spaces, we are looking at the
body of Jupiter, or only at lower layers of clouds, is not known.
But sometimes blacker spots show upon the dark cloud-belts, and this
seems rather as if they were only lower layers of clouds, the black
spots giving us peeps down into still lower and deeper layers, or else
perhaps to the planet itself. These appearances remind one strongly of
the sun-spots, each with its penumbra, umbra, and nucleus. Occasionally
bright white spots show, instead of dark ones. It is thought that they
may be caused by a violent upward rush of dense clouds of white vapor.
The white spots again recall the sun and his _faculæ_.
Jupiter’s bands are not fixed. Great changes go on constantly among
them. Sometimes a white band will turn dark-colored, or a dark band
will turn white. Sometimes few and sometimes many belts are to be seen.
Sometimes a dark belt will lie slanting across the others, nearly from
north to south. Once, in a single hour, an entirely new belt was seen
to come into shape. Another time, two whole belts vanished in one day.
The bands, in which such rapid movements are seen, are often thousands
of miles in breadth. Sometimes these wide zones of clouds will remain
for weeks the same. At another time a break or rift in them will be
seen to journey swiftly over the surface of the planet.
The winds on earth are often destructive. A hurricane, moving at the
rate of ninety miles an hour, will carry away whole buildings and level
entire plantations. Such hurricanes rarely, if ever, last more than
a few hours. But winds in Jupiter, judging from the movements of the
clouds, often travel at the rate of one hundred and fifty miles an
hour; and that, not for hours only, but for many weeks together. What
manner of living beings could stand such weather may well be questioned.
Another difficulty which arises is as to the cause of these tremendous
disturbances on Jupiter. Our earthly storms are brought about by the
heat of the sun acting on our atmosphere. But the sun-heat which
reaches Jupiter seems very far from enough to raise such vast clouds of
vapor, and to bring about such prolonged and tremendous hurricanes of
wind.
What if there is another cause? What if Jupiter is _not_ a cooled
body like our earth, but a liquid, seething, bubbling mass of fiery
heat--just as we believe our earth was once upon a time, in long past
ages, before her outside crust became cold enough for men and animals
to live thereon? _Then_, indeed, we could understand how, instead
of oceans lying on his surface, all the water of Jupiter would be
driven aloft to hang in masses of steam or be condensed into vast
cloud-layers. _Then_ we could understand why a perpetual stir of
rushing winds should disturb the planet’s atmosphere.
In that case would Jupiter be a planet at all? Certainly--in the sense
of obeying the sun’s control. Our earth was once, we believe, a globe
of melted matter, glowing with heat--and farther back still, possibly,
a globe of gas. Some people are very positive about these past changes;
but it is wise not to be over-positive where we can not know to a
certainty what has taken place. However, Jupiter _may_ have cooled down
only to the liquid state, and if he goes on cooling he may, by and by,
gain a solid crust like the earth.
This idea about Jupiter’s hot and molten state belongs quite to late
years. Certain other matters seem to bear it out, though of actual
proof we have none. It is thought, for instance, that the dull, coppery
red light, showing often in the dark bands, may be a red glow from the
heated body within. Also it has been calculated that Jupiter gives out
much more light than our earth would do, if increased to his size and
moved to his place--more, in fact, than we could reasonably expect him
to give out. If so, whence does he obtain the extra brightness? If he
does not shine by reflected light alone, he probably shines also in
some additional degree by his own light.
But what about Jupiter being inhabited? Would it in such a case be
quite impossible? “Impossible” is not a word for us to use about
matters where we are ignorant. We can only say that it is impossible
for us to _imagine_ any kind of living creatures finding a home there,
if our present notions about the present state of Jupiter are correct.
Then is the chief planet of the Solar System a huge, useless monument
of God’s power to create? Not so fast. Even as merely such a monument,
he could not be useless. And even if he were put to no present use at
all, it might be merely because this is a time of preparation for the
future. God has his times of long and slow preparation, alike with
worlds, with nations, and with individuals.
But now as to the five moons circling round Jupiter. There used to be
some very pretty ideas afloat about the wonderful beauty of the moons,
as seen from Jupiter, their united brilliancy so far surpassing the
shining of our one poor satellite, and making up for the dim light of
Jupiter’s sun.
A certain little difficulty was not quite enough considered. If
anybody were living on the surface of Jupiter, he would have, one is
inclined to think, small chance of often seeing the moons through the
cloud-laden atmosphere.
[Illustration: SATELLITES OF JUPITER COMPARED WITH THE EARTH AND MOON.]
The nearest of the four larger moons to Jupiter would, it is true,
appear--when visible at all--rather bigger than ours does to us; while
the two next would be almost half as large, and the farthest about a
quarter as large--supposing inhabitants of Jupiter to have our powers
of vision.
All taken together they would cover a considerably larger space in the
sky than does our moon. But it must be remembered that Jupiter’s moons,
like ours, shine merely by reflected sunlight. And so dim is the
sunshine at that distance compared with what it is at our distance,
that all the five moons together, even if full at the same time, could
only give about one-sixteenth part of the light which we obtain from
our one full moon.
Besides, they never are full together, seen from any one part of
Jupiter. The four inner moons are never to be seen “full” at all; for
just when they might be so, they are eclipsed or shaded by Jupiter’s
shadow. The fourth sometimes escapes this eclipse, from being so much
farther away.
A thought has been lately put forward, which may or may not have truth
in it. What if--instead of Jupiter being a world, inhabited by animals
and people, as is often supposed, with a small distant sun and five dim
moons to give them light--what if Jupiter is himself in some sort a
second sun to his moons, and what if those “moons” are really inhabited
planets? It may be so. That is all we can say. The idea is not an
impossible one.
The so-called “moons” are certainly small. But they are by no means
too small for such a purpose. Jupiter would in that case, with his
five moons circling round him, enjoying his light and warmth, be a
small picture of the sun, with his four inner planets and the asteroids
circling round him, basking in a more lavish amount of the same.
Picturing the moons as giving light to Jupiter, we find them seemingly
dim and weak for such a purpose--though, of course, we may here make a
grand mistake in supposing the eyesight of living creatures in Jupiter
to be no better than our own eyesight. Even upon earth a cat can see
plainly where a man has to grope his way in darkness.
But by picturing the moons as inhabited, and Jupiter as giving out some
measure of heat and light to make up for the lessened amount of light
and heat received from the sun, the matter becomes more easy to our
understanding.
[Illustration: THE SYSTEM OF JUPITER.]
The nearest moon has indeed a magnificent view of Jupiter as a huge
bright disk in its sky, no less than three thousand times as large
as our moon appears to us, shining brightly with reflected sunlight,
and it may be glowing with a red light of his own in addition. Even
the farthest off of the five sees him with a face sixty-five times
the size of our moon. And the varying colors and stormy changes in
the cloud-belts, viewed thus near at hand, must afford marvelously
beautiful effects.
Just as Mercury, Venus, Earth, and Mars travel round the sun, at
different distances, nearly in the same plane, so Jupiter’s five moons
travel round him, at different distances, nearly in the same plane.
Jupiter’s moons are always to be seen in a line, not one high and
another low, one near his pole and another near his equator.
The moon nearest to Jupiter, discovered in September, 1892, by
Professor Barnard, now of the Yerkes Observatory of the Chicago
University, has a probable diameter of one hundred miles, and revolves
round its primary in about twelve hours. The second satellite, named
Io, is said to be over two thousand miles in diameter, travels round
Jupiter in less than two of our days, and is eclipsed by Jupiter’s
shadow once in every forty-two hours.
The third moon, Europa, is rather smaller, takes over three days to its
journey, and suffers eclipse once in every eighty-five hours.
The fourth moon, Ganymede, is believed to be considerably larger than
Mercury, journeys round Jupiter once a week, and is eclipsed once every
hundred and seventy-one hours.
The fifth moon, Callisto, is also said to be slightly larger than
Mercury, performs its journey in something more than sixteen days, and
from its greater distance suffers eclipse less often than the other
four.
The distance of the nearest is more than one hundred thousand miles
from Jupiter; that of the farthest, more than one million miles.
The following table presents more minutely the results of the latest
discoveries concerning these satellites:
+-----------------------+----------------+------------+-----------+
| | DISTANCE FROM | | PERIOD OF |
| SATELLITES. | CENTER OF | DIAMETER. |REVOLUTION.|
| | JUPITER. | | |
+-----------------------+----------------+------------+-----------+
| | | |D. H. M. S.|
|1. Barnard’s Satellite,| 112,500 miles.| 100 miles.| 11 57 23|
|2. Io, | 266,000 ” |2,356 ” | 1 18 27 33|
|3. Europa, | 424,000 ” |2,046 ” | 3 13 13 42|
|4. Ganymede, | 676,000 ” |3,596 ” | 7 3 42 33|
|5. Callisto, |1,189,000 ” |2,728 ” |16 15 32 11|
+-----------------------+----------------+------------+-----------+
The fact of these eclipses, and of the shadow thrown by Jupiter’s body,
shows plainly that though he may give out some measure of light, as has
been suggested, yet that light can not be strong, or it would prevent
any shadow from being thrown by the sunlight. Also, the dense masses
of cloud around him, though reflecting sunlight brightly, would shut
in much of his own light. Possibly it is chiefly as heat-giver and as
sunlight-reflector that Jupiter serves his five satellites.
As with our own moon, so with Jupiter’s moons, the real center of
their orbit is the sun, and not Jupiter. They accompany Jupiter in his
journey, controlled by the sun, and immensely influenced by Jupiter.
At night the spectacle of the sky seen from Jupiter is, with reference
to the constellations, the same as that which we see from the earth.
There, as here, shine Orion, the Great Bear, Pegasus, Andromeda,
Gemini, and all the other constellations, as well as the diamonds of
our sky: Sirius, Vega, Capella, Procyon, Rigel, and their rivals. The
390,000,000 of miles which separate us from Jupiter _in no way_ alter
the celestial perspectives. But the most curious character of this sky
is unquestionably the spectacle of the five moons, each of which shows
a different motion. The second moves in the firmament with an enormous
velocity, and Barnard’s satellite still faster, and produce almost
every day total eclipses of the sun in the equatorial regions. The four
inner moons are eclipsed at each revolution, just at the hours when
they are at their “full.” The fifth alone attains the full phase.
Contrary to the generally received opinion, these bodies do not give to
Jupiter all the light which is supposed. We might think, in fact, as
has been so often stated, that these five moons illuminate the nights
five times better relatively than our single moon does in this respect,
and that they supplement in some measure the feebleness of the light
received from the sun. This result would be, assuredly, very agreeable,
but nature has not so arranged it. The five satellites cover, it is
true, an area of the sky greater than our moon, but they reflect the
light of a sun twenty-seven times smaller than ours; indeed, the total
light reflected is only equal to a sixteenth of that of our full moon,
even supposing the soil of these satellites to be as white as it
appears to be, especially the fifth satellite.
Jupiter appears to be a world still in process of formation, which
lately--some thousands of centuries ago--served as a sun to his
own system of five or perhaps more worlds. If the central body is
not at present inhabited, his satellites may be. In this case, the
magnificence of the spectacle presented by Jupiter himself to the
inhabitants of the satellites is worthy of our attention. Seen from
Barnard’s satellite, Jupiter’s disk has a diameter of 47°, or more than
half the distance from the horizon to the zenith. Seen from the second
satellite, the Jovian globe presents an immense disk of twenty degrees
in diameter, or 1,400 times larger than the full moon! What a body!
What a picture, with its belts, its cloud motions, and its glowing
coloration, seen from so near! What a nocturnal sun!--still warm,
perhaps. Add to this the aspect of the satellites themselves seen from
each other, and you have a spectacle of which no terrestrial night can
give an idea.
Such is the world of Jupiter from the double point of view of its vital
organization and of the spectacle of external nature, seen from this
immense observatory.
The attraction of the planets has always played an important part
in the motion of comets and the form of their orbits. The enormous
size of Jupiter gives it more influence than any other planet, and we
are not surprised that it should have seriously interfered with some
of the comets that belong to the Solar System. We have already seen
that Biela’s comet was captured by Jupiter, and that it was probably
dissolved into meteoric dust. We show in the cut the orbits of eight
others that circle around the sun, but retreat no farther away than
Jupiter. These are the orbits as now determined, but they vary from
age to age on account of disturbances of other planets. But it is not
likely that the comets themselves will ever escape from the control of
Jupiter.
[Illustration: ORBITS OF NINE COMETS CAPTURED BY JUPITER.]
CHAPTER XVIII.
SATURN.
The system of Jupiter is a simple system compared with that of Saturn,
next in order. For whereas Jupiter has only five moons, Saturn has
eight, and, in addition to these, he has three wonderful rings. Neither
rings nor moons can be seen without a telescope, on account of Saturn’s
great distance from us--more than three thousand times the distance of
the moon, or upwards of eight hundred millions of miles. Saturn’s mean
distance from the sun is eight hundred and seventy-six million seven
hundred and sixty-seven thousand miles.
Saturn does not equal his mighty brother Jupiter in size, though he
comes near enough in this respect to be called often his “twin”--just
as that small pair of worlds, Venus and Earth, are called “twins.”
While Jupiter is equal in size to over one thousand two hundred earths,
Saturn is equal to about seven hundred earths. And while Jupiter is
equal in weight to three hundred earths, Saturn is only equal in weight
to ninety earths. He appears to be made of very light materials--not
more than three-quarters as dense as water. This would show the present
state of Saturn to be very different from the present state of the
earth. We are under the same uncertainty in speaking of Saturn as in
speaking of Jupiter. Like Jupiter, Saturn is covered with dense masses
of varying clouds, occasionally opening and allowing the astronomer
peeps into lower cloud-levels; but rarely or never permitting the
actual body of the planet to be seen.
The same perplexities also come in here, to be answered much in the
same manner. We should certainly expect that in a vast globe like
Saturn the strong force of attraction would bind the whole into a
dense solid mass; instead of which, Saturn is about the least solid of
all the planets. He seems to be made up of a light, watery substance,
surrounded by vapor.
[Illustration: SATURN AND THE EARTH--COMPARATIVE SIZE.]
One explanation can be offered. What if the globe of Saturn be still
in a red-hot, molten state, keeping such water as would otherwise lie
in oceans on his surface, floating aloft in masses of steam, the outer
parts of which condense into clouds?
No one supposes that Jupiter and Saturn are in the same condition
of fierce and tempestuous heat as the sun. They may have been so
once, but they must now have cooled down very many stages from that
condition. Though no longer, however, a mass of far-reaching flames and
fiery cyclones, the body of each may have only so far cooled as to have
reached a stage of glowing molten red-heat, keeping all water in the
form of vapor, and sending up strong rushes of burning air to cause the
hurricanes which sweep to and fro the vast cloud-masses overhead.
And if this be the case, then, with Saturn as with Jupiter, comes the
question, Can Saturn be inhabited? And if--though we may not say it is
impossible, yet we feel it to be utterly unlikely--then again follows
the question, What if Saturn’s _moons_ are inhabited?
Telescopic observations tempt us to believe that there is on this
planet a quantity of heat greater than that which results from its
distance from the sun; for the day-star as seen from Saturn is, as we
have said, ninety times smaller in surface, and its heat and light
are reduced in the same proportion. Water could only exist in the
solid state of ice, and the vapor of water could not be produced so
as to form clouds similar to ours. Now, meteorological variations are
observed similar to those which we have noticed on Jupiter, but less
intense. Facts, then, combine with theory to show us that the world of
Saturn is at a temperature at least as high as ours, if not higher.
But the strangest feature of the Saturnian calendar is, unquestionably,
its being complicated, not only with the fabulous number of 25,060
days in a year, but, further, with eight different kinds of months, of
which the length varies from 22 hours to 79 days--that is to say, from
about two Saturnian days to 167. It is as if we had here _eight moons
revolving in eight different periods_.
The inhabitants of such a world must assuredly differ strangely
from us from all points of view. The specific lightness of the
Saturnian substances and the density of the atmosphere will have
conducted the vital organization in an extra-terrestrial direction,
and the manifestations of life will be produced and developed under
unimaginable forms. To suppose that there is nothing fixed, that the
planet itself is but a skeleton, that the surface is liquid, that the
living beings are gelatinous--in a word, that all is unstable--would be
to surpass the limits of scientific induction.
The diameter of the largest moon is about half the diameter of the
earth, or much larger than Mercury. The four inner satellites are
all nearer to Saturn than our moon to us, though the most distant of
the eight is ten times as far away. The inner moon takes less than
twenty-three hours to travel round Saturn, and the outer one over
seventy-nine days.
A great many charming descriptions have been worked up, with Saturn
as with Jupiter, respecting the magnificent appearance of the eight
radiant moons, joined to the glorious shining of the rings, as quite
making up for the diminished light and heat of the sun. But here again
comes in the doubt, whether really it is the moons who make up to
Saturn for lack of light, or whether it is Saturn who makes up to the
moons for lack of light.
Certainly, Saturn’s cloudy covering would a little interfere with
observations of the moons by any inhabitants of the solid body
within--supposing there be any solid body at all. And though it sounds
very wonderful to have eight moons instead of one moon, yet all the
eight together give Saturn only a very small part of the light which we
receive from our one full moon--so much more dimly does the sun light
them up at that enormous distance.
The same thing has been noticed with Saturn as with Jupiter--that he
seems to shine more brightly than is to be expected in his position,
from mere reflection of the sun’s rays. A glowing body within, sending
a certain amount of added light through or between the masses of
clouds, would explain away this difficulty.
One more possible proof of Saturn’s half-liquid state is to be found
in his occasional very odd changes of shape. Astronomers have been
startled by a peculiar bulging out on one side, taking off from his
roundness, and giving a square-shouldered aspect. We may not say it
is quite impossible that a solid globe should undergo such tremendous
upheavals and outbursts as to raise a great portion of its surface five
or six hundred miles above the usual level--the change being visible
at a distance of eight hundred millions of miles. But it would be
easier to understand the possibility of such an event, in the case of a
liquid, seething mass, than in the case of a solid ball.
On the other hand this alteration of outline may be caused simply by
a great upheaval not of the planet’s surface, but of the overhanging
layers of clouds. Some such changes, only much slighter, have been
remarked in Jupiter.
And now as to the rings. Nothing like them is to be seen elsewhere in
the Solar System. They are believed to be three in number; though some
would divide them into more than three. Passing completely round the
whole body of Saturn, they rise, one beyond another, to a height of
many thousands of miles.
The inner edge of the inner ring--an edge perhaps one hundred miles in
thickness--is more than ten thousand miles from the surface of Saturn
or more strictly speaking, from the outer surface of Saturn’s cloudy
envelope. A man standing exactly on the equator and looking up, even if
no clouds came between, would scarcely be able to see such a slender
dark line at such a height.
This dark, transparent ring, described sometimes as dusky, sometimes as
richly purple, rises upwards to a height or breadth of nine thousand
miles. Closely following it is a ring more brilliant than Saturn
himself, over eighteen thousand miles in breadth. When astronomers talk
of the “breadth” of these rings, it must be understood that they mean
the width of the band measured _upwards_, in a direction away from the
planet.
Beyond the broad, bright ring is a gap of about one thousand seven
hundred miles. Then follows the third ring, ten thousand miles in
breadth; its outermost edge being at a height of more than forty-eight
thousand miles from Saturn. The color of the third ring is grayish,
much like the gray markings often seen on Saturn.
What would not be our admiration, our astonishment, our stupor perhaps,
if it were granted us to be transported there alive, and, among all
these extra-terrestrial spectacles, to contemplate the strange aspect
of the rings, which stretch across the sky like a bridge suspended
in the heights of the firmament! Suppose we lived on the Saturnian
equator itself, these rings would appear to us as a thin line drawn
across the sky above our heads, and passing exactly through the zenith,
rising from the east and increasing in width, then descending to the
west and diminishing according to perspective. Only there have we
the rings precisely in the zenith. The traveler who journeys from
the equator towards either pole leaves the plane of the rings, and
these sink imperceptibly, at the same time that the two extremities
cease to appear diametrically opposite, and by degrees approach each
other. What an amazing effect would be produced by this gigantic
arch, which springs from the horizon and spans the sky! The celestial
arch diminishes in height as we approach the pole. When we reach the
sixty-third degree of latitude the summit of the arch has descended to
the level of our horizon and the marvelous system disappears from the
sky; so that the inhabitants of those regions know nothing of it, and
find themselves in a less favorable position to study their own world
than we, who are nearly 800,000,000 miles distant.
During one-half of the Saturnian year the rings afford an admirable
moonlight on one hemisphere of the planet, and during the other half
they illuminate the other hemisphere; but there is always a half-year
without “ringlight,” since the sun illuminates but one face at a time.
Notwithstanding their volume and number, the satellites do not give
as much nocturnal light as might be supposed; for they receive, on an
equal surface, only the ninetieth part of the solar light which our
moon receives. All the Saturnian satellites which can be at the same
time above the horizon and as near as possible to the full phase do not
afford more than the hundredth part of our lunar light. But the result
may be nearly the same, for the optic nerve of the Saturnians may be
ninety times more sensitive than ours.
But there are further strange features in this system. The rings are
so wide that their shadow extends over the greater part of the mean
latitudes. During fifteen years the sun is to the south of the rings,
and for fifteen years it is to the north. The countries of Saturn’s
world which have the latitude of Paris endure this shadow for more than
five years. At the equator the eclipse is shorter, and is only renewed
every fifteen years; but there are every night, so to say, eclipses of
the Saturnian moons by the rings and by themselves. In the circumpolar
regions the day-star is never eclipsed by the rings; but the satellites
revolve in a spiral, describing fantastic rounds, and the sun himself
disappears at the pole during a long night of fifteen years.
At one time it was supposed that the rings were solid, but they are now
believed to consist of countless myriads of meteorites, each whirling
in its own appointed pathway round the monster planet.
[Illustration: IDEAL VIEW OF SATURN’S RINGS AND SATELLITES FROM THE
PLANET.]
As already said--leaving out of the question the cloudy atmosphere--a
man standing on the equator would see nothing of the rings. A man
standing at the north pole or the south pole of Saturn could see
nothing either, since the rings would all lie below his horizon. But
if he traveled southward from the north pole, or northward from the
south pole, towards the equator, he would in time see the ringed arch
appearing above the horizon, rising higher and growing wider with every
mile of his journey; and when he was in a position to view the whole
broad expanse, the transparent half-dark belt below, the wide radiant
band rising upwards over that, and the grayish border surmounting all,
he would truly have a magnificent spectacle before him.
This magnificent spectacle is, however, by no means always visible,
even from those parts of Saturn where alone it ever can be seen. The
rings shine merely by reflected sunlight. Necessarily, therefore, while
the sunbeams make one side bright the other side is dark; and not only
this, but the rings throw broad and heavy shadows upon Saturn in the
direction away from the sunlight.
In the daytime they probably give out a faint shining, something like
our own moon when seen in sunlight. During the summer nights they
shine, no doubt, very beautifully. During the winter nights it so
happens that their bright side is turned away; and not only that, but
during the winter days the rings, while giving no light themselves to
the wintry hemisphere of Saturn, completely hide the sun.
When it is remembered that Saturn’s winter--that is, the winter of each
hemisphere in turn--lasts during fifteen of our years; and when we hear
of total eclipses of the sun lasting unbroken through eight years of
such a winter, with not even bright rings to make up for his absence,
we can not think of Saturn as a tempting residence. The sun gives
Saturn at his best only about one-ninetieth of the heat and light that
he gives to our earth; but to be deprived of even that little for eight
years at a time, does indeed sound somewhat melancholy.
Looking now at the other side of the question, the possible inhabitants
of the moons, especially those near at hand, would have splendid views
of Saturn and his rings in all their varying phases. For Saturn is a
beautiful globe, wrapped in his changeful envelope of clouds, which,
seen through a telescope, are lit up often with rainbow tints of blue
and gold; a creamy white belt lying usually on the equator; while
around extend the purple and shining and gray rings, sometimes rivaling
in bright colors Saturn himself.
We are compelled to assume that the continuous appearance of the rings
is not due to real continuity of substance, but that they are composed
of flights of disconnected satellites, so small and so closely packed
that, at the immense distance to which Saturn is removed, they appear
to form a continuous mass. There are analogous instances in the Solar
System. In the zone of asteroids we have an undoubted instance of a
flight of disconnected bodies traveling in a ring about a central
attracting mass. The existence of zones of meteorites traveling around
the sun has long been accepted as the only probable explanation of the
periodic returns of meteoric showers. Again, the singular phenomenon
called the Zodiacal Light is, in all probability, caused by a ring of
minute cosmical bodies surrounding the sun. In the Milky Way and in
the ring-nebulæ we have other illustrations of similar arrangements
in nature, belonging, however, to orders immeasurably vaster than any
within the Solar System.
The moons of Saturn do not, like those of Jupiter, travel in one plane.
CHAPTER XIX.
URANUS AND NEPTUNE.
Till the year 1781, Saturn was believed to be the outermost planet
of the Solar System, and nobody suspected the fact of two great,
lonely brother-planets wandering around the same sun at vast distances
beyond,--Uranus, nine hundred millions of miles from Saturn; Neptune,
nine hundred millions of miles from Uranus. No wonder they remained
long undiscovered.
Uranus can sometimes be seen by the unaided eye as a dim star of the
sixth magnitude. And when he was known for a planet, it was found that
he _had_ been often so seen and noted. Again and again he had been
taken for a fixed star, and as he moved on, disappearing from that
particular spot, it was supposed that the star had vanished.
One night, March 13, 1781, when William Herschel was busily exploring
the heavens with a powerful telescope, he noticed something which
he took for a comet without a tail. He saw it was no mere point of
light like the stars, but had a tiny round disk or face, which could
be magnified. So he watched it carefully, and found in the course of
a few nights that it moved--very slowly, certainly; but still it did
move. Further watching and calculation made it clear that, though the
newly-found heavenly body was at a very great distance from the sun,
yet it was moving slowly in an orbit _round_ the sun. Then it was
known to be a planet, and another member of the Solar System.
From that day the reputation of Herschel rapidly increased. King George
III, who loved the sciences and patronized them, had the astronomer
presented to him; charmed with the simple and modest account of his
efforts and his labors, he secured him a life pension and a residence
at Slough, in the neighborhood of Windsor Castle. His sister Caroline
assisted him as secretary, copied all his observations, and made all
his calculations; the king gave him the title and salary of assistant
astronomer. Before long the observatory at Slough surpassed in
celebrity the principal observatories of Europe; we may say that, in
the whole world, this is the place where the most discoveries have been
made.
Astronomers soon applied themselves to the observation of the new body.
They supposed that this “comet” would describe, as usually happens,
a very elongated ellipse, and that it would approach considerably
to the sun at its perihelion. But all the calculations made on this
supposition had to be constantly recommenced. They could never succeed
in representing all its positions, although the star moved very slowly:
the observations of one month would utterly upset the calculations of
the preceding month.
Several months elapsed without a suspicion that a veritable planet
was under observation, and it was not till after recognizing that all
the imaginary orbits for the supposed comet were contradicted by the
observations, and that it had probably a circular orbit much farther
from the sun than Saturn, till then the frontier of the system, that
astronomers came to consider it as a planet. Still, this was at first
but a provisional consent.
[Illustration: PROMINENT ASTRONOMERS OF FORMER TIMES. Tycho Brahe,
Hipparchos, Galilei, Newton, Kepler, Kant, Kopernikus, Laplace.]
It was, in fact, more difficult than we may think to increase without
scruple the family of the sun. Indeed, for reasons of expediency this
idea was opposed. Ancient ideas are tyrannical. Men had so long been
accustomed to consider old Saturn as the guardian of the frontiers,
that it required a rare boldness of spirit to decide on extending these
frontiers and marking them by a new world.
William Herschel proposed the name of _Georgium Sidus_ “the Star of
George,” just as Galileo had given the name of “Medicean stars” to
the satellites of Jupiter discovered by him, and as Horace had said
_Julium Sidus_. Others proposed the name of _Neptune_, in order to
maintain the mythological character, and to give to the new body the
trident of the English maritime power; others, _Uranus_, the most
ancient deity of all, and the father of Saturn, to whom reparation
was due for so many centuries of neglect. Lalande proposed the name
of _Herschel_, to immortalize the name of its discoverer. These last
two denominations prevailed. For a long time the planet bore the
name of Herschel; but custom has since declared for the mythological
appellation, and Jupiter, Saturn, and Uranus succeed each other in
order of descent--son, father, and grandfather.
The discovery of Uranus extended the radius of the Solar System from
885,000,000 to 1,765,000,000 of miles.
The apparent brightness of this planet is that of a star of the
sixth magnitude; observers whose sight is very piercing may succeed
in recognizing it with the naked eye when they know where to look for
it. Uranus moves slowly from west to east, and takes no less than
eighty-four years to make the complete circuit of the sky. In its
annual motion round the sun the earth passes between the sun and Uranus
every 369 days--that is to say, once in a year and four days. It is at
these times that this planet crosses the meridian at midnight. We can
observe it in the evening sky for about six months of every year.
Everybody supposed that now, at least, the outermost member of all was
discovered. But a very strange and remarkable thing happened.
Astronomers know with great exactness the paths of the planets in
the heavens. They can tell, years beforehand, precisely what spot in
space will be filled at any particular time by any particular planet.
I am speaking now of their movements round the sun and in the Solar
System--not of the movements of the whole family with the sun, about
which little is yet known.
Each planet has its own particular pathway; its own particular distance
from the sun, varying at each part of its pathway; its own particular
speed in traveling round the sun, changing constantly from faster to
slower or slower to faster, according to its distance from the sun, and
according to the pull backwards or forwards of our neighboring planets
in front or in rear. For as the orbits of all the planets are ovals,
with the sun not in the middle, but somewhat to one side of the middle,
it follows that all the planets, in the course of their years, are
sometimes nearer to and sometimes farther from the sun.
The astronomers of the present day understand this well, and can
describe with exactness the pathway of each planet. This knowledge
does not come merely from watching one year how the planets travel,
and remembering for another year, but is much more a matter of close
and difficult calculation. Many things have to be considered, such as
the planet’s distance from the sun, the sun’s power of attraction, the
planet’s speed, the nearness and weight of other neighboring planets.
All these questions were gone into, and astronomers sketched out the
pathway in the heavens which they expected Uranus to follow. He would
move in such and such an orbit, at such and such distances from the
sun, and at such and such rates of speed.
But Uranus would not keep to these rules. He quite discomfited the
astronomers. Sometimes he went fast, when, according to their notions,
he ought to have gone more slowly; and sometimes he went slowly, when
they would have looked for him to go more fast, and the line of his
orbit was quite outside the line of the orbit which they had laid
down. He was altogether a perplexing acquaintance, and difficult to
understand. However, astronomers felt sure of their rules and modes of
calculation, often before tested, and not found to fail. They made a
guess at an explanation. What if there were yet another planet beyond
Uranus, disturbing his motions--now drawing him on, now dragging him
back, now so far balancing the sun’s attraction by pulling in the
opposite direction as to increase the distance of Uranus from the sun?
It might be so. But who could prove it? Hundreds of years might pass
before any astronomer, in his star-gazing, should happen to light upon
such a dim and distant world. Nay, the supposed planet might be, like
Uranus, actually seen, and only be mistaken for a “variable star,”
shining but to disappear.
There the matter seemed likely to rest. There the matter probably would
have rested for a good while, had not two men set themselves to conquer
the difficulty. One was a young Englishman, a student of Cambridge,
John Couch Adams; the other, a young Frenchman, Urbain Jean Joseph
Leverrier,--both being astronomers.
Each worked independently of the other, neither knowing of the other’s
toil. The task which they had undertaken was no light one--that of
reaching out into the unknown depths of space to find an unknown planet.
Each of these silent searchers into the sky-depths calculated what the
orbit and speed of Uranus would be, without the presence of another
disturbing planet beyond. Each examined what the amount of disturbance
was, and considered the degree of attraction needful to produce that
disturbance, together with the direction from which it had come. Each,
in short, gradually worked his way through calculations far too deep
and difficult for ordinary minds to grasp, till he had found just that
spot in the heavens where a planet _ought_ to be, to cause, according
to known laws, just such an effect upon Uranus as had been observed.
Adams finished his calculation first, and sent the result to two
different observatories. Unfortunately, his report was not eagerly
taken up. It was, in fact, hardly believed. Leverrier finished his
calculation also, and sent the result to the Berlin Observatory. The
planet was actually seen in England first, but the discovery was
actually made known from Berlin first. The young Englishman had been
beforehand, but the young Frenchman gained foremost honor.
It was on the 23d of September, 1846, that Leverrier’s letter reached
the Berlin astronomers. The sky that night was clear, and we can easily
understand with what anxiety Dr. Galle directed his telescope to the
heavens. The instrument was pointed in accordance with Leverrier’s
instructions. The field of view showed, as does every part of the
heavens, a multitude of stars. One of these was really the planet. The
new chart--prepared by the Berlin Academy of Sciences, upon which the
place of every star, down even to those of the tenth magnitude, was
engraved--was unrolled, and, star by star, the heavens were compared
with the chart. As the process of identification went on, one object
after another was found in the heavens as engraved on the chart, and
was of course rejected. At length a star of the eighth magnitude--a
brilliant object--was brought into review. The map was examined, but
there was no star there. This object could not have been in its present
place when the map was formed. The object was therefore a wanderer--a
planet. Yet it is necessary to be excessively cautious in such a matter.
Many possibilities had to be guarded against. It was, for instance,
possible that the object was really a star which, by some mischance,
eluded the careful eye of the astronomer who constructed the map.
It was even possible that the star might be one of the large class
of variables which alternate in brightness, and it might have been
conceivable that it was too faint to be seen when the chart was made.
Even if neither of these explanations would answer, it was still
necessary to show that the object was now moving, and moving with that
particular velocity and in that particular direction which the theory
of Leverrier indicated. The lapse of a single day was sufficient to
dissipate all doubts. The next night the object was again observed.
It had moved, and when its motion was measured it was found to
accord precisely with what Leverrier had foretold. Indeed, as if no
circumstance in the confirmation should be wanting, it was ascertained
that the diameter of the planet, as measured by the micrometers at
Berlin, was practically coincident with that anticipated by Leverrier.
The world speedily rang with the news of this splendid achievement.
Instantly the name of Leverrier rose to a pinnacle hardly surpassed by
that of any astronomer of any age or country. For a moment it seemed
as if the French nation were to enjoy the undivided honor of this
splendid triumph. But in the midst of the pæans of triumph with which
the enthusiastic French nation hailed the discovery of Leverrier,
there appeared a letter from Sir John Herschel in the _Athenæum_ for
3d October, 1846, in which he announced the researches made by Adams,
and claimed for him a participation in the glory of the discovery.
Subsequent inquiry has shown that this claim was a just one, and it is
now universally admitted by all independent authorities.
This, however, was of slight comparative importance. The truly
wonderful part of the matter was, that these two men could have so
reasoned that, from the movements of one lately-discovered planet, they
could point out the exact spot where a yet more distant planet ought to
be, and that close to this very spot the planet was found. For when,
both in England and in Germany, powerful telescopes were pointed in the
direction named--_there the planet was_. No doubt about the matter. Not
a star, but a real new planet in the far distance, wandering slowly
round the sun.
Here we have the discovery of Neptune in its simple grandeur. This
discovery is splendid, and of the highest order from a philosophical
point of view, for it proves the security and the precision of the data
of modern astronomy. Considered from the point of view of practical
astronomy, it was but a simple exercise of calculation, and the most
eminent astronomers saw in it nothing else! It was only after its
verification, its public demonstration--it was only after the visual
discovery of Neptune--that they had their eyes opened, and felt for a
moment the dizziness of the infinite in view of the horizon revealed by
the Neptunian perspective. The author of the calculation himself, the
transcendent mathematician, did not even give himself the trouble to
take a telescope and look at the sky to see whether a planet was really
there! I even believe that he never saw it. For him, however, then
and always, to the end of his life, astronomy was entirely inclosed in
formulæ--the stars were but centers of force.
Very often I submitted to him the doubts of an anxious mind on the
great problems of infinitude: I asked him if he thought that the other
planets might be inhabited like ours, what might be especially the
strange vital conditions of a world separated from the sun by the
distance of Neptune, what might be the retinue of innumerable suns
scattered in immensity, what astonishing colored lights the double
stars should shed on the unknown planets which gravitate in these
distant systems. His replies always showed me that these questions had
no interest for him, and that, in his opinion, the essential knowledge
of the universe consisted in equations, formulæ, and logarithmic series
having for their object the mathematical theory of velocities and
forces.
But it is not the less surprising that he had not the _curiosity_ to
verify the position of his planet, which would have been easy, since
it shows a planetary disk; and, besides, he might have had the aid of
a chart, because he had only to ask for these charts from the Berlin
Observatory, where they had just been finished and _published_. It
is not the less surprising that Arago, who was more of a physicist
than a mathematician, more of a naturalist than a calculator, and
whose mind had so remarkable a synthetical character, had not himself
directed one of the telescopes of the observatory towards this point
of the sky, and that no other French astronomer had this idea. But
what surprises us still more is to know that _nearly a year before_,
in October, 1845, a young student of the University of Cambridge, Mr.
Adams, had sought the solution of the _same_ problem, obtained the
same results, and communicated these results to the director of the
Greenwich Observatory, and that the astronomer to whom these results
were confided had said nothing, and had not himself searched in the sky
for the optical verification of his compatriot’s solution!
This was indeed a triumph of human intellect. Yet it is no matter
for human pride, but rather of thankfulness to God, who gave to man
this marvelous reasoning power. And the very delight we have in such
a success may humble us in the recollection of the vast amount lying
beyond of the utterly unknown. But while the rare mind of a Newton
could meekly realize the littleness of all he knew, lesser minds are
very apt to be puffed up with the thought of what the human intellect
can accomplish.
Perhaps the chief feeling of lawful satisfaction in this particular
discovery arises from the fact that it gives marked and strong proof
of the truth of our present astronomical system and beliefs. Many
mistakes may be made, and much has often to be unlearned. Nevertheless,
if the general principles of modern astronomy were wrong, if the
commonly-received facts were a delusion, such complete success could
not have attended so delicate and difficult a calculation.
We do not know much about these two outer planets, owing to their
enormous distance from us. Uranus is in size equal to seventy-four
earths, and Neptune is in size equal to one hundred and five earths.
Both these planets are formed of somewhat heavier materials than
Saturn, being about as dense as water.
The size of the sun as seen from Uranus is about one
three-hundred-and-ninetieth part of the size of the sun we see. To
Neptune he shows a disk only one nine-hundredth part of the size
of that visible to us--no disk at all, in fact, but only starlike
brilliancy.
The Uranian year lasts about eighty-four of our years; and this, with
a very sloping axis, must cause most long and dreary winters, the
tiny sun being hidden from parts of the planet during half an earthly
life-time.
Uranus has at least four moons, traveling in very different planes from
the plane of the ecliptic. Once it was thought that he had eight, but
astronomers have since searched in vain for the other four, believed
for a while to exist. Neptune has one moon, and may possess others not
yet discovered.
Sir Henry Holland, the celebrated physician, and a devoted student of
astronomy, has left on record an incident of his life connected with
the planet Neptune of singular interest. I give it here, and in his own
words, because it is scarcely likely otherwise to fall under the notice
of astronomical readers. After stating that his interest in astronomy
had led him to take advantage of all opportunities of visiting foreign
observatories, he says:
“Some of these opportunities, indeed, arising out of my visits to
observatories both in Europe and America, have been remarkable enough
to warrant a more particular mention of them. That which most strongly
clings to my memory is an evening I passed with Encke and Galle in the
observatory at Berlin, some ten or twelve days after the discovery
of the planet Neptune on this very spot; and when every night’s
observations of its motions had still an especial value in denoting
the elements of its orbit. I had casually heard of the discovery
at Bremen, and lost no time in hurrying on to Berlin. The night in
question was one of floating clouds, gradually growing into cumuli; and
hour after hour passed away without sight of the planet which had just
come to our knowledge by so wonderful a method of predictive research.
Frustrated in this main point, it was some compensation to stay and
converse with Encke in his own observatory, one signalized by so many
discoveries, the stillness and darkness of the place broken only by
the solemn ticking of the astronomical clock, which, as the unfailing
interpreter of the celestial times and motions, has a sort of living
existence to the astronomer. Among other things discussed, while thus
sitting together in a sort of tremulous impatience, was the name to be
given to the new planet. Encke told me he had thought of ‘Vulcan,’ but
deemed it right to remit the choice to Leverrier, then supposed to be
the sole indicator of the planet and its place in the heavens; adding
that he expected Leverrier’s answer by the first post. Not an hour had
elapsed before a knock at the door of the observatory announced the
letter expected. Encke read it aloud; and, coming to the passage where
Leverrier proposed the name of ‘Neptune,’ exclaimed, ‘_So lass den
Namen Neptun sein._’
It was a midnight scene not easily to be forgotten. A royal baptism,
with its long array of titles, would ill compare with this simple
naming of the remote and solitary planet thus wonderfully discovered.
There is no place, indeed, where the grandeur and wild ambitions of the
world are so thoroughly rebuked and dwarfed into littleness as in the
astronomical observatory. As a practical illustration of this remark,
I would add that my own knowledge of astronomers--those who have
worked themselves with the telescope--has shown them to be generally
men of tranquil temperament, and less disturbed than others by worldly
affairs, or by the quarrels incident even to scientific research.”
The same reasoning which has been used in reference to Jupiter and his
moons, and to Saturn and his moons, might perhaps be applied also to
Uranus and Neptune and their moons.
We know too little yet of their condition to venture far in such
speculations. Still, taking the matter as a whole, there seem many
reasons to incline us to the idea that each of the four greater outside
planets may be a kind of secondary half-cooled sun to his satellites,
helping to make up to them for the small amount of light and heat which
they can obtain from the far-off sun.
CHAPTER XX.
COMETS AND METEORITES.
A curious discovery has been lately made. It is that some sort of
mysterious tie seems to exist between comets and meteorites. For a long
while this was never suspected. How should it be? The comets, so large,
so airy, so light; the meteorites, so small, so solid, so heavy,--how
could it possibly be supposed that the one had anything to do with
the other? But supposings often have to give in to facts. Astronomers
are gradually becoming convinced that there certainly is a connection
between the two.
Strange to say, comets and meteorites occupy--sometimes, at least--the
very same pathways in the heavens, the very same orbits round the
sun. A certain number of meteorite systems are now pretty well known
to astronomers as regularly met by our earth at certain points in
her yearly journey. Some of these systems or rings have each a comet
belonging to it--not merely journeying near, but actually in its midst,
on the same orbit.
Perhaps it would be more correct to say that the meteorites belong to
the comet, than that the comet belongs to the meteorites. We tread here
on uncertain ground; for whether the meteorites spring from the comet,
or whether the comet springs from the meteorites, or whether each has
been brought into existence independently of the other, no one can at
present say.
Though it is out of our power to explain the kind of connection, yet a
connection there plainly is. So many instances are now known of a comet
and a meteorite ring traveling together that it is doubtful whether any
such ring could be found without a comet in its midst. By and by the
doubt may spring up whether there ever exists a comet without a train
of meteorites following him.
Among the many different meteorite rings which are known, two of the
most important are the so-called August and November systems. Of these
two, the November system must claim our chief attention. Not that we
are at all sure of these being the most important meteorite rings in
the Solar System. On the contrary, as regards the November ring, we
have some reason to think that matters may lie just the other way.
The comet belonging to the November system is a small one--quite an
insignificant little comet--only visible through a telescope. We
do not, of course, know positively that larger comets and greater
meteorite systems generally go together, but, to say the least, it
seems likely. And if the greatness of a ring can at all be judged of by
the size of its comet, then the November system must be a third-rate
specimen of its kind. It is of particular importance to us, merely
because it happens to be the one into which our earth plunges most
deeply, and which we therefore see and know the best. The August ring
is, on the contrary, connected with a magnificent comet, and may be
a far grander system. But our pathway does not lead us into the midst
of the August meteors as into those of November. We pass, seemingly,
through its outskirts.
The meteorites of the November system are very small. They are believed
to weigh commonly only a few grains each. If they were larger and
heavier, some of them would fall to earth as aerolites, not more than
half-burnt in their rush through the atmosphere. But this they are
never found to do.
There are known meteorite systems, which our earth merely touches
or grazes in passing, from which drop aerolites of a very different
description--large, heavy, solid masses. It is well for us that we do
not plunge into the midst of any such ring, or we might find our air,
after all, a poor protection.
The last grand display of the November system of meteorites took place
in the years 1866 to 1869, being continued more or less during three or
four Novembers following. The next grand display will occur in the year
1899.
For this system--Leonides, as it is called, because the falling-stars
in this display seem all to shoot towards us from a spot in the
constellation Leo--seems to have a “time” or “year” of thirty-three and
a quarter earthly years. The shape of its orbit is a very long ellipse,
near one end of which is the sun, while the other end is believed to
reach farther away than the orbit of Uranus.
A great deal of curiosity has been felt about the actual length and
breadth and depth of the stream of meteorites through which our
solid earth has so often plowed her way. During many hours at a
time, lookers-on have watched the magnificent display of heavenly
fireworks,--not a mere shooting-star here and there, as on common
nights, but radiant meteors, flashing and dying by thousands through
the sky. In 1866 no less than eight thousand meteors, in two hours and
a quarter, were counted from the Greenwich Observatory in England. A
natural wonder sprang up in many minds as to the extent of the ring
from which they fell.
For not in one night only, but in several nights during three or four
years, and that not once only but once in every thirty-three years,
thousands and tens of thousands appear to have been stolen by our earth
from the meteorite-ring, never again to be restored. Yet each time we
touch the ring, we find the abundance of little meteorites in nowise
seemingly lessened.
When speaking of a “ring” of meteorites, it must not be supposed that
necessarily the meteorites form a whole unbroken ring all round the
long oval orbit. There may be no breaks. There may be a more or less
thin scattering throughout the entire length of the pathway. But the
meteorites certainly seem to cluster far more densely in some parts of
the orbit than in other parts, and it was about the size of the densest
cluster that so much curiosity was felt.
Little can be positively known, though it is very certain that the
cluster must be enormous in extent. Three or four years running, as our
earth, after journeying the whole way round the sun, came again to that
point in her orbit where she passes through the orbit of the Leonides,
she found the thick stream of meteorites still pouring on, though each
year lessening in amount. Taking into account this fact, and also the
numbers that were seen to fall night after night, and also the speed of
our earth, a “rough estimate” was formed.
The length of this dense cluster is supposed to reach to many
hundreds of millions of miles. The thickness or depth of the stream
is calculated to be in parts over five hundred thousand miles, and
the breadth about ten times as much as the depth. Each meteorite is
probably at a considerable distance from his neighbor; but the whole
mass of them, when in the near neighborhood of the sun, must form a
magnificent sight.
And if this be only a third-rate system, what must a first-rate system
be like? And how many such systems are there throughout the sun’s wide
domains? The most powerful telescope gives us no hint of the existence
of these rings till we find ourselves in their midst.
It may be that they are numbered by thousands, even by millions. The
whole of the Solar System--nay, the very depths of space beyond--may,
for aught we know, be crowded with meteorite systems. Every comet may
have his stream of meteorites following him. But though the comet is
visible to us, the meteorites are not. Billions upon billions of them
may be ever rushing round our sun, entirely beyond our ken, till one
or another straggler touches our atmosphere to flash and die as a
“shooting star” in our sight.
We have, and with our present powers we can have, no certainty as
to all this. But I may quote here the illustration of a well-known
astronomical writer on the subject.
Suppose a blind man were walking out of doors along a high road, and
during the course of a few miles were to feel rain falling constantly
upon him. Would it be reasonable on his part, if he concluded that a
small shower of rain had accompanied him along the road as he moved,
but that fine weather certainly existed on either side of the road? On
the contrary, he might be sure that the drops which he felt, were but a
few among millions falling all together.
Or, look at the raindrops on your window some dull day at home. Count
how many there are? Could you, with any show of common sense, decide
that those raindrops, and those alone, had fallen that day? So, when
we find these showers of meteorites falling to earth, we may safely
conclude that, for every one which touches our atmosphere, myriads rush
elsewhere in space, never coming near us.
CHAPTER XXI.
MORE ABOUT COMETS AND METEORITES.
The three prominent features of comets--a head, nucleus, and tail--are
seen only in particular instances. The great majority appear as faint
globular masses of vapor, with little or no central condensation, and
without tails. On the other hand, some have a strongly-defined nucleus,
which shines with a light as vivacious as that of the planets, so as to
be even visible in the daytime. This was the case with one seen at Rome
soon after the assassination of Julius Cæsar, believed by the populace
to be the soul of the dictator translated to the skies. The first comet
of the year 1442 was also so brilliant, that the light of the sun at
noon, at the end of March, did not prevent it from being seen; and the
second, which appeared in the summer, was visible for a considerable
time before sunset. In 1532, the people of Milan were alarmed by the
appearance of a star in the broad daylight; and as Venus was not then
in a position to be visible, the object is inferred to have been a
comet. Tycho Brahe discovered the comet of 1577, from his observatory
in the isle of Huene, in the sound, before sunset; and Chizeaux saw the
comet of 1744, at one o’clock in the afternoon, without a telescope.
The comet of 1843, already referred to in a previous chapter, was
distinctly seen at noon by many persons in the streets of Bologna
without the aid of glasses. The nucleus is generally of small size,
but the surrounding nebulosity, which forms the head, is often of
immense extent; and the hazy envelope, the “horrid hair” of poetry, is
sometimes seen separated from it by a dark space, encircling it like a
ring. This was the aspect of the comet of 1811. The entire diameter of
the head measured 1,250,000 miles, and hence its bulk was nearly three
times that of the sun, and four million times that of the earth. But
such extraordinary dimensions are quite exceptional.
[Illustration: THE GREAT COMET OF 1811.]
Popular impressions respecting the supposed terrestrial effects of
the comet of 1811--a fine object in the autumn of that year, long
remembered by many--are on record. It was gravely noted, that wasps
were very few in number; that flies became blind, and disappeared
early; and that twins were born more frequently than usual. The
season was remarkable for its bountiful harvest and abundant vintage.
Grapes, figs, melons, and other fruits, were not only produced in
extraordinary quantity, but of delicious flavor, so that “comet wines”
had distinct bins allotted to them in the cellars of merchants, and
were sold at high prices. There is, however, no fact better attested,
by a comparison of observations, than that comets have no influence
whatever in heightening or depressing the temperature of the seasons.
The fine fruits and ample harvest of the year in question were,
therefore, coincidences merely with the celestial phenomenon, without
the slightest physical connection with it.
Though the advanced civilization of recent times has led to juster
views of cometary apparitions than to regard them as divinely-appointed
omens of terrestrial calamity, yet society is apt to be nervous
respecting these bodies, as likely to cause some great natural
convulsion by collision with our globe.
The great comet, which in 1682 received first the name of Halley’s
comet, appeared last in 1835, his return having been foretold within
three days of its actually taking place. For Halley’s comet is a member
of the Solar System, having a yearly journey of seventy-six earthly
years. He journeys nearer the sun than Venus, and travels farther away
than Neptune.
In the years 1858, 1861, and 1862, three more comets appeared, all
visible without the help of a telescope. Of these three, Donati’s
comet, in 1858, was far superior to the rest. This fine object was
first discovered on the 2d of June by Dr. Donati, at Florence. It
appeared at first as a faint nebulous patch of light, without any
remarkable condensation; and as its motion towards the sun at that
time was slow, it was not till the beginning of September that it
became visible to the naked eye. A short tail was then seen on the side
opposite to the sun.
[Illustration: DONATI’S COMET, 1858.]
The development was afterwards rapid, and the comet became equally
interesting to the professional astronomer and to the unlearned gazer.
In the first week of October the tail attained its greatest length,
36°. At this time the tail was sensibly curved, and had the appearance
of a large ostrich-feather when waved gently in the hand. At the latter
end of October the comet was lost in the evening twilight. Its nearest
approach to the earth was about equal to half the distance of the earth
from the sun, and the nearest approach to Venus was about one-ninth
part of that distance. An elliptical orbit, with a periodic time of
more than 2,000 years, has been assigned to our late visitor.
It was a singular fact about this comet that at one time the star
Arcturus could be seen shining through the densest portion of the tail,
close to the nucleus. Now, although the faintest cloud-wreath of earth
would dim, if not hide, this star, yet the tail of the great comet was
of so transparent a nature that Arcturus shone undimmed, as if no veil
had come between. The exceedingly slight and airy texture of a comet’s
tail could hardly be more plainly shown. It was this gauzy appendage to
a little nucleus which men once thought could destroy our solid earth
at a single blow!
These cometary trains generally appear straight, or at least, by the
effect of perspective, they seem to be directed along the arcs of great
circles of the celestial sphere. Some are recorded, however, which
presented a different appearance. Thus, in 1689, a comet was seen whose
tail, according to the historians, was curved like a Turkish saber.
This tail had a total length of 68°. It was the same with the beautiful
comet of Donati, which we admired in 1858, and of which the tail had a
very decided curvature.
A comet can only be observed in the sky for a limited time. We
perceive it at first in a region where nothing was visible on preceding
days. The next day, the third day, we see it again; but it has changed
its place considerably among the constellations. We can thus follow it
in the sky for a certain number of days, often during several months;
then we cease to perceive it. Often the comet is lost to view because
it approaches the sun, and the vivid light of that body hides it
completely; but soon we observe it again on the other side of the sun,
and it is not till some time afterwards that it definitely disappears.
Some have been visible at noonday, quite near the sun, like that of
1882, on September 17th.
Yet, while taking care not to overrate, we must not underrate. True,
the comets are delicate and slight in structure. One comet, in 1770,
wandered into the very midst of Jupiter’s moons, and so small was its
weight that it had no power whatever, so far as has been detected, to
disturb the said moons in their orbits. Jupiter and his moons did very
seriously disturb the comet, however; and when he came out from their
midst, though none the worse for his adventure, he was forced to travel
in an entirely new orbit, and never managed to get back to his old
pathway again.
But there are comets _and_ comets, some being heavier than others. The
comet named after Donati, albeit too transparent to hide a star, was
yet so immense in size that his weight was calculated by one astronomer
to amount to as much as a mass of water forty thousand miles square and
one hundred and nine yards deep.
When first noticed, Donati’s comet had, like all large comets, a
bright envelope of light round the nucleus. After a while the one
envelope grew into three envelopes, and a new tail formed beside the
principal tail, which for a time was seen to bend gracefully into a
curve, like a splendid plume. A third, but much fainter tail, also
made its appearance, and many angry-looking jets were poured out from
the nucleus. These changes took place while the comet was passing
through the great heat of near neighborhood to the sun. Afterwards, as
he passed away, he seemed gradually to cool down and grow quiet. The
singular changes in the appearance of Newton’s comet have been earlier
noticed. No marvel that he did undergo some alterations. The tremendous
glare and burning heat which that comet had to endure in his rush past
the sun, were more than twenty-five thousand times as much as the glare
and heat of the fiercest tropical noonday ever known upon earth. Can we
wonder that he should have shown “signs of great excitement,” that his
head should have grown larger and his tail longer?
It certainly was amazing, and past comprehension, that the said tail,
over ninety millions of miles in length, should in four days have
seemingly swept round in a tremendous half-circle, so as first to point
in one direction, and then to point in just the opposite direction.
We are much in the habit of speaking about comets as traveling through
the heavens with their tails streaming behind them. But though this is
sometimes the case, it is by no means always.
The tails of comets always stream _away from the sun_--whether before
or behind the comet’s head seeming to be a matter of indifference. As
the comet comes hurrying along his orbit, with ever-increasing speed,
towards the sun, the head journeys first, and the long tail follows
after. But as the comet rounds the loop of his orbit near the sun--the
point nearest of all being called his _perihelion_--the head always
remains towards the sun, while the tail swings, or seems to swing, in
a magnificent sweep round, pointing always in the direction just away
from the sun.
Then, as the comet journeys with slackening speed, on the other side of
his orbit, towards the distant _aphelion_, or farthest point from the
sun, he still keeps his head towards the sun. So, at this part of his
passage, in place of the head going first and the tail following after,
the tail goes first and the head follows after. The comet thus appears
to be moving backwards; or, like an engine pushing instead of drawing a
train, the head seems to be driving the tail before it.
[Illustration: GREAT COMET OF 1744.]
[Illustration: THE FIRST COMET OF 1888--JUNE 4.]
The sun, then, acts on these bodies when they approach him, produces
in them important physical and chemical transformations, and exercises
on their developed atmosphere a repulsive force, the nature of which
is still unknown to us, but the effects of which coincide with the
formation and development of the tails. The tails are thus, in the
extension of the cometary atmosphere, driven back, either by the
solar heat, by the light, by electricity, or by other forces; and
this extension is rather a motion in the ether than a real transport
of matter, at least in the great comets which approach very near the
sun, and in their immense luminous appendages. The effects produced
and observed are not the same in all comets, which proves that they
differ from each other in several respects. The tails have sometimes
been seen to diminish before the perihelion passage, as in 1835.
Luminous envelopes have also been seen succeeding each other round the
head, concentrating themselves on the side opposite to the sun, and
leaving the central line of the tail darker than the two sides. This
is what happened in the Donati comet and in that of 1861. Sometimes a
secondary tail has been seen projected towards the sun, as in 1824,
1850, 1851, and 1880. Comets have been seen with the head enveloped in
phosphorescence, surrounding them with a sort of luminous atmosphere.
Comets have also been seen with three, four, five, and _six tails_,
like that of 1744, for example, which appeared like a splendid aurora
borealis rising majestically in the sky, until, the celestial fan being
raised to its full height, it was perceived that the six jets of light
all proceeded from the same point, which was nothing else but the
nucleus of a comet. On the other hand, the nuclei themselves show great
variations--some appear simply nebulous, and permit the faintest stars
to be visible through them; others seem to be formed of one or more
solid masses surrounded by an enormous atmosphere; in others, again, a
nucleus does not exist, as in the Southern comet of 1887. One of the
comets of 1888 showed a triple nucleus and a bristling coma, as may
be seen in the cut. We may, then, consider that the wandering bodies
collected under the name of comets are of several origins and _several
different species_.
_Why_ the tails of comets should so persistently avoid the sun it is
impossible to say. We do not even know whether the cause lies actually
in the sun or in the comet. Astronomers speak of the “repulsive
energy” with which the sun “sweeps away” from his neighborhood the
light vapory matter of which the tails are made. But the how and the
wherefore of this strange seeming repulsion they can not explain. For
at the selfsame time the sun appears to be attracting the comet towards
himself, and driving the comet’s tail away from himself.
A few of the more well-known comets have been mentioned; but a year
rarely passes in which at least one comet is not discovered, though
often only a small specimen, sometimes even tailless and hairless. No
doubt many others pass unseen. The large and grand ones only come to
view now and then.
What are comets and meteorites made of? Respecting comets, a good
many ideas are put forth, and a good many guesses are made, as to
“burning gas,” “luminous vapor,” “beams of light,” and so on. But in
truth, little is known about the matter. Respecting meteorites, we can
speak more certainly. A good many meteorites have fallen to earth as
aerolites, and have been carefully examined. They are found to contain
nickel, cobalt, iron, phosphorus, and sometimes at least, a large
supply of hydrogen gas.
Not only do solid aerolites fall half-burnt to the ground, but even
when the meteorites are quite consumed in the air, the fine dust
remaining still sinks earthward. This fine dust has been found upon
mountain-tops, and has been proved by close examination to be precisely
the same as the material of the solid aerolites.
A luminous body of sensible dimensions rapidly traverses space,
diffusing on all sides a vivid light--like a globe of fire of which
the apparent size is often comparable with that of the moon. This
body usually leaves behind it a perceptible luminous train. On or
immediately after its appearance it often produces an explosion, and
sometimes even several successive explosions, which are heard at great
distances. These explosions are also often accompanied by the division
of the globe of fire into luminous fragments, more or less numerous,
which seem to be projected in different directions. This phenomenon
constitutes what is called a _meteor_, properly speaking, or a
_bolide_. It is produced during the day as well as the night. The light
which it causes in the former case is greatly enfeebled by the presence
of the solar light, and it is only when it is developed with sufficient
intensity that it can be perceived.
In the British Museum there is a superb collection of meteorites. They
have been brought together from all parts of the earth, and vary in
size from bodies not much larger than a pin’s-head up to vast masses
weighing many hundred pounds. There are also many models of celebrated
meteorites, of which the originals are dispersed through various other
museums.
Many of these objects have nothing very remarkable in their external
appearance. If they were met with on the sea-beach, they would be
passed by without more notice than would be given to any other stone.
Yet what a history such a stone might tell us if we could only manage
to obtain it! It fell; it was seen to fall from the sky; but what
was its course anterior to that movement? Where was it one hundred
years ago, one thousand years ago? Through what regions of space has
it wandered? Why did it never fall before? Why has it actually now
fallen? Such are some of the questions which crowd upon us as we
ponder over these most interesting bodies. Some of these objects are
composed of very characteristic materials. Take, for example, one of
the more recent meteorites, known as the Rowton siderite. This body
differs very much from the more ordinary kind of stony meteorite. It
is an object which even a casual passer-by would hardly pass without
notice. Its great weight would also attract attention, while if it be
scratched or rubbed with a file, it would be found that it was not in
any sense a stone, but that it was a mass of nearly pure iron. We know
the circumstances under which that piece of iron fell to the earth.
It was on the 20th of April, 1876, about 3.40 P. M., that a strange
rumbling noise, followed by a startling explosion, was heard over an
area of several miles in extent among the villages in Shropshire. About
an hour after this occurrence, a farmer noticed the ground in one of
his grass-fields to have been disturbed, and he probed the hole which
the meteorite had made, and found it, still warm, about eighteen inches
below the surface. Some men working at no great distance had actually
heard the noise of its descent, but without being able to indicate
the exact locality. This remarkable object weighs 7¾ pounds. It is an
irregular angular mass of iron, though all its edges seem to have been
rounded by fusion in its transit through the air, and it is covered
with a thick black film of the magnetic oxide of iron, except at the
point where it first struck the ground.
This siderite is specially interesting on account of its distinctly
metallic character. Falls of the siderites, as they are called, are not
so common as those of the stony meteorites; in fact, there are only
a few known instances of meteoric irons having been actually seen to
fall, while the falls of stony meteorites are to be counted in scores
or in hundreds. The inference is that the iron meteorites are much less
frequent than the stony ones.
It is a wonderful thought that we should really have these visitants
from the sky, solid metal or showers of dust, coming to us from distant
space.
If the dust of thousands of meteorites is always thus falling
earthward, one would imagine that it must in time add something to the
weight of the earth. And this actually is the case. During the last
three thousand years, no less than one million tons of meteorite-dust
must, according to calculation, have fallen to earth out of the sky. A
million tons is of course a mere nothing compared with the size of the
world. Still, the fact is curious and interesting.
It has been suggested that, _perhaps_ the flames of the sun are partly
fed by vast showers of falling meteorites. It has even been suggested
that _perhaps_, in long past ages, the earth and the planets grew to
their present size under a tremendous downpour of meteorites; the
numbers which now drop to earth being merely the thin remains of what
once existed. But for this guess there is no real foundation.
Whether suns and worlds full-grown, created as in an instant;
whether tiny meteorites in countless myriads to be used as stones in
“building;” whether vast masses of flaming gas, to be gradually cooled
and “framed” into shape--which ever may have come first, and whichever
may have been the order of God’s working--still that “first” was made
by him; still he throughout was the Master-builder.
[Illustration: GREAT COMET OF 1680.]
A few words more about “comet-visitors.” Many comets, as already
stated, belong to our Solar System, though whether they have always so
belonged is another question. It is not impossible that they may once
upon a time have wandered hence from a vast distance, and, being caught
prisoner by the powerful attraction of Jupiter or one of his three
great brother-planets, have been compelled thenceforth to travel in a
closed pathway round the sun.
[Illustration: GREAT COMET OF 1769.]
There are also many comets which come once only to our system, flashing
round past the sun, and rushing away in quite another direction, never
to return. Where do these comets come from? And where do they go? From
other suns--brother suns to ours? It may be. One is almost disposed to
think that it must be so.
Do we ever think what an immense voyage they must have made to come
from there to here? Do we imagine for how many years they must have
flown through the dark immensity to plunge themselves into the fires
of our sun? If we take into account the directions from which certain
comets come to us, and if we assign to the stars situated in that
region the least distances consistent with known facts, we find that
these comets certainly left their last star more than _twenty millions
of years ago_.
In thus putting to us from the height of their celestial apparitions
so many notes of interrogation on the grandest problems of creation,
comets assume to our eyes an interest incomparably greater than that
with which superstition blindly surrounded them in past ages. When we
reflect for a moment that a certain comet which shines before us in
the sky came originally from the depths of the heavens, that it has
traveled during millions of years to arrive here, and that consequently
it is by millions of years that we must reckon its age if we wish to
form any idea of it, we can not refrain from respecting this strange
visitor as a witness of vanished eras, as an echo of the past, as
the most ancient testimony which we have of the existence of matter.
But what do we say? These bodies are neither old nor young. There
is nothing old, nothing new--all is present. The ages of the past
contemplate the ages of the future, which all work, all gravitate, all
circulate, in the eternal plan. Musing, you look at the river which
flows so gently at your feet, and you believe you see again the river
of your childhood; but the water of to-day is not that of yesterday, it
is not the same substance which you have before your eyes, and never,
never shall this union of molecules, which you behold at this moment,
come back there--never till the consummation of the ages!
If the appearance of comets forebodes absolutely nothing as to the
microscopical events of our ephemeral human history, it is not the same
with the effects which might be produced by their encounter with our
wandering planet. In such an encounter there is nothing impossible. No
law of celestial mechanics forbids that two bodies should come into
collision in their course, be broken up, pulverized, and mutually
reduced to vapor.
I have before described some of the tremendous outbursts seen on the
surface of our sun. It is believed that in these outbursts matter is
expelled, or driven forth, with such fearful violence as to send it
whirling through space, never to fall back to the sun.
Some meteorites may have their births thus. So also may some comets.
And if all the stars are suns--huge fiery globes like our sun, and
subject like him to tremendous eruptions--they too, probably, send out
comets and meteorites to wander through space. Whether or no comets
come into existence in any such manner, one thing seems pretty certain.
Those comets which come to us from outside our system must come from
some other system. And the nearest systems known are those of the stars.
The nearest star of all, whose distance has been measured, is Alpha
Centauri. It has been roughly calculated that a comet, passing direct
from Alpha Centauri to our sun, would take about twenty millions of
years for his journey. But here we tread upon very doubtful ground.
Many matters, at present unknown to us, might greatly affect the result
of such a calculation. Also it is by no means impossible that other
stars lie really much nearer to us than Alpha Centauri, whose distance
astronomers have not yet attempted to measure.
[Illustration: BARON ALEXANDER VON HUMBOLDT.]
“In order to complete our view,” says Alexander Von Humboldt, “of all
that we have learned to consider as appertaining to our Solar System,
which now, since the discovery of the small planets, of the interior
comets of short revolutions, and of the meteoric asteroids, is so rich
and complicated in its form, it remains for us to speak of the ring
of zodiacal light. Those who have lived for many years in the zone of
palms must retain a pleasing impression of the mild radiance with which
the zodiacal light, shooting pyramidally upward, illumines a part of
the uniform length of tropical nights. I have seen it shine with an
intensity of light equal to the Milky Way in Saggitarius, and that not
only in the rare and dry atmosphere of the summit of the Andes, at an
elevation of from thirteen to fifteen thousand feet, but even on the
boundless grassy plains of Venezuela, and on the seashore, beneath the
ever-clear sky of Cumana. This phenomenon was often rendered especially
beautiful by the passage of light, fleecy clouds, which stood out in
picturesque and bold relief from the luminous background.
“This phenomenon, whose primordial antiquity can scarcely be doubted,
is not the luminous solar atmosphere itself, which can not be diffused
beyond nine-tenths of the distance of Mercury. With much probability we
may regard the existence of a very compressed ring of nebulous matter,
revolving freely in space around the sun between the orbits of Venus
and Mars, as the material cause of the zodiacal light. These nebulous
particles may either be self-luminous or receive their light from the
sun.
“I have occasionally been astonished, in the tropical climates of
South America, to observe the variable intensity of the zodiacal
light. As I passed the nights, during many months, in the open air,
on the shores of rivers and on plains, I enjoyed ample opportunities
of carefully examining this phenomenon. When the zodiacal light had
been most intense, I have observed that it would be weakened for a
few minutes, until it again suddenly shone forth in full brilliancy.
In a few instances I have thought I could perceive, not exactly a
reddish coloration, nor the lower portion darkened in an arc-like
form, nor even a scintillation, but a kind of flickering and wavering
of the light. Must we suppose that changes are actually in progress
in the nebulous ring? Or is it not more probable that processes of
condensation may be going on in the uppermost strata of the air,
by means of which the transparency--or, rather, the reflection of
light--may be modified in some peculiar and unknown manner? An
assumption of the existence of such meteorological causes on the
confines of our atmosphere is strengthened by the sudden flash and
pulsations of light which have been observed to vibrate for several
seconds through the tail of a comet. During the continuation of these
pulsations it has been noticed that the comet’s tail was lengthened by
several degrees, and then again contracted.”
CHAPTER XXII.
MANY SUNS.
Once more we have to wing our flight far, far away from the busy Solar
System where we live; away from whirling planets, moons, meteorites,
all shining with reflected sunlight; away from the great central
sun himself, our own particular bright star. Once more we have, in
imagination, to cross the vast, black, empty space--_is_ it black, and
_is_ it empty, had we sight to see things as they are?--separating
our sun from other suns, our star from other stars. For the sun is a
star--only a star. And stars are suns--big, blazing suns. One is near,
and the others are far away. That is the difference.
We have no longer to do with bodies merely reflecting another’s
light--always dark on one side and bright on the other--but with
burning bodies, shining all round by their own light. We have no longer
to picture just one single star with his surrounding worlds, but we
have to fix our thoughts upon the great universe of stars or suns in
countless millions.
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
towards 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 our distance from
the sun; 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!
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!
The sun is center and ruler and king in his own system. But as a star,
he is only one among many stars, some greater, some less than himself.
From the far siderial heavens he is hardly recognized even as a star
among the constellations, so faint is his light.
Do the suns which surround us form a system with that which illuminates
us, as the planets form one round our solar focus, and does our sun
revolve round an attractive center? Does this center, the point of the
revolutions of many suns, itself revolve round a preponderating center?
In a word, is the visible universe organized in one or several systems?
No Divine revelation comes to instruct men on the mysteries which
interest them most--their personal or collective destinies. We have
now, as ever, but science and observation to answer us.
A problem so vast as this is still far from receiving even an
approximate solution. From whatever point of view we consider it,
we find ourselves face to face with the infinite in space and time.
The present aspect of the universe immediately brings into question
its past and its future state, and then the whole of united human
learning supplies us in this great research with but a pale light,
scarcely illuminating the first steps of the dark and unknown road on
which we are traveling. However, such a problem is worthy of engaging
our attention, and positive science has already made sufficient
discoveries in the knowledge of the laws of nature to permit us to
attempt to penetrate these great mysteries. What is it that the general
observation of the heavens--what is it that sidereal synthesis teaches
us on our real situation in infinitude?
[Illustration: A TELESCOPIC FIELD IN THE MILKY WAY.]
In the calm and silent hours of beautiful evenings, what pensive gaze
is not lost in the vague windings of the Milky Way, in the soft and
celestial gleam of that cloudy arch, which seems supported on two
opposite points of the horizon, and elevated more or less in the sky
according to the place of the observer and the hour of the night? While
one-half appears above the horizon, the other sinks below it, and if we
removed the earth, or if it were rendered transparent, we should see
the complete Milky Way, under the form of a great circle, making the
whole circuit of the sky. The scientific study of this trail of light,
and its comparison with the starry population of the heavens, begins
for us the solution of the great problem.
Let us point a telescope towards any point of this stupendous arch.
Suddenly hundreds, thousands of stars show themselves in the telescopic
field, like needle-points on the celestial vault. Let us wait for some
moments, that our eye may become accustomed to the darkness of the
background, and the little sparks shine out by thousands. Let us leave
the instrument pointed motionless towards the same region, and there
slowly passes before our dazzled vision the distant army of stars. In
a quarter of an hour we see them appear by thousands and thousands.
William Herschel counted three hundred and thirty-one thousand in a
width of 5° in the constellation Cygnus, so nebulous to the naked eye.
If we could see the whole of the Milky Way pass before us, we should
see eighteen millions of stars.
This seed-plot of stars is formed of objects individually invisible
to the naked eye, below the sixth magnitude; but so crowded that they
appear to touch each other, and form a nebulous gleam which all human
eyes, directed to the sky for thousands of years, have contemplated and
admired. Since it is developed like a girdle round the whole circuit of
the sky, we ourselves must be in the Milky Way. The first fact which
impresses our mind is, that _our sun is a star of the Milky Way_.
Thought travels fast--faster than a comet, faster than light. A rushing
comet would, it is believed, take twenty millions of years to cross the
chasm between the nearest known fixed star and us. Light, flashing
along at the rate of about one hundred and eighty-six thousand miles a
second, will perform the same journey in four years and a third. But
thought can overleap the boundary in less than a single moment.
Each star that we see in the heavens is to our eyesight simply one
point of light. The brighter stars are said to be of greater magnitude,
and the fainter stars of lesser magnitude; yet, one and all, they have
no apparent size. The most powerful telescope, though it can increase
their brilliancy, can not add to their size. A planet, which to the
naked eye may look like a star, will, under a telescope, show a disk,
the breadth of which can be measured or divided; but no star has any
real, visible disk in the most powerful telescope yet constructed. The
reason of this is the enormous distance of the stars. Far off as many
of the planets lie, yet the farthest of them is as a member of our
household compared with the nearest star.
I have already tried to make clear the fact of their vast distance.
Light, which comes to us from the sun in eight minutes and a half,
takes over four and a third years to reach us from Alpha Centauri. From
this four-years-and-a-third length of journey between Alpha Centauri
and earth, the numbers rise rapidly to twenty years, fifty years,
seventy years, even hundreds of years. The distance of most of the
stars is completely beyond our power to measure. The whole orbit of our
earth, nay, the whole wide orbit of the far-off Neptune, would dwindle
down to one single point, if seen from the greater number of the stars.
It used to be believed that, taking the stars generally, there was
probably no very marked difference in their size, their kind, their
brightness. Some of course would be rather larger, and others rather
smaller; still it was supposed that they might be roughly classed as
formed much on the same scale and the same plan. But doubts are now
felt about this notion. For a very similar idea used to be held with
regard to the Solar System. The wonderful variety of form and richness
in numbers, now known to abound within its limits, are discoveries of
late years. May not the same variety in kind and size be found also
among the stars? The more we look into the heavens, the more we find
that dull, blank uniformity is not to be seen there.
It is the same upon earth. Man builds his little rows of boxlike houses
side by side, each one exactly like all the rest, or dresses his
thousand soldiers in coats of the same cut and color, or repeats a neat
leaf-design hundreds of times on carpets or wall-papers; but God never
makes two leafs or two blades of grass alike. Wholesale turning out of
things after one pattern is quite a human idea, not divine.
We know so much about the stars as that some are at least considerably
larger and some considerably smaller than others. When one star is
seen to shine brightly, and another beside it shines dimly, we are
apt to think that the brightest must be the nearest. Yet it is often
impossible for us to say how much of the difference is owing to the
greater distance of one or the other, to the greater size of one or
the other, or to the greater brilliancy of one or the other. In many
instances we do know enough to be quite sure that there is a great
difference, not only in the distance of the stars, but in their size,
their kind, their brightness.
The stars have been lately classed by one or two astronomers into four
distinct orders or degrees--partly depending on their color. The first
class is that of the White Suns. These are said to be the grandest and
mightiest of all. The star Sirius belongs to the order of White Suns.
Secondly comes the class of Golden Suns. To these blazing furnaces of
yellow light, second only to the white-light stars, belongs our own
sun. Thirdly, there are numbers of stars called Variable Stars, the
light of which is constantly changing, now becoming more, now becoming
less. Fourthly, there is the class of small Red Suns, about which not
much is known.
These four orders or divisions do not by any means include all the
stars, or even all the single stars. Roughly speaking, however, the
greater number of single stars, and many also of the double stars,
belong to one or another of the above classes.
When we talk of the different sizes of the different stars, it should
be plainly understood that we have no means of directly measuring them.
A point of light showing no disk, no surface, no breadth, can not be
measured, for there is nothing to measure.
In certain cases we are not entirely without the power of judging. The
distances of a few of the stars from us have been found out. Knowing
how far off any particular star is, astronomers are able to calculate
exactly how bright our own sun would look at that same distance. If
they find that our sun would shine just as the star in question
shines, there is some reason for supposing that our sun and the star
may be of the same size. If our sun would shine more brightly than the
star shines, there is some reason for supposing that the star may be
smaller than our sun. If our sun would shine more dimly than the star
shines, there is some reason for supposing that the star may be larger
than our sun.
Other matters, however, have to be considered. Suppose we find a star
at a certain distance shining twice as brilliantly as our own sun would
shine at that same distance. Naturally, then, we say, That star must be
much larger than our sun.
The reasoning may be mistaken. We do not know the fact. What if,
instead of being a much _larger_ sun, it is only a much _brighter_
sun? This possibly must be allowed for. It has, indeed, been strongly
doubted by one astronomer, after close study of the sun, whether any
surface of any star _could_ exceed the surface of our sun in its power
of light and heat.
But Sirius sheds actually forty-eight times as much light around
it as does our sun; we are not exactly entitled to say that Sirius
is forty-eight times as big as the sun, but we can say that Sirius
is forty-eight times as brilliant or as splendid. In making this
calculation we have taken a lower determination of the brightness of
Sirius relatively to the sun than some other careful observations would
have warranted. It will thus be seen that if there be any uncertainty
in our result it must only be as to whether Sirius is not really more
than forty-eight times as bright as the sun.
When we picture to ourselves the star-depths, the boundless reaches of
heavenly space, with these countless blazing suns scattered broadcast
throughout, we have not to picture an universe in repose. On the
contrary, all is life, stir, energy. Just as in the busy whirl of our
Solar System no such thing as rest is to be found, so also it seems to
be in the wide universe.
Every star is in motion. “Fixed,” as we call them, they are not fixed.
Invisible as their movements are to our eyes through immensity of
distance, yet all are moving. Those silent, placid, twinkling specks of
light are, in reality, huge, roaring, seething, tumultuous furnaces of
fire and flame, heat and radiance.
Each, too, is hurrying along his appointed pathway in space. Some move
faster, some move more slowly. One mile per second; ten miles per
second; twenty, thirty, forty, fifty miles per second,--thus varying
are their rates of speed. But whether fast or whether slowly, still
onward and ever onward they press. Some are rushing towards us, some
are rushing away from us. Some are speeding to the right, some are
speeding to the left.
The fine star Arcturus, which any one may admire every evening on the
prolongation of the tail of the Great Bear, is slowly withdrawing from
the fixed point where the celestial charts placed it two thousand years
ago, and is moving towards the southwest. It takes eight hundred years
to describe a space in the sky equal to the apparent diameter of the
moon; nevertheless, this displacement was sufficiently perceptible to
attract attention more than a century and a half ago, for Halley had
noticed it in 1718, as well as those of Sirius and Aldebaran. However
slow it may appear at the distance we are from Arcturus, this motion
is, at a minimum, 1,637 millions of miles a year. Sirius takes 1,338
years to pass over the same angular space in the sky; at the distance
of that star this is, at a minimum, 397 millions of miles per annum.
The study of the proper motions of the stars has made the greatest
progress in the last half-century, and especially in recent years. All
the stars visible to the naked eye and a large number of telescopic
stars show displacements of this kind.
Where are they going? Does any single star ever return to his
starting-point--wherever that starting-point may have been? Do they
journey in vast circles or ellipses, round some far-distant center?
What controls them all? Is it the mighty power of some such center, or
does each star, by his faint and distant attraction, help to control
all his brother-stars, to guide them on their appointed path, to
preserve the delicate balance of a universe?
How little we know about the matter! Only so much we can tell--that
the controlling and restraining hand of God is over the whole. Whether
by the attraction of one great center or by the united influences of
a thousand fainter attractions, he steers each radiant sun upon its
heavenly path, “upholding all things by the words of his power.” There
is no blundering, no confusion, no entanglement. All is perfect order,
calm arrangement, restrained energy.
CHAPTER XXIII.
SOME PARTICULAR SUNS.
In the constellation of the Swan there is a little, dim,
sixth-magnitude star, scarcely to be seen without a telescope. This
star, 61 Cygni by name, is the first whose distance from us it was
found possible to determine.
We may think it strange that so faint a star was even attempted.
Would not astronomers have naturally supposed it to be one of the
farther-distant stars? No, they did not. For though 61 Cygni showed but
a dim light, yet his motion--not the daily apparent motion, but the
real motion as seen from earth--was found to be so much more rapid than
the motion of most other stars that they rightly guessed 61 Cygni to be
a rather near neighbor of ours.
Do not misunderstand me, when I speak of a “more rapid motion,” and of
“rather near neighborhood.” The real rate of 61 Cygni’s rush through
space is believed to be about forty miles each second, or one thousand
four hundred and fifty millions of miles each year. All we can perceive
of this quick motion is that, in the course of three hundred and fifty
years, 61 Cygni travels over a space in the sky about as long as the
breadth of the full moon. Little enough, yet very far beyond what can
be detected in the greater number of even the brightest stars.
Then, again, as to the near neighborhood of this star, 61 Cygni is
near enough to have his distance measured, and that is saying a good
deal. Alpha Centauri, the nearest star of which we know in the southern
heavens, is two hundred and seventy-five thousand times as far distant
as the sun. But 61 Cygni, the nearest star of which we know in the
northern heavens, is nearly twice as far away as Alpha Centauri.
[Illustration: CYGNUS.]
We call 61 Cygni a star, for so he appears to common observers. In
reality, instead of being only one star, the speck of light which
we call 61 Cygni consists of _two stars_. The two are separated by
a gap about half as wide again as the wide gap between the sun and
Neptune. Yet so great is their distance from us that to the naked eye
the two seem to be one. These two suns would together make a sun only
about one-third as large as our sun. They differ in size, the quicker
movements of one showing it to be the smaller; and it is by means of
their known distance from one another, and their known rate of motion,
that their size, or rather their weight, can be roughly calculated. Of
course neither of the two shows any actual measurable disk. So much and
so little is with tolerable certainty known about this particular pair
of suns.
Next let us turn to Alpha Centauri--named _Alpha_, the first
letter of the Greek alphabet, because it is the brightest star in
the constellation of the Centaur. The second brightest star in a
constellation is generally called Beta, the third Gamma, the fourth
Delta, and so on; just as if we were to name them A, B, C, D, in order
of brightness.
The constellation Centaur lies in the southern heavens, close to the
beautiful constellation called the Southern Cross, and is invisible in
the northern hemisphere. Of all the stars shining in the heavens round
our earth, two only--Sirius and Canopus--show greater brilliancy than
Alpha Centauri.
As in the case of 61 Cygni, astronomers were led to attempt the
measurement of Alpha Centauri’s distance, by noticing how much more
distinct were his movements than the movements of other stars, though
less rapid both to the eye and in reality than those of 61 Cygni. Alpha
Centauri’s rate of motion is some thirteen miles each second.
And this, so far as we yet know, is our sun’s nearest neighbor in the
heavens outside his own family circle.
Strange to say, Alpha Centauri, like 61 Cygni, consists, not of a
single star, but of a pair of stars. It is a two-sun system--whether or
no surrounded by planets can not be told. We can only reason from what
we see to what we do not see. And as God did not form our earth “in
vain,” and did not form our sun “in vain,” so we firmly believe that he
did not form “in vain” any one of his myriads of suns scattered through
space. What is, has been, or will be the particular use of each one,
it would be rash to attempt to say. But that many among them have,
like our own sun, systems of worlds revolving round them, we may safely
consider very probable.
[Illustration: STARS WHOSE DISTANCES ARE BEST KNOWN. (See Table.)]
The two suns of the Alpha Centauri double-star are separated by a
distance about twenty-two times as great as the distance of the earth
from the sun, yet to the naked eye they show as a single star. Here
again one is much smaller than the other; and the smaller revolves
round the larger in about eighty-five years.
It is believed that the two together would form a sun about twice as
large and heavy as our sun. This belief is strengthened by the great
brilliancy of Alpha Centauri. Our own sun, placed at that distance from
us, would shine only one-third as brightly as he does.
STARS WHOSE DISTANCES ARE BEST KNOWN.
+--+--------------------+----------+------------------+
| | NAME OF STAR. |MAGNITUDE.|DISTANCE IN MILES.|
+--+--------------------+----------+------------------+
| 1| Alpha Centauri, | 1.0 | 25 trillions. |
| 2| 61 Cygni, | 5.1 | 43 ” |
| 3| 2,398, Draco, | 8.2 | 55 ” |
| 4| Sirius, | 1.0 | 58 ” |
| 5| 9,352, Lacaille, | 7.5 | 66 ” |
| 6| Procyon, | 1.3 | 71 ” |
| 7| Lalande, 21,258, | 8.5 | 74 ” |
| 8| Œltzen, 11,677, | 9.0 | 74 ” |
| 9| Sigma Draconis, | 4.7 | 78 ” |
|10| Aldebaran, | 1.5 | 81 ” |
|11| Epsilon Indi, | 5.2 | 87 ” |
|12| Œltzen, 17,415, | 9.0 | 94 ” |
|13| 1,516, Draco, | 7.0 | 101 ” |
|14| Omicron Eridani, | 4.4 | 101 ” |
|15| Altair, | 1.6 | 101 ” |
|16| Bradley, 3,077, | 5.5 | 101 ” |
|17| Eta Cassiopeiæ, | 3.6 | 118 ” |
|18| Vega, | 1.0 | 128 ” |
|19| Capella, | 1.2 | 174 ” |
|20| Arcturus, | 1.0 | 204 ” |
|21| Pole Star, | 2.1 | 215 ” |
|22| Mu Cassiopeiæ, | 5.2 | 320 ” |
|23| 1,830, Groombridge,| 6.5 | 426 ” |
+--+--------------------+----------+------------------+
This table presents the most trustworthy data which we have yet
obtained with reference to stellar distances. As a great number of
attempts have been made on stars which, by their brightness or by the
magnitude of their proper motion, would appear to be the nearest to us,
we may believe that the star now considered as the nearest is really
so, and that there is no other less distant. Thus our sun, a star
in the immensity, is isolated in infinitude, and _the nearest_ sun
reigns at twenty-five trillions of miles from our terrestrial abode.
Notwithstanding its unimaginable velocity of 186,400 miles a second,
light moves, flies, during four years and one hundred and twenty-eight
days to come from this sun to us. Sound would take more than three
millions of years to cross the same abyss. At the constant velocity of
thirty-seven miles an hour, _an express train starting from the sun
Alpha Centauri would not arrive here till after an uninterrupted course
of nearly seventy-five millions of years_.
Here, then, are the nearest suns to us. These stars, twenty-three
in number, are almost the only ones which have shown a perceptible
parallax; still the result is very doubtful for the last four, of which
the parallax is less than a tenth of a second. Attempts have been made
to ascertain the parallax of all the stars of the first magnitude,
and the result has been negative for those which are not entered in
this list. Canopus, Rigel, Betelgeuse, Achernar, Alpha of the Cross,
Antares, Spica, and Fomalhaut, do not show a perceptible parallax. The
fine star Alpha Cygni, which shines near 61 Cygni, does not present to
the most accurate researches any trace of fluctuation: it is, then,
incomparably more distant than its modest neighbor--at least five
times, and perhaps twenty times, fifty times, one hundred times beyond
that. What must be the colossal size and amazing light of these suns,
of which the distance is greater than three hundred to four hundred
trillions of miles, and which nevertheless still shine with so splendid
a brightness!
We did not know the distance of any stars till the year 1840. This
shows how recent this discovery is; indeed, we have hardly now begun
to form an approximate idea of the real distances which separate the
stars from each other. The parallax of 61 Cygni, the first which was
known, was determined by Bessel, and resulted from observations made at
Königsberg from 1837 to 1840. Since then, the first figure obtained has
been corrected by a series of more recent observations.
Such distances are amazing and almost terrifying to the imagination.
The mind is bewildered and almost overwhelmed when attempting to form a
conception of such portions of immensity, and feels its own littleness,
the limited nature of its powers, and its utter incapacity for grasping
the amplitudes of creation. But though it were possible for us to wing
our flight to such a distant orb as 61 Cygni, we should still find
ourselves standing only on the verge of the starry firmament where ten
thousands of other orbs a thousand times more distant would meet our
view. We have reason to believe that a space equal to that which we are
now considering intervenes between most of the stars which diversify
our nocturnal sky. The stars appear of different magnitudes; but we
have the strongest reason to conclude that in the majority of instances
this is owing, not to the difference of their real magnitudes, but to
the different distances at which they are placed from our globe.
If, then, the distance of a star of the first or second magnitude, or
those which are nearest to us, be so immensely great, what must be the
distance of stars of the sixteenth or twentieth magnitude, which can
be distinguished only by the most powerful telescopes? Some of these
must be many millions of times more distant than the nearest star whose
distance now appears to be determined. And what shall we think of the
distance of those which lie beyond the reach of any telescope that has
yet been constructed, stretching beyond the utmost limits of mortal
vision, within the unexplored regions of immensity? Here even the most
vigorous imagination drops its wing, and owns itself utterly unable to
penetrate this mysterious and boundless unknown.
Turning now from the sun, whose distance was first measured, and from
the nearest star with which we are acquainted, let us think about the
most radiant star in the heavens--Sirius, “the blazing Dog-star of the
ancients;” named by one astronomer “the king of suns.”
First, as to the color of Sirius. He belongs to the order of “White
Suns,” and among all the white suns known to us, Sirius ranks as
chief. There may be many at greater distances far surpassing him in
size, and weight, and brilliancy; but we can only speak so far as we
know. Strange to say, Sirius was not always a “white sun;” for ancient
writers describe him as being, in their days, a _red_ star. This change
is very singular, and difficult to understand. But Sirius is not the
only example of the kind. Many alterations in color have been noticed
as taking place, often in a much shorter space of time. For instance,
one star, which was a white sun in the days of Herschel, is now a
golden sun.
Secondly, as to the distance of Sirius. Like a few other stars, Sirius
lies not quite so far away as to be beyond reach of measurement.
No base-line upon earth would cause the slightest seeming change of
position in him; but as our earth journeys round the sun, the line from
one side of her orbit to the other is found wide enough. A base-line
of one hundred and eighty-six millions of miles does cause just a tiny
seeming change.
It is very little even with Alpha Centauri, and with Sirius it is much
less. The “displacement” of Sirius is so slight that to measure his
distance with exactness is impossible. Sirius lies more than twice as
far away from earth as Alpha Centauri. To reach Sirius, the straight
line between earth and sun must be repeated six hundred and twenty-six
thousand times. Light, which reaches us from the sun in eight minutes
and a half, and from Alpha Centauri in four years and a third, can not
reach us from Sirius in less than nine years.
Thirdly, as to the size of Sirius. Here, of course, we are in
difficulties. Radiantly as Sirius shines on a clear night, and dazzling
as he looks through a powerful telescope, he shows no real disk or
round surface, but only appears as one point of brilliant light.
Some believe him to be much the same size as our sun, while others
believe him to be very much larger. But we have no certain ground to
rest upon. The only safe method of calculating the probable size--or,
rather, weight--of a star is through the discovery of a companion,
and a knowledge of its distance from the chief star, and its time of
revolution round the chief star.
Fourthly, as to the motions of Sirius--not his nightly apparent
movement, caused by the earth’s turning on her axis, but his real
journey through space. For a long while it was only possible to observe
the movements of a star when the star was traveling _sideways_ to us
across the sky. A star coming straight towards us, or going straight
away from us, would seem to be at rest. Lately, however, by means
of that remarkable instrument, the spectroscope, it has been found
possible to measure the motions of stars coming towards or going away
from us. This is possible as yet only in a few scattered cases, but
among those few is the brilliant Sirius.
The sideways motion of Sirius had been known before. It now appears
that he does not move exactly sideways to us, but is rushing away in a
slanting direction at the rate of thirty miles each second, or about
one thousand millions of miles a year.
How many millions upon millions of miles Sirius must now be farther
from us than in the days of the ancients! Yet he shines still, the most
brilliant star in the sky. So small a matter are all those millions of
miles, compared with the whole of his vast distance from us, that we do
not perceive any lessening of light.
It is an interesting fact that while Sirius is moving away from us,
we are also moving away from him. Our “first station ahead,” in the
constellation Hercules, lies exactly in the opposite direction from
Sirius. But the sun does not move so fast as Sirius. He is believed
to accomplish only about one hundred and fifty millions of miles each
year, drawing all his planets with him.
Fifthly, has Sirius a family, or system, like that of our sun?
Why not? Common sense and reason alike answer with a “Probably,
yes”--not only about Sirius, but about other stars also. No doubt
the systems--the number, size, weight, speed, distance, and kind of
planets--differ very greatly. No doubt there are boundless varieties of
beauty and grandeur.
But that our sun should be the head of so wonderful and complex a
system, that his rays should be the source of life and heat to so many
dependents, and that all the myriads of suns besides should be mere
solitary lamps shining into empty space, warming, lighting, controlling
_nothing_, is an idea scarcely to be looked in the face.
Of course, there may be other and different uses for some of these
suns, beyond our understanding. We must not be positive in the matter.
It seems, however, pretty certain that Sirius at least is not a
solitary, unattended sun.
Astronomers, carefully watching his movements, were somewhat perplexed.
As with Uranus, the attraction of a heavy body beyond was suspected
from the nature of the planet’s motions, so it happened again with the
far more distant Sirius. Astronomers could only explain his movements
by supposing the attraction of some large and near satellite. No one
knew anything about such a satellite, but it was felt that one must
exist.
Nearly twenty years had elapsed after Bessel had predicted the
disturber of Sirius before the telescopic discovery which confirmed it
was made. The circumstances under which that discovery was made are
not, indeed, so dramatic as those which attended the discovery of
Neptune, but yet they have an interest of their own. In February, 1862,
Alvan Clark & Sons, the celebrated telescope-makers, of Cambridge,
Massachusetts, were completing a superb eighteen-inch object-glass for
the Chicago Observatory. Turning the instrument on Sirius, for the
purpose of trying it, the practiced eye of Alvan Graham Clark, the
son, soon detected something unusual, and he exclaimed, “Why, father,
the star has a companion!” The father looked, and there was a faint
companion due east from the bright star, and distant about ten seconds
of a degree. This was exactly the predicted direction of the companion
of Sirius, and yet the observers knew nothing of the prediction. As
the news of this discovery spread, many great telescopes were pointed
on Sirius; and it was found that when observers knew exactly where to
look for the object, many instruments would show it. The new companion
star to Sirius lay in the true direction, and it was now watched with
the keenest interest, to see whether it also was moving in the way
it should move,--if it were really the body whose existence had been
foretold. Four years of observation showed that this was the case, so
that hardly any doubt could remain that the telescopic discovery had
been made of the star which had caused the inequality in the motion of
Sirius.
It is certainly a very remarkable fact that, out of the thousands of
stars with which the heavens are adorned, no single star has yet been
found which certainly shows an appreciable disk in the telescope. We
are aware that some skillful observers have thought that certain small
stars do show disks; but we may lay this aside, and appeal only to the
ordinary fact that our best telescopes turned on the brightest stars
show merely glittering points of light, so hopelessly small as to elude
our most delicate micrometers. The ideal astronomical telescope is,
indeed, one which will show Sirius, or any other bright star, as nearly
identical as possible with Euclid’s definition of a point, being that
which has no parts and no magnitude.
Lastly, what is Sirius made of? The late discovery of spectrum
analysis, or of the instrument called the spectroscope, helps us here.
A little further explanation on this subject will be given later. It
may be observed in passing that before the said discovery, astronomers
can scarcely be asserted to have known with any certainty that stars
were suns. They could, indeed, be sure that at the vast distances of
the fixed stars no bodies, shining by merely reflected light, could
possibly be visible to us. But there knowledge stopped.
Now we can say more. Now the spectroscope, by breaking up and dividing
for us the slender ray of light traveling from each star, has shown to
us something of the nature of the stars. Now we know that the stars,
like our sun, are burning bodies, surrounded by atmospheres full of
burning gas.
It has even been found out that in Sirius there are large quantities of
sodium and magnesium, besides other metals, and an abundant supply of
hydrogen gas.
CHAPTER XXIV.
DIFFERENT KINDS OF SUNS.
Various in kind, various in size, various in color, various in
position, various in motion, are the myriad suns scattered through
space. So far are they from being formed on the same plan, turned out
on the same model, that it may with reason be doubted whether any two
stars could be found exactly alike. Why should we expect to find them
so? No two oak-leaves, no two elm-leaves, precisely alike, are to be
found upon earth.
So some stars are large, some are small. Some are rapid in movement,
some are slow. Some are yellow, some white, some red, green, blue,
purple, or gray. Some are single stars; others are arranged in pairs,
trios, quartets, or groups. Some appear only for a time, and then
disappear altogether. Others are changeful, with a light that regularly
waxes and wanes in brightness.
We have now to give a little time and thought to variable stars and
temporary stars; afterwards to double stars and colored stars. There
are many stars which pass through gradual and steady changes--first
brightening, then lessening in light, then brightening again. One such
star is to be seen in the constellation of the Whale. It is named
“Mira,” or “The Marvelous,” and the time in which its changes take
place extends to eleven months. For about one fortnight it is a star
of the second magnitude. Through three months it grows slowly more and
more dim, till it becomes invisible, not only to the naked eye, but
through ordinary telescopes. About five months it remains thus. Then
again, during three months, it grows brighter and brighter, till it is
once more a second-magnitude star, and after a fortnight’s pause begins
anew to fade.
At its maximum, this star is yellow; when it is faint, it is reddish.
Spectrum analysis shows in it a striped spectrum of the third type,
and when its light diminishes it preserves all the principal bright
rays reduced to very fine threads. The most plausible explanation
of this variability is to suppose that it periodically emits vapors
similar to the eruptions observed in the solar photosphere. Instead
of its periodicity being like that of the sun--eleven years and
hardly perceptible--this variation of the sun of the Whale is three
hundred and thirty-one days and very considerable. It is subject to
oscillations and irregularities similar to those which we remark in our
sun. Of all the variable stars, this is the easiest to observe, and it
has been known for nearly three hundred years.
The most celebrated of all the variable stars is that known as Algol,
in the constellation of Perseus. This star is very conveniently placed
for observation, being visible every night in the Northern Hemisphere,
and its wondrous and regular changes can be observed without any
telescopic aid. Every one who desires to become acquainted with the
great truths of astronomy should be able to recognize this star, and
should have also followed it during one of its periods of change.
Algol is usually a star of the second magnitude; but in a period
between two or three days--or, more accurately, in an interval of two
days, twenty hours, forty-eight minutes, and fifty-five seconds--its
brilliancy goes through a most remarkable cycle of variations. The
series commences with a gradual decline of the star’s brightness,
which, in the course of three or four hours, falls from the second
magnitude down to the fourth. At this lowest stage of brightness, Algol
remains for about twenty minutes, and then begins to increase, until
in three or four hours it regains the second magnitude, at which it
continues for about two days, thirteen hours, when the same series
commences anew. It seems that the period required by Algol to go
through its changes is itself subject to a slow, but certain variation.
Another variable star, Betelgeuse in Orion, undergoes its variations in
about two hundred days; while yet another, Delta Cephei, takes only six
days. Our own sun is believed to be in some slight degree a variable
star, passing through his changes in eleven years. When the sun-spots
are most numerous, he would probably appear, if seen from a great
distance, more dim than when there are few or none of them.
Sometimes a star is not merely variable. Stars have appeared for a
brief space, and then utterly vanished. They are then named Temporary
Stars. Whether such a star really does go out of existence, or whether
it merely becomes too dim to be seen; whether the furnace-fires are
extinguished, and the glowing sun changes into a dark body, or whether
it merely turns a dark side towards us for a very long period,--these
are questions we can not answer.
An extraordinary specimen of a temporary star was seen in 1572. It was
not a comet, for it had no coma or tail, and it never moved from its
place. The brightness of the star was so great as to surpass Sirius and
Jupiter, and to equal Venus at her greatest brilliancy. Nay, it must
have surpassed even Venus, for it was plainly visible at midday in a
clear sky. Gradually the light faded and grew more dim, till at length
it entirely disappeared. As it lessened in brilliancy, it also changed
in color, passing from white to yellow, and from yellow to red. This
curiously agrees with the three tints of the first, second, and fourth
orders of suns, as lately classified.
This star was observed by Tycho Brahe, who gives the following curious
account:
“One evening, when I was contemplating, as usual, the celestial
vault, whose aspect was so familiar to me, I saw, with inexpressible
astonishment, near the zenith, in Cassiopeia, a radiant star of
extraordinary magnitude. Struck with surprise, I could hardly believe
my eyes. To convince myself that it was not an illusion, and to obtain
the testimony of other persons, I called out the workmen employed in my
laboratory, and asked them, as well as all passers-by, if they could
see, as I did, the star, which had appeared all at once. I learned
later on that in Germany carriers and other people had anticipated the
astronomers in regard to a great apparition in the sky, which gave
occasion to renew the usual railleries against men of science (as with
comets whose coming had not been predicted).
“The new star was destitute of a tail; no nebulosity surrounded it;
it resembled in every way other stars of the first magnitude. Its
brightness exceeded that of Sirius, of Lyra, and of Jupiter. It could
only be compared with that of Venus when it is at its nearest possible
to the earth. Persons gifted with good sight could distinguish this
star in daylight, even at noonday, when the sky was clear. At night,
with a cloudy sky, when other stars were veiled, the new star often
remained visible through tolerably thick clouds. The distance of
this star from the other stars of Cassiopeia, which I measured the
following year with the greatest care, has convinced me of its complete
immobility. From the month of December, 1572, its brightness began to
diminish; it was then equal to Jupiter. In January, 1573, it became
less brilliant than Jupiter; in February and March, equal to stars of
the first order; in April and May, of the brightness of stars of the
second order. The passage from the fifth to the sixth magnitude took
place between December, 1573, and February, 1574. The following month
the new star disappeared without leaving a trace visible to the naked
eye, having shone for seventeen months.”
These circumstantial details permit us to imagine the influence
which such a phenomenon must have exercised on the minds of men. Few
historical events have caused so much excitement as this mysterious
envoy of the sky. It first appeared on November 11, 1572. General
uneasiness, popular superstition, the fear of comets, the dread of the
end of the world, long since announced by the astrologers, were an
excellent setting for such an apparition. It was soon announced that
the new star was the same which had led the wise men to Bethlehem,
and that its arrival foretold the return of the Messiah and the last
judgment. For the hundredth time, perhaps, this sort of prognostication
was recognized as absurd. It did not, however, prevent the astrologers
from being believed, twelve years later, when they announced anew the
end of the world for the year 1588; these predictions exercised the
same influence on the public mind.
After the star of 1572 the most celebrated is that which appeared
in October, 1604, in Serpentarius, and which was observed by two
illustrious astronomers, Kepler and Galileo. As happened with the
preceding, its light imperceptibly faded. It remained visible for
fifteen months, and disappeared without leaving any traces. In 1670
another temporary star, blazing out in the head of the Fox (Vulpecula),
showed the singular phenomenon of being extinguished and reviving
several times before it completely vanished. We know of _twenty-four_
stars which, during the last two thousand years, have presented a
sudden increase of light; have been visible to the naked eye, often
brilliant; and have then again become invisible. The last apparitions
of this kind happened before our eyes in 1866, 1876, 1885, and 1892,
and spectrum analysis enabled us to ascertain, as we have seen, that
they were due to veritable combustion--a fire caused by a tremendous
expansion of incandescent hydrogen, and to phenomena analogous to
those which take place in the solar photosphere. A rather curious fact
about these stars is, that they do not blaze out indifferently in any
point of the sky, but in rather restricted regions, chiefly in the
neighborhood of the Milky Way.
Many other instances have been known, beside those referred to, of
variable and temporary stars.
There are two distinct kinds of double-stars. First, we have those
which merely seem to be double, because one lies almost directly behind
the other, though widely distant from it. Just as a church-tower,
two miles off, may appear to stand close side by side with another
church-tower two and a half miles off, though they are in fact
separated. Secondly, we have the real systems of two suns belonging to
one another; the smaller moving round the larger, or more correctly
both traveling round one central point called the center of gravity,
the smaller having the quicker rate of motion.
[Illustration: LYRA]
Alpha Centauri and 61 Cygni have been already described, as examples of
true double-stars. In the constellation Lyra a marked instance is to be
seen. The brightest star in the Lyre is Vega, and near Vega shines a
tiny star, which to people with particularly clear sight has sometimes
rather a longish look. If you examine this star through an opera-glass,
you find it to consist of two separate stars. But if you get a more
powerful telescope, and look again, you will find that _each star_ of
the couple actually consists of _two_ stars. The four are not at equal
distances. Two points of light seemingly close together are parted by
a wide gap from two other points equally close together. These four
stars are believed to have a double motion. Each of the separate pairs
revolves by itself, the two suns traveling round one center; and in
addition to this the two _couples_ of suns probably perform a long
journey round another center common to them all.
Many thousands of double stars have been discovered; and a large number
of these are now known to be, not merely two distinct suns lying in the
same line of sight, but two brother-suns, each probably the center of
his own system of planets.
We have not only to consider the number of suns, though of simple
numbers more yet remains to be said. Attention must also be given to
the varying colors of different stars; for all suns in the universe are
not made after the model of our sun. All suns are not yellow.
So far as single stars are concerned, colors seem rather limited. White
stars, golden or orange stars, ruby-red stars, placed alone, are often
seen; but blue stars, green stars, gray stars, silver stars, purple
stars, are seldom if ever visible to the naked eye, or known to exist
as single stars.
Take a powerful telescope and examine star-couples, and a very
different result you will find. Not white, yellow, and red alone, but
blue, purple, gray, green, fawn, buff, silvery white, and coppery hues,
will delight you in turn. As a rule, when the two stars of a couple
are alike in color, they are either white, or yellow, or red. Also in
the case of double-stars of different colors, the larger of the two is
almost invariably white or some shade of yellow or red.
There are, however, exceptions to all such rules. Blue stars are almost
never seen alone, and as one of a pair the blue star is generally, if
not invariably, the smaller. But instances are known of double-stars,
both of which are blue; and one group in the southern heavens is
entirely made up of a multitude of bluish suns.
It is when we come to consider double-stars of two colors that the
most striking effects are found. Now and then the two suns are nearly
the same in size; but more commonly one is a good deal larger than the
other. This is known by the brighter light of the largest, and the more
rapid movements of the smallest. The lesser star is often only small by
comparison, and may be in reality a very goodly and brilliant sun.
Among nearly six hundred “doubles” examined by one astronomer, there
were three hundred and seventy-five in which the two stars were of one
color, generally white, yellow, orange, or red. The rest were different
in tint, the difference between the two suns in about one hundred and
twenty cases being very marked.
For instance, a red “primary,” as the larger star is called, will
be seen with a small green satellite; or a white primary will have
a little brother-sun of purple, or of dark ruby, or of light red.
Sometimes the larger sun is orange, the companion being purple or
dark blue. Again, the chief star will be red with a blue satellite,
or yellow with a green satellite, or orange with an emerald satellite,
or golden with a reddish-green satellite. We hear of golden and lilac
couples, of cream and violet pairs, of white and green companions. But,
indeed, the variety is almost endless.
[Illustration: CONSTELLATIONS IN THE NORTHERN HEMISPHERE.]
There may be worlds circling around these suns--worlds, perhaps, with
living creatures on them. We know little about how such systems of suns
and worlds may be arranged. Probably each sun would have his own set
of planets, and both suns with their planets would travel round one
central point. Perhaps, where the second sun is much the smallest, it
might occasionally be like a big blazing satellite among the planets--a
kind of burning Jupiter-sun to the chief sun.
Among colored stars, single and double, a few may be mentioned by name
as examples. Sirius, as already observed, is a brilliant white sun; and
brilliant white also are Vega, Altair, Regulus, Spica, and many others.
Capella, Procyon, the Pole-star, and our own sun, are examples of
yellow stars. Aldebaran, Betelgeuse, and Pollux, are ruby-red. Antares
is a red star, with a greenish “scintillation” or change of hue in its
twinkling. A tiny green sun belonging to this great and brilliant red
sun has been discovered. Some have called Antares “the Sirius of Red
Suns.”
It is during the fine nights of winter that the constellation Taurus,
or The Bull, with the star Aldebaran marking its eye, 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 an 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 _Alpha_ of the
constellation of the Great Dog. 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 three thousand 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. But we shall see
farther on what it teaches us of the grandeur of the sidereal universe.
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 Alpha Procyon, no brilliant star distinguishes it.
The two double-stars, 61 Cygni and Alpha Centauri, are formed each of
two orange suns.
In the Southern Cross there is a wonderful group of stars, consisting
of about one hundred and ten suns, nearly all invisible to the naked
eye. Among the principal stars of this group, which Sir John Herschel
described as being, when viewed through a powerful telescope, like “a
casket of variously-colored precious stones,” are two red stars, two
bright green, three pale green, and one of a greenish blue.
The splendor of these natural illuminations can hardly be conceived
by our terrestrial imagination. The tints which we admire in these
stars from here can give but a distant idea of the real value of their
colors. Already, in passing from our foggy latitudes to the limpid
regions of the tropics, the colors of the stars are accentuated, and
the sky becomes a veritable casket of brilliant gems. What would it be
if we could transport ourselves beyond the limits of our atmosphere?
Seen from the moon, these colors would be splendid. Antares, Alpha
Herculis, Pollux, Aldebaran, Betelgeuse, Mars, shine like rubies; the
Polar Star, Capella, Castor, Arcturus, Procyon, are veritable celestial
topazes; while Sirius, Vega, and Altair are diamonds, eclipsing all
by their dazzling whiteness. How would it be if we could approach the
stars so as to perceive their luminous disks, instead of merely seeing
brilliant points destitute of all diameter?
Blue days, violet days, dazzling red days, livid green days! Could the
imagination of poets, could the caprice of painters, picture on the
palette of fancy a world of light more astounding than this? Could the
mad hand of the chimera, throwing on the receptive canvas the strange
lights of its fancy, erect by chance a more astonishing edifice?
CHAPTER XXV.
GROUPS AND CLUSTERS OF SUNS.
We have been thinking a good deal about single stars and double stars,
as seen from earth. Now we have to turn our attention to groups,
clusters, _masses_ of stars, in the far regions of space.
Have you ever noticed on a winter night, when the sky was clear and
dotted with twinkling stars, a band of faintly-glimmering light
stretching across the heavens from one horizon to the other? The band
is irregular in shape, sometimes broader, sometimes narrower; here more
bright, there more dim. If you were in the Southern Hemisphere you
would see the same soft belt of light passing all across the southern
heavens. This band or belt is called the Milky Way.
But what is the Milky Way? It is made up of stars. So much we know.
As the astronomer turns his telescope to the zone of faintly-gleaming
light, he finds stars appearing behind stars in countless multitudes;
and the stronger his telescope, the more the white light changes into
distant stars.
Our sun we believe to be one of the stars of the Milky Way; merely
one star among millions of stars; merely one golden grain among the
millions of sparkling gold-dust grains scattered lavishly through
creation. Scattered, not recklessly, not by chance, but placed,
arranged, and guided each by its Maker’s upholding hand.
The Milky Way, or the Galaxy, as it has been called, has great interest
for astronomers. Many have been the attempts made to discover its
actual size, its real shape, how many stars it contains, how far
it extends; but to all such questions the only safe answers to be
returned are fenced around with “perhaps” and “may be.” There are many
very remarkable clusters of stars to be seen in the heavens--some
few visible as faint spots of light to the naked eye, though the
greater number are only to be seen through a telescope. Either with
the naked eye, or in telescopes of varying power, they show first as
mere glimmers of light, which, viewed with a more powerful telescope,
separate into clusters of distant stars.
[Illustration: A CLUSTER OF STARS IN CENTAURUS.]
The most common shape of these clusters is globular--to the eye
appearing simply round. Stars gather densely near the center, and
gradually open out to a thin scattering about the edge. Thousands of
suns are often thus collected into one cluster.
The clusters are to be seen in all parts of the sky; but the greater
number seem to be gathered into the space covered by the Milky Way and
by the famous south Magellanic Clouds.
Some of them are beautifully colored; as, for instance, a cluster in
Toucan, not visible from England, the center of which is rose-colored,
bordered with white. No doubt it contains a large number of bright red
suns, surrounded by a scattering of white suns.
It used to be supposed that many of these clusters were other vast
gatherings or galaxies of stars, like the Milky Way, lying at enormous
distances from us. This now seems unlikely. The present idea rather is,
that each cluster, in place of being another “Milky Way” of millions of
widely-scattered suns, is one great _star-system_, consisting indeed
of thousands of suns, but all moving round one center. Probably most
of these clusters are themselves a part of the collection of stars to
which our sun belongs.
If this be so, and if worlds are traveling among the suns--as may well
be, since they are doubtless quite far enough apart for each sun to
have his own little or great system of planets--what sights must be
seen by the inhabitants of such planets!
We do not indeed know the distance between the separate suns of
a cluster, which may be far greater than appears to us. But if
astronomers calculate rightly, they are near enough together to shed
bright light on all sides of a planet revolving in their midst. The
said planet might perhaps not have within view a single sun equal in
apparent size to our sun as seen from earth; yet thousands of lesser
suns, shining brightly in the firmament night and day, would cause a
radiance which we never enjoy.
No, not _night_. In such a world there could be no night. Worlds in the
midst of a star-cluster must be regions of perpetual day. No night, no
starry heaven, no sunrise lights or sunset glories, no shadow mingling
with sunshine, but one continual, ceaseless blaze of brightness. We can
hardly picture, even in imagination, such a condition of things.
[Illustration: THE GREAT NEBULA IN ANDROMEDA, COMPARED WITH SIZE OF THE
SOLAR SYSTEM.]
Besides star-clusters there are also nebulæ. The word _nebula_ comes
from the Latin word for “cloud,” and the nebulæ are so named from their
cloudlike appearance.
It is not easy to draw a line of clear division between nebulæ and
distant star-clusters; for both have at first sight the same dim,
white, cloudy look. In past days the star-clusters were included
by astronomers under the general class of nebulæ. And as with the
star-clusters, so with many of the nebulæ, the more powerful telescopes
of modern days have shown them to be great clusters, or systems, or
galaxies of stars, at vast distances from us.
It was supposed with the nebulæ, as with some of the star-clusters,
that they were other “Milky Ways” of countless stars, far beyond the
outside boundaries of our Milky Way. This may still be true of some or
many nebulæ, but certainly not of all.
For there are different kinds of nebulæ. Some may, as just said, be
vast gatherings of stars lying at distances beyond calculation, almost
beyond imagination. Others appear rather to be clusters of stars, like
those already described, probably situated in our own galaxy of stars.
There is also a third kind of nebula. Many nebulæ, once supposed to
be clusters of stars, having been lately examined by means of the
spectroscope, are found to be enormous masses of glowing gas, and not
solid bodies at all.
The number of nebulæ known amounts to many thousands. They are
commonly divided into classes, according to their seeming shape. There
are nebulæ of regular form, and nebulæ of irregular form. There are
circular nebulæ, oval nebulæ, annular nebulæ, conical nebulæ, cometary
nebulæ, spiral nebulæ, and nebulæ of every imaginable description.
These shapes would no doubt entirely change, if we could see them
nearer; and indeed, even in more powerful telescopes, they are often
found to look quite different. While on the subject of clusters and
nebulæ, mention should be made of the famous Magellanic Clouds in the
southern heavens. Sometimes they are called the Cape Clouds. They
differ from other nebulæ in many points, and more particularly in their
apparent size. The Great Cloud is about two hundred times the size of
the full moon, while the Small Cloud is about one-quarter as large. In
appearance they are not unlike two patches of the Milky Way, separated
and moved to a distance from the main stream.
These clouds are surrounded by a very barren portion of the heavens,
containing few stars; but in themselves they are peculiarly rich.
Seen through a powerful telescope, they are found to abound with
stars. The Greater Cloud alone contains over six hundred from the
seventh to the tenth magnitudes, countless tiny star-points of lesser
magnitudes, star-clusters of all descriptions, and nearly three hundred
nebulæ,--all crowded into this seemingly limited space.
Among many famous nebulæ, the one in Orion and the one in Andromeda may
be particularly mentioned. The size of the latter, as compared with the
size of our Solar System, is shown in the cut of this nebula. On the
hypothesis of a complete resolvability into stars, the mind is lost in
numbering the myriads of suns, the agglomerated individual lights of
which produce these nebulous fringes of such different intensities.
What must be the extent of this universe, of which each sun is no more
than a grain of luminous dust!
CHAPTER XXVI.
THE PROBLEM OF SUN AND STAR DISTANCES.
One question had long occupied the minds of scientific men without the
finding of any satisfactory answer, and this was the distance of the
sun from our earth.
At length the sun had fully gained his rightful position in the minds
of men as center and controller of the Solar System, and earth was
fully dethroned from her old false position. In point of fact, the sun
had gained _more_ than his rightful position; since he was now looked
upon very much as the earth had been formerly looked upon; since he
was regarded practically as the motionless Universe-Center. The earth
might and did move--people had grown used to that idea--but the sun,
at all events, was fixed. The sun, beyond turning upon his axis, was
motionless.
Still, as to the true distance of the sun from ourselves, ignorance
reigned. Tycho Brahe had made a great advance on earlier notions by
placing the light-giver 4,000,000 miles away in imagination. Kepler,
a quarter of a century later, increased this to 14,000,000. Galileo
afterwards reverted to the 4,000,000. All this, however, was sheer
guesswork.
Not till well on in the seventeenth century did Cassini make a
definite attempt at actual measurement of the sun’s distance, and this
attempt gave as a result some 82,000,000 miles--not far removed from
the truth. But at the same period other observers gave results of
41,000,000 and 136,000,000; so for years the question still remained
swathed in mist. Better modes and better instruments were required
before it could receive settlement.
The measurement of the distance of objects a good way off is by means
of their “parallax,” or the apparent change in their position when
viewed from two different points at some distance from each other. To
this method of calculating distance we have already referred.
This kind of measurement of distances is not confined to our earth’s
surface alone. The distances of bright bodies in the heavens may be
calculated after just the same method, provided only that the heavenly
body is not too far distant to be made to change its seeming position
in the sky.
Suppose you wished to find out how far off the moon is. You would have
to make your observation of the moon’s exact position in the sky--of
just that spot precisely where she is to be seen among heavenly scenery
at a particular moment; and you would have to get somebody else to make
a similar observation, at the same time, from some other part of earth,
at a good distance from your post.
There are no hills or woods in the sky for the moon to be seen against;
but there are plenty of stars, bright points of light far beyond the
moon. If you look at her from one place, and your friend looks at her
from another place a good way off, there will be a difference in your
two “views.” You will see her very close to certain stars; and your
friend will see her not quite so close to those stars, but closer to
others. This difference of view would give the moon’s parallax.
If your observation were very careful, very exact, and if you had
the precise distance between the spot where you stood, and the spot
where your friend stood,--then, from that base-line, and from the two
angles formed by its junction with the lines from the two ends of it
straight to the moon, you might reckon how many miles away the moon is.
The greater the distance between the two places of observation, the
better,--as, for instance, at Greenwich and the Cape of Good Hope.
Practically, this is not nearly so simple a matter as it may sound,
because our earth is always on the move, and the moon herself is
perpetually journeying onward. All such motions have to be most
scrupulously allowed for. So the actual measurement of moon-distance
requires a great deal of knowledge and of study. Many other
difficulties and complications besides these enter into the question,
and have to be overcome.
Now, the sun is very much farther away than the moon, and the
stumbling-blocks in the way of finding out his distance become
proportionately greater and more numerous. To observe him from two
parts of England, or from two parts of Europe, would give him no
parallax. That is to say, there would be no change of position on his
part apparent to us.
I do not say that there would be no change at all; but only that it
would be so minute as to be quite unseen by human eyes, even with the
help of most careful and accurate measurement. A very long base-line
is needed to make the sun distinctly appear to change his position
ever so little. Only the very longest base-line which can be found
on earth will do for this,--nothing less than the earth’s whole
diameter of nearly eight thousand miles. And I think you will see that
the success of the calculation would then depend, not alone on most
careful observation from two posts at the opposite sides of earth; not
alone on mathematical gifts and powers of close reckoning; but also,
essentially, on a true knowledge of our earth’s diameter; that is, of
_the exact length of the base-line_ from which the whole calculation
would have to be made, and upon which the answer would largely depend.
For a long while the earth’s diameter was not well known. As time
went on, fresh measurements were again and again made of different
portions of earth’s surface, fresh calculations following therefrom;
and gradually clear conclusions were reached. A very important matter
it was that they should be reached; for the semi-diameter of our earth
has been adopted as a “standard measure” for the whole universe; and
the slightest error in that standard measure would affect all after
calculations.
When actual observations of sun-distance came to be made, innumerable
difficulties arose. Foremost stood the huge amount of that distance.
This made precise observations more difficult; and at the same time it
made every mistake in observation so much the worse. A little mistake
in observing the moon might mean only a hundred miles or so wrong in
the answer; but a mistake equally small in observing the sun would lead
to an error of many millions of miles.
Again, to observe the sun’s exact position among the stars, as with the
moon, was not possible; because when the sun is visible, the stars are
not visible. Then, too, the dazzling brightness of the sun balked the
needed exactitude.
Halley had a brilliant thought before he died. He could not carry
it out himself, but he left it to others as a legacy; that was, by
observing the transit of Venus from two different points on the earth’s
surface. At his suggestion, on the first opportunity--which was not
till after his death--the above mode was tried of measuring the sun’s
distance.
A certain observer, stationed on one part of earth, saw the tiny dark
body of Venus take one particular line across the sun’s bright face.
Another observer, standing on quite another and a far-off part of
earth, saw the little dark body take quite another line across the
sun’s bright face. Not that there were two little dark bodies, but that
the one body was seen by different men in different places.
From these separate views of the path of Venus across the face of the
sun, in connection with what was already known of the earth’s diameter,
and therefore of the length of base-line between the two places, the
distance of the sun was reckoned to be about 95,000,000 miles.
Since that date many fresh attempts have been made, and errors have
been set right. Mercury as well as Venus has been used in this matter,
and other newer modes of measurement have also been successfully tried.
We know now, with tolerable accuracy, that the sun’s greater distance
from earth is between 92,000,000 and 93,000,000 miles. A curious
illustration of sun-distance has been offered by one writer. Sound and
light, heat and sensation, all require _time_ for journeying. When a
child puts his finger into a candle-flame, he immediately shrieks with
pain. Yet, quickly as the cry follows the action, his brain is not
really aware of the burn until a certain interval has elapsed. True,
the interval is extremely minute; still it is a real interval. News of
the burn has to be telegraphed from the finger, through the nerves of
the arm, up to the brain; and it occupies time in transmission, though
so small a fraction of a second that we can not be conscious of it.
Now, try to imagine a child on earth with an arm long enough to reach
the sun. His fingers might be scorched by the raging fires there, while
yet his brain on earth would remain quite unaware of the fact for
about one hundred and thirty years. All through those years, sensation
would be darting along the arm exactly as fast as it darts from the
finger-tips of an ordinary child on earth to that child’s brain.
If the scorching began before he was one year old, he would have become
a very aged man, one hundred and thirty years old, before he could know
in his mind what was happening in the region of his hand. Moreover, if,
on receiving the intimation, he should decide to withdraw his hand from
that unpleasantly-hot neighborhood, another hundred and thirty years
would elapse before the fingers could receive and act upon the message,
telegraphed from the brain, through the nerves of the arm. So much for
the distance of the sun from our earth!
But the stars! How far off are the stars?
The distance of the moon is a mere nothing. The distances of the
planets have been found out. The distance of the sun has been measured.
But the stars--those wondrous points of light, twinkling on, night
after night, century after century, unchanged in position save by the
seeming nightly pilgrimage of them all across the sky in company,
caused only by our earth’s restless, continual whirl,--
What about the distances of the stars? Can we measure their distance
by means of their parallax? This mode of measurement was tried
successfully on the moon, on the planets, and on the sun. But when it
was tried upon the stars, from two stations as far apart as any two
stations on earth could be, the attempt was a failure. Not a ghost of
parallax could be detected with any one star. Not the faintest sign
of displacement was seen in the position of a single star when most
critically and carefully examined.
Then arose a brilliant thought! What of earth’s yearly journey round
the sun? If a base-line of 8,000 miles were not enough, compared with
the great distance of the stars, this at least remained. Our earth at
midsummer is somewhere about 185,000,000 miles away from where she
is at midwinter, comparing her position with that of the sun, and
reckoning him to be at rest.
In reality, the sun is not at rest, but is in ceaseless motion,
carrying with him, wherever he goes, the whole Solar System with as
much ease as a train carries its passengers. Those passengers are truly
in motion, yet, with regard merely to the train, they are at rest. So
each member of the Solar System--attached to the sun, not by Ptolemaic
bars, but by the bond of gravity--is borne along by him through space;
yet, with respect to each member of that system, the central sun is
always and absolutely at rest.
At one time of the year, our earth is on one side of the sun, over
92,000,000 miles distant. Six months later, the earth is on the other
side of the sun, not quite 93,000,000 miles away from him in that
opposite direction. Twice 93,000,000 comes to 186,000,000. This line,
therefore--the diameter of the earth’s whole yearly orbit, may be
roughly stated as about 185,000,000 of miles in length.
Here surely was a base-line fit to give parallax to any star--or,
rather, to make parallax visible in the case of any star. For it is,
after all, a question, not of fact, but of visibility; not of whether
the thing _is_, but of whether we are able to _see_ it.
As the earth journeys on her annual tour round the sun, following a
slightly elliptic pathway, the diameter of which is about 185,000,000
miles, each star in the sky must of necessity undergo a change of
position, however minute, performing a tiny apparent annual journey in
exact correspondence with the earth’s great annual journey.
The question is not, Does the star do this? but, Can we _see_ the star
do this? Its apparent change of position may be so infinitesimal,
through enormous distance, that no telescopes or instruments yet made
by man can possibly show it to us.
The star-motion of which I am now speaking is purely a seeming
movement, not real. Be very clear on this point in your mind.
_Real_ star-movements, though suspected earlier, were not definitely
surmised--one may even say “discovered”--until the year 1718, by
Halley. And though Cassini in 1728 referred to this discovery of
Halley’s, yet very little was heard about the matter until the days of
Herschel. Only _apparent_ star-motions were generally understood and
accepted.
The first and simplest of such seeming star-motions is one which we can
all see--the nightly journey of the whole host of stars, caused by our
earth’s whirl upon her axis.
The second is also simple, but by no means also easily seen.
Astronomers reasoned out the logical necessity for such an apparent
motion long before it could be perceived. As far back as the days of
Copernicus it was felt that if the Copernican System were true, if the
earth in very deed traveled round the sun, then the stars ought to
change their positions in the sky when viewed from different parts of
earth’s annual journey. Observations were taken, divided by six months
of time and by one hundred and eighty-five millions of miles of space.
And the stars stirred not!
Stupendous as was this base-line, it proved insufficient. So much
_more_ stupendous was the distance of the stars that the base-line sank
to nothing, and once again parallax could not be detected. Not that the
seeming change of position in the star did not take place, but that
human eyes were unable to see it, human instruments were unable to
register it.
There lies the gist of the matter. If the change of position can be
observed, well and good! The length of the base-line being known, the
distance of the star may be mathematically calculated. For the size of
the tiny apparent path, followed in a year by the star, is and must be
exactly proportioned to the distance of the star from that base-line.
If the tiny oval be so much larger, then the star is known to be so
much nearer. If the tiny oval be so much smaller, then the star is
known to be so much farther away.
But when not the minutest token could be discovered of a star’s
position in the sky being in the least degree affected by the earth’s
great annual change of position, astronomers were at a loss. There was
absolutely nothing to calculate from. The star was a motionless point
to earth. The whole yearly orbit of earth was a motionless point to the
star. One slender beam of light united the two. Reckoners had nothing
to stand upon.
It is interesting to know that Copernicus had actually, long
before, suggested this as a possible explanation of the absence of
star-parallax. He thought that astronomers might fail altogether
to find it, because the stars might be “at a practically infinite
distance” from our earth.
CHAPTER XXVII.
MEASUREMENT OF STAR DISTANCES.
Until the days of Sir William Herschel, little attention was bestowed
upon the universe of distant stars. Before his advent the interest
of astronomers had been mainly centered in the sun and his revolving
worlds. The “fixed stars” were indeed studied, as such, with a certain
amount of care, and numberless efforts were made to discover their
distances from earth; but the thought of a vast Starry System, in which
our little Solar System should sink to a mere point by comparison with
its immensity, had not yet dawned.
With larger and yet larger telescopes of his own making, Herschel
studied the whole Solar System, and especially the nature of sun-spots,
the rings of Saturn, the various motions of attendant moons, together
with countless other details. He enlarged the system by the discovery
of another planet outside Saturn--the planet Uranus, till then unknown.
These were only first steps. The work which he did in respect of the
Solar System was as nothing compared with the work which he did among
the stars. His work in our system was supplementary to other men’s
labors; his work among the stars was the beginning of a new era. Like
many before him, he, too, sought eagerly to find star-parallax, and he,
too, failed. Not yet had instruments reached a degree of finish which
should permit measuring operations of so delicate a nature.
Although he might not detect star-parallax, he sprang a mine upon the
older notions of star-fixity. He shook to its very foundations the
sidereal astronomy of the day,--the theory, long held, of a motionless
universe, motionless stars, and a rotating but otherwise motionless
sun. He did away with the mental picture, then widely believed in,
of vast interminable fixity and stillness, extending through space,
varied only by a few little wandering worlds. As he swept the skies,
and endeavored to gauge the fathomless depths, and vainly pursued the
search for the parallax of one star after another, he made a great and
unlooked-for discovery. This was in connection with double-stars.
Double-stars had long been known, and were generally recognized. But
whether the doubleness were purely accidental, due merely to the fact
of two stars happening to lie almost in the same line of sight, as
viewed from earth, or whether any real connection existed between
the two, no man living could say. Indeed, so long as all stars were
regarded as utter fixtures, the question was of no very great interest.
But light dawned as Herschel watched. He found the separate stars in a
certain pair to be moving. Each from time to time had slightly, very
slightly, changed its place. Then, at least, if all other stars in
the universe were fixed, those two were not fixed. He watched on, and
gradually he made out that their motions were steady, were systematic,
and were connected the one with the other. That was one of the first
steps towards breaking down the olden notion, so widely held, of a
fixed and unchanging universe.
Another double-star, and yet another, responded to Herschel’s intense
and careful searching. These other couples, too, were revolving, not
separately, but in company; journeying together round one center; bound
together apparently by bonds of gravitation, even as our sun and his
planets are bound together.
Other stars besides binary stars are found to move with a real and
not merely a seeming movement. Viewed carelessly, the stars do indeed
appear to remain fixed in changeless groups; fixed even through
centuries. But, to exceedingly close watching and accurate measurement,
many among them are distinctly _not_ fixed; many among them can be
actually seen to move.
Of course the observed motions are very small and slow. One may be
found to creep over a space as wide as the whole full moon in the
course of three hundred or four hundred years; and this is rapid
traveling for a star in earth’s sky! Another will perhaps cross a space
one-tenth or one-twentieth or one-fiftieth of the moon’s width in one
hundred years.
Such motions had never been carefully noted or examined, until Herschel
came to do away with the old received notions of star-fixity. Happily,
Herschel was no slave to “received ideas.” Like Galileo in earlier
times, he wished to “prove all things” personally, anxious only to find
out what was the truth. And he found that the stars were _not_ fixed!
He found that numbers of them were moving. He conjectured that probably
all the rest were moving also; that in place of a fixed universe of
changeless stars we have a whirling universe of rushing suns.
Herschel could not, of course, watch any one star for a hundred years,
much less for several centuries. But in a very few years he could, by
exceedingly close measurement, detect sufficient motion to be able to
calculate how long it would take a certain star to creep across a space
as wide as the full moon.
From step to step he passed on, never weary of his toil. He sought to
gauge the Milky Way, and to form some notion of its shape. He noted
a general drift of stars to right and to left, which seemed to speak
of a possible journey of our sun through space, with all the planets
of the Solar System. He flung himself with ardor into the study of
star-clusters and of nebulæ. He saw, with an almost prophetic eye, the
wondrous picture of a developing universe--of nebulæ growing slowly
into suns, and of suns cooling gradually into worlds--so far as to
liken the heavens to a piece of ground, containing trees and plants in
every separate stage of growth.
And still the search for star parallax went on, so long pursued in
vain. No longer base-line than that of the diameter of earth’s yearly
orbit lay within man’s reach. But again and yet again the attempt was
made. Instruments were improved, and measurements became ever more
delicate; and at length some small success crowned these persistent
endeavors. The tiny sounding-line of earth, lowered so often into the
mysterious depths of space, did at last “touch bottom.”
Herschel died, full of years and honors, in 1822; and some ten years
later three different attempts proved, all to some extent, and almost
at the same date, successful. Bessel, however, was actually the first
in point of time; and his attempt was upon the double-star, 61 Cygni;
not at all a bright star, but only just visible to the naked eye.
There are stars and stars, enough to choose from. The difficulty
always was, which to select as a subject for trial with any reasonable
prospect of a good result. Some astronomers held that the brightest
stars were the most hopeful, since they were probably the nearest; and
of course the nearer stars would show parallax more readily, because
their parallax would be the greater. But certain very bright stars
indeed are now known to be far more distant than certain very dim stars.
Again, some astronomers thought that such stars as could be perceived
to move most rapidly in the course of years would be the most hopeful
objects to attack, since the more rapid movement might be supposed to
mean greater nearness, and so greater ease of measurement. But some
stars, seen to move rapidly, are now known to be more distant than
others which are seen to move more sluggishly. So neither of these two
rules could altogether be depended on; yet both were, on the whole, the
best that could be followed, either separately or together.
The star 61 Cygni is not one of the brighter stars, but it is one of
those stars which can be seen to travel most quickly across the sky in
the course of a century. Therefore it was selected for a trial; and
that trial was the first to meet with success.
For 61 Cygni was found to have an apparent parallax; in other words,
its tiny seeming journey through the year, caused by our earth’s great
journey round the sun, could be detected. Small as the star-motion
was, it might, through careful measurement, be perceived. And, in
consequence, the distance of the star from earth could be measured.
Not measured with anything like such exactitude as the measurements
of sun-distance, but with enough to give a fair general notion of
star-distance.
One success was speedily followed by others. By a few others,--not by
many. Among the thousands of stars which can be seen by the naked eye,
one here and one there responded faintly to the efforts made. One here
and one there was found to stir slightly in the sky, when viewed from
earth’s summer and winter positions, or from her spring and autumn
positions, in her yearly pathway round the sun.
It was a very, very delicate stir on the part of the star. Somewhat
like the difference in position which you might see in a penny a
few miles off, if you looked at it first out of one window and then
out of another window in the same house. You may picture the penny
as radiantly bright, shining through pitch darkness; and you may
picture yourself as looking at it through a telescope. But even so,
the apparent change of position in the penny, viewed thus, would be
exceedingly minute, and exceedingly difficult to see.
There are two ways of noting this little seeming movement on the part
of a star in the sky.
Either its precise place in the sky may be observed--its exact
position, as in a map of the heavens--or else its place may be noted
as compared with another more distant star, near to it in the sky,
though really far beyond. Parallax, if visible, would make it alter its
precise place in the sky. Parallax, if visible, would bring it nearer
to or farther from any other star more distant than itself from earth.
The more distant star would have a much smaller parallax--probably
so small as to be invisible to us--and so it would do nicely for a
“comparison star.”
The first of these methods was the first tried; and the second was the
first successful.
To find star-distance through star-parallax sounds quite simple,
when one thinks of the general principle of it. Just merely the
question of a base-line, accurately measured; and of two angles,
accurately observed; and of another angle, a good way off, accurately
calculated,--all resting on the slight seeming change of position in
a certain distant object, watched from two different positions, about
185,000,000 miles apart.
Quite simple, is it not? Only, when one comes to realize that the
“change of position” is about equal to the change of position in a
penny piece, miles away, looked at from two windows in one house,--then
the difficulty grows.
Besides this, one has to remember all the “corrections” necessary,
before any true result can be reached.
A penny piece, miles away, seen from two windows, would be difficult
enough as a subject for measurement. But, at least, the penny would
be at rest; and you yourself would be at rest; and light would pass
instantaneously from it to you; and there would be no wobbling and
nodding motions of everything around to add to your perplexities.
In the measurement of star-parallax, all these things have to be
considered and allowed for; all have to be put out of the question, as
it were, before any correct answer is obtained.
The refraction of light must be considered; because that displaces the
star, and makes it seem to us to be where it is not. And the aberration
of light must be considered; because, in a different way, that does the
same thing, making the star seem to take a little journey in the course
of the year. And the precession, or forward motion, of the equinoxes,
and nutation, or vibratory motion, have both to be separately
considered; because they, too, affect the apparent position of every
star in the sky.
To watch the star in comparison with another star is easier than
merely to note its exact position in the sky; for the other star--the
“companion-star”--is equally with itself affected by refraction of
light and by aberration of light. But, then, another question comes in
seriously: whether or no the companion-star shows any parallax also;
since, if it does, that parallax must be carefully calculated and
allowed for.
So the measurement of star-distance, even when parallax can be
detected, is by no means a light or easy matter. On the contrary, it
bristles with difficulties. It is a most complicated operation, needing
profound knowledge, accurate observation, trained powers of reasoning
and calculation.
Nothing short of what has been termed “the terrific accuracy” of the
present day could grapple with the truly tremendous difficulties of
this problem of star-distance. It has, however, been grappled with,
and grappled with successfully. We now know, not indeed with anything
like exactitude, yet with reasonable certainty, the distances of a good
many stars, as expressed roughly in round number of “about” so many
trillions of miles.[3] We have at least learned enough to gain some
notion of the immeasurable distances of countless other stars, lying
far beyond reach of earth’s longest measuring-line.
[3] The diameter of the moon as seen from the earth is equal to
one thousand eight hundred and sixty eight seconds of space. When
the parallax of a star is ascertained, it is a simple matter to
calculate the distance of the star by the use of the following table.
If the star shows a displacement of one second in space, or one
eighteen-hundred-and-sixty-eighth part of the width of the moon, that
star is distant two hundred and six thousand two hundred and sixty-five
times the distance of the earth from the sun.
An angle of 1″ is equivalent to 206,265 times 93,000,000 of miles.
“ “ “ 0″.9 “ “ “ 229,183 “ “ “ “
“ “ “ 0″.8 “ “ “ 257,830 “ “ “ “
“ “ “ 0″.7 “ “ “ 294,664 “ “ “ “
“ “ “ 0″.6 “ “ “ 343,750 “ “ “ “
“ “ “ 0″.5 “ “ “ 412,530 “ “ “ “
“ “ “ 0″.4 “ “ “ 515,660 “ “ “ “
“ “ “ 0″.3 “ “ “ 687,500 “ “ “ “
“ “ “ 0″.2 “ “ “ 1,031,320 “ “ “ “
“ “ “ 0″.1 “ “ “ 2,062,650 “ “ “ “
“ “ “ 0″.0 “ “ “ Immeasurable.
The nearest star, Alpha Centauri, shows a parallax of less than one
second (0″.75). The farthest star, the distance of which has been
ascertained, is 1830 Groombridge, which shows a parallax of only
0″.045, and is distant 4,583,000 times earth’s distance from the sun,
or 426 trillions of miles.
The very thought of such unimaginable depths of space, of suns beyond
suns “in endless range,” toned down by simple distance to mere
quivering specks of light, is well-nigh overwhelming.
CHAPTER XXVIII.
THE MILKY WAY.
The Milky Way forms a soft band of light round the whole heavens. In
the Southern Hemisphere, as in the Northern Hemisphere, it is to be
seen. In some parts the band narrows; in some parts it widens. Here
it divides into two branches; there we find dark spaces in its midst.
One such space in the south is so black and almost starless as to have
been named the Coal-sack. All along, over the background of soft, dim
light, lies a scattering of brighter stars shining on its surface.
Much interest and curiosity have long been felt about this mysterious
Milky Way. That it consists of innumerable suns, and that our sun is
one among them, has been believed for a considerable time. But other
questions arise. How many stars does the Milky Way contain? What is its
shape? How far does it reach?
No harm in asking the questions, only we have to be satisfied in
astronomy to ask many questions which can not yet receive answers. No
harm for man to learn that the utmost reach of his intellect must fall
short in any attempt to sound the depths of God’s universe, even as the
arm of a child would fall short in seeking to sound the depths of the
ocean over the side of a little boat. For the attempt has been made to
sound the depths of our star-galaxy out of this little earth-boat.
The idea first occurred to the great Herschel, as we have already
mentioned, and a grand idea it was--only a hopeless one. He turned
his powerful telescope north, south, east, west. He counted the stars
visible at one time in this, in that, in the other directions. He found
a marked difference in the numbers. The portion of sky seen through
his telescope was about one quarter the size of that covered by the
moon. Sometimes he could merely perceive two or three bright points
on a black background. At other times the field of his telescope was
crowded. In the fuller portions of the Milky Way, he had four or five
hundred stars under view at once. In one place he saw about one hundred
and sixteen thousand stars pass before him in a single quarter of an
hour.
Herschel took it for granted that the stars of the Milky Way,
uncountable in numbers, are, as a rule, much the same in size--so that
brightest stars would, as a rule, be nearest, and dimmest stars would,
as a rule, be farthest off. Where he found stars clustering thickly,
beyond his power to penetrate, he believed that the Milky Way reached
very far in that direction. Where he found black space, unlighted by
stars or lighted by few stars, he decided that he had found the borders
of our galaxy in that direction.
Following these rules which he had laid down, he made a sort of rough
sketch of what he supposed might be the shape of the Milky Way. He
thought it was somewhat flat, extending to a good distance breadthways
and a much greater distance lengthways, and he placed our sun not far
from the middle. This imagined shape of the Milky Way is called “The
Cloven-disk Theory.” To explain the appearance of the Milky Way in the
sky, Herschel supposed it to be cloven or split through half of its
length, with a black space between the two split parts.
It seems that Herschel did not hold strongly to this idea in later
years, and doubts are now felt whether the rules on which he formed it
have sufficient foundation.
For how do we know that the stars of the Milky Way are, as a rule,
much the same in size? Certainly the planets of the Solar System are
very far from being uniform, and the few stars whose weight can with
any certainty be measured, seem to vary considerably. There is a
great difference also between the large and small suns in many of the
double-stars!
Again, how do we know that the bright stars are, as a rule, the
nearest, and the dim stars the farthest off? Here, also, late
discoveries make us doubtful. Look at Sirius and 61 Cygni--Sirius the
most radiant star in the heavens, and 61 Cygni almost invisible to the
naked eye. According to this rule, 61 Cygni ought to lie at an enormous
distance beyond Sirius. Yet in actual fact, Sirius is the farthest away
of the two.
The illustrious Herschel believed that he had penetrated, on one
occasion, into the star-cluster on the sword-hand in the constellation
of Perseus until he found himself among sidereal depths, from which the
light could not have reached him in less than four thousand years.
The distance of those stars had not, and has not, been mathematically
measured. Herschel judged of it by their dimness, by the strong power
needed to make them visible, and by the rules which he had adopted as
most likely true.
Again, when Herschel found black spaces in the heavens almost void of
stars, and believed that he had reached the outside borders of the
Milky Way, he may have been in the right, or he may have been mistaken.
The limit might lie there, or thousands more of small stars might
extend in that very direction, too far off for their little glimmer to
be seen through the most powerful telescope.
[Illustration: A CLUSTER OF STARS IN PERSEUS.]
If this latter idea about the Milky Way being formed of a great many
brilliant suns, and of vast numbers of lesser suns also, be true,
astronomers will, in time, be able to prove its truth. For in that
case, many faint telescopic stars being much nearer to us than bright
stars of the greater magnitudes, it will be found possible to measure
their distance. The journey of our earth round the sun must cause a
seeming change in their position between summer and winter.
The theory also that some of the nebulæ are other outlying Milky Ways,
or galaxies of stars, separated by tremendous distances from our own,
is interesting, and was long held as almost certain, yet we have no
distinct proof either one way or the other. Many of the nebulæ may be
such gatherings of countless stars outside our own, or every nebula
visible may be actually part and parcel of our galaxy.
Much attention has of late been paid to the arrangement of stars in the
sky. The more the matter is looked into, the more plainly it is seen
that stars are neither regular in size nor regular in distribution.
They are not merely scattered carelessly, as it were, here, there, and
anywhere, but certain laws and plans of arrangement seem to have been
followed which astronomers are only now beginning dimly to perceive.
Stars are not flung broadcast through the heavens, each one alone and
independent of the rest. They are placed often, as we have already
seen, in pairs, in triplets, in quartets, in clusters. Also, the great
masses of them in the heavens seem to be more or less arranged in
streams, and sprays, and spirals. So remarkable are the numbers and
forms of many of these streams that the idea has been suggested, with
regard to the Milky Way, whether it also may not be a vast stream of
stars, like a mighty river, collecting into itself hundreds of lesser
streams.
The contemplation of the heavens affords no spectacle so grand and so
eloquent as that of a cluster of stars. Most of them lie at such a
distance that the most powerful telescopes still show them to us like
star-dust. “Their distance from us is such that they are beyond, not
only all our means of measurement,” says Newcomb, “but beyond all our
powers of estimation. Minute as they appear, there is nothing that
we know of to prevent our supposing each of them to be the center of
a group of planets as extensive as our own, and each planet to be as
full of inhabitants as this one. We may thus think of them as little
colonies on the outskirts of creation itself, and as we see all the
suns which give them light condensed into one little speck, we might
be led to think of the inhabitants of the various systems as holding
intercourse with each other. Yet, were we transported to one of these
distant clusters, and stationed on a planet circling one of the suns
which compose it, instead of finding the neighboring suns in close
proximity, we should see a firmament of stars around us, such as we see
from the earth. Probably it would be a brighter firmament, in which
so many stars would glow, with more than the splendor of Sirius, as
to make the night far brighter than ours; but the inhabitants of the
neighboring worlds would as completely elude telescopic vision as the
inhabitants of Mars do here. Consequently, to the inhabitants of every
planet in the cluster, the question of the plurality of worlds might be
as insolvable as it is to us.”
These are clusters of stars of regular form in which attraction appears
to mark its secular stamp. Our mind, accustomed to order in the cosmos,
anxious for harmony in the organization of things, is satisfied with
these agglomerations of suns, with these distant universes, which
realize in their entirety an aspect approaching the spherical form.
More extraordinary, more marvelous still, are the clusters of stars
which appear organized in spirals.
In considering stars of the first six magnitudes only--stars visible
to the naked eye--a somewhat larger number is found in the Southern
Hemisphere than in the Northern Hemisphere. In both hemispheres there
are regions densely crowded with stars, and regions by comparison
almost empty.
It has been long questioned whether the number of bright stars is or is
not greater in the Milky Way than in other parts of the sky. Careful
calculations have at length been made. It appears that the whole of the
Milky Way--that zone of soft light passing round the earth--covers, if
we leave out the Coal-sack and other such gaps, between one-tenth and
one-eleventh of the whole heavens.
The entire number of naked-eye stars, or stars of the first six
magnitudes, does not exceed six thousand; and of these, eleven hundred
and fifteen lie scattered along the bed of the Milky Way stream. If the
brighter stars were scattered over all the sky as thickly as throughout
the Milky Way, their number would amount to twelve thousand instead of
only six thousand. This shows us that the higher-magnitude stars really
are collected along the Milky Way in greater numbers than elsewhere,
and is an argument used by those who believe the Milky Way to be a
mighty stream of streams of stars.
In the dark spaces of the Milky Way, on the contrary, bright stars are
so few that if they were scattered in the same manner over all the sky,
their present number of six thousand would come down to twelve hundred
and forty. This would be a serious loss.
We know in the sky 1,034 clusters of stars and more than 11,000
nebulæ. The former are composed of associated stars; the latter may
be divided into two classes: First, nebulæ which the ever-increasing
progress of optics will one day resolve into stars, or which in any
case are composed of stars, although their distances may be too great
to enable us to prove it; second, nebulæ properly so called, of which
spectrum analysis demonstrates their gaseous constitution. Here is
an instructive fact. The clusters of stars present the same general
distribution as the telescopic stars--they are more numerous in the
plane of the Milky Way, while it is the contrary which is presented
by the nebulæ properly so called; they are rare, thinly spread in the
Milky Way, and thickly scattered to the north as well as to the south
of this zone up to its poles. The constitution of the Milky Way--not
nebulous, but stellar--is a very significant fact. The nebulæ properly
so called are distributed, in a sense, contrary to the stars, being
more numerous towards the poles of the Milky Way and in regions poor
in stars, as if they had absorbed the matter of which the stars are
formed.
CHAPTER XXIX.
A WHIRLING UNIVERSE.
No rest, no quiet, no repose, in that great universe, which to our dim
eyesight looks so fixed and still, but one perpetual rush of moving
suns and worlds. For every star has its own particular motion, every
sun is pressing forward in its own appointed path. And among the
myriads of stars--bright, blazing furnaces of white or golden, red,
blue, or green flame--sweeping with steady rush through space, our sun
also hastens onwards.
The rate of his speed is not very certain, but it is generally believed
to be about one hundred and fifty million miles each year. Possibly he
moves in reality much faster.
When I speak of the sun’s movement, it must of course be understood
that the earth and planets all travel with him, much as a great steamer
on the sea might drag in his wake a number of little boats. From one
of the little boats you could judge of the steamer’s motion quite as
well as if you were on the steamer itself. Astronomers can only judge
of the sun’s motion by watching the seeming backward drift of stars to
the right and left of him; and the watching can be as well accomplished
from earth as from the sun himself.
After all, this mode of judging is, and must be, very uncertain. Among
the millions of stars visible, we only know the real distances of
about twenty-five; and every star has its own real motion, which has to
be separated from the apparent change of position caused by the sun’s
advance.
It seems now pretty clear that the sun’s course is directed towards a
certain point in the constellation Hercules. If the sun’s path were
straight, he might be expected by and by, after long ages, to enter
that constellation. But if orbits of suns, like orbits of planets, are
ellipses, he will curve away sideways long before he reaches Hercules.
[Illustration: HERCULES]
One German astronomer thought he had found the center of the sun’s
orbit. He believed the sun and the stars of the Milky Way to be
traveling round the chief star in the Pleiades, Alcyone. This is
not impossible; but it is now felt that much stronger proof will be
required before the idea can be accepted.
In the last chapter mention was made of star-streams as a late
discovery. Though a discovery still in its infancy, it is one of no
small importance. Briefly stated, the old theory as to the plan of
the Milky Way was as follows: Our sun was a single star among millions
of stars forming the galaxy, some comparatively near, some lying at
distances past human powers of calculation, and all formed upon much
the same model as to size and brightness. Where the band of milky
light showed, stars were believed to extend in countless thousands to
measureless distances. Where dark spaces showed, it was believed that
we looked beyond the limits of our universe into black space. Stars
scattered in other parts of the sky were supposed generally to be
outlying members of the same great Milky Way. Many of the nebulæ and
star-clusters were believed to be vast and distant gatherings of stars,
like the Milky Way itself, but separated by unutterably wide reaches of
space. Some of these views may yet be found to contain truth, though at
the present moment a different theory is afloat.
It is still thought probable that our sun is one among many millions
of suns, forming a vast system or collection of stars, called by
some a universe. It is also thought possible that other such mighty
collections of stars may exist outside and separate from our own at
immense distances. It is thought not impossible or improbable that some
among the nebulæ may be such far-off galaxies of stars; though, on the
other hand, it is felt that every star-cluster and nebula within reach
of man’s sight may form a part of our own “universe.”
According to this view of the question, the Milky Way, instead of
being an enormous universe of countless suns reaching to incalculable
distances, may rather be a vast and mighty star-stream, consisting
of hundreds of brilliant leading suns, intermixed with thousands or
even millions of lesser shining orbs. If this be the true view, the
lesser suns would often be nearer than the greater suns, although more
dim; and the Milky Way would not be itself a universe, though a very
wonderful and beautiful portion of our universe.
Which of these two different theories or opinions contains the most
truth remains to be found out. But respecting the arrangement of
stars into streams, interesting facts have lately been discovered. We
certainly see in the Solar System a tendency of heavenly bodies to
travel in the same lines and in companies. Not to speak of Jupiter and
his moons, or Saturn and his moons, we see it more remarkably in the
hundreds of asteroids pursuing one path, the millions of meteorites
whirling in herds. Would there be anything startling in the same
tendency appearing on a mightier scale? Should we be greatly astonished
to find streams of stars, as well as streams of planets?
For such, indeed, appears to be the case. Separated by abysses of
space, brother suns are plainly to be seen journeying side by side
through the heavens, towards the same goal.
The question of star-drift is too complex and difficult to be gone
into closely in a book of this kind. One example, however, may be
given. Almost everybody knows by sight the constellation of the Great
Bear--the seven principal stars of which are called also Charles’s
Wain, a corruption of the old Gothic _Karl Wagen_, the churl’s or
peasant’s wagon. It is also called the Great Dipper. Four bright stars
form a rough sort of oblong, and from one of the corners three more
bright stars stretch away in a curve, representing the Bear’s tail.
Many smaller stars are intermixed.
[Illustration: THE GREAT BEAR.]
These seven bright stars have always been bound together in men’s
minds, as if they belonged to one another. But who, through the
centuries past, since aught was known of the real distances of the
stars from ourselves and from one another, ever supposed that any among
the seven were _really_ connected together?
One of these seven stars, the middle one in the tail, has a tiny
companion-star, close to it, visible to the naked eye. For a good
while it was uncertain whether the two were a “real double,” or only
a seeming double. In time it became clear that the two did actually
belong to one another.
Mizar is the name of the chief star, and Alcor of the companion. Alcor
is believed to be about three thousand times as far away from Mizar,
as our earth from the sun. Now, if this be the width of space between
those two bright points, lying seemingly so close together as almost to
look to the naked eye like one, what must be the distance between the
seven leading stars of the Great Bear, separated by broad sky-spaces?
Who could imagine that one of these suns had aught to do with the rest?
Yet among other amazing discoveries of late years it has been found by
means of the new instrument, the spectroscope, that _five_ out of these
seven suns are traveling the same journey, with the same speed. Two of
the seven appear to be moving in another direction, but three of the
body-stars and two of the tail-stars are hastening in the direction
away from us, all in the same line of march, all rushing through space
at the rate of twenty miles each second. Some smaller stars close to
them are also moving in the same path. Is not this wonderful? We see
here a vast system of suns, all moving towards one goal, and each
probably bearing with him his own family of worlds.
Many such streams have been noticed, and many more will doubtless be
found. For aught we know, our own sun may be one among such a company
of brother-suns, traveling in company.
It is difficult to give any clear idea of the immensity of the
universe--even of that portion of the universe which lies within reach
of our most powerful telescopes. How far beyond such limits it may
reach, we lose ourselves in imagining.
Earlier in the book we have supposed possible models of the Solar
System, bringing down the sun and worlds to a small size, yet keeping
due proportions. What if we were to attempt to make a reduced model of
the universe; that is of just so much of it as comes within our ken?
Suppose a man were to set himself to form such a model, including
every star which has ever been seen. Let him have one tiny ball for
the sun, and another tiny ball for Alpha Centauri; and let him, as a
beginning, set the two _one yard apart_. That single yard represents
ninety-three millions of miles, two hundred and seventy-five thousand
times repeated. Then let him arrange countless multitudes of other tiny
balls, at due distances--some five times, ten times, twenty times,
fifty times, as far away from the sun as Alpha Centauri.
It is said that the known universe, made upon a model of these
proportions, would be many miles in length and breadth. But the model
would appear fixed as marble. The sizes and distances of the stars
being so enormously reduced, their rates of motion would be lessened
in proportion. Long intervals of time would need to pass before the
faintest motion in one of the millions of tiny balls could become
visible to a human eye.
CHAPTER XXX.
READING THE LIGHT.
[Illustration: THE PRISM AND SPECTRUM.]
Several times, in the course of this book, mention has been made of a
wonderful new instrument called the spectroscope. We now proceed to
give a brief account of the origin and achievements of this newest of
scientific appliances. The first step towards its formation was made
by Sir Isaac Newton, when he discovered the power of the prism to
decompose light. This consists in the fact that a ray of light, after
passing through a transparent prism, becomes expanded into an elongated
spectrum, no longer white, but presenting an invariable succession of
colors from red to violet. These are called the seven primary colors;
namely, red, orange, yellow, green, blue, indigo, and violet. The
rainbow is a familiar illustration of this spectrum with its various
colors; and the raindrops are the prisms which reflect and decompose
the light.
[Illustration: THE SPECTROSCOPE.]
Optical science was long satisfied with this glance into the interior
constitution of light, occupying itself with the phenomena of the
prismatic colors, and theorizing on the nature of white light. In
1802, Dr. W. H. Wollaston, in closely examining a spectrum, found it
to be crossed by at least four fine dark lines. It is only when an
extremely narrow slit is employed in admitting the sunlight that they
become visible. Dr. Wollaston, supposing them to be merely “natural
boundaries” of the different color-bands, inquired no further; and
there for a while the matter rested. Not many years later, in 1815,
the matter was taken up by a German optician and scientist of Munich,
Joseph von Fraunhofer. Applying more delicate means of observation, he
was surprised to find very numerous dark lines crossing the spectrum.
With patience he went into the question, using the telescope as well
as a very narrow slit, and soon he discovered that the dark lines were
to be numbered, not by units or by tens, but by hundreds--or, as we
now know, by thousands. Some were in the red, some were in the violet,
some were in the intermediate bands; but each one had, and has, its own
invariable position on the solar spectrum. For, be it understood, these
dark lines are constant, not variable. Where a line is seen, there it
remains. Whenever a ray of sunlight is properly examined, with slit and
with prism, that line will be found always occupying precisely the same
spot in the spectrum.
Some of the chief and more distinct lines were named by Fraunhofer
after certain letters of the alphabet, and by those letter-names they
are still known. He began with A in the red and went on to H in the
violet. Fraunhofer made a great many experiments connected with these
mysterious lines, anxious to discover, if possible, their meaning. For
although he now saw the lines, which had scarcely so much as been seen
before, he could not understand them--he could not read what they said.
They spoke to him, indeed, about the sun; but they spoke in a foreign
language, the key to which he did not possess. He tried making use of
prisms of different materials, thinking that perhaps the lines might be
due to something in the nature of the prism employed. But let the prism
be what it might, he found the lines still there. Then he examined
the light which shines from bright clouds, instead of capturing a ray
direct from the sun. And he found the lines still there. For cloudlight
is merely reflected sunlight.
Then he examined the light of the moon, to see if perchance the
spectrum might be clear of breaks. And he found the lines still
there. For moonlight is only reflected sunlight. Next he set himself
to examine the light which travels to us from some of the planets,
imagining that a different result might follow. And he found the lines
still there. For planet-light again is no more than reflected sunlight.
[Illustration: SPECTRA, SHOWING THE DARK LINES.]
Lastly, he turned his attention to some of the brighter stars,
examining, one by one, the ray which came from each. And, behold! he
found the lines _not_ there. For starlight is not reflected sunlight.
That is to say, the identical lines which distinguish sunlight were
not there. Each star had a spectrum as the sun has a spectrum, and
each star-spectrum was crossed by faint dark lines, more or less in
number. But the spectra of the stars differed from the spectrum of the
sun. Each particular star had its own particular spectrum of light,
different from that of the sun, and different from that of every other
star. For now, Fraunhofer was examining, not sunlight, but starlight;
not the light of our sun, either direct from himself or reflected from
some other body, such as planet or moon or cloud, but the light of
other suns very far distant, each one varying to some extent from the
rest in its make. The fact of the stars showing numerous sets of black
lines, all unlike those of our sun, showed conclusively that those
lines could not possibly be due to anything in our earthly atmosphere.
Sunlight and starlight travel equally through the air, and are equally
affected by it. If our atmosphere were the cause of the black lines in
sunlight, it would cause the _same_ lines in starlight. But the sun and
each individual star has its own individual lines, quite irrespective
of changeful states of the air.
So, also, the light from any metal sufficiently heated will give
a spectrum, just as sunlight gives a spectrum, under the needful
conditions. That is to say, there must be slit and prism, or slit and
diffraction-grating, for the light to pass through. But the kind of
spectrum is by no means always the same.
Putting aside for a few minutes the thought of sunlight and starlight,
let us look at the kind of rays or beams which are given forth by
heated earthly substances. Any very much heated substance sends forth
its light in rays or beams, and any such rays or beams may be passed
through a prism, and broken up or “analyzed,” just as easily as
sunlight may be “analyzed.”
Suppose that we have a solid substance first--a piece of iron or of
steel wire. If it is heated so far as to give out, not only heat,
but also light, and if that light is made to travel through the slit
and prism of a spectroscope, the ray will then be broken up into its
sub-rays. They, like the sub-rays of a sunbeam, will form a continuous
row of soft color-bands, one melting into another. This is the
characteristic spectrum of the light which is given forth by a burning
or glowing _solid_.
Next, suppose we take a liquid--some molten iron or some molten glass,
for instance. If you have ever been to a great plate-glass manufactory,
like that at St. Helens, not far from Manchester, you will have seen
streams of liquid glass pouring about, carried to and fro in huge
caldrons, bright with a living light of fire from its intensity of
heat. If a ray of _that_ light had been passed through slit and prism,
what do you think would have been the result? A continuous spectrum
once more, the same as with the glowing iron or steel. The light-ray
from a heated and radiant _liquid_, when broken up by a prism, lies in
soft bands of color, side by side.
Both of them a good deal like the solar spectrum, you will say. Only
here are no mysterious dark lines crossing the bright bands of color.
But how about gases? Suppose we have a substance in the state of gas
or vapor, as almost every known substance might be under the requisite
conditions, and suppose that substance to be heated to a glowing
brilliance. Then let its light be passed through the slit and prism of
a spectroscope. What result shall we find this time? Entirely different
from anything seen before. Instead of soft, continuous bands of color,
there are _bright lines_, well separated and sharply defined.
How many bright lines? Ah, that depends upon which particular gas
is having its ray analyzed. Try sodium first--one of the commonest
of earthly substances. Enormous quantities of it are distributed
broadcast in earth and air and water. More than two-thirds of the
surface of our globe lies under an enfolding vesture of water saturated
with salt, which is a compound of sodium. That is to say, sodium enters
largely into the make of salt. Every breeze which sweeps over the ocean
carries salt inland, to float through the atmosphere. Sir H. E. Roscoe
writes: “There is not a speck of dust, or a mote seen dancing in the
sunbeam, which does not contain chloride of sodium”--otherwise salt.
The very air which surrounds us is full of compounds of sodium, and
we can not breathe without taking some of it into our bodies. Sodium
is an “elementary substance.” By which I mean that it is one of a
number of substances called by us “simple,” because chemists have never
yet succeeded in breaking up those substances, by any means at their
command, into other and different materials.
Iron is, so far as we know, a simple substance. It is found as iron
in the earth. No chemist has ever been able to make iron by combining
other materials together. No chemist has ever managed to separate iron
into other materials unlike itself. Iron it is, and iron it remains,
whether as a solid, as a liquid, or as a gas. Gold is another simple
substance, and so is silver. Sodium is another. All these we know best
in the solid form. Mercury, another simple substance, we know best as a
liquid.
Water is not a simple substance; for it can be separated into
two different gases. Glass is not a simple substance; for it is
manufactured out of other substances combined together.
We can speak quite positively as to such substances as are not simple;
but with regard to so-called “elements,” we may only venture to assert
that, thus far, nobody has succeeded in breaking them up. Therefore, at
least for the present, they are to us “elementary.”
All the simple substances, and very many of the combinations of them,
though often known to us only in the solid form, may, under particular
conditions, be rendered liquid, and even gaseous. Every metal may be
either in the solid form, or the liquid form, or the vapor form. Iron,
as we commonly see it, is solid--in other words, it is frozen, like
ice. Just as increase of warmth will turn ice into water, so a certain
amount of heat will make solid iron become liquid iron. And just as yet
greater warmth will turn water into steam, so a very much increased
amount of heat will turn liquid iron into vapor of iron. A little heat
will do for ice what very great heat will do for iron.
Whether iron and other metals exist in the sun, as on earth, in a
hard and solid form, it is impossible to say. It is only in the form
of gas that man can become aware of their presence at that distance.
The intense, glowing, furnace-heat of the sun causes many metals to
be present in large quantities in the sun’s atmosphere in the form of
vapor.
Not only have we learned about some of the metals in the sun, but
this strange spectrum analysis has taught us about some of the metals
and gases in the stars as well. It is found that in Sirius, sodium
and magnesium, iron and hydrogen, exist. In Vega and Pollux there are
sodium, magnesium, and iron. In Aldebaran, these substances and many
others, including mercury, seem to abound. These are merely a few
examples among many stars, each being in some degree different from the
rest.
But how can we know all this? How could the wildest guessing reveal to
us the fact of iron in the sun, not to speak of the stars? We know it
by means of the spectrum analysis--or, as we may say, by means of the
spectroscope. This instrument may be looked upon as the twin-sister to
the telescope. The telescope gathers together the scattered rays of
light into a small spot or focus. The spectroscope tears up these rays
of light into ribbons, sorts them, sifts them, and enables us to read
in them hidden meanings.
When a ray of light reaches us from the sun, that ray is _white_; but
in the white ray there are bright colors concealed. Newton was the
first to discover that a ray of white light is really a bundle of
colored rays, so mixed up together as to appear white. If a ray of
sunlight is allowed to pass through a small round hole in a wall, it
will fall upon the opposite wall in a small round patch of white light.
But if a _prism_--a piece of glass cut in a particular shape--is put in
the path of the ray, it has power to do two curious things. First, it
bends the ray out of a straight course, causing the light to fall upon
a different part of the wall. Secondly, it breaks up or divides the
ray of white light into the several rays of colored light of which the
white ray is really composed. This breaking up, or dividing, is called
“analyzing.”
If in place of a round hole the ray of light is made to pass through
a very narrow slit, it is proved that the bright bands of color do not
overlap. Instead of this, dark lines, or gaps, show here and there.
Now, these lines or gaps are always to be seen in the spectrum or image
of bright colors formed by a broken-up ray of _sunlight_. There is
always a certain number of dark lines in each colored band--some near
together, some far apart; here one or two, there a great many. Where a
simple ray of sunlight is concerned, the exact arrangement of the lines
never changes.
When the stars were examined--when the rays of light coming from
various stars were split up and analyzed--it was found that they too,
like the sun, gave a spectrum of bright colors with dark lines. But
the lines were different in number and different in arrangement from
the sun’s lines. Each star has his own particular number and his own
particular arrangement, and that arrangement and number do not change.
If a white-hot metal is burnt, and the light of it as it burns is
allowed to pass through a prism, a row of bright colors appears as in
the sun’s spectrum, only there are no dark lines. If a gas is burnt,
and the light is allowed to pass through a prism, no bright color-bands
appear, and no dark lines either; but instead of this, there are
_bright lines_. Each gas or vapor has its own number of lines and its
own arrangement. Sodium shows two bright lines, side by side. Iron
shows sixty bright lines, arranged in a particular way.
Now you see how a row of bright colors without bands of color may
appear. But what about color-bands and dark lines together? That
discovery came latest. It was found that if a white-hot metal were
burned, and if its light were allowed before touching the prism to
shine through the flame of a burning gas, _then_ there were dark lines
showing in the colored bands. These dark lines changed in position and
number and arrangement with each different kind of gas, just as the
bright lines changed if the gases were burned alone. If the light of
the burning metal passed through a flame colored with gas of sodium,
two dark lines showed on one part of the spectrum; but if it passed
through a flame colored with vapor of iron, sixty dark lines showed on
another part of the spectrum.
So now, by means of this spectrum analysis, we know with all but
certainty that the sun and stars are solid, burning bodies, sending
their light through burning, gas-laden atmospheres. By examining the
little black lines which appear in the spectrum of one or another, it
is possible to say the names of many metals existing as gas in those
far-off heavenly bodies. Is not this a wonderful way of reading light?
The split-up rays tell us much more than the kinds of metals in
different stars. When a nebula is examined, and is found to give no
spectrum of bright bands and dark lines, but only a certain number of
bright lines, we know it to be formed of gas, unlike stars and other
nebulæ. Also, it is by means of the spectroscope that so much has
lately been discovered about the motions and speed of the stars coming
towards or going from us.
CHAPTER XXXI.
STELLAR PHOTOGRAPHY.
Photography is not only a useful handmaid to astronomy as a whole, but
also it is so, peculiarly, to that division of astronomy with which we
are now chiefly concerned--to spectroscopy. A few pages may therefore
with advantage be given to the subject of celestial photography.
Photography is, indeed, the greatest possible help to the astronomer.
It pictures for him those stars which his eyes can see but dimly, or
even can not see at all; it paints for him those light-rays of which he
would obtain but a passing glance, and which he could not accurately
remember in all their details; it maps out for him the wide heavens,
which he, unaided, could never do with anything like equal completeness
by eye and hand alone.
Only recently a vast photographic map of the whole sky was undertaken.
About eighteen different observatories, in divers parts of the world,
divided the task among them, stars down to the sixteenth magnitude
being most carefully registered in a complete series of something like
fifteen hundred separate photographs. The whole result, when finally
completed, will be a grand achievement of the present century. Each
individual star, in the entire heavens all around our earth, from the
first to the sixteenth magnitudes, will have its exact position in
the sky accurately known, and the smallest change in the position of
anyone of those stars may then be detected.
Even in the delicate and abstruse operation earlier described, the
measurement of star-distances through annual parallax, photography
again steps in. Dr. Pritchard, Savilian professor of Astronomy at
Oxford, pressed photography into the service of this task also.
Measurements for parallax were made under his direction upon
photographic plates--a work of no small interest. Here, as elsewhere,
peculiar advantages belong to the photographic method when it can be
followed. Its records are lasting, the limited number of hours which
are fully suitable for direct astronomical work may be employed in
obtaining those records, and in broad daylight the examination of them
can be carried on.
One very curious use is made of star-photographs, more especially of
the photographs of unseen stars--that is, of stars too distant or too
dim to be detected by the eye. These photographs, when taken, may
be afterwards looked into further with a microscope. So, first, the
far-off, invisible suns of the universe are photographed on a prepared
plate, with the help of a powerful telescope, this being needful to
secure sufficient starlight; and then the tiny picture of those suns is
examined more closely with the help of a powerful microscope.
Spectroscopy has much to say to us. It tells us about the positions
of the different stars. It tells us about the structure of the stars.
It tells us about the various classes to which the various stars
belong. And also it tells us about the motions of the stars--not mere
apparent motions, caused by movements of our own earth, but true onward
journeyings of the stars themselves through the depths of space.
For by means of photography we do not obtain simple pictures of
the stars themselves _only_, but pictures also of the _spectra_ of
the stars. An instrument is made uniting the spectroscope with the
photographic apparatus, and this is called a spectrograph. By its
means the disintegrated or broken-up star-ray is photographed in its
broken-up condition, so that an exact picture is obtained of the bands
and lines characteristic of any particular star.
No easy matter, as may be imagined, are these spectroscopic
observations and these spectroscopic photographings of the stars. To
bring the image of a star exactly opposite a slender opening or slit,
perhaps only about the _three-hundredth part of an inch in width_,
and to keep it there, is a task which might well be looked upon as
practically impossible.
No sooner is a star found in the field of a telescope than it vanishes
again. As the astronomer gazes, the ground beneath him is ever whirling
onward, leaving the star behind; and although clockwork apparatus in
all observatories of any importance is made to counteract this motion
of the earth, and to keep the heavenly object, whether star or moon, or
comet, within view by following its apparent motion, yet, as can easily
be imagined, to follow thus the seeming motion of a dim star through an
opening so minute, demands exceeding care and delicacy of adjustment.
It is not indeed, for purposes of analysis, _always_ needful to pass
the light of a star through a narrow slit. In the case of a nearer
body, such as the sun or moon or one of the planets, light flows
from all parts of the body, one ray crossing another; and for the
examination of such light, a slit is imperatively needed, all side-rays
having to be cut off. But the whole of the brightest star in the sky
is only one point of light to us, and a slitless telescope may be
used with no confused results. Where, however, direct comparison is
required, the star-lines being made to appear, side by side or above
and below, with the solar spectrum, or with the lines of earthly
metals, then the slit becomes unavoidable.
By such comparison of the two, side by side--the light from an
intensely-heated earthly substance and the light from a star, the rays
of both being broken up in the spectroscope--the oneness or difference
of lines in the rays can be easily made out.
One main difficulty in such observations arises from the diminution
of starlight, caused by its passage through a prism. If a rope of a
dozen strands is untwisted, each of those strands is far weaker than
the whole rope was; and each strand of color in the twisted rope of
light is of necessity much more feeble, seen by itself, than the whole
white ray seen in one. Another difficulty in our country generally
is the climate, which gives so few days or hours in the year for
effective work. Yet the full amount accomplished by seizing upon every
possible opportunity is, in the aggregate, astonishing. A third and
very pressing difficulty attendant upon examination of star-spectra is
caused by the incessant motion of our air, through which, as through
a veil, all observations have to be made. The astronomer can never
get away from the atmosphere; and unless the air be very still--that
is to say, as still as it ever can remain--the spectrum-lines are
so uncertainly seen as to make satisfactory results impossible. Dr.
Huggins has sometimes passed hours in the examination of a single line,
unable to determine whether or no it precisely coincided with the
comparison-line of some earthly substance. In _this_ matter, no leaping
at conclusions is admissible.
The photography of stars would be easy enough if one could just
expose a plate to the shining of the star, and there leave it to be
impressed--there leave the star-ray to sketch slowly its own image. But
this is hardly possible. The unceasing motion of the earth, causing
the star perpetually to pass away from the telescopic field, and the
exceeding narrowness of the slit opposite to which the star has to
be kept in a stationary position, make the most accurate adjustment
needful.
Clockwork alone can not be trusted. If it could, the star and the
photographic apparatus might be comfortably “fixed,” and left to do
their own work. Instead of which, while a photograph is proceeding,
it is desirable that an observer should sit gazing patiently at
the telescope-tube, where the image of the star is seen, ready at
any moment to correct by a touch the slightest irregularity in the
clockwork motion of the telescope, and so to prevent a blurred and
spoilt reproduction of the star, or of its spectrum.
One hour, two hours, three hours, at a stretch, this unceasing
watchfulness may have to be kept up, and no small amount of enthusiasm
in the cause of science is requisite for so monotonous and wearying a
vigil.
As noted earlier, it has been found possible, if the photograph of a
star or nebula is not completed in one night, to renew the work and
carry it on the next night, or even for many nights in succession. This
is especially practicable in spectroscopic photography.
From four to five hours, sometimes from eight to ten hours, may
be needful before the clear image of the star or of its spectrum
appears--a dim little star probably to us, yet perhaps in reality a
splendid sun, shedding warmth and light and life upon any number of
such worlds as ours.
Eight or ten hours of photography at one stretch with a star are
impossible; for the stars, ever seemingly on the move, do not remain
long enough in a good position. For three to six hours, a telescope may
be made, by means of its clockwork machinery, to keep a star steadily
in sight, and all that while the photograph is progressing. If further
exposure is needed, the process has to be resumed the next night.
The more one considers the matter, the more plainly one perceives how
enormously our powers of sky-observation are increased by photography.
It is not only that one photographer, with his apparatus, may
accomplish in a single night the work of many astronomers who have
to depend upon the power of the eye alone. It is not only that, with
the help of photography, as much can now be done in a lifetime as
formerly must have occupied many generations. It is not only that the
photograph, once taken, remains a permanent possession, instead of a
record, more or less imperfect, in which otherwise the astronomer would
have to trust.
It is not even only that in the photograph details come out which could
not be detected by the eye, and that stars are actually brought to our
knowledge which no man has ever seen, which perhaps no man ever will
see from this earth with the assistance of the most powerful telescope.
For the weak shining, which can by no possibility make itself felt by
the retina of a man’s eye, _can_ slowly impress its picture on the
photographic plate. Hundreds of stars, thousands of stars, utterly
invisible to man, have had their photographs taken as truly as you have
had your photograph taken, and by the same process, only it has been a
longer business.
But in addition to all this, we see reproduced upon the plate those
ultra-violet and infra-red portions of the spectrum of light which but
for the handmaid, photography, would still be to us as things which
have no existence. And by means of photography we can observe and study
in those same unseen portions of the spectra, when looking into a ray
from sun or star, the innumerable dark lines, every one of which has
its own tale to tell. To these tales we must have remained blind and
deaf, but for photographic aid to our limited powers.
Look at some dim star in the sky, and try how long you can gaze without
blinking. You will very soon find that you have done your best, and
that to gaze longer only means a sense of fatigue in your eyes, a
growing dimness in the star. How different with the photographic plate!
There, no exertion is wasted, no weariness is felt. Faint though the
light may be, which travels earthward and falls upon the plate, it is
all collected, all used.
It is easy to photograph the sun, and this has been done. When we come
to the moon the operation is more difficult on account of the feeble
intensity of its light, and the difference of tint of the various parts
of its surface. Skill and perseverance have, however, surmounted the
greatest difficulties, and now we have photographs of the moon enlarged
to more than a yard in diameter, which show the smallest details with a
truly admirable clearness.
In the first second of time, your eye receives as much light from the
star as does the photographic plate in the same time. But during the
course of one hour, the plate receives and stores up about thirty-six
hundred times as much light as it or you received in the first instant.
There is the secret of the matter. The photographic plate does not only
receive, it can also keep and treasure up the light, and that our eyes
are not able to do.
If you magnify the amount received in one hour by five or ten, and
remember that it is all retained, then you will begin to understand
how feeble stars, unseen by man, should become known to us through
photography.
CHAPTER XXXII.
THE DAWN OF ASTRONOMY.
The beginnings of astronomy lie in the distance of earliest historical
ages. It is no easy matter to say when first, in all probability, this
science grew into existence. Astronomy was alive before the British
nation was heard of; before Saxons or Franks had sprung into being;
before the Roman Empire extended its iron rule; before the Grecian
Empire began to flourish; before the Persian Empire gained power;
before the yet earlier Assyrian Empire held sway.
Among the ancient Chaldeans were devoted star-gazers--much more devoted
than the ordinary run of educated Anglo-Saxons in this present century.
They earnestly sought, in the dim light of those ages, to decipher the
meaning of heaven’s countless lamps. We have better instruments, and
more practiced modes of reasoning; also, we have the collected piles
of knowledge built up by our forefathers through tens of centuries.
Yet our search is in essence the same as was theirs, to know the truth
about the stars: not merely to start some attractive theory, and then
to prove that our theory must be right, because we have been so clever
as to start it; but to discover that which is in those regions of
space. No lower aim than this is worthy to be called scientific.
Mistakes, of course, are made; how should it be otherwise? A man
finding his way at night, for the first time, through a wild and
unknown country, will almost certainly take some wrong turns before
he discovers the right road. In astronomy, as in all other natural
sciences, blunders are a necessity if advance is to be made. We have
to grope our way to knowledge through observation and conjecture; in
simpler terms, through gazing and guessing. Observation of facts leads
to conjecture as to the possible causes for those facts; and each
conjecture is proved by later observation to be either right or wrong.
If proved to be right, it takes its place among accepted truths; if
proved to be wrong, it is flung aside. Some pet delusions die hard,
because of people’s love for them.
Astronomy, as an infant science, mixed up with astrology, existed
in the days when the Pyramids were juvenile, and when the Assyrian
sculptures were modern. How much farther back who shall say? By
astrology, those who studied the heavens endeavored to determine
the fate of men and of nations, to predict events, and to interpret
results. They paid particular attention to the aspect of the planets,
and the general appearance of the firmament.
These observations were at first simple remarks made by the shepherds
of the Himalayas after sunset and before sunrise: The phases of the
moon, and diurnal retrograde motion of that body with reference to
the sun and stars; the apparent motion of the starry sky accomplished
silently above our heads; the movements of the beautiful planets
through the constellations; the shooting star, which seems to fall
from the heavens; eclipses of the sun and moon, mysterious subjects
of terror; curious comets, which appear with disheveled hair in
the heights of heaven,--such were the first subjects of these old
observations made during thousands of years.
The ancient Chaldean star-gazers had rivals among the early Egyptians;
and the Chinese profess to have kept actual record of eclipses during
between three and four thousand years. The earlier records are not
trustworthy; but of thirty-six eclipses reckoned by the Chinese sage,
Confucius, no less than thirty-one have been proved true by modern
astronomers.
We can not name with certainty those people whose shepherds first gazed
with intelligent eyes upon the midnight sky, and noted the steady sweep
of stars across the firmament--intelligent eyes, that is to say, so
far as man then knew how to use his eyes intelligently; so far as man
had begun to note anything in nature, to watch, to compare, and to
conjecture causes.
In those times, and indeed for long afterwards, men thought much
more about the “influences” of stars upon their own lives, and about
supposed prophecies of the future to be read in the sky, than about the
actual physical condition of the stars themselves, or the causes and
meanings of the various phenomena observed in the heavens.
The desire to know, for the pure delight of knowing, had perhaps hardly
begun to dawn upon the mind of man. What people did want to know was
what might be going to happen to themselves; whether _they_ would be
happy or unhappy; and the stars were chiefly of interest as appearing
to tell beforehand of troubles or joys to come.
The wisest of men in those times knew less of the outspread heavens
than many a child now knows in our public schools. Earth and sky were
one vast bewildering puzzle. They had to discover everything for
themselves--how the sun rose each morning and set each evening; how the
seasons changed in steady sequence through the year; how the moon and
stars journeyed in the night; how the ocean-tides went and came; how
numberless every-day phenomena took place.
In olden days the daily rising and setting of the sun was a mystery,
accounted for by divers theories, none of which were right; and the
march of stars across the midnight sky was a complete puzzle; and an
eclipse of sun or moon was a fearful perplexity; and the tides of
ocean were a great bewilderment. These things are mysteries still to
barbarous nations; but they perplex us no longer, because we have found
out the mode in which such movements, or appearances of movement, are
brought about through the action of quite natural causes.
As a first step, in earliest times, the journey of the sun by day,
the journeys of moon and stars by night, across the sky, could hardly
fail to arrest attention. Very early, too, the stars were grouped into
constellations, definite figures and names being attached to each. Many
of the constellations are now known to us by names which belong to the
earliest historic ages.
The stars were known as “fixed,” because they continued unchangeably in
their relative positions--that is, in the position held by each star
with respect to its neighbor stars--although the whole array of them
moved nightly in company; constellations rising and setting at night,
as the sun rose and set in the day.
Ages may well have passed before the planets were recognized as
distinct from the fixed stars; ages more before any definite plan
was noted in their wanderings. In the course of time men’s attention
was directed to these matters; and one fact after another, of daily
or monthly or yearly occurrence, was observed, and commented upon,
and became familiar to the minds of people. Very slowly the first
beginnings of systematized knowledge took shape and grew, one discovery
being made after another, one explanation being offered after another,
one theory being started after another.
But an essential difference existed between the infantine science of
those primitive days and the matured astronomy of these later days.
The whole ancient science was built upon a huge mistake. Men held,
as a fact of absolutely unquestionable certainty, that this earth
of ours--this small whirling globe, less than eight thousand miles
in diameter--was the center, around which sun, moon, and stars all
revolved.
The Greek philosopher Thales, who lived about six hundred years before
Christ, is said to have laid the foundations of the Grecian astronomy;
and Pythagoras is stated to have been one of his disciples. Though, in
many respects, Thales wandered wide of the truth, he yet taught many
correct ideas; as, for instance, that the stars were made of fire; that
the moon had all her light from the sun; that the earth was a sphere
in shape, besides other facts respecting the earth’s zones and the
sun’s apparent path in the sky. He was also one of the three ancient
astronomers who were able to calculate and foretell eclipses.
After him came numerous astronomers, of greater or less merit, in the
Grecian and in other schools. They watched carefully; they discovered
many things of interest; they held divers theories. But one truth never
took firm root among them, although several of them dimly apprehended
it; and this was the very foundation-truth, for lack of which they were
all going hopelessly astray--the simple truth that our earth is _not_
the center of the universe, and that our earth _does_ move. Yet it is
not surprising. No wonder they were slow to grasp such a possibility.
Anything more bewildering to the mind of ancient man than the thought
of a solid, substantial world floating in empty space, supported upon
nothing, upheld by nothing, can hardly be imagined. As yet little was
known of the controlling laws or forces of nature, and that little was
with reference only to our earth. The very suspicion of gravitation
as a universal law lay in the far-distant future, waiting for the
intellect of a Newton to call it out of apparent chaos; and the
delicate balance of forces, by means of which the Solar System may
almost be said to exist, could not be so much as guessed at.
So men still clung to the thought of earth as the center of all things,
and still believed in a little sun, busily circling round her once in
every twenty-four hours.
Perhaps the greatest of all ancient astronomers was Hipparchus, about
150 B. C., who did more than any other in those early times to gather
together the scattered facts of astronomy, and to arrange them into
one united and orderly whole. He it was who discovered the precession
of the equinoxes. He studied eclipses and the motions of the various
planets. He made elaborate and valuable astronomical tables and
star-catalogues; but he, like others, failed to discover the gigantic
error which lay at the root of the whole ancient science.
Despite this great mistake, still persistently believed in, and despite
the crude notions of early astronomers on many points, it is marvelous
how much they did manage to observe and to learn for themselves,--as
to the sun and his apparent path, as to the moon and her path, as to
the five then known planets and their paths, as to eclipses and other
phenomena.
By all such careful watching, although they to some extent missed their
aim and fell into mistakes, yet they paved a way to later discoveries
and to fuller knowledge. Their work was not thrown away, their trouble
was not lost; for out of their very errors grew the fair form of truth.
Nearly three hundred years after the time of Hipparchus came the famous
Ptolemy--famous, not, like certain other astronomers, for stupendous
genius, or for the brilliancy and accuracy of his observations, but
rather noted for the ingenuity of his explanations, and for the adroit
manner in which he systematized such knowledge, on the subject of the
heavenly bodies, as was then in the possession of mankind.
Ptolemy’s name is best known in connection with what is commonly called
“The Ptolemaic System of the Universe,” and his greatest astronomical
work is best known as the _Almagest_.
A great many of Ptolemy’s leading notions, as well as the principal
mass of facts upon which he worked, were doubtless borrowed from
Hipparchus. The latter is said to have explained the movements of the
sun and of the moon by means of small epicycles, traveling round the
earth on circular orbits; and even Hipparchus did not originate this
fundamental idea of the so-called “Ptolemaic System,” which had indeed
been held by the ancients in much earlier days. Hipparchus worked it
out to some extent, and Ptolemy carried on the process much farther,
giving forth the system to the world in such wise that ever since it
has gone by his name.
The theory of cycles and epicycles lasted long--lasted from the time
of Ptolemy in the second century to the time of Copernicus in the
sixteenth century, of whom we have already spoken.
[For about two thousand years, astronomers observed attentively the
apparent revolutions of the heavenly bodies, and this attentive study
gradually showed them a large number of irregularities and inexplicable
complications, until at last they recognized that they were deceived as
to the earth’s position, in the same way that they had been deceived
as to its stability. The immortal Copernicus, in particular, discussed
with perseverance the earth’s motion, already previously suspected for
two thousand years, but always rejected by man’s self-love; and when
this learned Polish canon bid adieu to our world in the year 1543, he
bequeathed to science his great work, which demonstrated clearly the
long-standing error of mankind.--_F._]
The new theory of Copernicus had to make its way slowly against
the dead weight of unreasoning public opinion, and against
wrongly-reasoning hierarchical opposition. In time, gradually, despite
all resistance, the truth made its way, and was generally received,
though not till Galileo had been forced by ecclesiastical authority to
recant his opinions. All the same, Copernicus, Kepler, Galileo, were
right; public sentiment and the Church were wrong,--the earth did move,
and was not the fixed universe center, and, by and by, those who lived
on earth had to acknowledge the same.
The succession of these great men is interesting to note. Between the
two shining lights, Hipparchus and Ptolemy, nearly three hundred years
intervened. But as the history of the world advances, we find brilliant
scientific minds appearing more quickly, one following close upon
another, instead of their being divided by long intervals of blankness.
Within thirty years from the death of Copernicus, were born two mighty
men of Science: Kepler, who lived till 1630; and Galileo, who lived
till 1642, when England was plunged in civil strife.
Between Copernicus and Kepler came Tycho Brahe. Copernicus was a
Roman ecclesiastic, born in Poland. Tycho Brahe was a wealthy Danish
nobleman, an ardent lover of astronomy, and a most patient and accurate
observer. But Tycho held fast by the old astronomy. He was not
convinced by the arguments of Copernicus. To him, earth was still the
motionless center of all things. He was willing to allow that the rest
of the planets circled round the sun, as an explanation of things which
he could not help seeing; but the sun itself had still, in Tycho’s
imagination, to revolve daily, with planets and stars and the whole
sky, round our earth.
As an explainer of causes, therefore, Tycho rose to no lofty heights.
The real good which he did was in watching and noting actual phenomena,
not in trying to explain how those phenomena were brought about. This
was left for one of his young pupils, a sickly German lad, named John
Kepler.
In later years, Kepler made a grand use of his master’s mass of careful
and thoroughly dependable observations. These had really been gathered
together by Tycho, with a view of disproving the Copernican theory, and
of establishing the main features of the olden astronomy, with certain
improvements. But Kepler used the accumulated information to disprove
the old astronomy, and to establish the truth of the new Copernican
system which Tycho had rejected. He found a vast advantage, ready to
his hand, in the collection of careful observations systematically
worked out by Tycho in his observatory during many a long year. And
Kepler did not fail to use his advantage.
The planets still refused, as they always had refused, to keep to the
paths marked out for them by astronomers. Kepler grappled with the
difficulty. He particularly turned his attention to Mars, that near
neighbor of ours, in which we are all now so much interested. A long
series of close observations of Mars’s movements, made by Tycho in the
course of many years, lay before him; and he knew that he could depend
upon the absolute honesty and accuracy of Tycho’s work.
With immense labor and immense patience, he went into the matter, still
clinging to the old idea of a circular pathway round the sun. He tried
things this way and that way. He placed his orbit in imagination after
one mode and another mode; he conjectured various arrangements; he
tested and proved them in turn, comparing each plan with observations
made,--and still Mars defied all his efforts; still the little world
went persistently wrong, traveling contrary to every theory.
Thus far, nobody had ever thought of an orbit of any other shape than
a circle. When a straightforward journey round a circle was found
impossible, then epicycles were introduced to explain the planet’s
perplexing motions. No one had dreamt of an elliptic pathway. But
suddenly a gleam of daylight came upon this groping in the dark. What
if Mars traveled round the sun--_not_ in a circle, but in an oval?
Kepler tried this oval and that oval, comparing observations
past--testing, examining, proving or disproving, with unwearied
patience. And at length he found, beyond dispute, that Mars actually
did travel round the sun in a yearly pathway, which was shaped, not as
a circle, but as the kind of oval known as an _ellipse_; and, further,
that the sun was not in the exact center of this ellipse, but to one
side of the center, in one of the foci. Kepler’s success in detecting
the true shape of a planetary orbit is doubtless owing to the fact
that the orbit of Mars makes a wider departure from the circular form
than any of the other important planets.
One step further would have led Kepler to the knowledge that comets
also travel in elliptic orbits--only in very long and narrow ones
generally. But he had quite made up his mind that all comets merely
paid our sun one visit, and never by any chance returned; so he did not
trouble himself to enter into calculations with respect to them. This
taking for granted of ideas long held was a very common practice in
those days.
The above wonderful discovery of elliptic orbits, in place of
circular orbits, was only one among many made by the illustrious
Kepler--discoveries not carelessly hit upon, as one might pick up in
the street a valuable stone which somebody had dropped, but earnestly
sought for, and found with toil and diligence, as gems are first
found in foreign lands, after much seeking and long patience. Casual
discoveries in science are rare. Attainment is far more usually made
through intense study, through hard work, through profound thought,
through a gradual groping upward out of darkness to the point where
daylight breaks forth.
The three well-known “Laws of Kepler” still lie at the very foundation
of the modern system of astronomy. They are: First, every planet moves
in an elliptical orbit, in one focus of which the sun is situated;
second, the line drawn from the sun to a planet moves over equal areas
in equal times; and, third, the square of the periodic times of the
planets varies inversely as the cubes of their distances.
CHAPTER XXXIII.
MODERN ASTRONOMY.
Still, however, the world was not convinced of the truth of the
Copernican system. One man here or there saw and acknowledged its
reality. The mass of mankind continued firmly to believe that they
dwelt at the universe center, and utterly to scout the idea of a moving
earth. Indeed, it may be very much doubted whether, not merely the
mass of mankind, but even the most civilized and cultivated portion of
that mass, had as yet, to any wide extent, heard a whisper of the new
theories. Knowledge, in those days, traveled from place to place with
exceeding deliberation.
Another great man arose, contemporary with Kepler--a man whose story
seizes more strongly upon our imagination than that of the severe and
successful studies of Kepler. From an ordinary worldly point of view,
Kepler would hardly be looked upon as altogether a successful man. He
was poor, and in sickly health. His chief book was promptly prohibited
by Rome--then a power in the whole reading world of Europe to an extent
which can hardly be realized in these days of freedom. Kepler’s book
was placed in the forbidden category, side by side with the dying
publication of Copernicus. This checked all hope of literary gains, and
life with Kepler was one long struggle. But if he could have looked
ahead two or three hundred years; if he could have foreseen the time
when not a few wise men of science alone, but all the civilized world,
including the very Church which condemned him, should meekly have
accepted and indorsed his discoveries,--then he would have been able to
gauge the measure of his own real success.
Moreover, with all his troubles, Kepler was so happy as to escape
absolute persecution. He lived in a country which knew something of
liberty as an ideal; he was left to write and work freely; and, at
least, no public recantation of what he held to be truth was forced
from him.
Galileo’s was a sadder tale. A Florentine of noble birth, he turned his
attention early to science as the needle turns to the north, and before
the age of twenty-seven he already occupied a foremost position as
mathematical lecturer at Pisa. There it was that he direfully offended
the philosophers of the day by proving Aristotle to be in the wrong.
For Aristotle was the great master of the schoolmen. His philosophy
and theories were accepted as final, and whatsoever could not be
proved in accordance with them was regarded as untrue. Aristotle had
declared that if two weights--the one being ten times as heavy as the
other--were dropped together from a height, the heavier weight would
reach the ground ten times as quickly as the lighter.
Everybody believed this, and nobody had ever thought of putting it to
the test of actual trial. Aristotle had said so, and Aristotle was
indorsed as a whole by all the schools of Europe; and what more could
possibly be wanted by any reasonable human being? But Galileo was not
so easily satisfied. He looked into the matter independently, bent upon
finding out, not what Aristotle had thought or what the Church might
support, but simply what actually _was_.
[Illustration: PROMINENT ASTRONOMERS OF THE NINETEENTH CENTURY.]
Then, in view of many observers, called together for the occasion, he
dropped, at the same instant, from the top of the leaning tower of
Pisa, a shot weighing one pound and another shot weighing a hundred
pounds. And, behold! both reached the ground simultaneously. There was
not a fraction of difference between the two. To prove Aristotle wrong
was to prove everybody wrong, the authorities of the Church included.
Men calmly declined to believe their own eyes, and Galileo was in very
evil odor.
From falling bodies he went on to subjects of yet wider interest to
people in general. He read about and eagerly embraced the Copernican
system. Not only did he accept it himself, but he vigorously set
to work to teach and convince others also. This, as might only be
expected, aroused vehement opposition. But Galileo was still in the
heyday of his powers, and he was not to be easily silenced. In those
days he could afford to press onward in the teeth of resistance, and to
laugh at men who would not listen.
When at Venice, a report reached him of a certain optician in Holland
who had happened to hit upon a curious “combination of lenses,” through
which objects afar off could be actually seen as if they were near.
Galileo promptly seized upon the notion, and within twenty-four hours
he had rigged up a small, rough telescope out of an old organ-pipe and
two spectacle-glasses.
Thus the first telescope was made--a very elementary affair, formed of
two lenses, convex and concave, and capable of magnifying some three
times--a mere child’s toy compared with telescopes of the present day,
but of exceeding value and interest, because it was the very first;
because in all the history of the world no telescope had ever been
manufactured before.
This earliest effort was speedily improved upon. The next trial
produced a tube which could magnify seven or eight times; and in a
little while, Galileo had a telescope magnifying as much as thirty-two
times. Even thirty-two times does not sound very startling to us; but
in those days the revelations of such an instrument came upon men like
a thunderclap.
Through it could be seen clearly the broken nature of the moon’s
surface--the craters, the shadows, the mountains. Through it could be
seen the phases of Venus, which no man had ever yet detected, but which
Copernicus had declared must certainly exist, if indeed his system
were a reality. Through it could be seen four of the moons of Jupiter,
and their ceaseless journeyings. Through it could be seen spots upon
the face of the sun, the movements of which showed the rotation of the
sun upon his axis. Through it could be seen the very curious shape of
Saturn, caused by his rings, though a much stronger power was needed to
separate the rings from the body of the planet.
Of all these discoveries, and many others also made by Galileo, none
seemed worse to the bigoted schoolmen of his day than the preposterous
notion of black spots upon the sun’s face.
Rather curiously, no human being seems ever before then to have noticed
such spots, though often they are quite large enough to be detected by
the naked eye. A general belief had been held that the sun was and must
be perfect--an absolutely spotless and unblemished being, the purest
symbol of celestial incorruptibility--and the said discovery went right
in the teeth of deductions drawn from, and lessons founded upon, this
belief. Therefore the schoolmen held out and determinedly resisted, and
would have nought to do with Galileo or his telescopes, shutting their
eyes to marvels newly revealed.
But Kepler was a true scientist, a man who earnestly pursued truth,
and sought to discover it at any hazard. He and Galileo were
contemporaries, and they held communication on these subjects. Some
of Galileo’s discoveries went quite as much in the teeth of some of
Kepler’s most dearly-loved theories as they did in the teeth of the
schoolmen’s most dearly-loved doctrines. Yet, none the less, Kepler
welcomed them with great delight and eagerness, showing thereby his
true greatness of character.
Till after the age of fifty, Galileo was allowed to go on undisturbed,
or at least not materially disturbed, by opposition in his career of
success--observing, learning, theorizing, calculating, explaining,
teaching--a prominent figure indeed. Not only did he take a large share
in establishing firmly the Copernican system of astronomy, but some of
the principal laws of motion were discovered by him. He first found
the uses of a pendulum, and there is strong reason for believing that
the first microscope, as well as the first telescope, was from his
hands.
But after the age of fifty, reverses came, and one blow succeeded
another. Rome had been gradually waking up to the true sense of all
that was implied by his discoveries and his teaching, and at length she
let loose her thunders--impotent thunders enough, so far as regarded
any permanent checking of the progress of truth, but by no means
impotent to crush one defenseless victim, the champion of scientific
truth in that country and age.
Years of greater or less opposition, amounting often to persecution,
were followed by an imperious order commanding him to Rome. There he
was examined and severely “questioned,” and recantation was forced from
him, worth as much as such forced recantations commonly are worth.
Even then he did not cease from the work that he loved; even in prison
he carried it on; even when he might not write, nothing could stop him
from thinking. But his health was broken; his liberty was taken away;
illness followed illness; his favorite daughter died; his eyesight
gradually failed; the last book that he wrote might not for a long
while be printed; and at last he passed away, mercifully set free from
those who, in an essentially bigoted and intolerant age, were incapable
of appreciating his greatness.
They would fain _then_ have denied him even a respectable grave. Now,
all the civilized world unites to honor the name of Galileo.
CHAPTER XXXIII.
ISAAC NEWTON.
Even the great fame of Galileo must be relegated to a second place in
comparison with that of the philosopher who first saw the light in a
Lincolnshire farmhouse on the 25th of December, 1642, the very same
year of Galileo’s death. His father, Isaac Newton, had died before his
birth. The little Isaac was at first so frail and weakly that his life
was despaired of. The watchful mother, however, tended her delicate
child with such success that he ultimately acquired a frame strong
enough to outlast the ordinary span of human life.
In due time the boy was sent to the public school at Grantham. That
he might be near his work, he was boarded at the house of Mr. Clark,
an apothecary. He had at first a very low place in the class. His
first incentive to diligent study seems to have been derived from the
circumstance that he was severely kicked by one of the boys above
him in the class. This indignity had the effect of stimulating young
Newton’s activity to such an extent that he not only attained the
desired object of passing over the head of the boy who had maltreated
him, but continued to rise until he became the head of the school.
The play-hours of the great philosopher were devoted to pursuits very
different from those of most schoolboys. His chief amusement was
found in making mechanical toys, and various ingenious contrivances.
He watched, day by day, with great interest, the workmen engaged in
constructing a windmill in the neighborhood of the school, the result,
of which was that the boy made a working model of the windmill and of
its machinery, which seems to have been much admired as indicating his
aptitude for mechanics. We are told that he also indulged in somewhat
higher flights of mechanical enterprise. The first philosophical
instrument he made was a clock, which was actuated by water. He also
devoted much attention to the construction of paper kites, and his
skill in this respect was highly appreciated by his schoolfellows. Like
a true philosopher, even at this stage, he experimented on the best
methods of attaching the string, and on the proportions which the tail
ought to have.
When the schoolboy at Grantham was fourteen years of age, it was
thought necessary to recall him from the school. His recently-born
industry had been such that he had already made good progress in his
studies, and his mother hoped that he would now lay aside his books,
and those silent meditations to which, even at this early age, he had
become addicted. It was hoped that instead of such pursuits, which were
deemed quite useless, the boy would enter busily into the duties of
the farm and the details of a country life. But before long it became
manifest that the study of nature and the pursuit of knowledge had such
a fascination for the youth that he could give little attention to
aught else. It was plain that he would make but an indifferent farmer.
He greatly preferred experimenting on water-wheels, and working at
mathematics in the shadow of a hedge.
Fortunately for humanity, his mother, like a wise woman, determined
to let her boy’s genius have the scope which it required. He was
accordingly sent back to Grantham school, with the object of being
trained in the knowledge which would fit him for entering the
University of Cambridge.
It was the 5th of June, 1660, when Isaac Newton, a youth of eighteen,
was enrolled as an undergraduate of Trinity College, Cambridge. Little
did those who sent him there dream that this boy was destined to be the
most illustrious student who ever entered the portals of that great
seat of learning. Little could the youth himself have foreseen that the
rooms near the gateway which he occupied would acquire a celebrity from
the fact that he dwelt in them, or that the antechapel of his college
was in good time to be adorned by that noble statue of himself which
is regarded as the chief art treasure of Cambridge University, both
on account of its intrinsic beauty and the fact that it commemorates
the fame of her most distinguished alumnus, Isaac Newton, the immortal
astronomer. His advent at the university seemed to have been by
no means auspicious or brilliant. His birth was, as we have seen,
comparatively obscure, and though he had already given indication of
his capacity for reflecting on philosophical matters, yet he seems to
have been but ill-equipped with the routine knowledge which youths are
generally expected to take with them to the universities.
From the outset of his college career, Newton’s attention seems to have
been mainly directed to mathematics. Here he began to give evidence of
that marvelous insight into the deep secrets of nature which, more than
a century later, led so dispassionate a judge as Laplace to pronounce
Newton’s immortal work as pre-eminent above all the productions of
the human intellect. But though Newton was one of the very greatest
mathematicians that ever lived, he was never a mathematician for the
mere sake of mathematics. He employed his mathematics as an instrument
for discovering the laws of nature.
His industry and genius soon brought him under the notice of the
university authorities. It is stated in the university records that
he obtained a scholarship in 1664. Two years later we find that
Newton, as well as many residents in the university, had to leave
Cambridge temporarily on account of the breaking out of the plague. The
philosopher retired for a season to his old home at Woolsthorpe, and
there he remained until he was appointed a Fellow of Trinity College,
Cambridge, in 1667. From this time onwards, Newton’s reputation as a
mathematician and as a natural philosopher steadily advanced, so that
in 1669, while still but twenty-seven years of age, he was appointed to
the distinguished position of professor of Mathematics at Cambridge.
Here he found the opportunity to continue and develop that marvelous
career of discovery which formed his life’s work.
The earliest of Newton’s great achievements in natural philosophy was
his detection of the composite character of light. That a beam of
ordinary sunlight is, in fact, a mixture of a very great number of
different colored lights, is a doctrine now familiar to every one who
has the slightest education in physical science. We must, however,
remember that this discovery was really a tremendous advance in
knowledge at the time when Newton announced it.
[Illustration: SIR ISAAC NEWTON.]
A certain story is often told about Newton. It is said that as he
sat in a garden an apple fell from a tree, and that its fall set him
thinking, and that, in consequence of this train of thought, he found
out all about the law of attraction or gravitation.
But the fall of an apple could not possibly have set Newton thinking,
because he had already been thinking long and hard upon these
difficult questions. Indeed, it was _because_ he had been so thinking,
_because_ his mind was in a watchful and receptive condition, that so
slight a matter as a falling apple should, perhaps, have suggested to
him a useful train of ideas.
Newton was a man who really did think, and who really did think
intensely. He was not, like the ordinary run of people, one who merely
had a good many notions walking loosely through his brain. He would
bend his mind to a subject and be absolutely absorbed in it; later in
life so absorbed that he would forget to finish his dressing, and would
be unable to say whether or no he had dined. This sort of forgetfulness
does sometimes spring from vacancy of mind; but with Newton it arose
from fullness of thought.
An apple falls, of course, as a stone or any other body falls, because
the earth attracts it. Otherwise, it might as easily rise and float
away into the sky. But this was known perfectly well long before
Newton’s days. It was clearly understood that the earth had a curious
power of drawing all things towards herself. People could not explain
why or how it was so; neither can we explain now; but they were fully
aware of the fact.
So men knew something of the force called gravity; but they knew it
mainly, almost exclusively, as having to do with our earth, and with
the things upon our earth. They had not begun definitely to think of
gravity as having to do with bodies in the heavens; as being a chief
controlling force in the whole Solar System; much less as reigning
throughout the vast universe of stars. It had not come into their
minds with any distinctness that, just as the earth pulls towards her
own center a mountain, a horse, a man, so the sun pulls toward his
center the earth, the moon, and all the planets.
Certain glimmerings of the notion had, indeed, begun to dawn on the
horizon of learning. Horrocks, Borelli, Robert Hook, Wren, Halley, and
others had _glimpses_ of universal gravitation; but only glimpses. A
gifted English philosopher, late in the sixteenth century, Gilbert
by name, went so far as to speak of earth and moon acting one upon
the other “like two magnets.” He also saw the effect of the moon’s
attraction in bringing about ocean tides, though he oddly explained
that the said attraction was not so much exerted upon ocean waters as
upon “subterranean spirits and humors”--whatever he may have meant by
such an expression.
Kepler, not long after Gilbert, made further advance. He gained some
definite notions as to the nature of gravity; and he saw distinctly
that the moon’s attraction was the main cause of the tides. The
following remarkable statement is found in his writings: “If the earth
ceased to attract its waters, the whole sea would mount up and unite
itself with the moon. The sphere of the attracting force of the moon
extends even to the earth, and draws the waters towards the torrid
zone, so that they rise to the point which has the moon in the zenith.”
Here is a distinct enough grasp of the fact that gravity can and does
act through distance, between worlds separated by thousands of miles.
Yet, though both Gilbert and Kepler saw so much, they failed to go
farther. Neither of the two had any clear knowledge of the modes in
which gravitation acts, or of the laws which govern its working.
Gilbert and Kepler alike, while dimly recognizing, not only the moon’s
attraction for earth’s oceans, but the mutual attraction of earth and
moon each for the other, were utterly perplexed as to what force could
possibly hold the two bodies asunder. Gilbert could seriously talk of
“subterranean spirits and humors.” Kepler could soberly write of an
“animal force” or some tantamount power in the moon, which prevented
her nearer approach to earth.
Galileo was equally vague on the subject. With all his brilliant gifts,
and while acknowledging the earth’s attractive power over the moon, he
would not even admit the existence of _mutual_ attraction, and talked
of the moon as being compelled to “follow the earth.” Unquestionably
there are times when the moon does follow the earth, and that is when
the earth happens to go before. But quite as often it happens that the
moon goes before, and the earth follows after.
All these great men were groping near the truth, yet none managed to
find the clue that was wanted. They left the unfinished process of
thought to be carried on by the master-mind of Newton. And Newton set
himself to think the question out. He used Kepler’s observations.
He sought to penetrate the secrets of a Solar System in mysterious
and inviolable order. Some reason or cause for that order he knew
must exist--if only it might be grasped! He pondered deeply; lost in
profound cogitation, through stormy days of civil strife, of plague and
loss and suffering. The object of his life was to discover, if so he
might, what power or what forces retained the different worlds in their
respective pathways; in short, what manner of celestial guidance was
vouchsafed to the planets.
For although men had now learned the shape of the planet-orbits, and
even knew certain laws which helped to govern the planet-motions, they
had not discovered one supreme controlling law. They did not dream of
gravity in the heavens as a prevailing force. There was absolutely no
reason, with which men were acquainted, why each planet should preserve
its own particular distance from the sun; why Mars should not wander
as far away as Jupiter; why Mercury should not flee to the position of
Saturn; why our own earth should be always about as near as she always
is.
The resources of Newton’s genius seemed equal to almost any demand
that could be made upon it. He saw that each planet must disturb the
other, and in that way he was able to render a satisfactory account of
certain phenomena which had perplexed all preceding investigators. That
mysterious movement by which the pole of the earth sways about among
the stars, had been long an unsolved enigma; but Newton showed that the
moon grasped with its attraction the protuberant mass at the equatorial
regions of the earth, and thus tilted the earth’s axis in a way that
accounted for the phenomenon which had been known, but had never been
explained, for two thousand years. All these discoveries were brought
together in that immortal work, Newton’s “Principia,” the greatest work
on science that the world had yet seen.
It may well be that, as Newton pondered this perplexing question
while seated in the garden, an apple dropping to the ground might
have sufficed to turn his wide-awake and ready mind in the direction
of that familiar force, gravity, which draws all loose bodies towards
earth’s surface. The apple fell because of gravity. Quite naturally the
question might have arisen in Newton’s mind--“Why not this same gravity
in the heavens also? If the earth draws an apple towards herself, why
should not the earth draw the moon? Why should not the sun draw the
earth? Nay, more, why should not the sun draw all the planets?” Newton
set himself to work out this problem, allowing for the distance of the
moon from earth, and for the bulk and weight of both earth and moon.
And because the answer was _not_ precisely what he had looked for,
he put the calculation aside, and waited for years. It seems that he
mentioned his conjecture to no living person. After many years he took
it up again, and worked it out afresh. By this time new measurements of
earth’s surface, and new calculations of earth’s size had been made,
and Newton had fresh figures to go upon. This time the answer came
just as he had originally hoped, and the truth of his surmise was thus
proved.
Gravity held the moon near the earth, preventing her from wandering
off into unknown distances. Gravity held earth and moon, Mars, Venus,
Jupiter, each planet, in its own pathway, within a certain distance of
the sun. Gravity held many of the comets as permanent members of the
Solar System. So strong, indeed, was the drawing of gravity found to
be, that only the resisting force of each bright world’s rapid rush
round the sun could keep the sun and his planets apart.
Newton had discovered the long-sought secret. He had found that “every
particle of matter in the universe attracts every other particle?”
He had found also the main laws which govern the working of that
attraction--that it depends, first, upon the mass or amount of “matter”
in each body; that it depends, secondly, upon the distance of one body
from another. He found, too, how far attraction increases with greater
nearness, and decreases with greater distance. All these and countless
other questions he worked out in the course of years. But first he had
grasped the great leading principle, after which men had until then
groped in vain, that by the force of gravity the sun and his worlds
and their moons are all bound together into one large united family or
system.
Though Newton lived long enough to receive the honor that his
astonishing discoveries so justly merited, and though for many years
of his life his renown was much greater than that of any of his
contemporaries, yet it is not too much to say that, in the years which
have since elapsed, Newton’s fame has been ever steadily advancing.
We hardly know whether to admire more the sublime discoveries at
which he arrived, or the extraordinary character of the intellectual
processes by which those discoveries were reached. He died on Monday,
March 20, 1727, in the eighty-fifth year of his age.
CHAPTER XXXV.
LATER ASTRONOMY.
Sir William Herschel was born in the city of Hanover, November 15,
1738. He was the second son of Isaac Herschel, a musician, who brought
him up, with his four other sons, to his own profession.
A faithful chronicler has given us an interesting account of the way in
which Isaac Herschel educated his boys; the narrative is taken from the
recollections of one who, at the time, was a little girl of five or six
years old. She writes:
“My brothers were often introduced as solo performers and assistants
in the orchestra at the court, and I remember that I was frequently
prevented from going to sleep by the lively criticisms on music
on coming from a concert. Often I would keep myself awake that I
might listen to their animated remarks, for it made me happy to see
them so happy. But generally their conversation would branch out on
philosophical subjects, when my brother William and my father often
argued with such warmth that my mother’s interference became necessary
when the names Euler, Leibnitz, and Newton sounded rather too loud for
the repose of her little ones, who had to be at school by seven in the
morning.”
The child, whose reminiscences are here given, became afterwards
the famous Caroline Herschel. The narrative of her life is a most
interesting book, not only for the account it contains of the
remarkable woman herself, but also because it presents the best picture
we have of the great astronomer, to whom Caroline devoted her life.
This modest family circle was, in a measure, dispersed at the outbreak
of the Seven Years’ War in 1756. The French proceeded to invade
Hanover, which, it will be remembered, belonged at this time to
the British dominions. Young William Herschel had already obtained
the position of a regular performer in the regimental band of the
Hanoverian Guards, and it was his fortune to obtain some experience
of actual warfare in the disastrous battle of Hastenbeck. He was not
wounded, but he had to spend the night after the battle in a ditch, and
his meditations on the occasion convinced him that soldiering was not
the profession exactly suited to his tastes. He left his regiment by
the very simple, but somewhat risky, process of desertion. He had, it
would seem, to adopt disguises in order to effect his escape. By some
means he succeeded in eluding detection, and reached England in safety.
The young musician must have had some difficulty in providing for his
maintenance during the first few years of his abode in England.
It was not until he had reached the age of twenty-two that he succeeded
in obtaining any regular appointment. He was then made instructor of
music to the Durham Militia. Shortly afterwards, his talents being more
widely recognized, he was appointed as organist of the parish Church at
Halifax.
In 1766 we find that Herschel had received the further promotion of
organist in the Octagon Chapel, at Bath. Bath was then, as now, a
highly fashionable resort, and many notable personages patronized the
rising musician. Herschel had other points in his favor besides his
professional skill; his appearance was striking, his address superb,
and his conversation animated, interesting, and instructive; and
even his nationality was a distinct advantage, inasmuch as he was a
Hanoverian in the reign of King George the Third. From his earliest
youth, Herschel had been endowed with that invaluable characteristic,
an intense desire for knowledge. He naturally wished to perfect himself
in the theory of music, and thus he was led to study mathematics. When
he had once tasted the charms of mathematics, he saw vast regions of
knowledge unfolded before him, and in this way he was induced to direct
his attention to astronomy. More and more this pursuit engrossed his
attention until, at last, it had become an absorbing passion.
It was with quite a small telescope, which had been lent him by a
friend, that Herschel commenced his career as an observer. However, he
speedily discovered that to see all he wanted to see, a telescope of
far greater power would be necessary.
He commissioned a friend to procure for him in London a telescope with
high magnifying power. Fortunately for science the price was so great
that it precluded the purchase, and he set himself at work to construct
one. After many trials he succeeded in making a reflecting instrument
of five feet focal length, with which he was able to observe the rings
of Saturn and the satellites of Jupiter.
It was in 1774, when the astronomer was thirty-six years old, that he
obtained his first glimpse of the stars with an instrument of his own
construction. Night after night, as soon as his musical labors were
ended, his telescopes were brought out.
His sister Caroline, who occupies such a distinct place in scientific
history, the same little girl to whom we have already referred, was
his able assistant, and when mathematical work had to be done, she
was ready for it. She had taught herself sufficiently to enable her
to perform the kind of calculations--not, perhaps, very difficult
ones--that Herschel’s work required; indeed, it is not much to say that
the mighty life-work which this man was enabled to perform could never
have been accomplished had it not been for the self-sacrifice of this
ever-loving and faithful sister.
It was not until 1782 that the great achievement took place by which
Herschel at once sprang into fame. He appears to have formed a project
for making a close examination of all the stars above a certain
magnitude for the purpose of discovering, if possible, a parallax among
them. Star after star was brought to the center of the field of view of
his telescope, and after being carefully examined, was then displaced,
while another star was brought forward to be submitted to the same
process.
In the great review which Herschel undertook, he doubtless examined
hundreds, or perhaps thousands of stars, allowing them to pass away
without note or comment. But on an ever-memorable night in March,
1782, it happened that he was pursuing his task among the stars in the
constellation of Gemini. One star was noticed which, to Herschel’s
acute vision, seemed different from the stars which in so many
thousands are strewn over the sky. There was something in the starlike
object that immediately arrested his attention, and made him apply to
it a higher magnifying power. This at once disclosed the fact that the
object possessed a disk--that is, a definite, measurable size--and
that it was thus totally different from any of the hundreds and
thousands of stars which exist elsewhere in space. The organist at the
Octagon Chapel at Bath had discovered a new planet with his home-made
telescope. Great was the astonishment of the scientific world when the
Bath organist announced that the five planets, which had been known
from all antiquity, must now admit the company of a sixth.
The now great astronomer was invited to Windsor, and to bring his
famous telescope in order to exhibit the planet to George III, and
to tell his majesty all about it. The king took so great a fancy to
Herschel that he proposed to confer on him the title of “his majesty’s
own astronomer,” to assign him a residence near Windsor, to provide him
with a salary, and to furnish such funds as might be required for the
erection of great telescopes, and for the conduct of that mighty scheme
of celestial observation on which Herschel was so eager to enter.
No single discovery of Herschel’s later years was, however, of the same
momentous description as that which first brought him to fame. There
is no separate collection of his writings, and very scanty accounts of
his life have been published; but he who has written his name among the
stars needs no other testimonial to his fame. Prior to his time the
number of bodies known as belonging to the Solar System was eighteen,
including secondary planets and Halley’s comet. To these he added nine;
namely, Uranus and six satellites, and two satellites of Saturn.
Though no additional honors could add to his fame, Dr. Herschel, in
1816, received the decoration of the Guelphic Order of Knighthood.
In 1820 he was elected president of the Astronomical Society, and
among their Transactions, the next year, he published an interesting
memoir on the places of one hundred and forty-five double-stars. This
paper was the last which he lived to publish. His health had begun to
decline, and on the 24th of August, 1822, he sank under the infirmities
of age, having completed his eighty-fourth year. He was survived by his
widow, Lady Herschel, an only son, Sir John F. W. Herschel, and his
sister Caroline, who died in 1847, in her ninety-eighth year.
LAPLACE.
The author of “Celestial Mechanics” was born at Beaumont-en-Auge in
1749, just thirteen years later than his renowned friend Lagrange.
His father was a farmer, but appears to have been in a position to
provide a good education for a son who seemed promising. The subject
which first claimed his attention was theology. He was, however, soon
introduced to the study of mathematics, in which he presently became so
proficient that, while he was still no more than eighteen years old, he
obtained employment as a mathematical teacher in his native town.
Desiring wider opportunities for study, young Laplace started for
Paris, being provided with letters of introduction to D’Alembert,
who then occupied the most prominent positions as a mathematician in
France, if not in Europe. On presenting these letters, he seems to have
had no reply; whereupon, Laplace wrote to D’Alembert, submitting a
discussion of some point in dynamics.
This letter instantly produced the desired effect. D’Alembert thought
that such mathematical talent as the young man displayed was in itself
the best of introductions to his favor. It could not be overlooked, and
accordingly he invited Laplace to come and see him. Laplace, of course,
presented himself, and erelong D’Alembert obtained for the rising
philosopher a professorship of Mathematics in the military school in
Paris. This gave the brilliant young mathematician the opening for
which he sought, and he quickly availed himself of it.
Laplace’s most famous work is “Celestial Mechanics,” in which he
essayed a comprehensive attempt to carry out, in much greater detail,
the principles which Newton had laid down. The fact was that Newton
had not only to construct the theory of gravitation, but he had to
invent the mathematical processes by which his theory could be applied
to the explanation of the movements of the heavenly bodies. In the
course of the century which had elapsed between the time of Newton and
the time of Laplace, mathematics had been extensively developed. The
disturbances which one planet exercises upon the rest can only be fully
ascertained by the aid of long calculations, and for these calculations
analytical methods are required.
With an armament of mathematical methods which had been perfected
since the days of Newton by the labors of two or three generations of
mathematical inventors, Laplace essayed in his “Celestial Mechanics”
to unravel the mysteries of the heavens. It will hardly be disputed
that the book which he has produced is one of the most difficult
books to understand that has ever been written. The investigations of
Laplace are, generally speaking, of too technical a character to make
it possible to set forth any account of them in such a work as the
present. He did, however, publish one treatise, called the “System of
the Universe,” in which, without introducing mathematical symbols, he
was able to give a general account of the theories of the celestial
movements, and of the discoveries to which he and others had been led.
In this work Laplace laid down the principles of the nebular theory,
which, in modern days, has been generally accepted.
The nebular theory gives a physical account of the origin of the
Solar System, and has already been explained in this volume. Bach
advance in science seems to make it more certain that this hypothesis
substantially represents the way in which our Solar System has grown to
its present form.
Not satisfied with a career which was merely scientific, Laplace sought
to connect himself with public affairs. Napoleon appreciated his
genius, and desired to enlist him in the service of the state. Laplace
was appointed to the office of minister of the interior. The experiment
was not successful, for he was not by nature a statesman. In despair
of Laplace’s capacity as an administrator, Napoleon declared that he
carried the spirit of his infinitesimal calculus into the management
of business. Indeed, Laplace’s political conduct hardly admits of much
defense. While he accepted the honors which Napoleon showered on him in
the time of his prosperity, he seems to have forgotten all this when
Napoleon could no longer render him service. Laplace was made a marquis
by Louis XVIII. During the latter part of his life the philosopher
lived in a retired country-place at Arcueile. Here he pursued his
studies, and, by strict abstemiousness, preserved himself from many of
the infirmities of old age.
He was endowed with remarkable scientific sagacity; but above all
his powers his wonderful memory shone pre-eminent. His “Celestial
Mechanics” is, next to Newton’s “Principia,” the greatest of
astronomical works.
He died on March 5, 1827, in his seventy-eighth year, his last words
being, “What we know is but little, what we do not know is immense.”
LEVERRIER.
We are apt to identify the idea of an astronomer with that of a man
who looks through a telescope at the stars; but the word astronomer
has really a much wider significance. No man who ever lived has been
more entitled to be designated an astronomer than Urbain Jean Joseph
Leverrier, and yet it is certain that he never made a telescopic
discovery of any kind. In mathematics, however, he excelled, and
he simply used the observations of others as the basis of his
calculations.
These observations form, as it were, the raw material on which the
mathematician exercises his skill. It is for him to elicit from the
observed places the true laws which govern the movements of the
heavenly bodies. Here is indeed a task in which the highest powers of
the human intellect may be worthily employed. To Leverrier it has been
given to provide a superb illustration of the success with which the
mind of man can penetrate the deep things of nature.
The illustrious Frenchman was born on the 11th of March, 1811, at
Saint-Lô, in the department of Manche. In the famous polytechnic
school for education in the higher branches of science he acquired
considerable fame as a mathematician. His labors at school had revealed
to Leverrier that he was endowed with the powers requisite for dealing
with the subtlest instruments of mathematical analysis. When he was
twenty-eight years old his first great astronomical investigation was
brought forth. This was the profound calculation of the disturbances of
the planet Uranus and the causes of them.
The talent which his researches displayed brought Leverrier into
notice. At that time the Paris Observatory was presided over by
Arago, a savant who occupies a distinguished position in French
scientific annals. Arago at once perceived that Leverrier possessed the
qualifications suitable for undertaking a problem of great importance
and difficulty that had begun to force itself on the attention of
astronomers. What this great problem was, and how astonishing was the
solution it received, must now be considered.
Ever since Herschel brought himself into fame by the discovery of
Uranus, the movements of this new addition to the Solar System had been
scrutinized with care and attention. The position of Uranus was thus
accurately determined from time to time. When due allowance was made
for whatever influence the attraction of Jupiter and Saturn and all
the other planets could possibly produce, the movements of Uranus were
still inexplicable. It was perfectly obvious that there must be some
other influence at work besides that which could be attributed to the
planets already known.
Astronomers could only recognize one solution of such a difficulty.
It was impossible to doubt that there must be some other planet in
addition to the bodies at that time known, and that the perturbations
of Uranus, hitherto unaccounted for, were due to the disturbances
caused by the action of this unknown planet. Arago urged Leverrier
to undertake the great problem of searching for this body. But the
conditions of the search were such that it must be conducted on
principles wholly different from any search which had ever before been
undertaken for a celestial object. For this was not a case in which
mere survey with a telescope might be expected to lead to the discovery.
There are in the heavens many millions of stars, and the problem of
identifying the planet, if indeed it should lie among these stars,
seemed a very complex matter. Of course, it is the abundant presence of
the stars which causes the difficulty.
The materials available to the mathematician for the solution of
this problem were to be derived solely from the discrepancies between
the calculated places in which Uranus should be found, taking into
account the known causes of disturbances, and the actual places in
which observation had shown the planet to exist. Here was, indeed, an
unprecedented problem, and one of extraordinary difficulty. Leverrier,
however, faced it, and, to the astonishment of the world, succeeded in
carrying it through to a brilliant solution.
After many trials, Leverrier ascertained that, by assuming a certain
size, shape, and position for the unknown planet’s orbit, and a certain
value for the mass of the hypothetical body, it would be possible
to account for the observed disturbances of Uranus. Gradually it
became clear to his perception, not only that the difficulties in
the movements of Uranus could be thus explained, but that no other
explanation need be sought for. And now for an episode in this history
which will be celebrated so long as science shall endure. It is nothing
less than the telescopic confirmation of the existence of this new
planet, which had previously been indicated only by mathematical
calculation.
Great indeed was the admiration of the scientific world at this superb
triumph. Here was a mighty planet, whose very existence was revealed by
the indications afforded by refined mathematical calculation. At once
the name of Leverrier, already known to those conversant with the more
profound branches of astronomy, became everywhere celebrated.
When, in 1854, Arago’s place had to be filled at the head of the great
Paris Observatory, it was universally felt that the discoverer of
Neptune was the suitable man to assume the office. Leverrier died on
Sunday, September 23, 1877, in his sixty-seventh year.
* * * * *
FREDERICK WILLIAM BESSEL was born at Minden in 1784, and early turned
his attention to mathematical subjects. He devoted himself with
ardor to astronomy, and in 1804 he undertook the reduction of the
observations made on the comet of 1607. His results were communicated
to Olbers, who warmly praised the young astronomer, and in 1806
recommended him to Schroeter as an assistant in the observatory of
Lilienthal. In 1810 he was appointed director of the new observatory
then being founded by the king at Königsberg. He was admirably fitted
for this post, and is distinguished mainly for his discovery of the
parallax of the star 61 Cygni, which he accomplished by methods of
extreme ingenuity and delicacy.
Two kinds of telescopes are commonly used, the refractor and the
reflector. The first telescopes made were all refractors, and the very
first of them was made, as you know, by Galileo. The first reflector
was made in later days by Sir Isaac Newton.
In a refractor the rays of light, from a star or any other bright
body, reach first a large object-glass, through which they pass, and
by which, as they pass, they are caused to converge, narrowing to a
focus or point. From this point they widen slightly on their way to the
eye-piece. After passing through the eye-piece, by which they are once
more straightened into parallel rays, they arrive at the eye.
Speaking broadly, the eye receives--actually sees--nearly as much of
the starlight as if its pupil were the full size of the object-glass
of that telescope, and the power of such an instrument depends upon
the size of the object-glass. A refractor has been made with an
object-glass forty inches across, and this, for the observer, means
looking at a star with an eye pupil more than three feet in diameter.
This refractor is now in the Yerkes Observatory at Williams Bay, Lake
Geneva, Wisconsin.
There are several forms of reflectors. With one form, when it is
pointed at a star, the rays of starlight fall direct through the
telescope-tube upon a highly-polished mirror, so shaped that rays of
light reflected thence are caused to fall converging upon a small
plane-mirror in the center of the tube, from which they are thrown,
parallel to the side of the tube, to the eye-piece, which converges
them to the eye.
The mirror of a reflector can be made very much larger than the
object-glass of a refractor. The famous Lord Rosse telescope, which
has a tube sixty feet long, contains a mirror no less than six feet in
diameter. This, to an observer, is tantamount to looking at the stars
with an eye so huge that its pupil alone would be nearly equal in size
to the six-foot wheel of a steam-engine. It will readily be perceived
how much more starlight can be grasped by such a fishing-net than by
the tiny pupil of your eye or mine.
A main difference between the two kinds of telescope thus resides in
the fact that the star-rays--or any other kind of light-rays--when
first captured, pass, in one case, _through_ the glass on which they
fall, and, in the other case, are thrown off _from_ the mirror. In both
cases, the whole amount of light so captured is gathered into a small
compass, so as to be available for human sight.
[Illustration: EYE-PIECE OF THE LICK TELESCOPE.]
Once more, let me remind you, it should be always kept clearly in
mind that every object that is seen by us--from a mote of dust to a
sun, from a coal-scuttle to a star--is perceived purely and solely by
the light which it gives out, either intrinsic or reflected light.
Countless myriads of rays pass from the surface of the thing seen to
our eyes, picturing there, on the sensitive retina, a fleeting vision
of its form.
All the leading Governments in Europe have observatories for the study
of the heavens, which are furnished with the best instruments of modern
construction. The principal observatories are those at Paris, Berlin,
Vienna, Nice; Dorpat, the seat of a celebrated university founded by
Gustavus Adolphus, of Sweden, in 1630; Pulkowa, near St. Petersburg;
Lisbon, and at Greenwich in England. In America, though only in recent
years has astronomy been cultivated with ardor, there are observatories
of more or less importance,--the National Observatory at Washington,
D. C.; the Cincinnati Observatory, projected by the late General O. M.
Mitchel; the Dudley Observatory, founded by Mrs. Blandina Dudley in
honor of her husband, at Albany, N. Y.; the Lick Observatory, founded
by James Lick, on Mount Hamilton, Cal.; and the Yerkes Observatory,
connected with the Chicago University, founded by Charles T. Yerkes.
Besides these, there are observatories connected with some of our other
leading universities and colleges, as those of Harvard, at Cambridge,
Mass.; Yale University; Williams College; West Point Military Academy;
Amherst College; Princeton; Dartmouth; Michigan University; Hamilton
College, N. Y., and several others.
[Illustration: LICK OBSERVATORY IN WINTER, FROM THE EAST.]
The largest and best refracting telescopes in the world are those
at Lick Observatory and at Yerkes Observatory, now in charge
of Professor E. E. Barnard, who, while at the head of the Lick
Observatory, discovered the fifth satellite of Jupiter, and determined
the character of the inner ring of Saturn. The best reflecting
telescope is that of Lord Rosse, at Birr Castle, in Kings County,
Ireland.
[Illustration: E. E. BARNARD.]
In reviewing the path which we have traversed in this volume, though we
have penetrated the sidereal depths and wandered at will through space,
we feel that we have not yet reached the outskirts of creation. We
have never left the center. Beyond us still lies the circumference. We
see opened before us the infinite, of which the study is not yet begun!
We have seen nothing; we recoil in terror; we fall back astounded.
Indeed, we might fall into the yawning abyss--fall forever, during a
whole eternity; never, never should we reach the bottom, any more than
we have attained the summit. There is no east nor west, neither right
nor left. There is no up nor down; there is neither a zenith nor a
nadir. In whatever way we look, it is infinite in all directions.
In this infinitude of space, the associations of suns and of worlds
which constitute our visible universe form but an island, a vast
archipelago; and in the eternity of duration, the life of our proud
humanity, with all its concerns, its aspirations, and its achievements,
the whole life of our entire planet, is but the shadow of a dream! Well
might the Hebrew poet, as he looked forth upon nature, exclaim in his
amazement: “When I consider thy heavens, the work of thy fingers, the
moon and the stars which thou hast ordained, what is man that thou art
mindful of him, and the son of man that thou visitest him?”
The constellations, the charts of the sky; the catalogues of curious
stars--variable, double or multiple, and colored; the description of
instruments accessible to the observer of the heavens, and useful
tables to consult, are only touched upon in this volume. The reader
whose scientific desires are satisfied by the elements of astronomy,
may stop here. Few have time to pursue the subject further; but it is
sweet to live in the sphere of the mind; it is sweet to contemn the
rough noises of a vulgar world; it is sweet to soar in the ethereal
heights, and to devote the best moments of our life to the study of the
true, the infinite, and the eternal!
PUBLISHERS’ NOTE.
We have now reached the conclusion of “The Story of the Sun, Moon,
and Stars.” Miss Giberne’s work has been supplemented with a large
amount of matter from other sources. The value of the book has thus
been increased without diminishing its popular character. In most
admirable form it tells the story of the heavenly world, which is the
work of God’s hands, the moon and the stars which he has made. The
reverent reader, to whom the Word of God is “sweeter than honey and
the honey-comb,” may here find that “the heavens declare the glory of
God, and the firmament showeth his handiwork.” We confidently present
it to the reader as the best and completest work in print on the great
subject of which it treats.
INDEX
Adams, John Couch, 231, 236
Aërolites, 84
Size of, 86
Aldebaran, Movement of, 277
Alexander at Arbela, 146
Alpha Centauri, Distance of, 99
Duration of light journey, 102
Position, 279
Alps, Lunar Mountains, 179
Altai, Lunar Mountains, 179
Andromeda, Constellation of, 112
Apennines, Lunar Mountains, 179
Arago, 235, 402
Arcturus, Motion of, 114
Position, 276
Aristotle, 376
Asteroids, 53
Astronomy, The dawn of, 363
Attraction, 34
Aurora Borealis, where seen, 141
Axis of earth slanting, 44
Barnard, E. E., 53, 410
Bessel, F. W., 284
Discovery of star parallax, 324
Discovers star parallax, 405
Bolides, 87, 257
Borelli, 388
Capella, Duration of light journey, 103
Velocity of motion, 114
Caucasian Range, Lunar Mountains, 179
Carpathian, Lunar Mountains, 179
Carrington, 141
Cassini, Measurement of sun’s distance, 310
Cassiopeia, Constellation of, 112
Ceres, size of, 53
Chaldean star-gazers, 365
Chinese records of eclipses, 365
Cicero, 177
Clark, Alvan Graham, 289
Cloven-disk theory, 331
Coal sack, 329
Columbus in Jamaica, 146
Comet, Biela’s, 90, 93
Halley’s, 78, 248
Of 1843, 80
Newton’s, 78
With two tails, 77
Encke’s, 78
Heat endured by, 81
Length of tail, 81
Collision with, 82
Split in two, 91
Comets, 73, 240, 246
Number of, 75
Nature of, 74
Captured by Jupiter, 213
Composition of, 256
Composition of tails, 253
Orbits of, 129
With closed orbits, 76
Confucius as an astronomer, 365
Constellations, Grouping, 366
When named, 366
Copernicus, Author of “Celestial Spheres,” 177, 183, 370, 371, 375
Star parallax, 319
Copernican system of the universe, 175
Corona, Description of, 153
D’Alembert, 399
Dante, 121
Demos, Satellite of Mars, 191
Donati’s Comet, 248, 250
Draco, Constellation of, 112
Earth, Globular, 17
How formed, 21
Member of the Solar System, 14
In motion, 16
Motions, 18, 41, 126
One of a family, 14
Size of, 26
What is it? 13
Ecliptic, 43
Eclipses omens of evil, 145
Egyptian astronomers, 365
Encke, 238
Epicycles, 373
Equinox, 44
Fire-balls, Speed of, 88
Fabricius discovers sun-spots, 27
Fraunhofer, Joseph von, 345
Galle, 232, 238
Galileo, 28, 276, 371, 376, 378, 380
Gassendi, 184
George III, 226
Gilbert, 388
Great Bear, Constellation of, 108, 340
Halley, 388
Transit of Venus, 314
Star motions, 318
Hercules, Constellation of, 112
Herschel, Caroline, 226, 393
Herschel, Sir John, 81, 233, 303, 398
Herschel, William, 96, 119, 225, 271, 393
Hipparchus, 368, 371
Hodgson, 141
Holland, Sir Henry, 237
Hook, Robert, 388
Horrocks, 388
Huggins, Dr., 359
Humboldt, Alexander von, 265
Jupiter, 55
Comparative size, 56
Distance of, 122
His satellites, 56, 206, 210
Velocity, 199
Wind storms, 204
Kepler, 75, 296
Laws of, 371, 374, 380, 388
Kew Observatory, 141
Lacaille, Catalogue of, 105
Lalande, Catalogue of, 105
Laplace, 385, 398
Later astronomy, 393
Leverrier, Urbain J. J., 231, 401
Discovers Neptune, 321
Light, Rapidity of, 101
Time, Duration of, 331
Lowe at Santander, 147
Lunar craters, Antiquity of, 172
How formed, 173
Plato and Copernicus, 173
Ptolemy, 174
Schickard, 174
Tycho, 174
Lunar Mountains, How formed, 174
Lyra, Constellation, 297
Magellanic clouds, 309
Magnetism, 140, 141
Mars, 52, 373, 374
Moons of, 191
Continents of, 197
Distance from earth, 197
Maury, Lieutenant, 80
Mercury, Atmosphere, 181
Distance from sun, 49
Length of year, 180
Is it inhabited? 181
Orbit of, 180
Phases, 50
Size of, 49
Transit of, 184
Meteor, 84, 257
Meteorites, 240, 257
Around the sun, 94
August system, 90, 242
In Saturn’s rings, 94
November system, 90, 94, 241
Number of, 86
Protection from, 86
Shower of 1872, 92
Shower of 1885, 93
Milky Way, 270, 329
Mizar, 342
Modern Astronomy, 375
Moon, 62
Appearance at its full, 63
Atmosphere, 157, 169
Phases of, 160
Condition of, 65
Craters, 68, 170
Earth’s satellite, 164
Earth-shine, 160
Eclipse, 163
Influence on tides, 167
Landscape of, 68
Mountains, 67
Orbit of, 166
Temperature on, 72
Revolution, 64
Size of, 64
Surface, 67
Nasmyth, 173
Nebulæ, Number of, 336
Neptune, 59
Discovery of, 232, 234
Distance of, 123
Length of year, 59
Newton analyzes light, 385
Birth and childhood, 382
Nicetos of Syracuse, 177
Nicias, Athenian General, 146
Nicolas, Cardinal, 177
Observatories, 408
Orbits, Planetary, 47
Orion, Constellation of, 112
Parallax, 311
Phobos, Satellite of Mars, 191
Photography, Stellar, 355
Pius IX, 93
Pisa, Leaning Tower of, 378
Plane of the Ecliptic, 201
Planets, Distance from the sun, 123
First group, 48
How formed, 22
Length of years, 49
Orbits, 47
Order of formation, 23
Second group, 48,55
Plutarch, 177
Pole star, Duration of light journey, 103
Position, 108
Rate of motion, 115
Prism, 344
Pritchard, C., 356
Ptolemaic system of the universe, 175, 369
Ptolemy, 371, 369
Pythagoras, Grecian astronomer, 367
Rosse, Lord, Telescope, 406
Rowton siderite, 258
Saturn, 57
Composition of rings, 221
Distance of, 122
Distance from sun, 214
Length of year, 57
Moons, 217
Rings, 219
Satellites, 58
Size, 214
Weight, 214
Shickard, Crater on the moon, 174
Scheiner, 28
Secchi, 92
Shooting stars, 84
Sirius, Brightness of, 275, 285
Changing color, 285
Distance of, 100
Duration of light journey, 103
Movement of, 277
Solar cloud, 148
Solar System, 60, 122
Model of, 60
Spectroscope, 143, 345
Spectroscopic photography, 360
Spectrum analysis, 346, 354
Star clusters, 304, 336
Parallax, 284, 316
Spectroscopy, 356
Stars, Apparent motion, 107
Attraction, 21
Distance of, 99, 310, 320
Distance, 281, 282
Fixed, Why so called, 20
Fixed, Why, 366
Golden, 274
Magnitude of, 97
How many visible, 95
Measurement, 278
Red, 274
Variable, 274, 291
White, 274
Sun, 24
Attraction of, 156
Attraction of gravitation, 34
Chromatosphere, 32
Cyclones of 30, 136
Density, 26
Destination, 119
Distance of, 25
Eclipse of, 143
Flames, Their height, 31, 151
Flames, Their velocity, 138
Forces and influences, 39
Head of our family, 24
Magnetic power of, 139
Its mottled appearance, 142
Rate of motion, 119
Penumbra, 133
Photosphere, 31, 134
Power of, 39, 137
Rapidity of motion, 26
Revolution on axis, 28
Size, 26, 154
Solar prominence, 32
Spots, 27, 30, 133
Symbol of purity, 380
Umbra, 133
Weight, 27, 154
Swift, Voyage to Laputa, 194
Telescopes, 378, 379, 405
Refractors and reflectors, 405, 408, 410
Tides, 168
Thales, founder of Grecian Astronomy, 367
Toucan, 306
Tycho Brahe, 175, 371, 372, 373
Universe, Definition of, 19
Uranus, 58
Discovery of, 225
Distance of, 123
Venus, Orbit of 51
Is it inhabited? 189
Distance from earth, 186
Phases of, 51, 379
Transits of, 185
Vesta, Size of, 53
Voltaire, Prophecy of, 193
Wilson, 30
Wollaston, Dr. W. H., 345
Wren, 388
Yerkes Observatory, 406
Young, Chas. A., 142, 148
Zodiacal Light, 94, 224, 265
Transcriber's Notes:
In the original a link to Sir Isaac Newton was omitted from the
List of Illustrations. The transcriber has inserted one.
Italics are shown thus: _sloping_.
Variations in spelling and hyphenation are retained.
Perceived typographical errors have been changed.
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