The Project Gutenberg EBook of Curiosities of the Sky, by Garrett Serviss
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Title: Curiosities of the Sky
Author: Garrett Serviss
Posting Date: August 25, 2012 [EBook #6630]
Release Date: October, 2004
First Posted: January 6, 2003
Language: English
*** START OF THIS PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY ***
Produced by William McClain
Curiosities of the Sky
by Garrett Serviss
Curiosities of the Sky was first published in 1909 and the text is in
the public domain. The transcription was done by William McClain
([email protected]), 2002.
A printed version of this book is available from Sattre Press
(http://csky.sattre-press.com). It includes extensive annotations, a
new introduction and all the original photographs and diagrams.
_________________________________________________________________
Preface
What Froude says of history is true also of astronomy: it is the most
impressive where it transcends explanation. It is not the mathematics
of astronomy, but the wonder and the mystery that seize upon the
imagination. The calculation of an eclipse owes all its prestige to
the sublimity of its data; the operation, in itself, requires no more
mental effort than the preparation of a railway time-table.
The dominion which astronomy has always held over the minds of men is
akin to that of poetry; when the former becomes merely instructive and
the latter purely didactic, both lose their power over the
imagination. Astronomy is known as the oldest of the sciences, and it
will be the longest-lived because it will always have arcana that have
not been penetrated.
Some of the things described in this book are little known to the
average reader, while others are well known; but all possess the
fascination of whatever is strange, marvelous, obscure, or mysterious
-- magnified, in this case, by the portentous scale of the phenomena.
The idea of the author is to tell about these things in plain
language, but with as much scientific accuracy as plain language will
permit, showing the wonder that is in them without getting away from
the facts. Most of them have hitherto been discussed only in technical
form, and in treatises that the general public seldom sees and never
reads.
Among the topics touched upon are:
* The strange unfixedness of the ``fixed stars,'' the vast
migrations of the suns and worlds constituting the universe.
* The slow passing out of existence of those collocations of stars
which for thousands of years have formed famous
``constellations,'' preserving the memory of mythological heroes
and heroines, and perhaps of otherwise unrecorded history.
* The tendency of stars to assemble in immense clouds, swarms, and
clusters.
* The existence in some of the richest regions of the universe of
absolutely black, starless gaps, deeps, or holes, as if one were
looking out of a window into the murkiest night.
* The marvelous phenomena of new, or temporary, stars, which appear
as suddenly as conflagrations, and often turn into something else
as eccentric as themselves.
* The amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,''
and ``lace,'' or ``tress,'' nebulæ.
* The strange surroundings of the sun, only seen in particular
circumstances, but evidently playing a constant part in the daily
phenomena of the solar system.
* The mystery of the Zodiacal Light and the Gegenschein.
* The extraordinary transformations undergone by comets and their
tails.
* The prodigies of meteorites and masses of stone and metal fallen
from the sky.
* The cataclysms that have wrecked the moon.
* The problem of life and intelligence on the planet Mars.
* The problematical origin and fate of the asteroids.
* The strange phenomena of the auroral lights.
An attempt has been made to develop these topics in an orderly way,
showing their connection, so that the reader may obtain a broad
general view of the chief mysteries and problems of astronomy, and an
idea of the immense field of discovery which still lies, almost
unexplored, before it.
The Windows of Absolute Night
To most minds mystery is more fascinating than science. But when
science itself leads straight up to the borders of mystery and there
comes to a dead stop, saying, ``At present I can no longer see my
way,'' the force of the charm is redoubled. On the other hand, the
illimitable is no less potent in mystery than the invisible, whence
the dramatic effect of Keats' ``stout Cortez'' staring at the
boundless Pacific while all his men look at each other with a wild
surmise, ``silent upon a peak in Darien.'' It is with similar feelings
that the astronomer regards certain places where from the peaks of the
universe his vision seems to range out into endless empty space. He
sees there the shore of his little isthmus, and, beyond, unexplored
immensity.
The name, ``coal-sacks,'' given to these strange voids is hardly
descriptive. Rather they produce upon the mind the effect of blank
windows in a lonely house on a pitch-dark night, which, when looked at
from the brilliant interior, become appalling in their rayless murk.
Infinity seems to acquire a new meaning in the presence of these black
openings in the sky, for as one continues to gaze it loses its purely
metaphysical quality and becomes a kind of entity, like the ocean. The
observer is conscious that he can actually see the beginning of its
ebon depths, in which the visible universe appears to float like an
enchanted island, resplendent within with lights and life and gorgeous
spectacles, and encircled with screens of crowded stars, but with its
dazzling vistas ending at the fathomless sea of pure darkness which
encloses all.
The Galaxy, or Milky Way, surrounds the borders of our island in space
like a stellar garland, and when openings appear in it they are, by
contrast, far more impressive than the general darkness of the
interstellar expanse seen in other directions. Yet even that expanse
is not everywhere equally dark, for it contains gloomy deeps
discernable with careful watching. Here, too, contrast plays an
important part, though less striking than within the galactic region.
Some of Sir William Herschel's observations appear to indicate an
association between these tenebrious spots and neighboring star clouds
and nebulæ. It is an illuminating bit of astronomical history that
when he was sweeping the then virgin heavens with his great telescopes
he was accustomed to say to his sister who, note-book in hand, waited
at his side to take down his words, fresh with the inspiration of
discovery: ``Prepare to write; the nebulæ are coming; here space is
vacant.''
The most famous of the ``coal-sacks,'' and the first to be brought to
general attention before astronomers had awakened to the significance
of such things, lies adjacent to the ``Southern Cross,'' and is truly
an amazing phenomenon. It is not alone the conspicuousness of this
celestial vacancy, opening suddenly in the midst of one of the richest
parts of the Galaxy, that has given it its fame, but quite as much the
superstitious awe with which it was regarded by the early explorers of
the South Seas. To them, as well as to those who listened in rapt
wonder to their tales, the ``Coal-sack'' seemed to possess some occult
connection with the mystic ``Cross.'' In the eyes of the sailors it
was not a vacancy so much as a sable reality in the sky, and as,
shuddering, they stared at it, they piously crossed themselves. It was
another of the magical wonders of the unknown South, and as such it
formed the basis of many a ``wild surmise'' and many a sea-dog's yarn.
Scientific investigation has not diminished its prestige, and today no
traveler in the southern hemisphere is indifferent to its fascinating
strangeness, while some find it the most impressive spectacle of the
antarctic heavens.
All around, up to the very edge of the yawning gap, the sheen of the
Milky Way is surpassingly glorious; but there, as if in obedience to
an almighty edict, everything vanishes. A single faint star is visible
within the opening, producing a curious effect upon the sensitive
spectator, like the sight of a tiny islet in the midst of a black,
motionless, waveless tarn. The dimensions of the lagoon of darkness,
which is oval or pear-shaped, are eight degrees by five, so that it
occupies a space in the sky about one hundred and thirty times greater
than the area of the full moon. It attracts attention as soon as the
eye is directed toward the quarter where it exists, and by virtue of
the rarity of such phenomena it appears a far greater wonder than the
drifts of stars that are heaped around it. Now that observatories are
multiplying in the southern hemisphere, the great austral
``Coal-sack'' will, no doubt, receive attention proportioned to its
importance as one of the most significant features of the sky. Already
at the Sydney Observatory photographs have shown that the southern
portion of this Dead Sea of Space is not quite ``bottomless,''
although its northern part defies the longest sounding lines of the
astronomer.
There is a similar, but less perfect, ``coal-sack'' in the northern
hemisphere, in the constellation of ``The Swan,'' which, strange to
say, also contains a well-marked figure of a cross outlined by stars.
This gap lies near the top of the cross-shaped figure. It is best seen
by averted vision, which brings out the contrast with the Milky Way,
which is quite brilliant around it. It does not, however, exercise the
same weird attraction upon the eye as the southern ``Coal-sack,'' for
instead of looking like an absolute void in the sky, it rather appears
as if a canopy of dark gauze had been drawn over the stars. We shall
see the possible significance of this appearance later.
Just above the southern horizon of our northern middle latitudes, in
summer, where the Milky Way breaks up into vast sheets of nebulous
luminosity, lying over and between the constellations Scorpio and
Sagittarius, there is a remarkable assemblage of ``coal-sacks,''
though none is of great size. One of them, near a conspicuous
star-cluster in Scorpio, M80, is interesting for having been the first
of these strange objects noted by Herschel. Probably it was its
nearness to M80 which suggested to his mind the apparent connection of
such vacancies with star-clusters which we have already mentioned.
But the most marvelous of the ``coal-sacks'' are those that have been
found by photography in Sagittarius. One of Barnard's earliest and
most excellent photographs includes two of them, both in the
star-cluster M8. The larger, which is roughly rectangular in outline,
contains one little star, and its smaller neighbor is lune-shaped --
surely a most singular form for such an object. Both are associated
with curious dark lanes running through the clustered stars like
trails in the woods. Along the borders of these lanes the stars are
ranked in parallel rows, and what may be called the bottoms of the
lanes are not entirely dark, but pebbled with faint stellar points.
One of them which skirts the two dark gaps and traverses the cluster
along its greatest diameter is edged with lines of stars, recalling
the alignment of the trees bordering a French highway. This road of
stars cannot be less than many billions of miles in length!
All about the cluster the bed of the Galaxy is strangely disturbed,
and in places nearly denuded, as if its contents had been raked away
to form the immense stack and the smaller accumulations of stars
around it. The well-known ``Trifid Nebula'' is also included in the
field of the photograph, which covers a truly marvelous region, so
intricate in its mingling of nebulæ, star-clusters, star-swarms,
star-streams, and dark vacancies that no description can do it
justice. Yet, chaotic as it appears, there is an unmistakable
suggestion of unity about it, impressing the beholder with the idea
that all the different parts are in some way connected, and have not
been fortuitously thrown together. Miss Agnes M. Clerke made the
striking remark that the dusky lanes in M8 are exemplified on the
largest scale in the great rift dividing the Milky Way, from Cygnus in
the northern hemisphere all the way to the ``Cross'' in the southern.
Similar lanes are found in many other clusters, and they are generally
associated with flanking rows of stars, resembling in their
arrangement the thick-set houses and villas along the roadways that
traverse the approaches to a great city.
But to return to the black gaps. Are they really windows in the
star-walls of the universe? Some of them look rather as if they had
been made by a shell fired through a luminous target, allowing the eye
to range through the hole into the void space beyond. If science is
discretely silent about these things, what can the more venturesome
and less responsible imagination suggest? Would a huge ``runaway
sun,'' like Arcturus, for instance, make such an opening if it should
pass like a projectile through the Milky Way? It is at least a
stimulating inquiry. Being probably many thousands of times more
massive than the galactic stars, such a stellar missile would not be
stopped by them, though its direction of flight might be altered. It
would drag the small stars lying close to its course out of their
spheres, but the ultimate tendency of its attraction would be to sweep
them round in its wake, thus producing rather a star-swarm than a
vacancy. Those that were very close to it might be swept away in its
rush and become its satellites, careering away with it in its flight
into outer space; but those that were farther off, and they would, of
course, greatly outnumber the nearer ones, would tend inward from all
sides toward the line of flight, as dust and leaves collect behind a
speeding motor (though the forces operating would be different), and
would fill up the hole, if hole it were. A swarm thus collected should
be rounded in outline and bordered with a relatively barren ring from
which the stars had been ``sucked'' away. In a general sense the M8
cluster answers to this description, but even if we undertook to
account for its existence by a supposition like the above, the black
gaps would remain unexplained, unless one could make a further draft
on the imagination and suggest that the stars had been thrown into a
vast eddy, or system of eddies, whose vortices appear as dark holes.
Only a maelstrom-like motion could keep such a funnel open, for
without regard to the impulse derived from the projectile, the proper
motions of the stars themselves would tend to fill it. Perhaps some
other cause of the whirling motion may be found. As we shall see when
we come to the spiral nebulæ, gyratory movements are exceedingly
prevalent throughout the universe, and the structure of the Milky Way
is everywhere suggestive of them. But this is hazardous sport even for
the imagination -- to play with suns as if they were but thistle-down
in the wind or corks in a mill-race.
Another question arises: What is the thickness of the hedge of stars
through which the holes penetrate? Is the depth of the openings
proportionate to their width? In other words, is the Milky Way round
in section like a rope, or flat and thin like a ribbon? The answer is
not obvious, for we have little or no information concerning the
relative distances of the faint galactic stars. It would be easier,
certainly, to conceive of openings in a thin belt than in a massive
ring, for in the first case they would resemble mere rifts and breaks,
while in the second they would be like wells or bore-holes. Then, too,
the fact that the Milky Way is not a continuous body but is made up of
stars whose actual distances apart is great, offers another quandary;
persistent and sharply bordered apertures in such an assemblage are a
priori as improbable, if not impossible, as straight, narrow holes
running through a swarm of bees.
The difficulty of these questions indicates one of the reasons why it
has been suggested that the seeming gaps, or many of them, are not
openings at all, but opaque screens cutting off the light from stars
behind them. That this is quite possible in some cases is shown by
Barnard's later photographs, particularly those of the singular region
around the star Rho Ophiuchi. Here are to be seen somber lanes and
patches, apparently forming a connected system which covers an immense
space, and which their discoverer thinks may constitute a ``dark
nebula.'' This seems at first a startling suggestion; but, after all,
why should their not be dark nebulæ as well as visible ones? In truth,
it has troubled some astronomers to explain the luminosity of the
bright nebulæ, since it is not to be supposed that matter in so
diffuse a state can be incandescent through heat, and phosphorescent
light is in itself a mystery. The supposition is also in accord with
what we know of the existence of dark solid bodies in space. Many
bright stars are accompanied by obscure companions, sometimes as
massive as themselves; the planets are non-luminous; the same is true
of meteors before they plunge into the atmosphere and become heated by
friction; and many plausible reasons have been found for believing
that space contains as many obscure as shining bodies of great size.
It is not so difficult, after all, then, to believe that there are
immense collections of shadowy gases and meteoric dust whose presence
is only manifested when they intercept the light coming from shining
bodies behind them.
This would account for the apparent extinguishment of light in open
space, which is indicated by the falling off in relative number of
telescopic stars below the tenth magnitude. Even as things are, the
amount of light coming to us from stars too faint to be seen with the
naked eye is so great that the statement of it generally surprises
persons who are unfamiliar with the inner facts of astronomy. It has
been calculated that on a clear night the total starlight from the
entire celestial sphere amounts to one-sixtieth of the light of the
full moon; but of this less than one-twenty-fifth is due to stars
separately distinguished by the eye. If there were no obscuring medium
in space, it is probable that the amount of starlight would be
noticeably and perhaps enormously increased.
But while it seems certain that some of the obscure spots in the Milky
Way are due to the presence of ``dark nebulæ,'' or concealing veils of
one kind or another, it is equally certain that there are many which
are true apertures, however they may have been formed, and by whatever
forces they may be maintained. These, then, are veritable windows of
the Galaxy, and when looking out of them one is face to face with the
great mystery of infinite space. There the known universe visibly
ends, but manifestly space itself does not end there. It is not within
the power of thought to conceive an end to space, for the instant we
think of a terminal point or line the mind leaps forward to the
beyond. There must be space outside as well as inside. Eternity of
time and infinity of space are ideas that the intellect cannot fully
grasp, but neither can it grasp the idea of a limitation to either
space or time. The metaphysical conceptions of hypergeometry, or
fourth-dimensional space, do not aid us.
Having, then, discovered that the universe is a thing contained in
something indefinitely greater than itself; having looked out of its
windows and found only the gloom of starless night outside -- what
conclusions are we to draw concerning the beyond? It seems as empty as
a vacuum, but is it really so? If it be, then our universe is a single
atom astray in the infinite; it is the only island in an ocean without
shores; it is the one oasis in an illimitable desert. Then the Milky
Way, with its wide-flung garland of stars, is afloat like a tiny
smoke-wreath amid a horror of immeasurable vacancy, or it is an
evanescent and solitary ring of sparkling froth cast up for a moment
on the viewless billows of immensity. From such conclusions the mind
instinctively shrinks. It prefers to think that there is something
beyond, though we cannot see it. Even the universe could not bear to
be alone -- a Crusoe lost in the Cosmos! As the inhabitants of the
most elegant château, with its gardens, parks, and crowds of
attendants, would die of loneliness if they did not know that they
have neighbors, though not seen, and that a living world of indefinite
extent surrounds them, so we, when we perceive that the universe has
limits, wish to feel that it is not solitary; that beyond the hedges
and the hills there are other centers of life and activity. Could
anything be more terrible than the thought of an isolated universe?
The greater the being, the greater the aversion to seclusion. Only the
infinite satisfies; in that alone the mind finds rest.
We are driven, then, to believe that the universal night which
envelopes us is not tenantless; that as we stare out of the
star-framed windows of the Galaxy and see nothing but uniform
blackness, the fault is with our eyes or is due to an obscuring
medium. Since our universe is limited in extent, there must be other
universes beyond it on all sides. Perhaps if we could carry our
telescopes to the verge of the great ``Coal-sack'' near the ``Cross,''
being then on the frontier of our starry system, we could discern,
sparkling afar off in the vast night, some of the outer galaxies. They
may be grander than ours, just as many of the suns surrounding us are
immensely greater than ours. If we could take our stand somewhere in
the midst of immensity and, with vision of infinite reach, look about
us, we should perhaps see a countless number of stellar systems, amid
which ours would be unnoticeable, like a single star among the
multitude glittering in the terrestial sky on a clear night. Some
might be in the form of a wreath, like our own; some might be
globular, like the great star-clusters in Hercules and Centaurus; some
might be glittering circles, or disks, or rings within rings. If we
could enter them we should probably find a vast variety of
composition, including elements unknown to terrestrial chemistry; for
while the visible universe appears to contain few if any substances
not existing on the earth or in the sun, we have no warrant to assume
that others may not exist in infinite space.
And how as to gravitation? We do not know that gravitation acts beyond
the visible universe, but it is reasonable to suppose that it does. At
any rate, if we let go its sustaining hand we are lost, and can only
wander hopelessly in our speculations, like children astray. If the
empire of gravitation is infinite, then the various outer systems must
have some, though measuring by our standards an imperceptible,
attractive influence upon each other, for gravitation never lets go
its hold, however great the space over which it is required to act.
Just as the stars about us are all in motion, so the starry systems
beyond our sight may be in motion, and our system as a whole may be
moving in concert with them. If this be so, then after interminable
ages the aspect of the entire system of systems must change, its
various members assuming new positions with respect to one another. In
the course of time we may even suppose that our universe will approach
relatively close to one of the others; and then, if men are yet living
on the earth, they may glimpse through the openings which reveal
nothing to us now, the lights of another nearing star system, like the
signals of a strange squadron, bringing them the assurance (which can
be but an inference at present) that the ocean of space has other
argosies venturing on its limitless expanse.
There remains the question of the luminiferous ether by whose agency
the waves of light are borne through space. The ether is as mysterious
as gravitation. With regard to ether we only infer its existence from
the effects which we ascribe to it. Evidently the ether must extend as
far as the most distant visible stars. But does it continue on
indefinitely in outer space? If it does, then the invisibility of the
other systems must be due to their distance diminishing the quantity
of light that comes from them below the limit of perceptibility, or to
the interposition of absorbing media; if it does not, then the reason
why we cannot see them is owing to the absence of a means of
conveyance for the light waves, as the lack of an interplanetary
atmosphere prevents us from hearing the thunder of sun-spots. (It is
interesting to recall that Mr Edison was once credited with the
intention to construct a gigantic microphone which should render the
roar of sun-spots audible by transforming the electric vibrations into
sound-waves). On this supposition each starry system would be
enveloped in its own globule of ether, and no light could cross from
one to another. But the probability is that both the ether and
gravitation are ubiquitous, and that all the stellar systems are
immersed in the former like clouds of phosphorescent organisms in the
sea.
So astronomy carries the mind from height to greater height. Men were
long in accepting the proofs of the relative insignificance of the
earth; they were more quickly convinced of the comparative littleness
of the solar system; and now the evidence assails their reason that
what they had regarded as the universe is only one mote gleaming in
the sunbeams of Infinity.
Star-Clouds, Star-Clusters, and Star-Streams
In the preceding chapter we have seen something of the strangely
complicated structure of the Galaxy, or Milky Way. We now proceed to
study more comprehensively that garlanded ``Pathway of the Gods.''
Judged by the eye alone, the Milky Way is one of the most delicately
beautiful phenomena in the entire realm of nature -- a shimmer of
silvery gauze stretched across the sky; but studied in the light of
its revelations, it is the most stupendous object presented to human
ken. Let us consider, first, its appearance to ordinary vision. Its
apparent position in the sky shifts according to the season. On a
serene, cloudless summer evening, in the absence of the moon, whose
light obscures it, one sees the Galaxy spanning the heavens from north
to southeast of the zenith like a phosphorescent arch. In early spring
it forms a similar but, upon the whole, less brilliant arch west of
the zenith. Between spring and summer it lies like a long, faint,
twilight band along the northern horizon. At the beginning of winter
it again forms an arch, this time spanning the sky from east to west,
a little north of the zenith. These are its positions as viewed from
the mean latitude of the United States. Even the beginner in
star-gazing does not have to watch it throughout the year in order to
be convinced that it is, in reality, a great circle, extending
entirely around the celestial sphere. We appear to be situated near
its center, but its periphery is evidently far away in the depths of
space.
Although to the casual observer it seems but a delicate scarf of
light, brighter in some places than in others, but hazy and indefinite
at the best, such is not its appearance to those who study it with
care. They perceive that it is an organic whole, though marvelously
complex in detail. The telescope shows that it consists of stars too
faint and small through excess of distance to be separately visible.
Of the hundred million suns which some estimates have fixed as the
probable population of the starry universe, the vast majority (at
least thirty to one) are included in this strange belt of misty light.
But they are not uniformly distributed in it; on the contrary, they
are arrayed in clusters, knots, bunches, clouds, and streams. The
appearance is somewhat as if the Galaxy consisted of innumerable
swarms of silver-winged bees, more or less intermixed, some massed
together, some crossing the paths of others, but all governed by a
single purpose which leads them to encircle the region of space in
which we are situated.
From the beginning of the systematic study of the heavens, the fact
has been recognized that the form of the Milky Way denotes the scheme
of the sidereal system. At first it was thought that the shape of the
system was that of a vast round disk, flat like a cheese, and filled
with stars, our sun and his relatively few neighbors being placed near
the center. According to this view, the galactic belt was an effect of
perspective; for when looking in the direction of the plane of the
disk, the eye ranged through an immense extension of stars which
blended into a glimmering blur, surrounding us like a ring; while when
looking out from the sides of the disk we saw but few stars, and in
those directions the heavens appeared relatively blank. Finally it was
recognized that this theory did not correspond with the observed
appearances, and it became evident that the Milky Way was not a mere
effect of perspective, but an actual band of enormously distant stars,
forming a circle about the sphere, the central opening of the ring
(containing many scattered stars) being many times broader than the
width of the ring itself. Our sun is one of the scattered stars in the
central opening.
As already remarked, the ring of the Galaxy is very irregular, and in
places it is partly broken. With its sinuous outline, its pendant
sprays, its graceful and accordant curves, its bunching of masses, its
occasional interstices, and the manifest order of a general plan
governing the jumble of its details, it bears a remarkable resemblance
to a garland -- a fact which appears the more wonderful when we recall
its composition. That an elm-tree should trace the lines of beauty
with its leafy and pendulous branches does not surprise us; but we can
only gaze with growing amazement when we behold a hundred million suns
imitating the form of a chaplet! And then we have to remember that
this form furnishes the ground-plan of the universe.
As an indication of the extraordinary speculations to which the
mystery of the Milky Way has given rise, a theory recently (1909)
proposed by Prof. George C. Comstock may be mentioned. Starting with
the data (first) that the number of stars increases as the Milky Way
is approached, and reaches a maximum in its plane, while on the other
hand the number of nebulæ is greatest outside the Milky Way and
increases with distance from it, and (second) that the Milky Way,
although a complete ring, is broad and diffuse on one side through
one-half its course -- that half alone containing nebulæ -- and
relatively narrow and well defined on the opposite side, the author of
this singular speculation avers that these facts can best be explained
by supposing that the invisible universe consists of two
interpenetrating parts, one of which is a chaos of indefinite extent,
strewn with stars and nebulous dust, and the other a long, broad but
comparatively thin cluster of stars, including the sun as one of its
central members. This flat star-cluster is conceived to be moving
edgewise through the chaos, and, according to Professor Comstock, it
acts after the manner of a snow-plough sweeping away the cosmic dust
and piling it on either hand above and below the plane of the moving
cluster. It thus forms a transparent rift, through which we see
farther and command a view of more stars than through the intensified
dust-clouds on either hand. This rift is the Milky Way. The dust
thrown aside toward the poles of the Milky Way is the substance of the
nebulæ which abound there. Ahead, where the front of the star-plough
is clearing the way, the chaos is nearer at hand, and consequently
there the rift subtends a broader angle, and is filled with primordial
dust, which, having been annexed by the vanguard of the star-swarm,
forms the nebulæ seen only in that part of the Milky Way. But behind,
the rift appears narrow because there we look farther away between
dust-clouds produced ages ago by the front of the plough, and no
scattered dust remains in that part of the rift.
In quoting an outline of this strikingly original theory the present
writer should not be understood as assenting to it. That it appears
bizarre is not, in itself, a reason for rejecting it, when we are
dealing with so problematical and enigmatical a subject as the Milky
Way; but the serious objection is that the theory does not
sufficiently accord with the observed phenomena. There is too much
evidence that the Milky Way is an organic system, however fantastic
its form, to permit the belief that it can only be a rift in chaotic
clouds. As with every organism, we find that its parts are more or
less clearly repeated in its ensemble. Among all the strange things
that the Milky Way contains there is nothing so extraordinary as
itself. Every astronomer must many times have found himself marveling
at it in those comparatively rare nights when it shows all its beauty
and all its strangeness. In its great broken rifts, divisions, and
spirals are found the gigantic prototypes of similar forms in its
star-clouds and clusters. As we have said, it determines the general
shape of the whole sidereal system. Some of the brightest stars in the
sky appear to hang like jewels suspended at the ends of tassels
dropped from the Galaxy. Among these pendants are the Pleiades and the
Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a loop of
light'' thrown out from it. The majority of the great first-magnitude
stars seem related to it, as if they formed an inner ring inclined at
an angle of some twenty degrees to its plane. Many of the long curves
that set off from it on both sides are accompanied by corresponding
curves of lucid stars. In a word, it offers every appearance of
structural connection with the entire starry system. That the universe
should have assumed the form of a wreath is certainly a matter for
astonishment; but it would have been still more astonishing if it had
been a cube, a rhomboid, or a dodecahedron, for then we should have
had to suppose that something resembling the forces that shape
crystals had acted upon the stars, and the difficulty of explaining
the universe by the laws of gravitation would have been increased.
From the Milky Way as a whole we pass to the vast clouds, swarms, and
clusters of stars of which it is made up. It may be, as some
astronomers hold, that most of the galactic stars are much smaller
than the sun, so that their faintness is not due entirely to the
effect of distance. Still, their intrinsic brilliance attests their
solar character, and considering their remoteness, which has been
estimated at not less than ten thousand to twenty thousand light-years
(a light-year is equal to nearly six thousand thousand million miles)
their actual masses cannot be extremely small. The minutest of them
are entitled to be regarded as real suns, and they vary enormously in
magnitude. The effects of their attractions upon one another can only
be inferred from their clustering, because their relative movements
are not apparent on account of the brevity of the observations that we
can make. But imagine a being for whom a million years would be but as
a flitting moment; to him the Milky Way would appear in a state of
ceaseless agitation -- swirling with ``a fury of whirlpool motion.''
The cloud-like aspect of large parts of the Galaxy must always have
attracted attention, even from naked-eye observers, but the true
star-clouds were first satisfactorily represented in Barnard's
photographs. The resemblance to actual clouds is often startling. Some
are close-packed and dense, like cumuli; some are wispy or mottled,
like cirri. The rifts and modulations, as well as the general
outlines, are the same as those of clouds of vapor or dust, and one
notices also the characteristic thinning out at the edges. But we must
beware of supposing that the component suns are thickly crowded as the
particles forming an ordinary cloud. They look, indeed, as if they
were matted together, because of the irradiation of light, but in
reality millions and billions of miles separate each star from its
neighbors. Nevertheless they form real assemblages, whose members are
far more closely related to one another than is our sun to the stars
around him, and if we were in the Milky Way the aspect of the
nocturnal sky would be marvelously different from its present
appearance.
Stellar clouds are characteristic of the Galaxy and are not found
beyond its borders, except in the ``Magellanic Clouds'' of the
southern hemisphere, which resemble detached portions of the Milky
Way. These singular objects form as striking a peculiarity of the
austral heavens as does the great ``Coal-sack'' described in Chapter
1. But it is their isolation that makes them so remarkable, for their
composition is essentially galactic, and if they were included within
its boundaries they would not appear more wonderful than many other
parts of the Milky Way. Placed where they are, they look like masses
fallen from the great stellar arch. They are full of nebulæ and
star-clusters, and show striking evidences of spiral movement.
Star-swarms, which are also characteristic features of the Galaxy,
differ from star-clouds very much in the way that their name would
imply -- i.e., their component stars are so arranged, even when they
are countless in number, that the idea of an exceedingly numerous
assemblage rather than that of a cloud is impressed on the observer's
mind. In a star-swarm the separate members are distinguishable because
they are either larger or nearer than the stars composing a ``cloud.''
A splendid example of a true star-swarm is furnished by Chi Persei, in
that part of the Milky Way which runs between the constellations
Perseus and Cassiopeia. This swarm is much coarser than many others,
and can be seen by the naked eye. In a small telescope it appears
double, as if the suns composing it had divided into two parties which
keep on their way side by side, with some commingling of their members
where the skirts of the two companies come in contact.
Smaller than either star-clouds or star-swarms, and differing from
both in their organization, are star-clusters. These, unlike the
others, are found outside as well as inside the Milky Way, although
they are more numerous inside its boundaries than elsewhere. The term
star-cluster is sometimes applied, though improperly, to assemblages
which are rather groups, such, for instance, as the Pleiades. In their
most characteristic aspect star-clusters are of a globular shape --
globes of suns! A famous example of a globular star-cluster, but one
not included in the Milky Way, is the ``Great Cluster in Hercules.''
This is barely visible to the naked eye, but a small telescope shows
its character, and in a large one it presents a marvelous spectacle.
Photographs of such clusters are, perhaps, less effective than those
of star-clouds, because the central condensation of stars in them is
so great that their light becomes blended in an indistinguishable
blur. The beautiful effect of the incessant play of infinitesimal rays
over the apparently compact surface of the cluster, as if it were a
globe of the finest frosted silver shining in an electric beam, is
also lost in a photograph. Still, even to the eye looking directly at
the cluster through a powerful telescope, the central part of the
wonderful congregation seems almost a solid mass in which the stars
are packed like the ice crystals in a snowball.
The same question rises to the lips of every observer: How can they
possibly have been brought into such a situation? The marvel does not
grow less when we know that, instead of being closely compacted, the
stars of the cluster are probably separated by millions of miles; for
we know that their distances apart are slight as compared with their
remoteness from the Earth. Sir William Herschel estimated their number
to be about fourteen thousand, but in fact they are uncountable. If we
could view them from a point just within the edge of the assemblage,
they would offer the appearance of a hollow hemisphere emblazoned with
stars of astonishing brilliancy; the near-by ones unparalleled in
splendor by any celestial object known to us, while the more distant
ones would resemble ordinary stars. An inhabitant of the cluster would
not know, except by a process of ratiocination, that he was dwelling
in a globular assemblage of suns; only from a point far outside would
their spherical arrangement become evident to the eye. Imagine
fourteen-thousand fire-balloons with an approach to regularity in a
spherical space -- say, ten miles in diameter; there would be an
average of less than thirty in every cubic mile, and it would be
necessary to go to a considerable distance in order to see them as a
globular aggregation; yet from a point sufficiently far away they
would blend into a glowing ball.
Photographs show even better than the best telescopic views that the
great cluster is surrounded with a multitude of dispersed stars,
suggestively arrayed in more or less curving lines, which radiate from
the principle mass, with which their connection is manifest. These
stars, situated outside the central sphere, look somewhat like vagrant
bees buzzing round a dense swarm where the queen bee is sitting. Yet
while there is so much to suggest the operation of central forces,
bringing and keeping the members of the cluster together, the
attentive observer is also impressed with the idea that the whole
wonderful phenomenon may be the result of explosion. As soon as this
thought seizes the mind, confirmation of it seems to be found in the
appearance of the outlying stars, which could be as readily explained
by the supposition that they have been blown apart as that they have
flocked together toward a center. The probable fact that the stars
constituting the cluster are very much smaller than our sun might be
regarded as favoring the hypothesis of an explosion. Of their real
size we know nothing, but, on the basis of an uncertain estimate of
their parallax, it has been calculated that they may average
forty-five thousand miles in diameter -- something more than half the
diameter of the planet Jupiter. Assuming the same mean density,
fourteen thousand such stars might have been formed by the explosion
of a body about twice the size of the sun. This recalls the theory of
Olbers, which has never been altogether abandoned or disproved, that
the Asteroids were formed by the explosion of a planet circulating
between the orbits of Mars and Jupiter. The Asteroids, whatever their
manner of origin, form a ring around the sun; but, of course, the
explosion of a great independent body, not originally revolving about
a superior center of gravitational force, would not result in the
formation of a ring of small bodies, but rather of a dispersed mass of
them. But back of any speculation of this kind lies the problem, at
present insoluble: How could the explosion be produced? (See the
question of explosions in Chapters 6 and 14).
Then, on the other hand, we have the observation of Herschel, since
abundantly confirmed, that space is unusually vacant in the immediate
neighborhood of condensed star-clusters and nebulæ, which, as far as
it goes, might be taken as an indication that the assembled stars had
been drawn together by their mutual attractions, and that the tendency
to aggregation is still bringing new members toward the cluster. But
in that case there must have been an original condensation of stars at
that point in space. This could probably have been produced by the
coagulation of a great nebula into stellar nuclei, a process which
seems now to be taking place in the Orion Nebula.
A yet more remarkable globular star-cluster exists in the southern
hemisphere, Omega Centauri. In this case the central condensation of
stars presents an almost uniform blaze of light. Like the Hercules
cluster, that in Centaurus is surrounded with stars scattered over a
broad field and showing an appearance of radial arrangement. In fact,
except for its greater richness, Omega Centauri is an exact duplicate
of its northern rival. Each appears to an imaginative spectator as a
veritable ``city of suns.'' Mathematics shrinks from the task of
disentangling the maze of motions in such an assemblage. It would seem
that the chance of collisions is not to be neglected, and this idea
finds a certain degree of confirmation in the appearance of
``temporary stars'' which have more than once blazed out in, or close
by, globular star-clusters.
This leads up to the notable fact, first established by Professor
Bailey a few years ago, that such clusters are populous with variable
stars. Omega Centauri and the Hercules cluster are especially
remarkable in this respect. The variables found in them are all of
short period and the changes of light show a noteworthy tendency to
uniformity. The first thought is that these phenomena must be due to
collisions among the crowded stars, but, if so, the encounters cannot
be between the stars themselves, but probably between stars and meteor
swarms revolving around them. Such periodic collisions might go on for
ages without the meteors being exhausted by incorporation with the
stars. This explanation appears all the more probable because one
would naturally expect that flocks of meteors would abound in a close
aggregation of stars. It is also consistent with Perrine's discovery
-- that the globular star clusters are powdered with minute stars
strewn thickly among the brighter ones.
In speaking of Professor Comstock's extraordinary theory of the Milky
Way, the fact was mentioned that, broadly speaking, the nebulæ are
less numerous in the galactic belt than in the comparatively open
spaces on either side of it, but that they are, nevertheless, abundant
in the broader half of the Milky Way which he designates as the front
of the gigantic ``plough'' supposed to be forcing its way through the
enveloping chaos. In and around the Sagittarius region the
intermingling of nebulæ and galactic star clouds and clusters is
particularly remarkable. That there is a causal connection no
thoughtful person can doubt. We are unable to get away from the
evidence that a nebula is like a seed-ground from which stars spring
forth; or we may say that nebulæ resemble clouds in whose bosom
raindrops are forming. The wonderful aspect of the admixtures of
nebulæ and star-clusters in Sagittarius has been described in Chapter
1. We now come to a still more extraordinary phenomenon of this kind
-- the Pleiades nebulæ.
The group of the Pleiades, although lying outside the main course of
the Galaxy, is connected with it by a faint loop, and is the scene of
the most remarkable association of stars and nebulous matter known in
the visible universe. The naked eye is unaware of the existence of
nebulæ in the Pleiades, or, at the best, merely suspects that there is
something of the kind there; and even the most powerful telescopes are
far from revealing the full wonder of the spectacle; but in
photographs which have been exposed for many hours consecutively, in
order to accumulate the impression of the actinic rays, the revelation
is stunning. The principle stars are seen surrounded by, and, as it
were, drowned in, dense nebulous clouds of an unparalleled kind. The
forms assumed by these clouds seem at first sight inexplicable. They
look like fleeces, or perhaps more like splashes and daubs of luminous
paint dashed carelessly from a brush. But closer inspection shows that
they are, to a large extent, woven out of innumerable threads of filmy
texture, and there are many indications of spiral tendencies. Each of
the bright stars of the group -- Alcyone, Merope, Maia, Electra,
Taygeta, Atlas -- is the focus of a dense fog (totally invisible,
remember, alike to the naked eye and to the telescope), and these
particular stars are veiled from sight behind the strange mists.
Running in all directions across the relatively open spaces are
nebulous wisps and streaks of the most curious forms. On some of the
nebular lines, which are either straight throughout, or if they change
direction do so at an angle, little stars are strung like beads. In
one case seven or eight stars are thus aligned, and, as if to
emphasize their dependence upon the chain which connects them, when it
makes a slight bend the file of stars turns the same way. Many other
star rows in the group suggest by their arrangement that they, too,
were once strung upon similar threads which have now disappeared,
leaving the stars spaced along their ancient tracks. We seem forced to
the conclusion that there was a time when the Pleiades were embedded
in a vast nebula resembling that of Orion, and that the cloud has now
become so rare by gradual condensation into stars that the merest
trace of it remains, and this would probably have escaped detection
but for the remarkable actinic power of the radiant matter of which it
consists. The richness of many of these faint nebulous masses in
ultra-violet radiations, which are those that specifically affect the
photographic plate, is the cause of the marvelous revelatory power of
celestial photography. So the veritable unseen universe, as
distinguished from the ``unseen universe'' of metaphysical
speculation, is shown to us.
A different kind of association between stars and nebulæ is shown in
some surprising photographic objects in the constellation Cygnus,
where long, wispy nebulæ, billions of miles in length, some of them
looking like tresses streaming in a breeze, lie amid fields of stars
which seem related to them. But the relation is of a most singular
kind, for notwithstanding the delicate structure of the long nebulæ
they appear to act as barriers, causing the stars to heap themselves
on one side. The stars are two, three, or four times as numerous on
one side of the nebulæ as on the other. These nebulæ, as far as
appearance goes, might be likened to rail fences, or thin hedges,
against which the wind is driving drifts of powdery snow, which, while
scattered plentifully all around, tends to bank itself on the leeward
side of the obstruction. The imagination is at a loss to account for
these extraordinary phenomena; yet there they are, faithfully giving
us their images whenever the photographic plate is exposed to their
radiations.
Thus the more we see of the universe with improved methods of
observation, and the more we invent aids to human senses, each
enabling us to penetrate a little deeper into the unseen, the greater
becomes the mystery. The telescope carried us far, photography is
carrying us still farther; but what as yet unimagined instrument will
take us to the bottom, the top, and the end? And then, what hitherto
untried power of thought will enable us to comprehend the meaning of
it all?
Stellar Migrations
To the untrained eye the stars and the planets are not
distinguishable. It is customary to call them all alike ``stars.'' But
since the planets more or less rapidly change their places in the sky,
in consequence of their revolution about the sun, while the stars
proper seem to remain always in the same relative positions, the
latter are spoken of as ``fixed stars.'' In the beginnings of
astronomy it was not known that the ``fixed stars'' had any motion
independent of their apparent annual revolution with the whole sky
about the earth as a seeming center. Now, however, we know that the
term ``fixed stars'' is paradoxical, for there is not a single really
fixed object in the whole celestial sphere. The apparent fixity in the
positions of the stars is due to their immense distance, combined with
the shortness of the time during which we are able to observe them. It
is like viewing the plume of smoke issuing from a steamer, hull down,
at sea: if one does not continue to watch it for a long time it
appears to be motionless, although in reality it may be traveling at
great speed across the line of sight. Even the planets seem fixed in
position if one watches them for a single night only, and the more
distant ones do not sensibly change their places, except after many
nights of observation. Neptune, for instance, moves but little more
than two degrees in the course of an entire year, and in a month its
change of place is only about one-third of the diameter of the full
moon.
Yet, fixed as they seem, the stars are actually moving with a speed in
comparison with which, in some cases, the planets might almost be said
to stand fast in their tracks. Jupiter's speed in his orbit is about
eight miles per second, Neptune's is less than three and one-half
miles, and the earth's is about eighteen and one-half miles; while
there are ``fixed stars'' which move two hundred or three hundred
miles per second. They do not all, however, move with so great a
velocity, for some appear to travel no faster than the planets. But in
all cases, notwithstanding their real speed, long-continued and
exceedingly careful observations are required to demonstrate that they
are moving at all. No more overwhelming impression of the frightful
depths of space in which the stars are buried can be obtained than by
reflecting upon the fact that a star whose actual motion across the
line of sight amounts to two hundred miles per second does not change
its apparent place in the sky, in the course of a thousand years,
sufficiently to be noticed by the casual observer of the heavens!
There is one vast difference between the motions of the stars and
those of the planets to which attention should be at once called: the
planets, being under the control of a central force emanating from
their immediate master, the sun, all move in the same direction and in
orbits concentric about the sun; the stars, on the other hand, move in
every conceivable direction and have no apparent center of motion, for
all efforts to discover such a center have failed. At one time, when
theology had finally to accept the facts of science, a grandiose
conception arose in some pious minds, according to which the Throne of
God was situated at the exact center of His Creation, and, seated
there, He watched the magnificent spectacle of the starry systems
obediently revolving around Him. Astronomical discoveries and
speculations seemed for a time to afford some warrant for this view,
which was, moreover, an acceptable substitute for the abandoned
geocentric theory in minds that could only conceive of God as a
superhuman artificer, constantly admiring his own work. No longer ago
than the middle of the nineteenth century a German astronomer,
Maedler, believed that he had actually found the location of the
center about which the stellar universe revolved. He placed it in the
group of the Pleiades, and upon his authority an extraordinary
imaginative picture was sometimes drawn of the star Alcyone, the
brightest of the Pleiades, as the very seat of the Almighty. This idea
even seemed to gain a kind of traditional support from the mystic
significance, without known historical origin, which has for many
ages, and among widely separated peoples, been attached to the
remarkable group of which Alcyone is the chief. But since Maedler's
time it has been demonstrated that the Pleiades cannot be the center
of revolution of the universe, and, as already remarked, all attempts
to find or fix such a center have proved abortive. Yet so powerful was
the hold that the theory took upon the popular imagination, that even
today astronomers are often asked if Alcyone is not the probable site
of ``Jerusalem the Golden.''
If there were a discoverable center of predominant gravitative power,
to which the motions of all the stars could be referred, those motions
would appear less mysterious, and we should then be able to conclude
that the universe was, as a whole, a prototype of the subsidiary
systems of which it is composed. We should look simply to the law of
gravitation for an explanation, and, naturally, the center would be
placed within the opening enclosed by the Milky Way. If it were there
the Milky Way itself should exhibit signs of revolution about it, like
a wheel turning upon its hub. No theory of the star motions as a whole
could stand which failed to take account of the Milky Way as the basis
of all. But the very form of that divided wreath of stars forbids the
assumption of its revolution about a center. Even if it could be
conceived as a wheel having no material center it would not have the
form which it actually presents. As was shown in Chapter 2, there is
abundant evidence of motion in the Milky Way; but it is not motion of
the system as a whole, but motion affecting its separate parts.
Instead of all moving one way, the galactic stars, as far as their
movements can be inferred, are governed by local influences and
conditions. They appear to travel crosswise and in contrary
directions, and perhaps they eddy around foci where great numbers have
assembled; but of a universal revolution involving the entire mass we
have no evidence.
Most of our knowledge of star motions, called ``proper motions,''
relates to individual stars and to a few groups which happen to be so
near that the effects of their movements are measurable. In some cases
the motion is so rapid (not in appearance, but in reality) that the
chief difficulty is to imagine how it can have been imparted, and what
will eventually become of the ``runaways.'' Without a collision, or a
series of very close approaches to great gravitational centers, a star
traveling through space at the rate of two hundred or three hundred
miles per second could not be arrested or turned into an orbit which
would keep it forever flying within the limits of the visible
universe. A famous example of these speeding stars is ``1830
Groombridge,'' a star of only the sixth magnitude, and consequently
just visible to the naked eye, whose motion across the line of sight
is so rapid that it moves upon the face of the sky a distance equal to
the apparent diameter of the moon every 280 years. The distance of
this star is at least 200,000,000,000,000 miles, and may be two or
three times greater, so that its actual speed cannot be less than two
hundred, and may be as much as four hundred, miles per second. It
could be turned into a new course by a close approach to a great sun,
but it could only be stopped by collision, head-on, with a body of
enormous mass. Barring such accidents it must, as far as we can see,
keep on until it has traversed our stellar system, whence in may
escape and pass out into space beyond, to join, perhaps, one of those
other universes of which we have spoken. Arcturus, one of the greatest
suns in the universe, is also a runaway, whose speed of flight has
been estimated all the way from fifty to two hundred miles per second.
Arcturus, we have every reason to believe, possesses hundreds of times
the mass of our sun -- think, then, of the prodigious momentum that
its motion implies! Sirius moves more moderately, its motion across
the line of sight amounting to only ten miles per second, but it is at
the same time approaching the sun at about the same speed, its actual
velocity in space being the resultant of the two displacements.
What has been said about the motion of Sirius brings us to another
aspect of this subject. The fact is, that in every case of stellar
motion the displacement that we observe represents only a part of the
actual movement of the star concerned. There are stars whose motion
carries them straight toward or straight away from the earth, and such
stars, of course, show no cross motion. But the vast majority are
traveling in paths inclined from a perpendicular to our line of sight.
Taken as a whole, the stars may be said to be flying about like the
molecules in a mass of gas. The discovery of the radial component in
the movements of the stars is due to the spectroscope. If a star is
approaching, its spectral lines are shifted toward the violet end of
the spectrum by an amount depending upon the velocity of approach; if
it is receding, the lines are correspondingly shifted toward the red
end. Spectroscopic observation, then, combined with micrometric
measurements of the cross motion, enables us to detect the real
movement of the star in space. Sometimes it happens that a star's
radial movement is periodically reversed; first it approaches, and
then it recedes. This indicates that it is revolving around a near-by
companion, which is often invisible, and superposed upon this motion
is that of the two stars concerned, which together may be approaching
or receding or traveling across the line of sight. Thus the
complications involved in the stellar motions are often exceedingly
great and puzzling.
Yet another source of complication exists in the movement of our own
star, the sun. There is no more difficult problem in astronomy than
that of disentangling the effects of the solar motion from those of
the motions of the other stars. But the problem, difficult as it is,
has been solved, and upon its solution depends our knowledge of the
speed and direction of the movement of the solar system through space,
for of course the sun carries its planets with it. One element of the
solution is found in the fact that, as a result of perspective, the
stars toward which we are going appear to move apart toward all points
of the compass, while those behind appear to close up together. Then
the spectroscopic principle already mentioned is invoked for studying
the shift of the lines, which is toward the violet in the stars ahead
of us and toward the red in those that we are leaving behind. Of
course the effects of the independent motions of the stars must be
carefully excluded. The result of the studies devoted to this subject
is to show that we are traveling at a speed of twelve to fifteen miles
per second in a northerly direction, toward the border of the
constellations Hercules and Lyra. A curious fact is that the more
recent estimates show that the direction is not very much out of a
straight line drawn from the sun to the star Vega, one of the most
magnificent suns in the heavens. But it should not be inferred from
this that Vega is drawing us on; it is too distant for its gravitation
to have such an effect.
Many unaccustomed thoughts are suggested by this mighty voyage of the
solar system. Whence have we come, and whither do we go? Every year of
our lives we advance at least 375,000,000 miles. Since the traditional
time of Adam the sun has led his planets through the wastes of space
no less than 225,000,000,000 miles, or more than 2400 times the
distance that separates him from the earth. Go back in imagination to
the geologic ages, and try to comprehend the distance over which the
earth has flown. Where was our little planet when it emerged out of
the clouds of chaos? Where was the sun when his ``thunder march''
began? What strange constellations shone down upon our globe when its
masters of life were the monstrous beasts of the ``Age of Reptiles''?
A million years is not much of a span of time in geologic reckoning,
yet a million years ago the earth was farther from its present place
in space than any of the stars with a measurable parallax are now. It
was more than seven times as far as Sirius, nearly fourteen times as
far as Alpha Centauri, three times as far as Vega, and twice as far as
Arcturus. But some geologists demand two hundred, three hundred, even
one thousand million years to enable them to account for the
evolutionary development of the earth and its inhabitants. In a
thousand million years the earth would have traveled farther than from
the remotest conceivable depths of the Milky Way!
Other curious reflections arise when we think of the form of the
earth's track as it follows the lead of the sun, in a journey which
has neither known beginning nor conceivable end. There are probably
many minds which have found a kind of consolation in the thought that
every year the globe returns to the same place, on the same side of
the sun. This idea may have an occult connection with our traditional
regard for anniversaries. When that period of the year returns at
which any great event in our lives has occurred we have the feeling
that the earth, in its annual round, has, in a manner, brought us back
to the scene of that event. We think of the earth's orbit as a
well-worn path which we traverse many times in the course of a
lifetime. It seems familiar to us, and we grow to have a sort of
attachment to it. The sun we are accustomed to regard as a fixed
center in space, like the mill or pump around which the harnessed
patient mule makes his endless circuits. But the real fact is that the
earth never returns to the place in space where it has once quitted.
In consequence of the motion of the sun carrying the earth and the
other planets along, the track pursued by our globe is a vast spiral
in space continually developing and never returning upon its course.
It is probable that the tracks of the sun and the others stars are
also irregular, and possibly spiral, although, as far as can be at
present determined, they appear to be practically straight. Every
star, wherever it may be situated, is attracted by its fellow-stars
from many sides at once, and although the force is minimized by
distance, yet in the course of many ages its effects must become
manifest.
Looked at from another side, is there not something immensely
stimulating and pleasing to the imagination in the idea of so
stupendous a journey, which makes all of us the greatest of travelers?
In the course of a long life a man is transported through space thirty
thousand million miles; Halley's Comet does not travel one-quarter as
far in making one of its immense circuits. And there are adventures on
this voyage of which we are just beginning to learn to take account.
Space is full of strange things, and the earth must encounter some of
them as it advances through the unknown. Many singular speculations
have been indulged in by astronomers concerning the possible effects
upon the earth of the varying state of the space that it traverses.
Even the alternation of hot and glacial periods has sometimes been
ascribed to this source. When tropical life flourished around the
poles, as the remains in the rocks assure us, the needed high
temperature may, it has been thought, have been derived from the
presence of the earth in a warm region of space. Then, too, there is a
certain interest for us in the thought of what our familiar planet has
passed through. We cannot but admire it for its long journeying as we
admire the traveler who comes to us from remote and unexplored lands,
or as we gaze with a glow of interest upon the first locomotive that
has crossed a continent, or a ship that has visited the Arctic or
Antarctic regions. If we may trust the indications of the present
course, the earth, piloted by the sun, has come from the Milky Way in
the far south and may eventually rejoin that mighty band of stars in
the far north.
While the stars in general appear to travel independently of one
another, except when they are combined in binary or trinary systems,
there are notable exceptions to this rule. In some quarters of the sky
we behold veritable migrations of entire groups of stars whose members
are too widely separated to show any indications of revolution about a
common center of gravity. This leads us back again to the wonderful
group of the Pleiades. All of the principle stars composing that group
are traveling in virtually parallel lines. Whatever force set them
going evidently acted upon all alike. This might be explained by the
assumption that when the original projective force acted upon them
they were more closely united than they are at present, and that in
drifting apart they have not lost the impulse of the primal motion. Or
it may be supposed that they are carried along by some current in
space, although it would be exceedingly difficult, in the present
state of our knowledge, to explain the nature of such a current. Yet
the theory of a current has been proposed. As to an attractive center
around which they might revolve, none has been found. Another instance
of similar ``star-drift'' is furnished by five of the seven stars
constituting the figure of the ``Great Dipper.'' In this case the
stars concerned are separated very widely, the two extreme ones by not
less than fifteen degrees, so that the idea of a common motion would
never have been suggested by their aspect in the sky; and the case
becomes the more remarkable from the fact that among and between them
there are other stars, some of the same magnitude, which do not share
their motion, but are traveling in other directions. Still other
examples of the same phenomenon are found in other parts of the sky.
Of course, in the case of compact star-clusters, it is assumed that
all the members share a like motion of translation through space, and
the same is probably true of dense star-swarms and star-clouds.
The whole question of star-drift has lately assumed a new phase, in
consequence of the investigations of Kapteyn, Dyson, and Eddington on
the ``systematic motions of the stars.'' This research will, it is
hoped, lead to an understanding of the general law governing the
movements of the whole body of stars constituting the visible
universe. Taking about eleven hundred stars whose proper motions have
been ascertained with an approach to certainty, and which are
distributed in all parts of the sky, it has been shown that there
exists an apparent double drift, in two independent streams, moving in
different and nearly opposed directions. The apex of the motion of
what is called ``Stream I'' is situated, according to Professor
Kapteyn, in right ascension 85°, declination south 11°, which places
it just south of the constellation Orion; while the apex of ``Stream
II'' is in right ascension 260°, declination south 48°, placing it in
the constellation Ara, south of Scorpio. The two apices differ very
nearly 180° in right ascension and about 120° in declination. The
discovery of these vast star-streams, if they really exist, is one of
the most extraordinary in modern astronomy. It offers the correlation
of stellar movements needed as the basis of a theory of those
movements, but it seems far from revealing a physical cause for them.
As projected against the celestial sphere the stars forming the two
opposite streams appear intermingled, some obeying one tendency and
some the other. As Professor Dyson has said, the hypothesis of this
double movement is of a revolutionary character, and calls for further
investigation. Indeed, it seems at first glance not less surprising
than would be the observation that in a snow-storm the flakes over our
heads were divided into two parties and driving across each other's
course in nearly opposite directions, as if urged by interpenetrating
winds.
But whatever explanation may eventually be found for the motions of
the stars, the knowledge of the existence of those motions must always
afford a new charm to the contemplative observer of the heavens, for
they impart a sense of life to the starry system that would otherwise
be lacking. A stagnant universe, with every star fixed immovably in
its place, would not content the imagination or satisfy our longing
for ceaseless activity. The majestic grandeur of the evolutions of the
celestial hosts, the inconceivable vastness of the fields of space in
which they are executed, the countless numbers, the immeasurable
distances, the involved convolutions, the flocking and the scattering,
the interpenetrating marches and countermarches, the strange community
of impulsion affecting stars that are wide apart in space and causing
them to traverse the general movement about them like aides and
despatch-bearers on a battle-field -- all these arouse an intensity of
interest which is heightened by the mystery behind them.
The Passing of the Constellations
From a historical and picturesque point of view, one of the most
striking results of the motions of the stars described in the last
chapter is their effect upon the forms of the constellations, which
have been watched and admired by mankind from a period so early that
the date of their invention is now unknown. The constellations are
formed by chance combinations of conspicuous stars, like figures in a
kaleidoscope, and if our lives were commensurate with the æons of
cosmic existence we should perceive that the kaleidoscope of the
heavens was ceaselessly turning and throwing the stars into new
symmetries. Even if the stars stood fast, the motion of the solar
system would gradually alter the configurations, as the elements of a
landscape dissolve and recombine in fresh groupings with the
traveler's progress amid them. But with the stars themselves all in
motion at various speeds and in many directions, the changes occur
more rapidly. Of course, ``rapid'' is here understood in a relative
sense; the wheel of human history to an eye accustomed to the majestic
progression of the universe would appear to revolve with the velocity
of a whirling dynamo. Only the deliberation of geological movements
can be contrasted with the evolution and devolution of the
constellations.
And yet this secular fluctuation of the constellation figures is not
without keen interest for the meditative observer. It is another
reminder of the swift mutability of terrestial affairs. To the passing
glance, which is all that we can bestow upon these figures, they
appear so immutable that they have been called into service to form
the most lasting records of ancient thought and imagination that we
possess. In the forms of the constellations, the most beautiful, and,
in imaginative quality, the finest, mythology that the world has ever
known has been perpetuated. Yet, in a broad sense, this scroll of
human thought imprinted on the heavens is as evanescent as the summer
clouds. Although more enduring than parchment, tombs, pyramids, and
temples, it is as far as they from truly eternizing the memory of what
man has fancied and done.
Before studying the effects that the motions of the stars have had and
will have upon the constellations, it is worth while to consider a
little further the importance of the stellar pictures as archives of
history. To emphasize the importance of these effects it is only
necessary to recall that the constellations register the oldest
traditions of our race. In the history of primeval religions they are
the most valuable of documents. Leaving out of account for the moment
the more familiar mythology of the Greeks, based on something older
yet, we may refer for illustration to that of the mysterious Maya race
of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group
of ruins perched, after the Mexican and Central-American plan, on the
summits of pyramidal mounds which mark the site of an ancient
theogonic center of the Mayas. Here the temples all evidently refer to
a cult based upon the constellations as symbols. The figures and the
names, of course, were not the same as those that we have derived from
our Aryan ancestors, but the star groups were the same or nearly so.
For instance, the loftiest of the temples at Izamal was connected with
the sign of the constellation known to us as Cancer, marking the place
of the sun at the summer solstice, at which period the sun was
supposed to descend at noon like a great bird of fire and consume the
offerings left upon the altar. Our Scorpio was known to the Mayas as a
sign of the ``Death God.'' Our Libra, the ``Balance,'' with which the
idea of a divine weighing out of justice has always been connected,
seems to be identical with the Mayan constellation Teoyaotlatohua,
with which was associated a temple where dwelt the priests whose
special business it was to administer justice and to foretell the
future by means of information obtained from the spirits of the dead.
Orion, the ``Hunter'' of our celestial mythology, was among the Mayas
a ``Warrior,'' while Sagittarius and others of our constellations were
known to them (under different names, of course), and all were endowed
with a religious symbolism. And the same star figures, having the same
significance, were familiar to the Peruvians, as shown by the temples
at Cuzco. Thus the imagination of ancient America sought in the
constellations symbols of the unchanging gods.
But, in fact, there is no nation and no people that has not recognized
the constellations, and at one period or another in its history
employed them in some symbolic or representative capacity. As handled
by the Greeks from prehistoric times, the constellation myths became
the very soul of poetry. The imagination of that wonderful race
idealized the principal star groups so effectively that the figures
and traditions thus attached to them have, for civilized mankind,
displaced all others, just as Greek art in its highest forms stands
without parallel and eclipses every rival. The Romans translated no
heroes and heroines of the mythical period of their history to the
sky, and the deified Cæsars never entered that lofty company, but the
heavens are filled with the early myths of the Greeks. Herakles
nightly resumes his mighty labors in the stars; Zeus, in the form of
the white ``Bull,'' Taurus, bears the fair Europa on his back through
the celestial waves; Andromeda stretches forth her shackled arms in
the star-gemmed ether, beseeching aid; and Perseus, in a blaze of
diamond armor, revives his heroic deeds amid sparkling clouds of
stellar dust. There, too, sits Queen Cassiopeia in her dazzling chair,
while the Great King, Cepheus, towers gigantic over the pole.
Professor Young has significantly remarked that a great number of the
constellations are connected in some way or other with the Argonautic
Expedition -- that strangely fascinating legend of earliest Greek
story which has never lost its charm for mankind. In view of all this,
we may well congratulate ourselves that the constellations will
outlast our time and the time of countless generations to follow us;
and yet they are very far from being eternal. Let us now study some of
the effects of the stellar motions upon them.
We begin with the familiar figure of the ``Great Dipper.'' He who has
not drunk inspiration from its celestial bowl is not yet admitted to
the circle of Olympus. This figure is made up of seven conspicuous
stars in the constellation Ursa Major, the ``Greater Bear.'' The
handle of the ``Dipper'' corresponds to the tail of the imaginary
``Bear,'' and the bowl lies upon his flank. In fact, the figure of a
dipper is so evident and that of a bear so unevident, that to most
persons the ``Great Dipper'' is the only part of the constellation
that is recognizable. Of the seven stars mentioned, six are of nearly
equal brightness, ranking as of the second magnitude, while the
seventh is of only the third magnitude. The difference is very
striking, since every increase of one magnitude involves an increase
of two-and-a-half times in brightness. There appears to be little
doubt that the faint star, which is situated at the junction of the
bowl and the handle, is a variable of long period, since three hundred
years ago it was as bright as its companions. But however that may be,
its relative faintness at the present time interferes but little with
the perfection of the ``Dipper's'' figure. In order the more readily
to understand the changes which are taking place, it will be well to
mention both the names and the Greek letters which are attached to the
seven stars. Beginning at the star in the upper outer edge of the rim
of the bowl and running in regular order round the bottom and then out
to the end of the handle, the names and letters are as follows: Dubhe
({\alpha}), Merak ({\beta}), Phaed ({\gamma}), Megrez ({\delta}),
Alioth ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez
is the faint star already mentioned at the junction of the bowl and
handle, and Mizar, in the middle of the handle, has a close, naked-eye
companion which is named Alcor. The Arabs called this singular pair of
stars ``The Horse and Rider.'' Merak and Duhbe are called ``The
Pointers,'' because an imaginary line drawn northward through them
indicates the Pole Star.
Now it has been found that five of these stars -- viz., Merak, Phaed,
Megrez, Alioth, and Mizar (with its comrade) -- are moving with
practically the same speed in an easterly direction, while the other
two, Dubhe and Benetnasch, are simultaneously moving westward, the
motions of Benetnasch being apparently more rapid. The consequence of
these opposed motions is, of course, that the figure of the ``Dipper''
cannot always have existed and will not continue to exist. In the
accompanying diagrams it has been thought interesting to show the
relative positions of these seven stars, as seen from the point which
the earth now occupies, both in the past and in the future. Arrows
attached to the stars in the figure representing the present
appearance of the ``Dipper'' indicate the directions of the motions
and the distances over which they will carry the stars in a period of
about five hundred centuries. The time, no doubt, seems long, but
remember the vast stretch of ages through which the earth has passed,
and then reflect that no reason is apparent why our globe should not
continue to be a scene of animation for ten thousand centuries yet to
come. The fact that the little star Alcor placed so close to Mizar
should accompany the latter in its flight is not surprising, but that
two of the principal stars of the group should be found moving in a
direction directly opposed to that pursued by the other five is
surprising in the highest degree; and it recalls the strange theory of
a double drift affecting all the stars, to which attention was called
in the preceding chapter. It would appear that Benetnasch and Dubhe
belong to one ``current,'' and Merak, Phaed, Megrez, Alioth, and Mizar
to the other. As far as is known, the motion of the seven stars are
not shared by the smaller stars scattered about them, but on the
theory of currents there should be such a community of motion, and
further investigation may reveal it.
From the ``Great Dipper'' we turn to a constellation hardly less
conspicuous and situated at an equal distance from the pole on the
other side -- Cassiopeia. This famous star-group commemorating the
romantic Queen of Ethiopia whose vain boasting of her beauty was
punished by the exposure of her daughter Andromeda to the ``Sea
Monster,'' is well-marked by five stars which form an irregular letter
``W'' with its open side toward the pole. Three of these stars are
usually ranked as of the second magnitude, and two of the third; but
to ordinary observation they appear of nearly equal brightness, and
present a very striking picture. They mark out the chair and a part of
the figure of the beautiful queen. Beginning at the right-hand, or
western, end of the ``W,'' their Greek letter designations are: Beta
({\beta}), Alpha ({\alpha}), Gamma ({\gamma}), Delta ({\delta}), and
Epsilon ({\epsilon}). Four of them, Beta, Alpha, Delta, and Epsilon
are traveling eastwardly at various speeds, while the fifth, Gamma,
moves in a westerly direction. The motion of Beta is more rapid than
that of any of the others. It should be said, however, that no little
uncertainty attaches to the estimates of the rate of motion of stars
which are not going very rapidly, and different observers often vary
considerably in their results.
In the beautiful ``Northern Crown,'' one of the most perfect and
charming of all the figures to be found in the stars, the alternate
combining and scattering effects of the stellar motions are shown by
comparing the appearance which the constellation must have had five
hundred centuries ago with that which it has at present and that which
it will have in the future. The seven principle stars of the asterism,
forming a surprisingly perfect coronet, have movements in three
directions at right angles to one another. That in these circumstances
they should ever have arrived at positions giving them so striking an
appearance of definite association is certainly surprising; from its
aspect one would have expected to find a community of movement
governing the brilliants of the ``Crown,'' but instead of that we find
evidence that they will inevitably drift apart and the beautiful
figure will dissolve.
A similar fate awaits such asterisms as the ``Northern Cross'' in
Cygnus; the ``Crow'' (Corvus), which stands on the back of the great
``Sea Serpent,'' Hydra, and pecks at his scales; ``Job's Coffin''
(Delphinus); the ``Great Square of Pegasus''; the ``Twins'' (Gemini);
the beautiful ``Sickle'' in Leo; and the exquisite group of the Hyades
in Taurus. In the case of the Hyades, two controlling movements are
manifest: one, affecting five of the stars which form the well-known
figure of a letter ``V,'' is directed northerly; the other, which
controls the direction of two stars, has an easterly trend. The chief
star of the group, Aldebaran, one of the finest of all stars both for
its brilliance and its color, is the most affected by the easterly
motion. In time it will drift entirely out of connection with its
present neighbors. Although the Hyades do not form so compact a group
as the Pleiades in the same constellation, yet their appearance of
relationship is sufficient to awaken a feeling of surprise over the
fact that, as with the stars of the ``Dipper,'' their association is
only temporary or apparent.
The great figure of Orion appears to be more lasting, not because its
stars are physically connected, but because of their great distance,
which renders their movements too deliberate to be exactly
ascertained. Two of the greatest of its stars, Betelgeuse and Rigel,
possess, as far as has been ascertained, no perceptible motion across
the line of sight, but there is a little movement perceptible in the
``Belt.'' At the present time this consists of an almost perfect
straight line, a row of second-magnitude stars about equally spaced
and of the most striking beauty. In the course of time, however, the
two right-hand stars, Mintaka and Alnilam (how fine are these Arabic
star names!) will approach each other and form a naked-eye double, but
the third, Alnita, will drift away eastward, so that the ``Belt'' will
no longer exist.
For one more example, let us go to the southern hemisphere, whose most
celebrated constellation, the ``Southern Cross,'' has found a place in
all modern literatures, although it has no claim to consideration on
account of association with ancient legends. This most attractive
asterism, which has never ceased to fascinate the imagination of
Christendom since it was first devoutly described by the early
explorers of the South, is but a passing collocation of brilliant
stars. Yet even in its transfigurations it has been for hundreds of
centuries, and will continue to be for hundreds of centuries to come,
a most striking object in the sky. Our figures show its appearance in
three successive phases: first, as it was fifty thousand years ago
(viewed from the earth's present location); second, as it is in our
day; and, third, as it will be an equal time in the future. The
nearness of these bright stars to one another -- the length of the
longer beam of the ``Cross'' is only six degrees -- makes this group
very noticeable, whatever the arrangement of its components may be.
The largest star, at the base of the ``Cross,'' is of the first
magnitude, two of the others are of the second magnitude, and the
fourth is of the third. Other stars, not represented in the figures,
increase the effect of a celestial blazonry, although they do not help
the resemblance to a cross.
But since the motion of the solar system itself will, in the course of
so long a period as fifty thousand years, produce a great change in
the perspective of the heavens as seen from the earth, by carrying us
nearly nineteen trillion miles from our present place, why, it may be
asked, seek to represent future appearances of the constellations
which we could not hope to see, even if we could survive so long? The
answer is: Because these things aid the mind to form a picture of the
effects of the mobility of the starry universe. Only by showing the
changes from some definite point of view can we arrive at a due
comprehension of them. The constellations are more or less familiar to
everybody, so that impending changes of their forms must at once
strike the eye and the imagination, and make clearer the significance
of the movements of the stars. If the future history of mankind is to
resemble its past and if our race is destined to survive yet a million
years, then our remote descendents will see a ``new heavens'' if not a
``new earth,'' and will have to invent novel constellations to
perpetuate their legends and mythologies.
If our knowledge of the relative distances of the stars were more
complete, it would be an interesting exercise in celestial geometry to
project the constellations probably visible to the inhabitants of
worlds revolving around some of the other suns of space. Our sun is
too insignificant for us to think that he can make a conspicuous
appearance among them, except, perhaps, in a few cases. As seen, for
instance, from the nearest known star, Alpha Centauri, the sun would
appear of the average first magnitude, and consequently from that
standpoint he might be the gem of some little constellation which had
no Sirius, or Arcturus, or Vega to eclipse him with its superior
splendor. But from the distance of the vast majority of the stars the
sun would probably be invisible to the naked eye, and as seen from
nearer systems could only rank as a fifth or sixth magnitude star,
unnoticed and unknown except by the star-charting astronomer.
Conflagrations in the Heavens
Suppose it were possible for the world to take fire and burn up -- as
some pessimists think that it will do when the Divine wrath shall have
sufficiently accumulated against it -- nobody out of our own little
corner of space would ever be aware of the catastrophe! With all their
telescopes, the astronomers living in the golden light of Arcturus or
the diamond blaze of Canopus would be unable to detect the least
glimmer of the conflagration that had destroyed the seat of Adam and
his descendents, just as now they are totally ignorant of its
existence.
But at least fifteen times in the course of recorded history men
looking out from the earth have beheld in the remote depths of space
great outbursts of fiery light, some of them more splendidly luminous
than anything else in the firmament except the sun! If they were
conflagrations, how many million worlds like ours were required to
feed their blaze?
It is probable that ``temporary'' or ``new'' stars, as these wonderful
apparitions are called, really are conflagrations; not in the sense of
a bonfire or a burning house or city, but in that of a sudden eruption
of inconceivable heat and light, such as would result from the
stripping off the shell of an encrusted sun or the crashing together
of two mighty orbs flying through space with a hundred times the
velocity of the swiftest cannon-shot.
Temporary stars are the rarest and most erratic of astronomical
phenomena. The earliest records relating to them are not very clear,
and we cannot in every instance be certain that it was one of these
appearances that the ignorant and superstitious old chroniclers are
trying to describe. The first temporary star that we are absolutely
sure of appeared in 1572, and is known as ``Tycho's Star,'' because
the celebrated Danish astronomer (whose remains, with his
gold-and-silver artificial nose -- made necessary by a duel -- still
intact, were disinterred and reburied in 1901) was the first to
perceive it in the sky, and the most assiduous and successful in his
studies of it. As the first fully accredited representative of its
class, this new star made its entry upon the scene with becoming
éclat. It is characteristic of these phenomena that they burst into
view with amazing suddenness, and, of course, entirely unexpectedly.
Tycho's star appeared in the constellation Cassiopeia, near a now
well-known and much-watched little star named Kappa, on the evening of
November 11, 1572. The story has often been repeated, but it never
loses interest, how Tycho, going home that evening, saw people in the
street pointing and staring at the sky directly over their heads, and
following the direction of their hands and eyes he was astonished to
see, near the zenith, an unknown star of surpassing brilliance. It
outshone the planet Jupiter, and was therefore far brighter than the
first magnitude. There was not another star in the heavens that could
be compared with it in splendor. Tycho was not in all respects free
from the superstitions of his time -- and who is? -- but he had the
true scientific instinct, and immediately he began to study the
stranger, and to record with the greatest care every change in its
aspect. First he determined as well as he could with the imperfect
instruments of his day, many of which he himself had invented, the
precise location of the phenomena in the sky. Then he followed the
changes that it underwent. At first it brightened until its light
equaled or exceeded that of the planet Venus at her brightest, a
statement which will be appreciated at its full value by anyone who
has ever watched Venus when she plays her dazzling rôle of ``Evening
Star,'' flaring like an arc light in the sunset sky. It even became so
brilliant as to be visible in full daylight, since, its position being
circumpolar, it never set in the latitude of Northern Europe. Finally
it began to fade, turning red as it did so, and in March, 1574, it
disappeared from Tycho's searching gaze, and has never been seen again
from that day to this. None of the astronomers of the time could make
anything of it. They had not yet as many bases of speculation as we
possess today.
Tycho's star has achieved a romantic reputation by being fancifully
identified with the ``Star of Bethlehem,'' said to have led the
wondering Magi from their eastern deserts to the cradle-manger of the
Savior in Palestine. Many attempts have been made to connect this
traditional ``star'' with some known phenomenon of the heavens, and
none seems more idle than this. Yet it persistently survives, and no
astronomer is free from eager questions about it addressed by people
whose imagination has been excited by the legend. It is only necessary
to say that the supposition of a connection between the phenomenon of
the Magi and Tycho's star is without any scientific foundation. It was
originally based on an unwarranted assumption that the star of Tycho
was a variable of long period, appearing once every three hundred and
fifteen years, or thereabout. If that were true there would have been
an apparition somewhere near the traditional date of the birth of
Christ, a date which is itself uncertain. But even the data on which
the assumption was based are inconsistent with the theory. Certain
monkish records speak of something wonderful appearing in the sky in
the years 1264 and 945, and these were taken to have been outbursts of
Tycho's star. Investigation shows that the records more probably refer
to comets, but even if the objects seen were temporary stars, their
dates do not suit the hypothesis; from 945 to 1264 there is a gap of
319 years, and from 1264 to 1572 one of only 308 years; moreover 337
years have now (1909) elapsed since Tycho saw the last glimmer of his
star. Upon a variability so irregular and uncertain as that, even if
we felt sure that it existed, no conclusion could be found concerning
an apparition occurring 2000 years ago.
In the year 1600 (the year in which Giordano Bruno was burned at the
stake for teaching that there is more than one physical world), a
temporary star of the third magnitude broke out in the constellation
Cygnus, and curiously enough, considering the rarity of such
phenomena, only four years later another surprisingly brilliant one
appeared in the constellation Ophiuchus. This is often called
``Kepler's star,'' because the great German astronomer devoted to it
the same attention that Tycho had given to the earlier phenomenon. It,
too, like Tycho's, was at first the brightest object in the stellar
heavens, although it seems never to have quite equaled its famous
predecessor in splendor. It disappeared after a year, also turning of
a red color as it became more faint. We shall see the significance of
this as we go on. Some of Kepler's contemporaries suggested that the
outburst of this star was due to a meeting of atoms in space, and idea
bearing a striking resemblance to the modern theory of ``astronomical
collisions.''
In 1670, 1848, and 1860 temporary stars made their appearance, but
none of them was of great brilliance. In 1866 one of the second
magnitude broke forth in the ``Northern Crown'' and awoke much
interest, because by that time the spectroscope had begun to be
employed in studying the composition of the stars, and Huggins
demonstrated that the new star consisted largely of incandescent
hydrogen. But this star, apparently unlike the others mentioned, was
not absolutely new. Before its outburst it had shown as a star of the
ninth magnitude (entirely invisible, of course, to the naked eye), and
after about six weeks it faded to its original condition in which it
has ever since remained. In 1876 a temporary star appeared in the
constellation Cygnus, and attained at one time the brightness of the
second magnitude. Its spectrum and its behavior resembled those of its
immediate predecessor. In 1885, astronomers were surprised to see a
sixth-magnitude star glimmering in the midst of the hazy cloud of the
great Andromeda Nebula. It soon absolutely disappeared. Its spectrum
was remarkable for being ``continuous,'' like that of the nebula
itself. A continuous spectrum is supposed to represent a body, or a
mass, which is either solid or liquid, or composed of gas under great
pressure. In January, 1892, a new star was suddenly seen in the
constellation Auriga. It never rose much above the fourth magnitude,
but it showed a peculiar spectrum containing both bright and dark
lines of hydrogen.
But a bewildering surprise was now in store; the world was to behold
at the opening of the twentieth century such a celestial spectacle as
had not been on view since the times of Tycho and Kepler. Before
daylight on the morning of February 22, 1901, the Rev. Doctor
Anderson, of Edinburgh, an amateur astronomer, who had also been the
first to see the new star in Auriga, beheld a strange object in the
constellation Perseus not far from the celebrated variable star Algol.
He recognized its character at once, and immediately telegraphed the
news, which awoke the startled attention of astronomers all over the
world. When first seen the new star was no brighter than Algol (less
than the second magnitude), but within twenty-four hours it was
ablaze, outshining even the brilliant Capella, and far surpassing the
first magnitude. At the spot in the sky where it appeared nothing
whatever was visible on the night before its coming. This is known
with certainty because a photograph had been made of that very region
on February 21, and this photograph showed everything down to the
twelfth magnitude, but not a trace of the stranger which burst into
view between the 21st and the 22nd like the explosion of a rocket.
Upon one who knew the stars the apparition of this intruder in a
well-known constellation had the effect of a sudden invasion. The new
star was not far west of the zenith in the early evening, and in that
position showed to the best advantage. To see Capella, the hitherto
unchallenged ruler of that quarter of the sky, abased by comparison
with this stranger of alien aspect, for there was always an unfamiliar
look about the ``nova,'' was decidedly disconcerting. It seemed to
portend the beginning of a revolution in the heavens. One could
understand what the effect of such an apparition must have been in the
superstitious times of Tycho. The star of Tycho had burst forth on the
northern border of the Milky Way; this one was on its southern border,
some forty-five degrees farther east.
Astronomers were well-prepared this time for the scientific study of
the new star, both astronomical photography and spectroscopy having
been perfected, and the results of their investigations were
calculated to increase the wonder with which the phenomenon was
regarded. The star remained at its brightest only a few days; then,
like a veritable conflagration, it began to languish; and, like the
reflection of a dying fire, as it sank it began to glow with the red
color of embers. But its changes were spasmodic; once about every
three days it flared up only to die away again. During these
fluctuations its light varied alternately in the ratio of one to six.
Finally it took a permanent downward course, and after a few months
the naked eye could no longer perceive it; but it remained visible
with telescopes, gradually fading until it had sunk to the ninth
magnitude. Then another astonishing change happened: in August
photographs taken at the Yerkes Observatory and at Heidelberg showed
that the ``nova'' was surrounded by a spiral nebula! The nebula had
not been there before, and no one could doubt that it represented a
phase of the same catastrophe that had produced the outburst of the
new star. At one time the star seemed virtually to have disappeared,
as if all its substance had been expanded into the nebulous cloud, but
always there remained a stellar nucleus about which the misty spiral
spread wider and ever wider, like a wave expanding around a center of
disturbance. The nebula too showed a variability of brightness, and
four condensations which formed in it seemed to have a motion of
revolution about the star. As time went on the nebula continued to
expand at a rate which was computed to be not less than twenty
thousand miles per second! And now the star itself, showing
indications of having turned into a nebula, behaved in a most erratic
manner, giving rise to the suspicion that it was about to burst out
again. But this did not occur, and at length it sunk into a state of
lethargy from which it has to the present time not recovered. But the
nebulous spiral has disappeared, and the entire phenomena as it now
(1909) exists consists of a faint nebulous star of less than the ninth
magnitude.
The wonderful transformations just described had been forecast in
advance of the discovery of the nebulous spiral encircling the star by
the spectroscopic study of the latter. At first there was no
suggestion of a nebular constitution, but within a month or two
characteristic nebular lines began to appear, and in less than six
months the whole spectrum had been transformed to the nebular type. In
the mean time the shifting of the spectral lines indicated a
complication of rapid motions in several directions simultaneously.
These motions were estimated to amount to from one hundred to five
hundred miles per second.
The human mind is so constituted that it feels forced to seek an
explanation of so marvelous a phenomenon as this, even in the absence
of the data needed for a sound conclusion. The most natural
hypothesis, perhaps, is that of a collision. Such a catastrophe could
certainly happen. It has been shown, for instance, that in infinity of
time the earth is sure to be hit by a comet; in the same way it may be
asserted that, if no time limit is fixed, the sun is certain to run
against some obstacle in space, either another star, or a dense meteor
swarm, or one of the dark bodies which there is every reason to
believe abound around us. The consequences of such a collision are
easy to foretell, provided that we know the masses and the velocities
of the colliding bodies. In a preceding chapter we have discussed the
motions of the sun and stars, and have seen that they are so swift
that an encounter between any two of them could not but be disastrous.
But this is not all; for as soon as two stars approached within a few
million miles their speed would be enormously increased by their
reciprocal attractions and, if their motion was directed radially with
respect to their centers, they would come together with a crash that
would reduce them both to nebulous clouds. It is true that the chances
of such a ``head-on'' collision are relatively very small; two stars
approaching each other would most probably fall into closed orbits
around their common center of gravity. If there were a collision it
would most likely be a grazing one instead of a direct front-to-front
encounter. But even a close approach, without any actual collision,
would probably prove disastrous, owing to the tidal influence of each
of the bodies on the other. Suns, in consequence of their enormous
masses and dimensions and the peculiarities of their constitution, are
exceedingly dangerous to one another at close quarters. Propinquity
awakes in them a mutually destructive tendency. Consisting of matter
in the gaseous, or perhaps, in some cases, liquid, state, their tidal
pull upon each other if brought close together might burst them
asunder, and the photospheric envelope being destroyed the internal
incandescent mass would gush out, bringing fiery death to any planets
that were revolving near. Without regard to the resulting disturbance
of the earth's orbit, the close approach of a great star to the sun
would be in the highest degree perilous to us. But this is a danger
which may properly be regarded as indefinitely remote, since, at our
present location in space, we are certainly far from every star except
the sun, and we may feel confident that no great invisible body is
near, for if there were one we should be aware of its presence from
the effects of its attraction. As to dark nebulæ which may possibly
lie in the track that the solar system is pursuing at the rate of
375,000,000 miles per year, that is another question -- and they, too,
could be dangerous!
This brings us directly back to ``Nova Persei,'' for among the many
suggestions offered to explain its outburst, as well as those of other
temporary stars, one of the most fruitful is that of a collision
between a star and a vast invisible nebula. Professor Seeliger, of
Munich, first proposed this theory, but it afterward underwent some
modifications from others. Stated in a general form, the idea is that
a huge dark body, perhaps an extinguished sun, encountered in its
progress through space a widespread flock of small meteors forming a
dark nebula. As it plunged into the swarm the friction of the
innumerable collisions with the meteors heated its surface to
incandescence, and being of vast size it then became visible to us as
a new star. Meanwhile the motion of the body through the nebula, and
its rotation upon itself, set up a gyration in the blazing atmosphere
formed around it by the vaporized meteors; and as this atmosphere
spread wider, under the laws of gyratory motion a rotation in the
opposite direction began in the inflamed meteoric cloud outside the
central part of the vortex. Thus the spectral lines were caused to
show motion in opposite directions, a part of the incandescent mass
approaching the earth simultaneously with the retreat of another part.
So the curious spectroscopic observations before mentioned were
explained. This theory might also account for the appearance of the
nebulous spiral first seen some six months after the original
outburst. The sequent changes in the spectrum of the ``nova'' are
accounted for by this theory on the assumption, reasonable enough in
itself, that at first the invading body would be enveloped in a
vaporized atmosphere of relatively slight depth, producing by its
absorption the fine dark lines first observed; but that as time went
on and the incessant collisions continued, the blazing atmosphere
would become very deep and extensive, whereupon the appearance of the
spectral lines would change, and bright lines due to the light of the
incandescent meteors surrounding the nucleus at a great distance would
take the place of the original dark ones. The vortex of meteors once
formed would protect the flying body within from further immediate
collisions, the latter now occurring mainly among the meteors
themselves, and then the central blaze would die down, and the
original splendor of the phenomenon would fade.
But the theories about Nova Persei have been almost as numerous as the
astronomers who have speculated about it. One of the most startling of
them assumed that the outburst was caused by the running amuck of a
dark star which had encountered another star surrounded with planets,
the renewed outbreaks of light after the principal one had faded being
due to the successive running down of the unfortunate planets! Yet
another hypothesis is based on what we have already said of the tidal
influence that two close approaching suns would have upon each other.
Supposing two such bodies which had become encrusted, but remained
incandescent and fluid within, to approach within almost striking
distance; they would whirl each other about their common center of
gravity, and at the same time their shells would burst under the tidal
strain, and their glowing nuclei being disclosed would produce a great
outburst of light. Applying this theory to a ``nova,'' like that of
1866 in the ``Northern Crown,'' which had been visible as a small star
before the outbreak, and which afterward resumed its former aspect, we
should have to assume that a yet shining sun had been approached by a
dark body whose attraction temporarily burst open its photosphere. It
might be supposed that in this case the dark body was too far advanced
in cooling to suffer the same fate from the tidal pull of its victim.
But a close approach of that kind would be expected to result in the
formation of a binary system, with orbits of great eccentricity,
perhaps, and after the lapse of a certain time the outburst should be
renewed by another approximation of the two bodies. A temporary star
of that kind would rather be ranked as a variable.
The celebrated French astronomer, Janssen, had a different theory of
Nova Persei, and of temporary stars in general. According to his idea,
such phenomena might be the result of chemical changes taking place in
a sun without interference by, or collision with, another body.
Janssen was engaged for many years in trying to discover evidence of
the existence of oxygen in the sun, and he constructed his observatory
on the summit of Mount Blanc specially to pursue that research. He
believed that oxygen must surely exist in the sun since we find so
many other familiar elements included in the constitution of the solar
globe, and as he was unable to discover satisfactory evidence of its
presence he assumed that it existed in a form unknown on the earth. If
it were normally in the sun's chromosphere, or coronal atmosphere, he
said, it would combine with the hydrogen which we know is there and
form an obscuring envelope of water vapor. It exists, then, in a
special state, uncombined with hydrogen; but let the temperature of
the sun sink to a critical point and the oxygen will assume its normal
properties and combine with the hydrogen, producing a mighty outburst
of light and heat. This, Janssen thought, might explain the phenomena
of the temporary stars. It would also, he suggested, account for their
brief career, because the combination of the elements would be quickly
accomplished, and then the resulting water vapor would form an
atmosphere cutting off the radiation from the star within.
This theory may be said to have a livelier human interest than some of
the others, since, according to it, the sun may carry in its very
constitution a menace to mankind; one does not like to think of it
being suddenly transformed into a gigantic laboratory for the
explosive combination of oxygen and hydrogen! But while Janssen's
theory might do for some temporary stars, it is inadequate to explain
all the phenomena of Nova Persei, and particularly the appearance of
the great spiral nebula that seemed to exhale from the heart of the
star. Upon the whole, the theory of an encounter between a star and a
dark nebula seems best to fit the observations. By that hypothesis the
expanding billow of light surrounding the core of the conflagration is
very well accounted for, and the spectroscopic peculiarities are also
explained.
Dr Gustov Le Bon offers a yet more alarming theory, suggesting that
temporary stars are the result of atomic explosion; but we shall touch
upon this more fully in Chapter 14.
Twice in the course of this discussion we have called attention to the
change of color invariably undergone by temporary stars in the later
stages of their career. This was conspicuous with Nova Persei which
glowed more and more redly as it faded, until the nebulous light began
to overpower that of the stellar nucleus. Nothing could be more
suggestive of the dying out of a great fire. Moreover, change of color
from white to red is characteristic of all variable stars of long
period, such as ``Mira'' in Cetus. It is also characteristic of stars
believed to be in the later stages of evolution, and consequently
approaching extinction, like Antares and Betelgeuse, and still more
notably certain small stars which ``gleam like rubies in the field of
the telescope.'' These last appear to be suns in the closing period of
existence as self-luminous bodies. Between the white stars, such as
Sirius and Rigel, and the red stars, such as Aldebaran and Alpha
Herculis, there is a progressive series of colors from golden yellow
through orange to deep red. The change is believed to be due to the
increase of absorbing vapors in the stellar atmosphere as the body
cools down. In the case of ordinary stars these changes no doubt
occupy many millions of years, which represent the average duration of
solar life; but the temporary stars run through similar changes in a
few months: they resemble ephemeral insects -- born in the morning and
doomed to perish with the going down of the sun.
Explosive and Whirling Nebulæ
One of the most surprising triumphs of celestial photography was
Professor Keeler's discovery, in 1899, that the great majority of the
nebulæ have a distinctly spiral form. This form, previously known in
Lord Rosse's great ``Whirlpool Nebula,'' had been supposed to be
exceptional; now the photographs, far excelling telescopic views in
the revelation of nebular forms, showed the spiral to be the typical
shape. Indeed, it is a question whether all nebulæ are not to some
extent spiral. The extreme importance of this discovery is shown in
the effect that it has had upon hitherto prevailing views of solar and
planetary evolution. For more than three-quarters of a century
Laplace's celebrated hypothesis of the manner of origin of the solar
system from a rotating and contracting nebula surrounding the sun had
guided speculation on that subject, and had been tentatively extended
to cover the evolution of systems in general. The apparent forms of
some of the nebulæ which the telescope had revealed were regarded, and
by some are still regarded, as giving visual evidence in favor of this
theory. There is a ``ring nebula'' in Lyra with a central star, and a
``planetary nebula'' in Gemini bearing no little resemblance to the
planet Saturn with its rings, both of which appear to be practical
realizations of Laplace's idea, and the elliptical rings surrounding
the central condensation of the Andromeda Nebula may be cited for the
same kind of proof.
But since Keeler's discovery there has been a decided turning away of
speculation another way. The form of the spiral nebulæ seems to be
entirely inconsistent with the theory of an originally globular or
disk-shaped nebula condensing around a sun and throwing or leaving off
rings, to be subsequently shaped into planets. Some astronomers,
indeed, now reject Laplace's hypothesis in toto, preferring to think
that even our solar system originated from a spiral nebula. Since the
spiral type prevails among the existing nebulæ, we must make any
mechanical theory of the development of stars and planetary systems
from them accord with the requirements which that form imposes. A
glance at the extraordinary variations upon the spiral which Professor
Keeler's photographs reveal is sufficient to convince one of the
difficulty of the task of basing a general theory upon them. In truth,
it is much easier to criticize Laplace's hypothesis than to invent a
satisfactory substitute for it. If the spiral nebulæ seem to oppose it
there are other nebulæ which appear to support it, and it may be that
no one fixed theory can account for all the forms of stellar evolution
in the universe. Our particular planetary system may have originated
very much as the great French mathematician supposed, while others
have undergone, or are now undergoing, a different process of
development. There is always a too strong tendency to regard an
important new discovery and the theories and speculations based upon
it as revolutionizing knowledge, and displacing or overthrowing
everything that went before. Upon the plea that ``Laplace only made a
guess'' more recent guesses have been driven to extremes and treated
by injudicious exponents as ``the solid facts at last.''
Before considering more recent theories than Laplace's, let us see
what the nature of the photographic revelations is. The vast celestial
maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be
taken as the leading type of the spiral nebulæ, although there are
less conspicuous objects of the kind which, perhaps, better illustrate
some of their peculiarities. Lord Rosse's nebula appears far more
wonderful in the photographs than in his drawings made with the aid of
his giant reflecting telescope at Parsonstown, for the photographic
plate records details that no telescope is capable of showing. Suppose
we look at the photograph of this object as any person of common sense
would look at any great and strange natural phenomenon. What is the
first thing that strikes the mind? It is certainly the appearance of
violent whirling motion. One would say that the whole glowing mass had
been spun about with tremendous velocity, or that it had been set
rotating so rapidly that it had become the victim of ``centrifugal
force,'' one huge fragment having broken loose and started to gyrate
off into space. Closer inspection shows that in addition to the
principal focus there are various smaller condensations scattered
through the mass. These are conspicuous in the spirals. Some of them
are stellar points, and but for the significance of their location we
might suppose them to be stars which happen to lie in a line between
us and the nebula. But when we observe how many of them follow most
faithfully the curves of the spirals we cannot but conclude that they
form an essential part of the phenomenon; it is not possible to
believe that their presence in such situations is merely fortuitous.
One of the outer spirals has at least a dozen of these star-like
points strung upon it; some of them sharp, small, and distinct, others
more blurred and nebulous, suggesting different stages of
condensation. Even the part which seems to have been flung loose from
the main mass has, in addition to its central condensation, at least
one stellar point gleaming in the half-vanished spire attached to it.
Some of the more distant stars scattered around the ``whirlpool'' look
as if they too had been shot out of the mighty vortex, afterward
condensing into unmistakable solar bodies. There are at least two
curved rows of minute stars a little beyond the periphery of the
luminous whirl which clearly follow lines concentric with those of the
nebulous spirals. Such facts are simply dumbfounding for anyone who
will bestow sufficient thought upon them, for these are suns, though
they may be small ones; and what a birth is that for a sun!
Look now again at the glowing spirals. We observe that hardly have
they left the central mass before they begin to coagulate. In some
places they have a ``ropy'' aspect; or they are like peascods filled
with growing seeds, which eventually will become stars. The great
focus itself shows a similar tendency, especially around its
circumference. The sense that it imparts of a tremendous shattering
force at work is overwhelming. There is probably more matter in that
whirling and bursting nebula than would suffice to make a hundred
solar systems! It must be confessed at once that there is no
confirmation of the Laplacean hypothesis here; but what hypothesis
will fit the facts? There is one which it has been claimed does so,
but we shall come to that later. In the meanwhile, as a preparation,
fix in the memory the appearance of that second spiral mass spinning
beside its master which seems to have spurned it away.
For a second example of the spiral nebulæ look at the one in the
constellation Triangulum. God, how hath the imagination of puny man
failed to comprehend Thee! Here is creation through destruction with a
vengeance! The spiral form of the nebula is unmistakable, but it is
half obliterated amid the turmoil of flying masses hurled away on all
sides with tornadic fury. The focus itself is splitting asunder under
the intolerable strain, and in a little while, as time is reckoned in
the Cosmos, it will be gyrating into stars. And then look at the
cyclonic rain of already finished stars whirling round the outskirts
of the storm. Observe how scores of them are yet involved in the
fading streams of the nebulous spirals; see how they have been thrown
into vast loops and curves, of a beauty that half redeems the terror
of the spectacle enclosed within their lines -- like iridescent cirri
hovering about the edges of a hurricane. And so again are suns born!
Let us turn to the exquisite spiral in Ursa Major; how different its
aspect from that of the other! One would say that if the terrific coil
in Triangulum has all but destroyed itself in its fury, this one on
the contrary has just begun its self-demolition. As one gazes one
seems to see in it the smooth, swift, accelerating motion that
precedes catastrophe. The central part is still intact, dense, and
uniform in texture. How graceful are the spirals that smoothly rise
from its oval rim and, gemmed with little stars, wind off into the
darkness until they have become as delicate as threads of gossamer!
But at bottom the story told here is the same -- creation by gyration!
Compare with the above the curious mass in Cetus. Here the plane of
the whirling nebula nearly coincides with our line of sight and we see
the object at a low angle. It is far advanced and torn to shreds, and
if we could look at it perpendicularly to its plane it is evident that
it would closely resemble the spectacle in Triangulum.
Then take the famous Andromeda Nebula (see Frontispiece), which is so
vast that notwithstanding its immense distance even the naked eye
perceives it as an enigmatical wisp in the sky. Its image on the
sensitive plate is the masterpiece of astronomical photography; for
wild, incomprehensible beauty there is nothing that can be compared
with it. Here, if anywhere, we look upon the spectacle of creation in
one of its earliest stages. The Andromeda Nebula is apparently less
advanced toward transformation into stellar bodies than is that in
Triangulum. The immense crowd of stars sprinkled over it and its
neighborhood seem in the main to lie this side of the nebula, and
consequently to have no connection with it. But incipient stars (in
some places clusters of them) are seen in the nebulous rings, while
one or two huge masses seem to give promise of transformation into
stellar bodies of unusual magnitude. I say ``rings'' because although
the loops encompassing the Andromeda Nebula have been called spirals
by those who wish utterly to demolish Laplace's hypothesis, yet they
are not manifestly such, as can be seen on comparing them with the
undoubted spirals of the Lord Rosse Nebula. They look quite as much
like circles or ellipses seen at an angle of, say, fifteen or twenty
degrees to their plane. If they are truly elliptical they accord
fairly well with Laplace's idea, except that the scale of magnitude is
stupendous, and if the Andromeda Nebula is to become a solar system it
will surpass ours in grandeur beyond all possibility of comparison.
There is one circumstance connected with the spiral nebulæ, and
conspicuous in the Andromeda Nebula on account of its brightness,
which makes the question of their origin still more puzzling; they all
show continuous spectra, which, as we have before remarked, indicate
that the mass from which the light comes is either solid or liquid, or
a gas under heavy pressure. Thus nebulæ fall into two classes: the
``white'' nebulæ, giving a continuous spectrum; and the ``green''
nebulæ whose spectra are distinctly gaseous. The Andromeda Nebula is
the great representative of the former class and the Orion Nebula of
the latter. The spectrum of the Andromeda Nebula has been interpreted
to mean that it consists not of luminous gas, but of a flock of stars
so distant that they are separately indistinguishable even with
powerful telescopes, just as the component stars of the Milky Way are
indistinguishable with the naked eye; and upon this has been based the
suggestion that what we see in Andromeda is an outer universe whose
stars form a series of elliptical garlands surrounding a central mass
of amazing richness. But this idea is unacceptable if for no other
reason than that, as just said, all the spiral nebulæ possess the same
kind of spectrum, and probably no one would be disposed to regard them
all as outer universes. As we shall see later, the peculiarity of the
spectra of the spiral nebulæ is appealed to in support of a modern
substitute for Laplace's hypothesis.
Finally, without having by any means exhausted the variety exhibited
by the spiral nebulæ, let us turn to the great representative of the
other species, the Orion Nebula. In some ways this is even more
marvelous than the others. The early drawings with the telescope
failed to convey an adequate conception either of its sublimity or of
its complication of structure. It exists in a nebulous region of
space, since photographs show that nearly the whole constellation is
interwoven with faintly luminous coils. To behold the entry of the
great nebula into the field even of a small telescope is a startling
experience which never loses its novelty. As shown by the photographs,
it is an inscrutable chaos of perfectly amazing extent, where spiral
bands, radiating streaks, dense masses, and dark yawning gaps are
strangely intermingled without apparent order. In one place four
conspicuous little stars, better seen in a telescope than in the
photograph on account of the blurring produced by over-exposure, are
suggestively situated in the midst of a dark opening, and no observer
has ever felt any doubt that these stars have been formed from the
substance of the surrounding nebula. There are many other stars
scattered over its expanse which manifestly owe their origin to the
same source. But compare the general appearance of this nebula with
the others that we have studied, and remark the difference. If the
unmistakably spiral nebulæ resemble bursting fly-wheels or grindstones
from whose perimeters torrents of sparks are flying, the Orion Nebula
rather recalls the aspect of a cloud of smoke and fragments produced
by the explosion of a shell. This idea is enforced by the look of the
outer portion farthest from the bright half of the nebula, where
sharply edged clouds with dark spaces behind seem to be billowing away
as if driven by a wind blowing from the center.
Next let us consider what scientific speculation has done in the
effort to explain these mysteries. Laplace's hypothesis can certainly
find no standing ground either in the Orion Nebula or in those of a
spiral configuration, whatever may be its situation with respect to
the grand Nebula of Andromeda, or the ``ring'' and ``planetary''
nebulæ. Some other hypothesis more consonant with the appearances must
be found. Among the many that have been proposed the most elaborate is
the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton.
It is to be remarked that it applies to the spiral nebulæ
distinctively, and not to an apparently chaotic mass of gas like the
vast luminous cloud in Orion. The gist of the theory is that these
curious objects are probably the result of close approaches to each
other of two independent suns, reminding us of what was said on this
subject when we were dealing with temporary stars. Of the previous
history of these appulsing suns the theory gives us no account; they
are simply supposed to arrive within what may be called an effective
tide-producing distance, and then the drama begins. Some of the
probable consequences of such an approach have been noticed in Chapter
5; let us now consider them a little more in detail.
Tides always go in couples; if there is a tide on one side of a globe
there will be a corresponding tide on the other side. The cause is to
be found in the law that the force of gravitation varies inversely as
the square of the distance; the attraction on the nearest surface of
the body exercised by another body is greater than on its center, and
greater yet than on its opposite surface. If two great globes attract
each other, each tends to draw the other out into an ellipsoidal
figure; they must be more rigid than steel to resist this -- and even
then they cannot altogether resist. If they are liquid or gaseous they
will yield readily to the force of distortion, the amount of which
will depend upon their distance apart, for the nearer they are the
greater becomes the tidal strain. If they are encrusted without and
liquid or gaseous in the interior, the internal mass will strive to
assume the figure demanded by the tidal force, and will, if it can,
burst the restraining envelope. Now this is virtually the predicament
of the body we call a sun when in the immediate presence of another
body of similarly great mass. Such a body is presumably gaseous
throughout, the component gases being held in a state of rigidity by
the compression produced by the tremendous gravitational force of
their own aggregate mass. At the surface such a body is enveloped in a
shell of relatively cool matter. Now suppose a great attracting body,
such as another sun, to approach near enough for the difference in its
attraction on the two opposite sides of the body and on its center to
become very great; the consequence will be a tidal deformation of the
whole body, and it will lengthen out along the line of the
gravitational pull and draw in at the sides, and if its shell offers
considerable resistance, but not enough to exercise a complete
restraint, it will be violently burst apart, or blown to atoms, and
the internal mass will leap out on the two opposite sides in great
fiery spouts. In the case of a sun further advanced in cooling than
ours the interior might be composed of molten matter while the
exterior crust had become rigid like the shell of an egg; then the
force of the ``tidal explosion'' produced by the appulse of another
sun would be more violent in consequence of the greater resistance
overcome. Such, then, is the mechanism of the first phase in the
history of a spiral nebula according to the Planetesimal Hypothesis.
Two suns, perhaps extinguished ones, have drawn near together, and an
explosive outburst has occured in one or both. The second phase calls
for a more agile exercise of the imagination.
To simplify the case, let us suppose that only one of the tugging suns
is seriously affected by the strain. Its vast wings produced by the
outburst are twisted into spirals by their rotation and the contending
attractions exercised upon them, as the two suns, like battleships in
desperate conflict, curve round each other, concentrating their
destructive energies. Then immense quantities of débris are scattered
about in which eddies are created, and finally, as the sun that caused
the damage goes on its way, leaving its victim to repair its injuries
as it may, the dispersed matter cools, condenses, and turns into
streams of solid particles circling in elliptical paths about their
parent sun. These particles, or fragments, are the ``planetesimals''
of the theory. In consequence of the inevitable intersection of the
orbits of the planetesimals, nodes are formed where the flying
particles meet, and at these nodes large masses are gradually
accumulated. The larger the mass the greater its attraction, and at
last the nodal points become the nuclei of great aggregations from
which planets are shaped.
This, in very brief form, is the Planetesimal Hypothesis which we are
asked to substitute for that based on Laplace's suggestion as an
explanation of the mode of origin of the solar system; and the
phenomena of the spiral nebulæ are appealed to as offering evident
support to the new hypothesis. We are reminded that they are
elliptical in outline, which accords with the hypothesis; that their
spectra are not gaseous, which shows that they may be composed of
solid particles like the planetesimals; and that their central masses
present an oval form, which is what would result from the tidal
effects, as just described. We also remember that some of them, like
the Lord Rosse and the Andromeda nebulæ, are visually double, and in
these cases we might suppose that the two masses represent the
tide-burst suns that ventured into too close proximity. It may be
added that the authors of the theory do not insist upon the appulse of
two suns as the only way in which the planetesimals may have
originated, but it is the only supposition that has been worked out.
But serious questions remain. It needs, for instance, but a glance at
the Triangulum monster to convince the observer that it cannot be a
solar system which is being evolved there, but rather a swarm of
stars. Many of the detached masses are too vast to admit of the
supposition that they are to be transformed into planets, in our sense
of planets, and the distances of the stars which appear to have been
originally ejected from the focal masses are too great to allow us to
liken the assemblage that they form to a solar system. Then, too, no
nodes such as the hypothesis calls for are visible. Moreover, in most
of the spiral nebulæ the appearances favor the view that the
supposititious encountering suns have not separated and gone each
rejoicing on its way, after having inflicted the maximum possible
damage on its opponent, but that, on the contrary, they remain in
close association like two wrestlers who cannot escape from each
other's grasp. And this is exactly what the law of gravitation
demands; stars cannot approach one another with impunity, with regard
either to their physical make-up or their future independence of
movement. The theory undertakes to avoid this difficulty by assuming
that in the case of our system the approach of the foreign body to the
sun was not a close one -- just close enough to produce the tidal
extrusion of the relatively insignificant quantity of matter needed to
form the planets. But even then the effect of the appulse would be to
change the direction of flight, both of the sun and of its visitor,
and there is no known star in the sky which can be selected as the
sun's probable partner in their ancient pas deux. That there are
unconquered difficulties in Laplace's hypothesis no one would deny,
but in simplicity of conception it is incomparably more satisfactory,
and with proper modifications could probably be made more consonant
with existing facts in our solar system than that which is offered to
replace it. Even as an explanation of the spiral nebulæ, not as solar
systems in process of formation, but as the birthplaces of stellar
clusters, the Planetesimal Hypothesis would be open to many
objections. Granting its assumptions, it has undoubtedly a strong
mathematical framework, but the trouble is not with the mathematics
but with the assumptions. Laplace was one of the ablest mathematicians
that ever lived, but he had never seen a spiral nebula; if he had, he
might have invented a hypothesis to suit its phenomena. His actual
hypothesis was intended only for our solar system, and he left it in
the form of a ``note'' for the consideration of his successors, with
the hope that they might be able to discover the full truth, which he
confessed was hidden from him. It cannot be said that that truth has
yet been found, and when it is found the chances are that intuition
and not logic will have led to it.
The spiral nebulæ, then, remain among the greatest riddles of the
universe, while the gaseous nebulæ, like that of Orion, are no less
mysterious, although it seems impossible to doubt that both forms give
birth to stars. It is but natural to look to them for light on the
question of the origin of our planetary system; but we should not
forget that the scale of the phenomena in the two cases is vastly
different, and the forces in operation may be equally different. A
hill may have been built up by a glacier, while a mountain may be the
product of volcanic forces or of the upheaval of the strata of the
planet.
The Banners of the Sun
As all the world knows, the sun, a blinding globe pouring forth an
inconceivable quantity of light and heat, whose daily passage through
the sky is caused by the earth's rotation on its axis, constitutes the
most important phenomenon of terrestial existence. Viewed with a dark
glass to take off the glare, or with a telescope, its rim is seen to
be a sharp and smooth circle, and nothing but dark sky is visible
around it. Except for the interference of the moon, we should probably
never have known that there is any more of the sun than our eyes
ordinarily see.
But when an eclipse of the sun occurs, caused by the interposition of
the opaque globe of the moon, we see its immediate surroundings, which
in some respects are more wonderful than the glowing central orb.
These surroundings, although not in the sense in which we apply the
term to the gaseous envelope of the earth, may be called the sun's
atmosphere. They consist of two very different parts -- first, the red
``prominences,'' which resemble tongues of flame ascending thousands
of miles above the sun's surface; and, second, the ``corona,'' which
extends to distances of millions of miles from the sun, and shines
with a soft, glowing light. The two combined, when well seen, make a
spectacle without parallel among the marvels of the sky. Although many
attempts have been made to render the corona visible when there is no
eclipse, all have failed, and it is to the moon alone that we owe its
revelation. To cover the sun's disk with a circular screen will not
answer the purpose because of the illumination of the air all about
the observer. When the moon hides the sun, on the other hand, the
sunlight is withdrawn from a great cylinder of air extending to the
top of the atmosphere and spreading many miles around the observer.
There is then no glare to interfere with the spectacle, and the corona
appears in all its surprising beauty. The prominences, however,
although they were discovered during an eclipse, can now, with the aid
of the spectroscope, be seen at any time. But the prominences are
rarely large enough to be noticed by the naked eye, while the
streamers of the corona, stretching far away in space, like ghostly
banners blown out from the black circle of the obscuring moon, attract
every eye, and to this weird apparition much of the fear inspired by
eclipses has been due. But if the corona has been a cause of terror in
the past it has become a source of growing knowledge in our time.
The story of the first scientific observation of the corona and the
prominences is thrillingly interesting, and in fact dramatic. The
observation was made during the eclipse of 1842, which fortunately was
visible all over Central and Southern Europe so that scores of
astronomers saw it. The interest centers in what happened at Pavia in
Northern Italy, where the English astronomer Francis Baily had set up
his telescope. The eclipse had begun and Bailey was busy at his
telescope when, to quote his own words in the account which he wrote
for the Memoirs of the Royal Astronomical Society:
I was astounded by a tremendous burst of applause from the streets
below, and at the same moment was electrified by the sight of one
of the most brilliant and splendid phenomena that can well be
imagined; for at that instant the dark body of the moon was
suddenly surrounded with a corona, or kind of bright glory, similar
in shape and magnitude to that which painters draw round the heads
of saints...
Pavia contains many thousand inhabitants, the major part of whom
were at this early hour walking about the streets and squares or
looking out of windows in order to witness this long-talked-of
phenomenon; and when the total obscuration took place, which was
instantaneous, there was a universal shout from every observer
which ``made the welkin ring,'' and for the moment withdrew my
attention from the object with which I was immediately occupied. I
had, indeed, expected the appearance of a luminous circle round the
moon during the time of total obscurity; but I did not expect, from
any of the accounts of preceding eclipses that I had read, to
witness so magnificent an exhibition as that which took place...
Splendid and astonishing, however, as this remarkable phenomenon
really was, and although it could not fail to call forth the
admiration and applause of every beholder, yet I must confess that
there was at the same time something in its singular and wonderful
appearance that was appalling...
But the most remarkable circumstance attending the phenomenon was
the appearance of three large protuberances apparently emanating
from the circumference of the moon, but evidently forming a portion
of the corona. They had the appearance of mountains of a prodigious
elevation; their color was red tinged with lilac or purple; perhaps
the color of the peach-blossom would more nearly represent it. They
somewhat resembled the tops of the snowy Alpine mountains when
colored by the rising or the setting sun. They resembled the Alpine
mountains in another respect, inasmuch as their light was perfectly
steady, and had none of that flickering or sparkling motion so
visible in other parts of the corona...
The whole of these protuberances were visible even to the last
moment of total obscuration, and when the first ray of light was
admitted from the sun they vanished, with the corona, altogether,
and daylight was instantly restored.
I have quoted nearly all of this remarkable description not alone for
its intrinsic interest, but because it is the best depiction that can
be found of the general phenomena of a total solar eclipse. Still, not
every such eclipse offers an equally magnificent spectacle. The
eclipses of 1900 and 1905, for instance, which were seen by the
writer, the first in South Carolina and the second in Spain, fell far
short of that described by Bailey in splendor and impressiveness. Of
course, something must be allowed for the effect of surprise; Bailey
had not expected to see what was so suddenly disclosed to him. But
both in 1900 and 1905 the amount of scattered light in the sky was
sufficient in itself to make the corona appear faint, and there were
no very conspicuous prominences visible. Yet on both occasions there
was manifest among the spectators that mingling of admiration and awe
of which Bailey speaks. The South Carolinians gave a cheer and the
ladies waved their handkerchiefs when the corona, ineffably delicate
of form and texture, melted into sight and then in two minutes melted
away again. The Spaniards, crowded on the citadel hill of Burgos, with
their king and his royal retinue in their midst, broke out with a
great clapping of hands as the awaited spectacle unfolded itself in
the sky; and on both occasions, before the applause began, after an
awed silence a low murmur ran through the crowds. At Burgos it is said
many made the sign of the cross.
It was not long before Bailey's idea that the prominences were a part
of the corona was abandoned, and it was perceived that the two
phenomena were to a great extent independent. At the eclipse of 1868,
which the astronomers, aroused by the wonderful scene of 1842, and
eager to test the powers of the newly invented spectroscope, flocked
to India to witness, Janssen conceived the idea of employing the
spectroscope to render the prominences visible when there was no
eclipse. He succeeded the very next day, and these phenomena have been
studied in that way ever since.
There are recognized two kinds of prominences -- the ``erruptive'' and
the ``quiescent.'' The latter, which are cloud-like in form, may be
seen almost anywhere along the edge of the sun; but the former, which
often shoot up as if hurled from mighty volcanoes, appear to be
associated with sun-spots, and appear only above the zones where spots
abound. Either of them, when seen in projection against the brilliant
solar disk, appears white, not red, as against a background of sky.
The quiescent prominences, whose elevation is often from forty
thousand to sixty thousand miles, consist, as the spectroscope shows,
mainly of hydrogen and helium. The latter, it will be remembered, is
an element which was known to be in the sun many years before the
discovery that it also exists in small quantities on the earth. A fact
which may have a significance which we cannot at present see is that
the emanation from radium gradually and spontaneously changes into
helium, an alchemistical feat of nature that has opened many curious
vistas to speculative thinkers. The eruptive prominences, which do not
spread horizontally like the others, but ascend with marvelous
velocity to elevations of half a million miles or more, are apparently
composed largely of metallic vapors -- i.e. metals which are usually
solid on the earth, but which at solar temperatures are kept in a
volatilized state. The velocity of their ascent occasionally amounts
to three hundred or four hundred miles per second. It is known from
mathematical considerations that the gravitation of the sun would not
be able to bring back any body that started from its surface with a
velocity exceeding three hundred and eighty-three miles per second; so
it is evident that some of the matter hurled forth in eruptive
prominences may escape from solar control and go speeding out into
space, cooling and condensing into solid masses. There seems to be no
reason why some of the projectiles from the sun might not reach the
planets. Here, then, we have on a relatively small scale, explosions
recalling those which it has been imagined may be the originating
cause of some of the sudden phenomena of the stellar heavens.
Of the sun-spots it is not our intention here specifically to speak,
but they evidently have an intimate connection with eruptive
prominences, as well as some relation, not yet fully understood, with
the corona. Of the real cause of sun-spots we know virtually nothing,
but recent studies by Professor Hale and others have revealed a
strange state of things in the clouds of metallic vapors floating
above them and their surroundings. Evidences of a cyclonic tendency
have been found, and Professor Hale has proved that sun-spots are
strong magnetic fields, and consist of columns of ionized vapors
rotating in opposite directions in the two hemispheres. A fact which
may have the greatest significance is that titanium and vanadium have
been found both in sun-spots and in the remarkable variable Mira Ceti,
a star which every eleven months, or thereabout, flames up with great
brilliancy and then sinks back to invisibility with the naked eye. It
has been suggested that sun-spots are indications of the beginning of
a process in the sun which will be intensified until it falls into the
state of such a star as Mira. Stars very far advanced in evolution,
without showing variability, also exhibit similar spectra; so that
there is much reason for regarding sunspots as emblems of advancing
age.
The association of the corona with sun-spots is less evident than that
of the eruptive prominences; still such an association exists, for the
form and extent of the corona vary with the sun-spot period of which
we shall presently speak. The constitution of the corona remains to be
discovered. It is evidently in part gaseous, but it also probably
contains matter in the form of dust and small meteors. It includes one
substance altogether mysterious -- ``coronium.'' There are reasons for
thinking that this may be the lightest of all the elements, and
Professor Young, its discoverer, said that it was ``absolutely unique
in nature; utterly distinct from any other known form of matter,
terrestial, solar, or cosmical.'' The enormous extent of the corona is
one of its riddles. Since the development of the curious subject of
the ``pressure of light'' it has been proposed to account for the
sustentation of the corona by supposing that it is borne upon the
billows of light continually poured out from the sun. Experiment has
proved, what mathematical considerations had previously pointed out as
probable, that the waves of light exert a pressure or driving force,
which becomes evident in its effects if the body acted upon is
sufficiently small. In that case the light pressure will prevail over
the attraction of gravitation, and propel the attenuated matter away
from the sun in the teeth of its attraction. The earth itself would be
driven away if, instead of consisting of a solid globe of immense
aggregate mass, it were a cloud of microscopic particles. The reason
is that the pressure varies in proportion to the surface of the body
acted upon, while the gravitational attraction is proportional to the
volume, or the total amount of matter in the body. But the surface of
any body depends upon the square of its diameter, while the volume
depends upon the cube of the diameter. If, for instance, the diameter
is represented by 4, the surface will be proportional to 4 × 4, or 16,
and the volume to 4 × 4 × 4, or 64; but if the diameter is taken as 2,
the surface will be 2 × 2, or 4, and the volume 2 × 2 × 2, or 8. Now,
the ratio of 4 to 8 is twice as great as that of 16 to 64. If the
diameter is still further decreased, the ratio of the surface to the
volume will proportionally grow larger; in other words, the pressure
will gain upon the attraction, and whatever their original ratio may
have been, a time will come, if the diminution of size continues, when
the pressure will become more effective than the attraction, and the
body will be driven away. Supposing the particles of the corona to be
below the critical size for the attraction of a mass like that of the
sun to control them, they would be driven off into the surrounding
space and appear around the sun like the clouds of dust around a mill.
We shall return to this subject in connection with the Zodiacal Light,
the Aurora, and Comets.
On the other hand, there are parts of the corona which suggest by
their forms the play of electric or magnetic forces. This is
beautifully shown in some of the photographs that have been made of
the corona during recent eclipses. Take, for instance, that of the
eclipse of 1900. The sheaves of light emanating from the poles look
precisely like the ``lines of force'' surrounding the poles of a
magnet. It will be noticed in this photograph that the corona appears
to consist of two portions: one comprising the polar rays just spoken
of, and the other consisting of the broader, longer, and less-defined
masses of light extending out from the equatorial and middle-latitude
zones. Yet even in this more diffuse part of the phenomenon one can
detect the presence of submerged curves bearing more or less
resemblance to those about the poles. Just what part electricity or
electro-magnetism plays in the mechanism of the solar radiation it is
impossible to say, but on the assumption that it is a very important
part is based the hypothesis that there exists a direct solar
influence not only upon the magnetism, but upon the weather of the
earth. This hypothesis has been under discussion for half a century,
and still we do not know just how much truth it represents. It is
certain that the outbreak of great disturbances on the sun,
accompanied by the formation of sun-spots and the upshooting of
eruptive prominences (phenomena which we should naturally expect to be
attended by action), have been instantly followed by corresponding
``magnetic storms'' on the earth and brilliant displays of the auroral
lights. There have been occasions when the influence has manifested
itself in the most startling ways, a great solar outburst being
followed by a mysterious gripping of the cable and telegraph systems
of the world, as if an invisible and irresistible hand had seized
them. Messages are abruptly cut off, sparks leap from the telegraph
instruments, and the entire earth seems to have been thrown into a
magnetic flurry. These occurrences affect the mind with a deep
impression of the dependence of our planet on the sun, such as we do
not derive from the more familiar action of the sunlight on the growth
of plants and other phenomena of life depending on solar influences.
Perhaps the theory of solar magnetic influence upon the weather is
best known in connection with the ``sun-spot cycle.'' This, at any
rate, is, as already remarked, closely associated with the corona. Its
existence was discovered in 1843 by the German astronomer Schwabe. It
is a period of variable length, averaging about eleven years, during
which the number of spots visible on the sun first increases to a
maximum, then diminishes to a minimum, and finally increases again to
a maximum. For unknown reasons the period is sometimes two or three
years longer than the average and sometimes as much shorter.
Nevertheless, the phenomena always recur in the same order. Starting,
for instance, with a time when the observer can find few or no spots,
they gradually increase in number and size until a maximum, in both
senses, is reached, during which the spots are often of enormous size
and exceedingly active. After two or three years they begin to
diminish in number, magnitude, and activity until they almost or quite
disappear. A strange fact is that when a new period opens, the spots
appear first in high northern and southern latitudes, far from the
solar equator, and as the period advances they not only increase in
number and size, but break out nearer and nearer to the equator, the
last spots of a vanishing period sometimes lingering in the equatorial
region after the advance-guard of its successor has made its
appearance in the high latitudes. Spots are never seen on the equator
nor near the poles. It was not very long after the discovery of the
sun-spot cycle that the curious observation was made that a striking
coincidence existed between the period of the sun-spots and another
period affecting the general magnetic condition of the earth. When a
curved line representing the varying number of sun-spots was compared
with another curve showing the variations in the magnetic state of the
earth the two were seen to be in almost exact accord, a rise in one
curve corresponding to a rise in the other, and a fall to a fall.
Continued observation has proved that this is a real coincidence and
not an accidental one, so that the connection, although as yet
unexplained, is accepted as established. But does the influence extend
further, and directly affect the weather and the seasons as well as
the magnetic elements of the earth? A final answer to this question
cannot yet be given, for the evidence is contradictory, and the
interpretations put upon it depend largely on the predilections of the
judges.
But, in a broad sense, the sun-spots and the phenomena connected with
them must have a relation to terrestial meteorology, for they prove
the sun to be a variable star. Reference was made, a few lines above,
to the resemblance of the spectra of sun-spots to those of certain
stars which seem to be failing through age. This in itself is
extremely suggestive; but if this resemblance had never been
discovered, we should have been justified in regarding the sun as
variable in its output of energy; and not only variable, but probably
increasingly so. The very inequalities in the sun-spot cycle are
suspicious. When the sun is most spotted its total light may be
reduced by one-thousandth part, although it is by no means certain
that its outgiving of thermal radiations is then reduced. A loss of
one-thousandth of its luminosity would correspond to a decrease of
.0025 of a stellar magnitude, considering the sun as a star viewed
from distant space. So slight a change would not be perceptible; but
it is not alone sun-spots which obscure the solar surface, its entire
globe is enveloped with an obscuring veil. When studied with a
powerful telescope the sun's surface is seen to be thickly mottled
with relatively obscure specks, so numerous that it has been estimated
that they cut off from one-tenth to one-twentieth of the light that we
should receive from it if the whole surface were as brilliant as its
brightest parts. The condition of other stars warrants the conclusion
that this obscuring envelope is the product of a process of
refrigeration which will gradually make the sun more and more variable
until its history ends in extinction. Looking backward, we see a time
when the sun must have been more brilliant than it is now. At that
time it probably shone with the blinding white splendor of such stars
as Sirius, Spica, and Vega; now it resembles the relatively dull
Procyon; in time it will turn ruddy and fall into the closing cycle
represented by Antares. Considering that once it must have been more
radiantly powerful than at present, one is tempted to wonder if that
could have been the time when tropical life flourished within the
earth's polar circles, sustained by a vivific energy in the sun which
it has now lost.
The corona, as we have said, varies with the sun-spot cycle. When the
spots are abundant and active the corona rises strong above the
spotted zones, forming immense beams or streamers, which on one
occasion, at least, had an observed length of ten million miles. At
the time of a spot minimum the corona is less brilliant and has a
different outline. It is then that the curved polar rays are most
conspicuous. Thus the vast banners of the sun, shaken out in the
eclipse, are signals to tell of its varying state, but it will
probably be long before we can read correctly their messages.
The Zodiacal Light Mystery
There is a singular phenomenon in the sky -- one of the most puzzling
of all -- which has long arrested the attention of astronomers,
defying their efforts at explanation, but which probably not one in a
hundred, and possibly not one in a thousand, of the readers of this
book has ever seen. Yet its name is often spoken, and it is a
conspicuous object if one knows when and where to look for it, and
when well seen it exhibits a mystical beauty which at the same time
charms and awes the beholder. It is called ``The Zodiacal Light,''
because it lies within the broad circle of the Zodiac, marking the
sun's apparent annual path through the stars. What it is nobody has
yet been able to find out with certainty, and books on astronomy
usually speak of it with singular reserve. But it has given rise to
many remarkable theories, and a true explanation of it would probably
throw light on a great many other celestial mysteries. The Milky Way
is a more wonderful object to look upon, but its nature can be
comprehended, while there is a sort of uncanniness about the Zodiacal
Light which immediately impresses one upon seeing it, for its part in
the great scheme of extra-terrestrial affairs is not evident.
If you are out-of-doors soon after sunset -- say, on an evening late
in the month of February -- you may perceive, just after the angry
flush of the dying winter's day has faded from the sky, a pale ghostly
presence rising above the place where the sun went down. The writer
remembers from boyhood the first time it was pointed out to him and
the unearthly impression that it made, so that he afterward avoided
being out alone at night, fearful of seeing the spectral thing again.
The phenomenon brightens slowly with the fading of the twilight, and
soon distinctly assumes the shape of an elongated pyramid of pearly
light, leaning toward the south if the place of observation is in the
northern hemisphere. It does not impress the observer at all in the
same manner as the Milky Way; that looks far off and is clearly among
the stars, but the Zodiacal Light seems closer at hand, as if it were
something more intimately concerning the earth. To all it immediately
suggests a connection, also, with the sunken sun. If the night is
clear and the moon absent (and if you are in the country, for city
lights ruin the spectacles of the sky), you will be able to watch the
apparition for a long time. You will observe that the light is
brightest near the horizon, gradually fading as the pyramidal beam
mounts higher, but in favorable circumstances it may be traced nearly
to the meridian south of the zenith, where its apex at last vanishes
in the starlight. It continues visible during the evenings of March
and part of April, after which, ordinarily, it is seen no more, or if
seen is relatively faint and unimpressive. But when autumn comes it
appears again, this time not like a wraith hovering above the westward
tomb of the day-god, but rather like a spirit of the morning
announcing his reincarnation in the east.
The reason why the Zodiacal Light is best seen in our latitudes at the
periods just mentioned is because at those times the Zodiac is more
nearly perpendicular to the horizon, first in the west and then in the
east; and, since the phenomenon is confined within the borders of the
Zodiac, it cannot be favorably placed for observation when the
zodiacal plane is but slightly inclined to the horizon. Its faint
light requires the contrast of a background of dark sky in order to be
readily perceptible. But within the tropics, where the Zodiac is
always at a favorable angle, the mysterious light is more constantly
visible. Nearly all observant travelers in the equatorial regions have
taken particular note of this phenomenon, for being so much more
conspicuous there than in the temperate zones it at once catches the
eye and holds the attention as a novelty. Humboldt mentions it many
times in his works, for his genius was always attracted by things out
of the ordinary and difficult of explanation, and he made many careful
observations on its shape, its brilliancy, and its variations; for
there can be no doubt that it does vary, and sometimes to an
astonishing degree. It is said that it once remained practically
invisible in Europe for several years in succession. During a trip to
South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a
remarkable difference between the appearance of the Zodiacal Light on
his going and coming voyages. In fact, when crossing the equator going
south he did not see it at all; but on returning he had, on March 6th,
when one degree south of the equator, a memorable view of it.
It was a bright, clear night, and the Zodiacal Light was
extraordinarily brilliant -- brighter than he had ever seen it
before. The Milky Way was not to be compared with it. The brightest
part extended 75° from the sun. There was a faint and much narrower
extension which they could just make out beyond the Pleiades along
the ecliptic, but the greater part of the Zodiacal Light showed as
a broad truncated column, and it did not appear nearly as conical
as he had before seen it.
When out of the brief twilight of intertropical lands, where the sun
drops vertically to the horizon and night rushes on like a wave of
darkness, the Zodiacal Light shoots to the very zenith, its color is
described as a golden tint, entirely different from the silvery sheen
of the Milky Way. If I may venture again to refer to personal
experiences and impressions, I will recall a view of the Zodiacal
Light from the summit of the cone of Mt Etna in the autumn of the year
1896 (more briefly described in Astronomy with the Naked Eye). There
are few lofty mountains so favorably placed as Etna for observations
of this kind. It was once resorted to by Prof. George E. Hale, in an
attempt to see the solar corona without an eclipse. Rising directly
from sea-level to an elevation of nearly eleven thousand feet, the
observer on its summit at night finds himself, as it were, lost in the
midst of the sky. But for the black flanks of the great cone on which
he stands he might fancy himself to be in a balloon. On the occasion
to which I refer the world beneath was virtually invisible in the
moonless night. The blaze of the constellations overhead was
astonishingly brilliant, yet amid all their magnificence my attention
was immediately drawn to a great tapering light that sprang from the
place on the horizon where the sun would rise later, and that seemed
to be blown out over the stars like a long, luminous veil. It was the
finest view of the Zodiacal light that I had ever enjoyed -- thrilling
in its strangeness -- but I was almost disheartened by the
indifference of my guide, to whom it was only a light and nothing
more. If he had no science, he had less poetry -- rather a remarkable
thing, I thought, for a child of his clime. The Light appeared to me
to be distinctly brighter than the visible part of the Milky Way which
included the brilliant stretches in Auriga and Perseus, and its color,
if one may speak of color in connection with such an object, seemed
richer than that of the galactic band; but I did not think of it as
yellow, although Humboldt has described it as resembling a golden
curtain drawn over the stars, and Du Chaillu in Equatorial Africa
found it of a bright yellow color. It may vary in color as in
conspicuousness. The fascination of that extraordinary sight has never
faded from my memory. I turned to regard it again and again, although
I had never seen the stellar heavens so brilliant, and it was one of
the last things I looked for when the morning glow began softly to
mount in the east, and Sicily and the Mediterranean slowly emerged
from the profound shadow beneath us.
The Zodiacal Light seems never to have attracted from astronomers in
general the amount of careful attention that it deserves; perhaps
because so little can really be made of it as far as explanation is
concerned. I have referred to the restraint that scientific writers
apparently feel in speaking of it. The grounds for speculation that it
affords may be too scanty to lead to long discussions, yet it piques
curiosity, and as we shall see in a moment has finally led to a most
interesting theory. Once it was the subject of an elaborate series of
studies which carried the observer all round the world. That was in
1845--46, during the United States Exploring Expedition that visited
the then little known Japan. The chaplain of the fleet, the Rev. Mr
Jones, went out prepared to study the mysterious light in all its
phases. He saw it from many latitudes on both sides of the equator,
and the imagination cannot but follow him with keen interest in his
world-circling tour, keeping his eyes every night fixed upon the
phantasm overhead, whose position shifted with that of the hidden sun.
He demonstrated that the flow extends at times completely across the
celestial dome, although it is relatively faint directly behind the
earth. On his return the government published a large volume of his
observations, in which he undertook to show that the phenomenon was
due to the reflection of sunlight from a ring of meteoric bodies
encircling the earth. But, after all, this elaborate investigation
settled nothing.
Prof. E. E. Barnard has more recently devoted much attention to the
Zodiacal Light, as well as to a strange attendant phenomenon called
the ``Gegenschein,'' or Counterglow, because it always appears at that
point in the sky which is exactly opposite the sun. The Gegenschein is
an extremely elusive phenomenon, suitable only for eyes that have been
specially trained to see it. Professor Newcomb has cautiously remarked
that
it is said that in that point of the heavens directly opposite the
sun there is an elliptical patch of light... This phenomenon is so
difficult to account for that its existence is sometimes doubted;
yet the testimony in its favor is difficult to set aside.
It certainly cannot be set aside at all since the observations of
Barnard. I recall an attempt to see it under his guidance during a
visit to Mount Hamilton, when he was occupied there with the Lick
telescope. Of course, both the Gegenschein and the Zodiacal Light are
too diffuse to be studied with telescopes, which, so to speak, magnify
them out of existence. They can only be successfully studied with the
naked eye, since every faintest glimmer that they afford must be
utilized. This is especially true of the Gegenschein. At Mount
Hamilton, Mr Barnard pointed out to me its location with reference to
certain stars, but with all my gazing I could not be sure that I saw
it. To him, on the contrary, it was obvious; he had studied it for
months, and was able to indicate its shape, its boundaries, its
diameter, and the declination of its center with regard to the
ecliptic. There is not, of course, the shadow of a doubt of the
existence of the Gegenschein, and yet I question if one person in a
million has ever seen or ever will see it. The Zodiacal Light, on the
other hand, is plain enough, provided that the time and the
circumstances of the observation are properly chosen.
In the attempts to explain the Zodiacal Light, the favorite hypothesis
has been that it is an appendage of the sun -- perhaps simply an
extension of the corona in the plane of the ecliptic, which is not
very far from coinciding with that of the sun's equator. This idea is
quite a natural one, because of the evident relation of the light to
the position of the sun. The vast extension of the equatorial wings of
the corona in 1878 gave apparent support to this hypothesis; if the
substance of the corona could extend ten million miles from the sun,
why might it not extend even one hundred million, gradually fading out
beyond the orbit of the earth? A variation of this hypothesis assumes
that the reflection is due to swarms of meteors circling about the
sun, in the plane of its equator, all the way from its immediate
neighborhood to a distance exceeding that of the earth. But in neither
form is the hypothesis satisfactory; there is nothing in the
appearance of the corona to indicate that it extends even as far as
the planet Mercury, while as to meteors, the orbits of the known
swarms do not accord with the hypothesis, and we have no reason to
believe that clouds of others exist traveling in the part of space
where they would have to be in order to answer the requirements of the
theory. The extension of the corona in 1878 did not resemble in its
texture the Zodiacal Light.
Now, it has so often happened in the history of science that an
important discovery in one branch has thrown unexpected but most
welcome light upon some pending problem in some other branch, that a
strong argument might be based upon that fact alone against the too
exclusive devotion of many investigators to the narrow lines of their
own particular specialty; and the Zodiacal Light affords a case in
point, when it is considered in connection with recent discoveries in
chemistry and physics. From the fact that atoms are compound bodies
made up of corpuscles at least a thousand times smaller than the
smallest known atom -- a fact which astounded most men of science when
it was announced a few years ago -- a new hypothesis has been
developed concerning the nature of the Zodiacal Light (as well as
other astronomical riddles), and this hypothesis comes not from an
astronomer, but from a chemist and physicist, the Swede, Svante
Arrhenius. In considering an outline of this new hypothesis we need
neither accept nor reject it; it is a case rather for suspension of
judgment.
To begin with, it carries us back to the ``pressure of light''
mentioned in the preceding chapter. The manner in which this pressure
is believed generally to act was there sufficiently explained, and it
only remains to see how it is theoretically extended to the particles
of matter supposed to constitute the Zodiacal Light. We know that
corpuscles, or ``fragments of atoms'' negatively electrified, are
discharged from hot bodies. Streams of these ``ions'' pour from many
flames and from molten metals; and the impact of the cathode and
ultra-violet rays causes them to gush even from cold bodies. In the
vast laboratory of the sun it is but reasonable to suppose that
similar processes are taking place. ``As a very hot metal emits these
corpuscles,'' says Prof. J. J. Thomson, ``it does not seem an
improbable hypothesis that they are emitted by that very hot body, the
sun.'' Let it be assumed, then, that the sun does emit them; what
happens next? Negatively charged corpuscles, it is known, serve as
nuclei to which particles of matter in the ordinary state are
attracted, and it is probable that those emitted from the sun
immediately pick up loads in this manner and so grow in bulk. If they
grow large enough the gravitation of the sun draws them back, and they
produce a negative charge in the solar atmosphere. But it is probable
that many of the particles do not attain the critical size which,
according to the principles before explained, would enable the
gravitation of the sun to retain them in opposition to the pressure of
the waves of light, and with these particles the light pressure is
dominant. Clouds of them may be supposed to be continually swept away
from the sun into surrounding space, moving mostly in or near the
plane of the solar equator, where the greatest activity, as indicated
by sunspots and related phenomena, is taking place. As they pass
outward into space many of them encounter the earth. If the earth,
like the moon, had no atmosphere the particles would impinge directly
on its surface, giving it a negative electric charge. But the presence
of the atmosphere changes all that, for the first of the flying
particles that encounter it impart to it their negative electricity,
and then, since like electric charges repel like, the storm of
particles following will be sheered off from the earth, and will
stream around it in a maze of hyperbolic paths. Those that continue on
into space beyond the earth may be expected to continue picking up
wandering particles of matter until their bulk has become so great
that the solar attraction prevails again over the light pressure
acting upon them, and they turn again sunward. Passing the earth on
their return they will increase the amount of dust-clouds careering
round it; and these will be further increased by the action of the
ultra-violet rays of the sunlight causing particles to shoot radially
away from the earth when the negative charge of the upper atmosphere
has reached a certain amount, which particles, although starting
sunward, will be swept back to the earth with the oncoming streams. As
the final result of all this accumulation of flying and gyrating
particles in the earth's neighborhood, we are told that the latter
must be transformed into the semblance of a gigantic solid-headed
comet provided with streaming tails, the longest of them stretching
away from the direction of the sun, while another shorter one extends
toward the sun. This shorter tail is due to the particles that we have
just spoken of as being driven sunward from the earth by the action of
ultra-violet light. No doubt this whole subject is too technical for
popular statement; but at any rate the general reader can understand
the picturesque side of the theory, for its advocates assure us that
if we were on the moon we would doubtless be able to see the
comet-like tails of the earth, and then we could appreciate the part
that they play in producing the phenomenon of the Zodiacal Light.
That the Light as we see it could be produced by the reflection of
sunlight from swarms of particles careering round the earth in the
manner supposed by Arrhenius' hypothesis is evident enough; and it
will be observed that the new theory, after all, is only another
variant of the older one which attributes the Zodiacal Light to an
extension of the solar corona. But it differs from the older theory in
offering an explanation of the manner in which the extension is
effected, and it differentiates between the corona proper and the
streams of negative particles shot away from the sun. In its details
the hypothesis of Arrhenius also affords an explanation of many
peculiarities of the Zodiacal Light, such as that it is confined to
the neighborhood of the ecliptic, and that it is stronger on the side
of the earth which is just turning away from a position under the sun
than on the other side; but it would carry us beyond our limits to go
into these particulars. The Gegenschein, according to this theory, is
a part of the same phenomenon as the Zodiacal Light, for by the laws
of perspective it is evident that the reflection from the streams of
particles situated at a point directly opposite to the sun would be at
a maximum, and this is the place which the Gegenschein occupies. Apart
from its geometrical relations to the position of the sun, the
variability of the Zodiacal Light appears to affirm its solar
dependence, and this too would be accounted for by Arrhenius'
hypothesis better than by the old theory of coronal extension. The
amount of corpuscular discharge from the sun must naturally be
governed by the state of relative activity or inactivity of the
latter, and this could not but be reflected in the varying splendor of
the Zodiacal Light. But much more extended study than has yet been
given to the subject will be required before we can feel that we know
with reasonable certainty what this mysterious phenomenon really is.
By the hypothesis of Arrhenius every planet that has an atmosphere
must have a Zodiacal Light attending it, but the phenomenon is too
faint for us to be able to see it in the case, for instance, of Venus,
whose atmosphere is very abundant. The moon has no corresponding
``comet's tail'' because, as already explained, of the lack of a lunar
atmosphere to repel the streams by becoming itself electrified; but if
there were a lunar Zodiacal Light, no doubt we could see it because of
the relative nearness of our satellite.
Marvels of the Aurora
One of the most vivid recollections of my early boyhood is that of
seeing my father return hastily into the house one evening and call
out to the family: ``Come outside and look at the sky!'' Ours was a
country house situated on a commanding site, and as we all emerged
from the doorway we were dumbfounded to see the heavens filled with
pale flames which ran licking and quivering over the stars. Instantly
there sprang into my terrified mind the recollection of an awful
description of ``the Day of Judgment'' (the Dies Iræ), which I had
heard with much perturbation of spirit in the Dutch Reformed church
from the lips of a tall, dark-browed, dreadfully-in-earnest preacher
of the old-fashioned type. My heart literally sank at sight of the
spectacle, for it recalled the preacher's very words; it was just as
he had said it would be, and it needed the assured bearing of my
elders finally to convince me that
That Day of Wrath, O dreadful day,
When Heaven and Earth shall pass away,
As David and the Sibyl say
had not actually come upon us. And even the older members of the
household were not untouched with misgivings when menacing spots of
crimson appeared, breaking out now here, now there, in the shuddering
sky. Toward the north the spectacle was appalling. A huge arch spanned
an unnaturally dark segment resting on the horizon, and above this
arch sprang up beams and streamers in a state of incessant agitation,
sometimes shooting up to the zenith with a velocity that took one's
breath, and sometimes suddenly falling into long ranks, and marching,
marching, marching, like an endless phalanx of fiery specters, and
moving, as I remember, always from east to west. The absolute silence
with which these mysterious evolutions were performed and the
quavering reflections which were thrown upon the ground increased the
awfulness of the exhibition. Occasionally enormous curtains of lambent
flame rolled and unrolled with a majestic motion, or were shaken to
and fro as if by a mighty, noiseless wind. At times, too, a sudden
billowing rush would be made toward the zenith, and for a minute the
sky overhead would glow so brightly that the stars seemed to have been
consumed. The spectacle continued with varying intensity for hours.
This exhibition occurred in Central New York, a latitude in which the
Aurora Borealis is seldom seen with so much splendor. I remember
another similar one seen from the city of New York in November, 1882.
On this last occasion some observers saw a great upright beam of light
which majestically moved across the heavens, stalking like an
apparition in the midst of the auroral pageant, of whose general
movements it seemed to be independent, maintaining always its upright
posture, and following a magnetic parallel from east to west. This
mysterious beam was seen by no less than twenty-six observers in
different parts of the country, and a comparison of their observations
led to a curious calculation indicating that the apparition was about
one hundred and thirty-three miles tall and moved at the speed of ten
miles per second!
But, as everybody knows, it is in the Arctic regions that the Aurora,
or the ``Northern Lights,'' can best be seen. There, in the long polar
night, when for months together the sun does not rise, the strange
coruscations in the sky often afford a kind of spectral daylight in
unison with the weird scenery of the world of ice. The pages in the
narratives of Arctic exploration that are devoted to descriptions of
the wonderful effects of the Northern Lights are second to none that
man has ever penned in their fascination. The lights, as I have
already intimated, display astonishing colors, particularly shades of
red and green, as they flit from place to place in the sky. The
discovery that the magnetic needle is affected by the Aurora,
quivering and darting about in a state of extraordinary excitement
when the lights are playing in the sky, only added to the mystery of
the phenomenon until its electro-magnetic nature had been established.
This became evident as soon as it was known that the focus of the
displays was the magnetic pole; and when the far South was visited the
Aurora Australis was found, having its center at the South Magnetic
Pole. Then, if not before, it was clear that the earth was a great
globular magnet, having its poles of opposite magnetism, and that the
auroral lights, whatever their precise cause might be, were
manifestations of the magnetic activity of our planet. After the
invention of magnetic telegraphy it was found that whenever a great
Aurora occurred the telegraph lines were interrupted in their
operation, and the ocean cables ceased to work. Such a phenomenon is
called a ``magnetic storm.''
The interest excited by the Aurora in scientific circles was greatly
stimulated when, in the last half of the nineteenth century, it was
discovered that it is a phenomenon intimately associated with
disturbances on the sun. The ancient ``Zurich Chronicles,'' extending
from the year 1000 to the year 1800, in which both sun-spots visible
to the naked eye and great displays of the auroral lights were
recorded, first set Rudolf Wolf on the track of this discovery. The
first notable proof of the suspected connection was furnished with
dramatic emphasis by an occurrence which happened on September 1,
1859. Near noon on that day two intensely brilliant points suddenly
broke out in a group of sun-spots which were under observation by Mr
R. C. Carrington at his observatory at Redhill, England. The points
remained visible for not more than five minutes, during which interval
they moved thirty-five thousand miles across the solar disk. Mr R.
Hodgson happened to see the same phenomenon at his observatory at
Highgate, and thus all possibility of deception was removed. But
neither of the startled observers could have anticipated what was to
follow, and, indeed, it was an occurrence which has never been
precisely duplicated. I quote the eloquent account given by Miss
Clerke in her History of Astronomy During the Nineteenth Century.
This unique phenomenon seemed as if specially designed to
accentuate the inference of a sympathetic relation between the
earth and the sun. From August 28 to September 4, 1859, a magnetic
storm of unparalleled intensity, extent, and duration was in
progress over the entire globe. Telegraphic communication was
everywhere interrupted -- except, indeed, that it was in some cases
found practicable to work the lines without batteries by the agency
of the earth-currents alone; sparks issued from the wires; gorgeous
auroras draped the skies in solemn crimson over both hemispheres,
and even in the tropics; the magnetic needle lost all trace of
continuity in its movements and darted to and fro as if stricken
with inexplicable panic. The coincidence was even closer. At the
very instant of the solar outburst witnessed by Carrington and
Hodgson the photographic apparatus at Kew registered a marked
disturbance of all the three magnetic elements; while shortly after
the ensuing midnight the electric agitation culminated, thrilling
the whole earth with subtle vibrations, and lighting up the
atmosphere from pole to pole with coruscating splendors which
perhaps dimly recall the times when our ancient planet itself shone
as a star.
If this amazing occurrence stood alone, and as I have already said it
has never been exactly duplicated, doubt might be felt concerning some
of the inferences drawn from it; but in varying forms it has been
repeated many times, so that now hardly anyone questions the reality
of the assumed connection between solar outbursts and magnetic storms
accompanied by auroral displays on the earth. It is true that the late
Lord Kelvin raised difficulties in the way of the hypothesis of a
direct magnetic action of the sun upon the earth, because it seemed to
him that an inadmissible quantity of energy was demanded to account
for such action. But no calculation like that which he made is final,
since all calculations depend upon the validity of the data; and no
authority is unshakable in science, because no man can possess
omniscience. It was Lord Kelvin who, but a few years before the thing
was actually accomplished, declared that aerial navigation was an
impracticable dream, and demonstrated its impracticability by
calculation. However the connection may be brought about, it is as
certain as evidence can make it that solar outbursts are coincident
with terrestial magnetic disturbances, and coincident in such a way as
to make the inference of a causal connection irresistible. The sun is
only a little more than a hundred times its own diameter away from the
earth. Why, then, with the subtle connection between them afforded by
the ether which conveys to us the blinding solar light and the
life-sustaining solar heat, should it be so difficult to believe that
the sun's enormous electric energies find a way to us also? No doubt
the impulse coming from the sun acts upon the earth after the manner
of a touch upon a trigger, releasing energies which are already stored
up in our planet.
But besides the evidence afforded by such occurrences as have been
related of an intimate connection between solar outbreaks and
terrestial magnetic flurries, attended by magnificent auroral
displays, there is another line of proof pointing in the same
direction. Thus, it is known that the sun-spot period, as remarked in
a preceding chapter, coincides in a most remarkable manner with the
periodic fluctuations in the magnetic state of the earth. This
coincidence runs into the most astonishing details. For instance, when
the sun-spot period shortens, the auroral period shortens to precisely
the same extent; as the short sun-spot periods usually bring the most
intense outbreaks of solar activity, so the corresponding short
auroral periods are attended by the most violent magnetic storms; a
secular period of about two hundred and twenty-two years affecting
sun-spots is said to have its auroral duplicate; a shorter period of
fifty-five and a half years, which some observers believe that they
have discovered appears also to be common to the two phenomena; and
yet another ``superposed'' period of about thirty-five years, which
some investigators aver exists, affects sun-spots and aurora alike. In
short, the coincidences are so numerous and significant that one would
have to throw the doctrine of probability to the winds in order to be
able to reject the conclusion to which they so plainly lead.
But still the question recurs: How is the influence transmitted? Here
Arrhenius comes once more with his hypothesis of negative corpuscles,
or ions, driven away from the sun by light-pressure -- a hypothesis
which seems to explain so many things -- and offers it also as an
explanation of the way in which the sun creates the Aurora. He would
give the Aurora the same lineage with the Zodiacal Light. To
understand the application of this theory we must first recall the
fact that the earth is a great magnet having its two opposite poles of
magnetism, one near the Arctic and the other near the Antarctic
Circle. Like all magnets, the earth is surrounded with ``lines of
force,'' which, after the manner of the curved rays we saw in the
photograph of a solar eclipse, start from a pole, rising at first
nearly vertically, then bend gradually over, passing high above the
equator, and finally descending in converging sheaves to the opposite
pole. Now the axis of the earth is so placed in space that it lies at
nearly a right angle to the direction of the sun, and as the streams
of negatively charged particles come pouring on from the sun (see the
last preceding chapter), they arrive in the greatest numbers over the
earth's equatorial regions. There they encounter the lines of magnetic
force at the place where the latter have their greatest elevation
above the earth, and where their direction is horizontal to the
earth's surface. Obeying a law which has been demonstrated in the
laboratory, the particles then follow the lines of force toward the
poles. While they are above the equatorial regions they do not become
luminescent, because at the great elevation that they there occupy
there is virtually no atmosphere; but as they pass on toward the north
and the south they begin to descend with the lines of force, curving
down to meet at the poles; and, encountering a part of the atmosphere
comparable in density with what remains in an exhausted Crookes tube,
they produce a glow of cathode rays. This glow is conceived to
represent the Aurora, which may consequently be likened to a gigantic
exhibition of vacuum-tube lights. Anybody who recalls his student days
in the college laboratory and who has witnessed a display of Northern
Lights will at once recognize the resemblance between them in colors,
forms, and behavior. This resemblance had often been noted before
Arrhenius elaborated his hypothesis.
Without intending to treat his interesting theory as more than a
possibly correct explanation of the phenomena of the Aurora, we may
call attention to some apparently confirmatory facts. One of the most
striking of these relates to a seasonal variation in the average
number of auroræ. It has been observed that there are more in March
and September than at any other time of the year, and fewer in June
and December; moreover (and this is a delicate test as applied to the
theory), they are slightly rarer in June than in December. Now all
these facts seem to find a ready explanation in the hypothesis of
Arrhenius, thus: (1) The particles issuing from the sun are supposed
to come principally from the regions whose excitement is indicated by
the presence of sun-spots (which accords with Hale's observation that
sun-spots are columns of ionized vapors), and these regions have a
definite location on either side of the solar equator, seldom
approaching it nearer than within 5° or 10° north or south, and never
extending much beyond 35° toward either pole; (2) The equator of the
sun is inclined about 7° to the plane of the earth's orbit, from which
it results that twice in a year -- viz., in June and December -- the
earth is directly over the solar equator, and twice a year -- viz., in
March and September -- when it is farthest north or south of the solar
equator, it is over the inner edge of the sun-spot belts. Since the
corpuscles must be supposed to be propelled radially from the sun, few
will reach the earth when the latter is over the solar equator in June
and December, but when it is over, or nearly over, the spot belts, in
March and September, it will be in the line of fire of the more active
parts of the solar surface, and relatively rich streams of particles
will reach it. This, as will be seen from what has been said above, is
in strict accord with the observed variations in the frequency of
auroræ. Even the fact that somewhat fewer auroræ are seen in June than
in December also finds its explanation in the known fact that the
earth is about three million miles nearer the sun in the winter than
in the summer, and the number of particles reaching it will vary, like
the intensity of light, inversely as the square of the distance. These
coincidences are certainly very striking, and they have a cumulative
force. If we accept the theory, it would appear that we ought to
congratulate ourselves that the inclination of the sun's equator is so
slight, for as things stand the earth is never directly over the most
active regions of the sun-spots, and consequently never suffers from
the maximum bombardment of charged particles of which the sun is
capable. Incessant auroral displays, with their undulating draperies,
flitting colors, and marching columns might not be objectionable from
the point of view of picturesqueness, but one magnetic storm of
extreme intensity following closely upon the heels of another, for
months on end, crazing the magnetic needle and continually putting the
telegraph and cable lines out of commission, to say nothing of their
effect upon ``wireless telegraphy'', would hardly add to the charms of
terrestrial existence.
One or two other curious points in connection with Arrhenius'
hypothesis may be mentioned. First, the number of auroræ, according to
his explanation, ought to be greatest in the daytime, when the face of
the earth on the sunward side is directly exposed to the atomic
bombardment. Of course visual observation can give us no information
about this, since the light of the Aurora is never sufficiently
intense to be visible in the presence of daylight, but the records of
the magnetic observatories can be, and have been, appealed to for
information, and they indicate that the facts actually accord with the
theory. Behind the veil of sunlight in the middle of the afternoon,
there is good reason to believe, auroral exhibitions often take place
which would eclipse in magnificence those seen at night if we could
behold them. Observation shows, too, that auroræ are more frequent
before than after midnight, which is just what we should expect if
they originate in the way that Arrhenius supposes. Second, the theory
offers an explanation of the alleged fact that the formation of clouds
in the upper air is more frequent in years when auroræ are most
abundant, because clouds are the result of the condensation of
moisture upon floating particles in the atmosphere (in an absolutely
dustless atmosphere there would be no clouds), and it has been proved
that negative ions like those supposed to come from the sun play a
master part in the phenomena of cloud formation.
Yet another singular fact, almost mystical in its suggestions, may be
mentioned. It seems that the dance of the auroral lights occurs most
frequently during the absence of the moon from the hemisphere in which
they appear, and that they flee, in greater part, to the opposite
hemisphere when the moon's revolution in an orbit considerably
inclined to the earth's equator brings her into that where they have
been performing. Arrhenius himself discovered this curious relation of
auroral frequency to the position of the moon north or south of the
equator, and he explains it in this way. The moon, like the earth, is
exposed to the influx of the ions from the sun; but having no
atmosphere, or almost none, to interfere with them, they descend
directly upon her surface and charge her with an electric negative
potential to a very high degree. In consequence of this she affects
the electric state of the upper parts of the earth's atmosphere where
they lie most directly beneath her, and thus prevents, to a large
extent, the negative discharges to which the appearance of the Aurora
is due. And so ``the extravagant and erring spirit'' of the Aurora
avoids the moon as Hamlet's ghost fled at the voice of the cock
announcing the awakening of the god of day.
There are even other apparent confirmations of the hypothesis, but we
need not go into them. We shall, however, find one more application of
it in the next chapter, for it appears to be a kind of cure-all for
astronomical troubles; at any rate it offers a conceivable solution of
the question, How does the sun manage to transmit its electric
influence to the earth? And this solution is so grandiose in
conception, and so novel in the mental pictures that it offers, that
its acceptance would not in the least detract from the impression that
the Aurora makes upon the imagination.
Strange Adventures of Comets
The fears and legends of ancient times before Science was born, and
the superstitions of the Dark Ages, sedulously cultivated for
theological purposes by monks and priests, have so colored our ideas
of the influence that comets have had upon the human mind that many
readers may be surprised to learn that it was the apparition of a
wonderful comet, that of 1843, which led to the foundation of our
greatest astronomical institution, the Harvard College Observatory. No
doubt the comet superstition existed half a century ago, as, indeed,
it exists yet today, but in this case the marvelous spectacle in the
sky proved less effective in inspiring terror than in awakening a
desire for knowledge. Even in the sixteenth century the views that
enlightened minds took of comets tended powerfully to inspire popular
confidence in science, and Halley's prediction, after seeing and
studying the motion of the comet which appeared in 1682, that it would
prove to be a regular member of the sun's family and would be seen
returning after a period of about seventy-six years, together with the
fulfillment of that prediction, produced a revulsion from the
superstitious notions which had so long prevailed.
Then the facts were made plain that comets are subject to the law of
gravitation equally with the planets; that there are many which
regularly return to the neighborhood of the sun (perihelion); and that
these travel in orbits differing from those of the planets only in
their greater eccentricity, although they have the peculiarity that
they do not, like the planets, all go round the sun in the same
direction, and do not keep within the general plane of the planetary
system, but traverse it sometimes from above and sometimes from below.
Other comets, including most of the ``great'' ones, appear to travel
in parabolic or, in a few cases, hyperbolic orbits, which, not being
closed curves, never bring them back again. But it is not certain that
these orbits may not be extremely eccentric ellipses, and that after
the lapse of hundreds, or thousands, of years the comets that follow
them may not reappear. The question is an interesting one, because if
all orbits are really ellipses, then all comets must be permanent
members of the solar system, while in the contrary case many of them
are simply visitors, seen once and never to be seen again. The
hypothesis that comets are originally interlopers might seem to derive
some support from the fact that the certainly periodic ones are
associated, in groups, with the great outer planets, whose attraction
appears to have served as a trap for them by turning them into
elliptical orbits and thus making them prisoners in the solar system.
Jupiter, owing to his great mass and his commanding situation in the
system, is the chief ``comet-catcher;'' but he catches them not for
himself, but for the sun. Yet if comets do come originally from
without the borders of the planetary system, it does not, by any
means, follow that they were wanderers at large in space before they
yielded to the overmastering attraction of the sun. Investigation of
the known cometary orbits, combined with theoretical considerations,
has led some astronomers to the conclusion that as the sun travels
onward through space he ``picks up en route'' cometary masses which,
without belonging strictly to his empire, are borne along in the same
vast ``cosmical current'' that carries the solar system.
But while no intelligent person any longer thinks that the appearance
of a great comet is a token from the heavenly powers of the
approaching death of a mighty ruler, or the outbreak of a devastating
war, or the infliction of a terrible plague upon wicked mankind,
science itself has discovered mysteries about comets which are not
less fascinating because they are more intellectual than the
irrational fancies that they have displaced. To bring the subject
properly before the mind, let us see what the principal phenomena
connected with a comet are.
At the present day comets are ordinarily ``picked up'' with the
telescope or the photographic plate before any one except their
discoverer is aware of their existence, and usually they remain so
insignificant in appearance that only astronomers ever see them. Yet
so great is the prestige of the word ``comet'' that the discovery of
one of these inconspicuous wanderers, and its subsequent movements,
become items of the day's news which everybody reads with the feeling,
perhaps, that at least he knows what is going on in the universe even
if he doesn't understand it. But a truly great comet presents quite a
different proposition. It, too, is apt to be detected coming out of
the depths of space before the world at large can get a glimpse of it,
but as it approaches the sun its aspect undergoes a marvelous change.
Agitated apparently by solar influence, it throws out a long streaming
tail of nebulous light, directed away from the sun and looking as if
blown out like a pennon by a powerful wind. Whatever may be the
position of the comet with regard to the sun, as it circles round him
it continually keeps its tail on the off side. This, as we shall soon
see, is a fact of capital importance in relation to the probable
nature of comets' tails. Almost at the same time that the formation of
the tail is observed a remarkable change takes place in the comet's
head, which, by the way, is invariably and not merely occasionally its
most important part. On approaching the sun the head usually
contracts. Coincidently with this contraction a nucleus generally
makes its appearance. This is a bright, star-like point in the head,
and it probably represents the totality of solid matter that the comet
possesses. But it is regarded as extremely unlikely that even the
nucleus consists of a uniformly solid mass. If it were such, comets
would be far more formidable visitors when they pass near the planets
than they have been found to be. The diameter of the nucleus may vary
from a few hundred up to several thousand miles; the heads, on the
average, are from twenty-five thousand to one hundred thousand miles
in diameter, although a few have greatly exceeded these dimensions;
that of the comet of 1811, one of the most stupendous ever seen, was a
million and a quarter miles in diameter! As to the tails, not
withstanding their enormous length -- some have been more than a
hundred million miles long -- there is reason to believe that they are
of extreme tenuity, ``as rare as vacuum.'' The smallest stars have
been seen shining through their most brilliant portions with
undiminished luster.
After the nucleus has been formed it begins to throw out bright jets
directed toward the sun. A stream, and sometimes several streams, of
light also project sunward from the nucleus, occasionally appearing
like a stunted tail directed oppositely to the real tail. Symmetrical
envelopes which, seen in section, appear as half circles or parabolas,
rise sunward from the nucleus, forming a concentric series. The ends
of these stream backward into the tail, to which they seem to supply
material. Ordinarily the formation of these ejections and envelopes is
attended by intense agitation of the nucleus, which twists and turns,
swinging and gyrating with an appearance of the greatest violence.
Sometimes the nucleus is seen to break up into several parts. The
entire heads of some comets have been split asunder in passing close
around the sun; The comet of 1882 retreated into space after its
perihelion passage with five heads instead of the one that it had
originally, and each of these heads had its own tail!
The possession of the spectroscope has enabled astronomers during
later years to study the chemical composition of comets by analyzing
their light. At first the only substances thus discovered in them were
hydro-carbon compounds, due evidently to the gaseous envelopes in
which some combination of hydrogen with carbon existed. Behind this
gaseous spectrum was found a faint continuous spectrum ascribed to the
nucleus, which apparently both reflects the sunlight and gives forth
the light of a glowing solid or liquid. Subsequently sodium and iron
lines were found in cometary spectra. The presence of iron would seem
to indicate that some of these bodies may be much more massive than
observations on their attractive effects have indicated. In some
recent comets, such as Morehouse's, in 1908, several lines have been
found, the origin of which is unknown.
Without going back of the nineteenth century we may find records of
some of the most extraordinary comets that man has ever looked upon.
In 1811, still spoken of as ``the year of the comet,'' because of the
wonderful vintage ascribed to the skyey visitor, a comet shaped like a
gigantic sword amazed the whole world, and, as it remained visible for
seventeen months, was regarded by superstitious persons as a symbol of
the fearful happenings of Napoleon's Russian campaign. This comet, the
extraordinary size of whose head, greatly exceeding that of the sun
itself, has already been mentioned, was also remarkable for exhibiting
so great a brilliancy without approaching even to the earth's distance
from the sun. But there was once a comet (and only once -- in the year
1729) which never got nearer to the sun than four times the distance
of the earth and yet appeared as a formidable object in the sky. As
Professor Young has remarked, ``it must have been an enormous comet to
be visible from such a distance.'' And we are to remember that there
were no great telescopes in the year 1729. That comet affects the
imagination like a phantom of space peering into the solar system,
displaying its enormous train afar off (which, if it had approached as
near as other comets, would probably have become the celestial wonder
of all human memory), and then turning away and vanishing in the
depths of immensity.
In 1843 a comet appeared which was so brilliant that it could be seen
in broad day close beside the sun! This was the first authenticated
instance of that kind, but the occurrence was to be repeated, as we
shall see in a moment, less than forty years later.
The splendid comet of 1858, usually called Donati's, is remembered by
many persons yet living. It was, perhaps, both as seen by the naked
eye and with the telescope, the most beautiful comet of which we have
any record. It too marked a rich vintage year, still remembered in the
vineyards of France, where there is a popular belief that a great
comet ripens the grape and imparts to the wine a flavor not attainable
by the mere skill of the cultivator. There are ``comet wines,''
carefully treasured in certain cellars, and brought forth only when
their owner wishes to treat his guests to a sip from paradise.
The year 1861 saw another very remarkable comet, of an aspect
strangely vast and diffuse, which is believed to have swept the earth
with its immense tail when it passed between us and the sun on the
night of June 30th, an event which produced no other known effect than
the appearance of an unwonted amount of scattered light in the sky.
The next very notable comet was the ``Great Southern Comet'' of 1880,
which was not seen from the northern hemisphere. It mimicked the
aspect of the famous comet of 1843, and to the great surprise of
astronomers appeared to be traveling in the same path. This proved to
be the rising of the curtain for an astronomical sensation
unparalleled in its kind; for two years later another brilliant comet
appeared, first in the southern hemisphere, and it too followed the
same track. The startling suggestion was now made that this comet was
identical with those of 1843 and 1880, its return having been hastened
by the resistance experienced in passing twice through the coronal
envelope, and there were some who thought that it would now swing
swiftly round and then plunge straight into the sun, with consequences
that might be disastrous to us on account of the ``flash of heat''
that would be produced by the impact. Nervous people were frightened,
but observation soon proved that the danger was imaginary, for
although the comet almost grazed the sun, and must have rushed through
two or three million miles of the coronal region, no retardation of
its immense velocity was perceptible, and it finally passed away in a
damaged condition, as before remarked, and has never since appeared.
Then the probable truth was perceived -- viz., that the three comets
(1843, 1880, and 1882) were not one identical body, but three separate
ones all traveling in the same orbit. It was found, too, that a comet
seen in 1668 bore similar insignia of relationship. The natural
inference was that these four bodies had once formed a single mass
which had been split apart by the disruptive action of the sun.
Strength was lent to this hypothesis by the fact that the comet of
1882 was apparently torn asunder during its perihelion passage,
retreating into space in a dissevered state. But Prof. George Forbes
has a theory that the splitting of the original cometary mass was
effected by an unknown planet, probably greater than Jupiter, situated
at a hundred times the earth's distance from the sun, and revolving in
a period of a thousand years. He supposes that the original comet was
not that of 1668, but one seen in 1556, which has since been
``missing,'' and that its disruption occurred from an encounter with
the supposititious planet about the year 1700. Truly from every point
of view comets are the most extraordinary of adventurers!
The comet of 1882 was likewise remarkable for being visible, like its
predecessor of 1843, in full daylight in close proximity to the sun.
The story of its detection when almost in contact with the solar disk
is dramatic. It had been discovered in the southern hemisphere only a
couple of weeks before its perihelion, which occurred on September
17th, and on the forenoon of that day it was seen by Doctor Common in
England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope,
almost touching the sun. It looked like a dazzling white bird with
outspread wings. The southern observers watched it go right into the
sun, when it instantly disappeared. What had happened was that the
comet in passing its perihelion point had swung exactly between the
earth and the sun. On the following morning it was seen from all parts
of the world close by the sun on the opposite side, and it remained
thus visible for three days, gradually receding from the solar disk.
It then became visible for northern observers in the morning sky
before sunrise, brandishing a portentous sword-shaped tail which, if
it had been in the evening sky, would have excited the wonder of
hundreds of millions, but situated where it was, comparatively few
ever saw it.
The application of photography to the study of comets has revealed
many curious details which might otherwise have escaped detection, or
at best have remained subject to doubt. It has in particular shown not
only the precise form of the tails, but the remarkable vicissitudes
that they undergo. Professor Barnard's photographs of Brooks' comet in
1893 suggested, by the extraordinary changes in the form of the tail
which they revealed, that the comet was encountering a series of
obstructions in space which bent and twisted its tail into fantastic
shapes. The reader will observe the strange form into which the tail
was thrown on the night of October 21st. A cloud of meteors through
which the comet was passing might have produced such deformations of
its tail. In the photograph of Daniels' comet of 1907, a curious
striping of the tail will be noticed. The short bright streaks seen in
the photograph, it may be explained, are the images of stars which are
drawn out into lines in consequence of the fact that the photographic
telescope was adjusted to follow the motion of the comet while the
stars remained at rest.
But the adventures of comets are not confined to possible encounters
with unknown obstacles. We have referred to the fact that the great
planets, and especially Jupiter, frequently interfere with the motions
of comets. This interference is not limited to the original alteration
of their orbits from possible parabolas to ellipses, but is sometimes
exercised again and again, turning the bewildered comets into
elliptical paths of all degrees of eccentricity. A famous example of
this kind of planetary horse-play is furnished by the story of
Lexell's missing comet. This comet was first seen in 1770.
Investigation showed that it was moving in an orbit which should bring
it back to perihelion every five and a half years; yet it had never
been seen before and, although often searched for, has never been seen
since. Laplace and Leverrier proved mathematically that in 1767 it had
approached so close to Jupiter as to be involved among the orbits of
his satellites. What its track had been before is not known, but on
that occasion the giant planet seized the interloper, threw it into a
short elliptic orbit and sent it, like an arrested vagrant, to receive
sentence at the bar of the sun. On this journey it passed within less
than 1,500,000 miles of the earth. The form of orbit which Jupiter had
impressed required, as we have said, its return in about five and a
half years; but soon after 1770 it had the misfortune a second time to
encounter Jupiter at close range, and he, as if dissatisfied with the
leniency of the sun, or indignant at the stranger's familiarity,
seized the comet and hurled it out of the system, or at any rate so
far away that it has never since been able to rejoin the family circle
that basks in the immediate rays of the solar hearth. Nor is this the
only instance in which Jupiter has dealt summarily with small comets
that have approached him with too little deference.
The function which Jupiter so conspicuously fulfills as master of the
hounds to the sun is worth considering a little more in detail. To
change the figure, imagine the sun in its voyage through space to be
like a majestic battleship surrounded by its scouts. Small vessels
(the comets, as they are overhauled by the squadron, are taken in
charge by the scouts, with Jupiter for their chief, and are forced to
accompany the fleet, but not all are impressed. If a strange comet
undertakes to run across Jupiter's bows the latter brings it to, and
makes prize of it by throwing it into a relatively small ellipse with
the sun for its focus. Thenceforth, unless, as happened to the unhappy
comet of Lexell, it encounters Jupiter again in such a way as to be
diverted by him into a more distant orbit, it can never get away.
About thirty comets are now known to have thus been captured by the
great planet, and they are called ``Jupiter's Comet Family.'' But, on
the other hand, if a wandering comet crosses the wake of the chief
planetary scout the latter simply drives it away by accelerating its
motion and compels it to steer off into open space. The transformation
of comets into meteors will be considered in the next chapter, but
here, in passing, mention may be made of the strange fate of one
member of Jupiter's family, Biela's comet, which, having become over
bold in its advances to its captor, was, after a few revolutions in is
impressed orbit, torn to pieces and turned into a flock of meteors.
And now let us return to the mystery of comets' tails. That we are
fully justified in speaking of the tails of comets as mysterious is
proved by the declaration of Sir John Herschel, who averred, in so
many words, that ``there is some profound secret and mystery of nature
concerned in this phenomenon,'' and this profound secret and mystery
has not yet been altogether cleared up. Nevertheless, the
all-explaining hypothesis of Arrhenius offers us once more a certain
amount of aid. Comets' tails, Arrhenius assures us, are but another
result of the pressure of light. The reader will recall the
applications of this theory to the Zodiacal Light and the Aurora. In
the form in which we now have to deal with it, the supposition is made
that as a comet approaches the sun eruptions of vapor, due to the
solar heat, occur in its nucleus. These are naturally most active on
the side which is directly exposed to the sun, whence the appearance
of the immense glowing envelopes that surround the nucleus on the
sunward side. Among the particles of hydro-carbon, and perhaps solid
carbon in the state of fine dust, which are thus set free there will
be many whose size is within the critical limit which enables the
light-waves from the sun to drive them away. Clouds of such particles,
then, will stream off behind the advancing comet, producing the
appearance of a tail. This accounts for the fact that the tails of
comets are always directed away from the sun, and it also explains the
varying forms of the tails and the extraordinary changes that they
undergo. The speed of the particles driven before the light-waves must
depend upon their size and weight, the lightest of a given size
traveling the most swiftly. By accretion certain particles might grow,
thus losing velocity and producing the appearance of bunches in the
tail, such as have been observed. The hypothesis also falls in with
the researches of Bredichin, who has divided the tails of comets into
three principal classes -- viz.: (1) Those which appear as long,
straight rays; (2) Those which have the form of curved plumes or
scimitars; (3) Those which are short, brushy, and curved sharply
backward along the comet's path. In the first type he calculates the
repulsive force at from twelve to fifteen times the force of gravity;
in the second at from two to four times; and in the third at about one
and a half times. The straight tails he ascribes to hydrogen because
the hydrogen atom is the lightest known; the sword-shaped tails to
hydro-carbons; and the stumpy tails to vaporized iron. It will be seen
that, if the force driving off the tails is that which Arrhenius
assumes it to be, the forms of those appendages would accord with
those that Bredichin's theory calls for. At the same time we have an
explanation of the multiple tails with which some comets have adorned
themselves. The comet of 1744, for instance, had at one time no less
than seven tails spread in a wide curved brush behind it. Donati's
comet of 1858 also had at least two tails, the principal one
sword-shaped and the other long, narrow, and as straight as a rule.
According to Bredichin, the straight tail must have been composed of
hydrogen, and the other of some form of hydro-carbon whose atoms are
heavier than those of hydrogen, and, consequently, when swept away by
the storm of light-waves, followed a curvature depending upon the
resultant of the forces operating upon them. The seven tails of the
comet of 1744 presented a kind of diagram graphically exhibiting its
complex composition, and, if we knew a little more about the
constituents of a comet, we might be able to say from the amount of
curvature of the different tails just what were the seven substances
of which that comet consisted.
If these theories seem to the reader fantastic, at any rate they are
no more fantastic than the phenomena that they seek to explain.
Meteors, Fire-Balls, and Meteorites
One of the most terrorizing spectacles with which the heavens have
ever caused the hearts of men to quake occurred on the night of
November 13, 1833. On that night North America, which faced the storm,
was under a continual rain of fire from about ten o'clock in the
evening until daybreak.
The fragments of a comet had struck the earth.
But the meaning of what had happened was not discovered until long
afterward. To the astronomers who, with astonishment not less than
that of other people, watched the wonderful scene, it was an
unparalleled ``shower of meteors.'' They did not then suspect that
those meteors had once formed the head of a comet. Light dawned when,
a year later, Prof. Denison Olmsted, of Yale College, demonstrated
that the meteors had all moved in parallel orbits around the sun, and
that these orbits intersected that of the earth at the point where our
planet happened to be on the memorable night of November 13th.
Professor Olmsted even went so far as to suggest that the cloud of
meteors that had encountered the earth might form a diffuse comet; but
full recognition of the fact that they were cometary débris came
later, as the result of further investigation. The key to the secret
was plainly displayed in the spectacle itself, and was noticed without
being understood by thousands of the terror-stricken beholders. It was
an umbrella of fire that had opened overhead and covered the heavens;
in other words, the meteors all radiated from a particular point in
the constellation Leo, and, being countless as the snowflakes in a
winter tempest, they ribbed the sky with fiery streaks. Professor
Olmsted showed that the radiation of the meteors from a fixed point
was an effect of perspective, and in itself a proof that they were
moving in parallel paths when they encountered the earth. The fact was
noted that there had been a similar, but incomparably less brilliant,
display of meteors on the same day of November, 1832, and it was
rightly concluded that these had belonged to the same stream, although
the true relationship of the phenomena was not immediately
apprehended. Olmsted ascribed to the meteors a revolution about the
sun once in every six months, bringing them to the intersection of
their orbit with that of the earth every November 13th; but later
investigators found that the real period was about thirty-three and
one-quarter years, so that the great displays were due three times in
a century, and their return was confidently predicted for the year
1866. The appearance of the meteors in 1832, a year before the great
display, was ascribed to the great length of the stream which they
formed in space -- so great that they required more than two years to
cross the earth's orbit. In 1832 the earth had encountered a
relatively rare part of the stream, but in 1833, on returning to the
crossing-place, it found there the richest part of the stream pouring
across its orbit. This explanation also proved to be correct, and the
predicted return in 1866 was duly witnessed, although the display was
much less brilliant than in 1833. It was followed by another in 1867.
In the mean time Olmsted's idea of a cometary relationship of the
meteors was demonstrated to be correct by the researches of
Schiaparelli and others, who showed that not only the November
meteors, but those of August, which are seen more or less abundantly
every year, traveled in the tracks of well-known comets, and had
undoubtedly an identical origin with those comets. In other words the
comets and the meteor-swarms were both remnants of original masses
which had probably been split up by the action of the sun, or of some
planet to which they had made close approaches. The annual periodicity
of the August meteors was ascribed to the fact that the separation had
taken place so long ago that the meteors had become distributed all
around the orbit, in consequence of which the earth encountered some
of them every year when it arrived at the crossing-point. Then
Leverrier showed that the original comet associated with the November
meteors was probably brought into the system by the influence of the
planet Uranus in the year 126 of the Christian era. Afterward
Alexander Herschel identified the tracks of no less than seventy-six
meteor-swarms (most of them inconspicuous) with those of comets. The
still more recent researches of Mr W. F. Denning make it probable that
there are no meteors which do not belong to a flock or system probably
formed by the disintegration of a cometary mass; even the apparently
sporadic ones which shoot across the sky, ``lost souls in the night,''
being members of flocks which have become so widely scattered that the
earth sometimes takes weeks to pass through the region of space where
their paths lie.
The November meteors should have exhibited another pair of spectacles
in 1899 and 1900, and their failure to do so caused at first much
disappointment, until it was made plain that a good reason existed for
their absence. It was found that after their last appearance, in 1867,
they had been disturbed in their movements by the planets Jupiter and
Saturn, whose attractions had so shifted the position of their orbit
that it no longer intersected that of the earth, as it did before.
Whether another planetary interference will sometime bring the
principal mass of the November meteors back to the former point of
intersection with the earth's orbit is a question for the future to
decide. It would seem that there may be several parallel streams of
the November meteors, and that some of them, like those of August, are
distributed entirely around the orbit, so that every mid-November we
see a few of them.
We come now to a very remarkable example of the disintegration of a
comet and the formation of a meteor-stream. In 1826 Biela, of
Josephstadt, Austria, discovered a comet to which his name was given.
Calculation showed that it had an orbital period of about six and a
half years, belonging to Jupiter's ``family.'' On one of its returns,
in 1846, it astonished its watchers by suddenly splitting in two. The
two comets thus formed out of one separated to a distance of about one
hundred and sixty thousand miles, and then raced side by side,
sometimes with a curious ligature connecting them, like Siamese twins,
until they disappeared together in interplanetary space. In 1852 they
came back, still nearly side by side, but now the distance between
them had increased to a million and a quarter of miles. After that, at
every recurrence of their period, astronomers looked for them in vain,
until 1872, when an amazing thing happened. On the night of November
28th, when the earth was crossing the plane of the orbit of the
missing comet, a brilliant shower of meteors burst from the northern
sky, traveling nearly in the track which the comet should have
pursued. The astronomers were electrified. Klinkerfues, of Göttingen,
telegraphed to Pogson, of Madras: ``Biela touched earth; search near
Theta Centauri.'' Pogson searched in the place indicated and saw a
cometary mass retreating into the southern heavens, where it was soon
swallowed from sight!
Since then the Biela meteors have been among the recognized periodic
spectacles of the sky, and few if any doubt that they represent a
portion of the missing comet whose disintegration began with the
separation into two parts in 1846. The comet itself has never since
been seen. The first display of these meteors, sometimes called the
``Andromedes,'' because they radiate from the constellation Andromeda,
was remarkable for the great brilliancy of many of the fire-balls that
shot among the shower of smaller sparks, some of which were described
as equaling the full moon in size. None of them is known to have
reached the earth, but during the display of the same meteors in 1885
a meteoric mass fell at Mazapil in Northern Mexico (it is now in the
Museum at Vienna), which many have thought may actually be a piece of
the original comet of Biela. This brings us to the second branch of
our subject.
More rare than meteors or falling stars, and more startling, except
that they never appear in showers, are the huge balls of fire which
occasionally dart through the sky, lighting up the landscapes beneath
with their glare, leaving trains of sparks behind them, often
producing peals of thunder when they explode, and in many cases
falling upon the earth and burying themselves from a few inches to
several feet in the soil, from which, more than once, they have been
picked up while yet hot and fuming. These balls are sometimes called
bolides. They are not really round in shape, although they often look
so while traversing the sky, but their forms are fragmentary, and
occasionally fantastic. It has been supposed that their origin is
different from that of the true meteors; it has even been conjectured
that they may have originated from the giant volcanoes of the moon or
have been shot out from the sun during some of the tremendous
explosions that accompany the formation of eruptive prominences. By
the same reasoning some of them might be supposed to have come from
some distant star. Others have conjectured that they are wanderers in
space, of unknown origin, which the earth encounters as it journeys
on, and Lord Kelvin made a suggestion which has become classic because
of its imaginative reach -- viz., that the first germs of life may
have been brought to the earth by one of these bodies, ``a fragment of
an exploded world.''
It is a singular fact that astronomers and scientific men in general
were among the last to admit the possibility of solid masses falling
from the sky. The people had believed in the reality of such phenomena
from the earliest times, but the savants shook their heads and talked
of superstition. This was the less surprising because no
scientifically authenticated instance of such an occurrence was known,
and the stones popularly believed to have fallen from the sky had
become the objects of worship or superstitious reverence, a fact not
calculated to recommend them to scientific credence. The celebrated
``black stone'' suspended in the Kaaba at Mecca is one of these
reputed gifts from heaven; the ``Palladium'' of ancient Troy was
another; and a stone which fell near Ensisheim, in Germany, was placed
in a church as an object to be religiously venerated. Many legends of
falling stones existed in antiquity, some of them curiously
transfigured by the imagination, like the ``Lion of the
Peloponnesus,'' which was said to have sprung down from the sky upon
the Isthmus of Corinth. But near the beginning of the nineteenth
century, in 1803, a veritable shower of falling stones occurred at
L'Aigle, in Northern France, and this time astronomers took note of
the phenomenon and scientifically investigated it. Thousands of the
strange projectiles came from the sky on this occasion, and were
scattered over a wide area of country, and some buildings were hit.
Four years later another shower of stones occurred at Weston, Conn.,
numbering thousands of individuals. The local alarm created in both
cases was great, as well it might be, for what could be more
intimidating than to find the blue vault of heaven suddenly hurling
solid missiles at the homes of men? After these occurrences it was
impossible for the most skeptical to doubt any longer, and the regular
study of ``aerolites,'' or ``meteorites,'' began.
One of the first things recognized was the fact that fire-balls are
solid meteorites in flight, and not gaseous exhalations in the air, as
some had assumed. They burn in the air during their flight, and
sometimes, perhaps, are entirely consumed before reaching the ground.
Their velocity before entering the earth's atmosphere is equal to that
of the planets in their orbits -- viz., from twenty to thirty miles
per second -- a fact which proves that the sun is the seat of the
central force governing them. Their burning in the air is not
difficult to explain; it is the heat of friction which so quickly
brings them to incandescence. Calculation shows that a body moving
through the air at a velocity of about a mile per second will be
brought, superficially, to the temperature of ``red heat'' by friction
with the atmosphere. If its velocity is twenty miles per second the
temperature will become thousands of degrees. This is the state of
affairs with a meteorite rushing into the earth's atmosphere; its
surface is liquefied within a few seconds after the friction begins to
act, and the melted and vaporized portion of its mass is swept
backward, forming the train of sparks that follows every great
fire-ball. However, there is one phenomenon connected with the trains
of meteorites which has never been satisfactorily explained: they
often persist for long periods of time, drifting and turning with the
wind, but not ceasing to glow with a phosphorescent luminosity. The
question is, Whence comes this light? It must be light without heat,
since the fine dust or vapor of which the train can only consist would
not retain sufficient heat to render it luminous for so long a time.
An extremely remarkable incident of this kind occurred on February 22,
1909, when an immense fire-ball that passed over southern England left
a train that remained visible during two hours, assuming many curious
shapes as it was drifted about by currents in the air.
But notwithstanding the enormous velocity with which meteorites enter
the air they are soon slowed down to comparatively moderate speed, so
that when they disappear they are usually traveling not faster than a
mile a second. The courses of many have been traced by observers
situated along their track at various points, and thus a knowledge has
been obtained of their height above the ground during their flight and
of the length of their visible courses. They generally appear at an
elevation of eighty or a hundred miles, and are seldom visible after
having descended to within five miles of the ground, unless the
observer happens to be near the striking-point, when he may actually
witness the fall. Frequently they burst while high in the air and
their fragments are scattered like shrapnel over the surface of the
ground, sometimes covering an area of several square miles, but of
course not thickly; different fragments of the same meteorite may
reach the ground at points several miles apart. The observed length of
their courses in the atmosphere varies from fifty to five hundred
miles. If they continued a long time in flight after entering the air,
even the largest of them would probably be consumed to the last scrap,
but their fiery career is so short on account of their great speed
that the heat does not have time to penetrate very deeply, and some
that have been picked up immediately after their fall have been found
cold as ice within. Their size after reaching the ground is variable
within wide limits; some are known which weigh several tons, but the
great majority weigh only a few pounds and many only a few ounces.
Meteorites are of two kinds: stony meteorites and iron meteorites. The
former outnumber the latter twenty to one; but many stone meteorites
contain grains of iron. Nickel is commonly found in iron meteorites,
so that it might be said that that redoubtable alloy nickel-steel is
of cosmical invention. Some twenty-five chemical elements have been
found in meteorites, including carbon and the ``sun-metal,'' helium.
The presence of the latter is certainly highly suggestive in
connection with the question of the origin of meteorites. The iron
meteorites, besides metallic iron and nickel, of which they are almost
entirely composed, contain hydrogen, helium, and carbonic oxide, and
about the only imaginable way in which these gases could have become
absorbed in the iron would be through the immersion of the latter
while in a molten or vaporized state in a hot and dense atmosphere
composed of them, a condition which we know to exist only in the
envelopes of the sun and the stars.
The existence of carbon in the Canyon Diablo iron meteorites is
attended by a circumstance of the most singular character -- a very
``fairy tale of science.'' In some cases the carbon has become
diamond! These meteoric diamonds are very small; nevertheless, they
are true diamonds, resembling in many ways the little black gems
produced by Moissan's method with the aid of the electric furnace. The
fact that they are found embedded in these iron meteorites is another
argument in favor of the hypothesis of the solar or stellar origin of
the latter. To appreciate this it is necessary to recall the way in
which Moissan made his diamonds. It was by a combination of the
effects of great heat, great pressure, and sudden or rapid superficial
cooling on a mass of iron containing carbon. When he finally broke
open his iron he found it a pudding stuffed with miniature black
diamonds. When a fragment of the Canyon Diablo meteoric iron was
polished in Philadelphia over fifteen years ago it cut the emery-wheel
to pieces, and examination showed that the damage had been effected by
microscopic diamonds peppered through the mass. How were those
diamonds formed? If the sun or Sirius was the laboratory that prepared
them, we can get a glimpse at the process of their formation. There is
plenty of heat, plenty of pressure, and an abundance of vaporized iron
in the sun and the stars. When a great solar eruption takes place,
masses of iron which have absorbed carbon may be shot out with a
velocity which forbids their return. Plunged into the frightful cold
of space, their surfaces are quickly cooled, as Moissan cooled his
prepared iron by throwing it into water, and thus the requisite stress
is set up within, and, as the iron solidifies, the included carbon
crystallizes into diamonds. Whether this explanation has a germ of
truth in it or not, at any rate it is evident that iron meteorites
were not created in the form in which they come to us; they must once
have been parts of immeasurably more massive bodies than themselves.
The fall of meteorites offers an appreciable, though numerically
insignificant, peril to the inhabitants of the earth. Historical
records show perhaps three or four instances of people being killed by
these bodies. But for the protection afforded by the atmosphere, which
acts as a very effective shield, the danger would doubtless be very
much greater. In the absence of an atmosphere not only would more
meteorites reach the ground, but their striking force would be
incomparably greater, since, as we have seen, the larger part of their
original velocity is destroyed by the resistance of the air. A
meteorite weighing many tons and striking the earth with a velocity of
twenty or thirty miles per second, would probably cause frightful
havoc.
It is a singular fact that recent investigations seem to have proved
that an event of this kind actually happened in North America --
perhaps not longer than a thousand or two thousand years ago. The
scene of the supposed catastrophe is in northern central Arizona, at
Coon Butte, where there is a nearly circular crater in the middle of a
circular elevation or small mountain. The crater is somewhat over four
thousand feet in diameter, and the surrounding rim, formed of upturned
strata and ejected rock fragments, rises at its highest point one
hundred and sixty feet above the plain. The crater is about six
hundred feet in depth -- that is, from the rim to the visible floor or
bottom of the crater. There is no evidence that volcanic action has
ever taken place in the immediate neighborhood of Coon Butte. The rock
in which the crater has been made is composed of horizontal sandstone
and limestone strata. Between three hundred and four hundred million
tons of rock fragments have been detached, and a large portion hurled
by some cause out of the crater. These fragments lie concentrically
distributed around the crater, and in large measure form the elevation
known as Coon Butte. The region has been famous for nearly twenty
years on account of the masses of meteoric iron found scattered about
and known as the ``Canyon Diablo'' meteorites. It was one of these
masses, which consist of nickel-iron containing a small quantity of
platinum, and of which in all some ten tons have been recovered for
sale to the various collectors throughout the world, that as before
mentioned destroyed the grinding-tool at Philadelphia through the
cutting power of its embedded diamonds. These meteoric irons are
scattered about the crater-hill, in concentric distribution, to a
maximum distance of about five miles. When the suggestion was first
made in 1896 that a monster meteorite might have created by its fall
this singular lone crater in stratified rocks, it was greeted with
incredulous smiles; but since then the matter has assumed a different
aspect. The Standard Iron Company, formed by Messrs. D. M. Barringer,
B. C. Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in
1903, the owner of this freak of nature, sunk shafts and bored holes
to a great depth in the interior of the crater, and also trenched the
slopes of the mountain, and the result of their investigations has
proved that the meteoric hypothesis of origin is correct. (See the
papers published in the Proceedings of the Academy of Natural Sciences
of Philadelphia, December, 1905, wherein it is proved that the United
States Geological Survey was wrong in believing this crater to have
been due to a steam explosion. Since that date there has been
discovered a great amount of additional confirmatory proof). Material
of unmistakably meteoric origin was found by means of the drills,
mixed with crushed rock, to a depth of six hundred to seven hundred
feet below the floor of the crater, and a great deal of it has been
found admixed with the ejected rock fragments on the outer slopes of
the mountain, absolutely proving synchronism between the two events,
the formation of this great crater and the falling of the meteoric
iron out of the sky. The drill located in the bottom of the crater was
sent, in a number of cases, much deeper (over one thousand feet) into
unaltered horizontal red sandstone strata, but no meteoric material
was found below this depth (seven hundred feet, or between eleven and
twelve hundred feet below the level of the surrounding plain), which
has been assumed as being about the limit of penetration. It is not
possible to sink a shaft at present, owing to the water which has
drained into the crater, and which forms, with the finely pulverized
sandstone, a very troublesome quicksand encountered at about two
hundred feet below the visible floor of the crater. As soon as this
water is removed by pumping it will be easy to explore the depths of
the crater by means of shafts and drifts. The rock strata (sandstone
and limestone) of which the walls consist present every appearance of
having been violently upturned by a huge body penetrating the earth
like a cannon-ball. The general aspect of the crater strikingly
resembles the impression made by a steel projectile shot into an
armor-plate. Mr Tilghman has estimated that a meteorite about five
hundred feet in diameter and moving with a velocity of about five
miles per second would have made just such a perforation upon striking
rocks of the character of those found at this place. There was some
fusion of the colliding masses, and the heat produced some steam from
the small amount of water in the rocks. As a result there has been
found at depth a considerable amount of fused quartz (original
sandstone), and with it innumerable particles or sparks of fused
nickel-iron (original meteorite). A projectile of that size
penetrating eleven to twelve hundred feet into the rocky shell of the
globe must have produced a shock which was perceptible several hundred
miles away.
The great velocity ascribed to the supposed meteorite at the moment of
striking could be accounted for by the fact that it probably plunged
nearly vertically downward, for it formed a circular crater in the
rocky crust of the earth. In that case it would have been less
retarded by the resistance of the atmosphere than are meteorites which
enter the air at a lower angle and shoot ahead hundreds of miles until
friction has nearly destroyed their original motion when they drop
upon the earth. Some meteoric masses of great size, such as Peary's
iron meteorite found at Cape York, Greenland, and the almost equally
large mass discovered at Bacubirito, Mexico, appear to have penetrated
but slightly on striking the earth. This may be explained by supposing
that they pursued a long, horizontal course through the air before
falling. The result would be that, their original velocity having been
practically destroyed, they would drop to the ground with a velocity
nearly corresponding to that which gravity would impart within the
perpendicular distance of their final fall. A
six-hundred-and-sixty-pound meteorite, which fell at Knyahinya,
Hungary, striking at an angle of 27° from the vertical, penetrated the
ground to a depth of eleven feet.
It has been remarked that the Coon Butte meteorite may have fallen not
longer ago than a few thousand years. This is based upon the fact that
the geological indications favor the supposition that the event did
not occur more than five thousand years ago, while on the other hand
the rings of growth in the cedar-trees growing on the slopes of the
crater show that they have existed there about seven hundred years.
Prof. William H. Pickering has recently correlated this with an
ancient chronicle which states that at Cairo, Egypt, in the year 1029,
``many stars passed with a great noise.'' He remarks that Cairo is
about 100°, by great circle, from Coon Butte, so that if the meteorite
that made the crater was a member of a flock of similar bodies which
encountered the earth moving in parallel lines, some of them might
have traversed the sky tangent to the earth's surface at Cairo. That
the spectacle spoken of in the chronicle was caused by meteorites he
deems exceedingly probable because of what is said about ``a great
noise;'' meteorites are the only celestial phenomena attended with
perceptible sounds. Professor Pickering conjectures that this supposed
flock of great meteorites may have formed the nucleus of a comet which
struck the earth, and he finds confirmation of the idea in the fact
that out of the ten largest meteorites known, no less than seven were
found within nine hundred miles of Coon Butte. It would be interesting
if we could trace back the history of that comet, and find out what
malicious planet caught it up in its innocent wanderings and hurled it
with so true an aim at the earth! This remarkable crater is one of the
most interesting places in the world, for there is absolutely no
record of such a mass, possibly an iron-headed comet, from outer space
having come into collision with our earth. The results of the future
exploration of the depths of the crater will be awaited with much
interest.
The Wrecking of the Moon
There are sympathetic moods under whose influence one gazes with a
certain poignant tenderness at the worn face of the moon; that little
``fossil world'' (the child of our mother earth, too) bears such
terrible scars of its brief convulsive life that a sense of pity is
awakened by the sight. The moon is the wonder-land of the telescope.
Those towering mountains, whose ``proud aspiring peaks'' cast
silhouettes of shadow that seem drawn with india-ink; those vast
plains, enchained with gentle winding hills and bordered with giant
ranges; those oval ``oceans,'' where one looks expectant for the flash
of wind-whipped waves; those enchanting ``bays'' and recesses at the
seaward feet of the Alps; those broad straits passing between guardian
heights incomparably mightier than Gibraltar; those locket-like
valleys as secluded among their mountains as the Vale of Cashmere;
those colossal craters that make us smile at the pretensions of
Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with
the unconcern of Roman roads across mountain, gorge, and valley -- all
these give the beholder an irresistible impression that it is truly a
world into which he is looking, a world akin to ours, and yet no more
like our world than Pompeii is like Naples. Its air, its waters, its
clouds, its life are gone, and only a skeleton remains -- a mute but
eloquent witness to a cosmical tragedy without parallel in the range
of human knowledge.
One cannot but regret that the moon, if it ever was the seat of
intelligent life, has not remained so until our time. Think what the
consequences would have been if this other world at our very door had
been found to be both habitable and inhabited! We talk rather airily
of communicating with Mars by signals; but Mars never approaches
nearer than 35,000,000 miles, while the moon when nearest is only a
little more than 220,000 miles away. Given an effective magnifying
power of five thousand diameters, which will perhaps be possible at
the mountain observatories as telescopes improve, and we should be
able to bring the moon within an apparent distance of about forty
miles, while the corresponding distance for Mars would be more than
seven thousand miles. But even with existing telescopic powers we can
see details on the moon no larger than some artificial constructions
on the earth. St Peter's at Rome, with the Vatican palace and the
great piazza, if existing on the moon, would unquestionably be
recognizable as something else than a freak of nature. Large cities,
with their radiating lines of communication, would at once betray
their real character. Cultivated tracts, and the changes produced by
the interference of intelligent beings, would be clearly recognizable.
The electric illumination of a large town at night would probably be
markedly visible. Gleams of reflected sunlight would come to us from
the surfaces of the lakes and oceans, and a huge ``liner'' traversing
a lunar sea could probably be followed by its trail of smoke. As to
communications by ``wireless'' signals, which certain enthusiasts have
thought of in connection with Mars, in the case of the moon they
should be a relatively simple matter, and the feat might actually be
accomplished. Think what a literature would grow up about the moon if
it were a living world! Its very differences from the earth would only
accentuate its interest for us. Night and day on the moon are each two
weeks in length; how interesting it would be to watch the manner in
which the lunarians dealt with such a situation as that. Lunar and
terrestrial history would keep step with each other, and we should
record them both. Truly one might well wish to have a neighbor world
to study; one would feel so much the less alone in space.
It is not impossible that the moon did at one time have inhabitants of
some kind. But, if so, they vanished with the disappearance of its
atmosphere and seas, or with the advent of its cataclysmic age. At the
best, its career as a living world must have been brief. If the water
and air were gradually absorbed, as some have conjectured, by its
cooling interior rocks, its surface might, nevertheless, have retained
them for long ages; but if, as others think, their disappearance was
due to the escape of their gaseous molecules in consequence of the
inability of the relatively small lunar gravitation to retain them,
then the final catastrophe must have been as swift as it was
inevitable. Accepting Darwin's hypothesis, that the moon was separated
from the earth by tidal action while both were yet plastic or
nebulous, we may reasonably conclude that it began its career with a
good supply of both water and air, but did not possess sufficient mass
to hold them permanently. Yet it may have retained them long enough
for life to develop in many forms upon its surface; in fact, there are
so many indications that air and water have not always been lacking to
the lunar world that we are driven to invent theories to explain both
their former presence and their present absence.
But whatever the former condition of the moon may have been, its
existing appearance gives it a resistless fascination, and it bears so
clearly the story of a vast catastrophe sculptured on its rocky face
that the thoughtful observer cannot look upon it without a feeling of
awe. The gigantic character of the lunar features impresses the
beholder not less than the universality of the play of destructive
forces which they attest. Let us make a few comparisons. Take the
lunar crater called ``Tycho'', which is a typical example of its kind.
In the telescope Tycho appears as a perfect ring surrounding a
circular depression, in the center of which rises a group of
mountains. Its superficial resemblance to some terrestrial volcanic
craters is very striking. Vesuvius, seen from a point vertically
above, would no doubt look something like that (the resemblance would
have been greater when the Monte del Cavallo formed a more complete
circuit about the crater cone). But compare the dimensions. The
remains of the outer crater ring of Vesuvius are perhaps half a mile
in diameter, while the active crater itself is only two or three
hundred feet across at the most; Tycho has a diameter of fifty-four
miles! The group of relatively insignificant peaks in the center of
the crater floor of Tycho is far more massive than the entire mountain
that we call Vesuvius. The largest known volcanic crater on the earth,
Aso San, in Japan, has a diameter of seven miles; it would take sixty
craters like Aso San to equal Tycho in area! And Tycho, though one of
the most perfect, is by no means the largest crater on the moon.
Another, called ``Theophilus,'' has a diameter of sixty-four miles,
and is eighteen thousand feet deep. There are hundreds from ten to
forty miles in diameter, and thousands from one to ten miles. They are
so numerous in many places that they break into one another, like the
cells of a crushed honeycomb.
The lunar craters differ from those of the earth more fundamentally
than in the matter of mere size; they are not situated on the tops of
mountains. If they were, and if all the proportions were the same, a
crater like Tycho might crown a conical peak fifty or one hundred
miles high! Instead of being cavities in the summits of mountains, the
lunar craters are rather gigantic sink-holes whose bottoms in many
cases lie two or three miles below the general surface of the lunar
world. Around their rims the rocks are piled up to a height of from a
few hundred to two or three thousand feet, with a comparatively gentle
inclination, but on the inner side they fall away in gigantic broken
precipices which make the dizzy cliffs of the Matterhorn seem but
``lover's leaps.'' Down they drop, ridge below ridge, crag under crag,
tottering wall beneath wall, until, in a crater named ``Newton,'' near
the south lunar pole, they attain a depth where the rays of the sun
never reach. Nothing more frightful than the spectacle which many of
these terrible chasms present can be pictured by the imagination. As
the lazy lunar day slowly advances, the sunshine, unmitigated by
clouds or atmospheric veil of any kind, creeps across their rims and
begins to descend the opposite walls. Presently it strikes the ragged
crest of a ridge which had lain hidden in such darkness as we never
know on the earth, and runs along it like a line of kindling fire.
Rocky pinnacles and needles shoot up into the sunlight out of the
black depths. Down sinks the line of light, mile after mile, and
continually new precipices and cliffs are brought into view, until at
last the vast floor is attained and begins to be illuminated. In the
meanwhile the sun's rays, darting across the gulf, have touched the
summits of the central peaks, twenty or thirty miles from the crater's
inmost edge, and they immediately kindle and blaze like huge stars
amid the darkness. So profound are some of these awful craters that
days pass before the sun has risen high enough above them to chase the
last shadows from their depths.
Although several long ranges of mountains resembling those of the
earth exist on the moon, the great majority of its elevations assume
the crateriform aspect. Sometimes, instead of a crater, we find an
immense mountain ring whose form and aspect hardly suggest volcanic
action. But everywhere the true craters are in evidence, even on the
sea-beds, although they attain their greatest number and size on those
parts of the moon -- covering sixty per cent of its visible surface --
which are distinctly mountainous in character and which constitute its
most brilliant portions. Broadly speaking, the southwestern half of
the moon is the most mountainous and broken, and the northeastern half
the least so. Right down through the center, from pole to pole, runs a
wonderful line of craters and crateriform valleys of a magnitude
stupendous even for the moon. Another similar line follows the western
edge. Three or four ``seas'' are thrust between these mountainous
belts. By the effects of ``libration'' parts of the opposite
hemisphere of the moon which is turned away from the earth are from
time to time brought into view, and their aspect indicates that that
hemisphere resembles in its surface features the one which faces the
earth. There are many things about the craters which seem to give some
warrant for the hypothesis which has been particularly urged by Mr G.
K. Gilbert, that they were formed by the impact of meteors; but there
are also many things which militate against that idea, and, upon the
whole, the volcanic theory of their origin is to be preferred.
The enormous size of the lunar volcanoes is not so difficult to
account for when we remember how slight is the force of lunar gravity
as compared with that of the earth. With equal size and density,
bodies on the moon weigh only one-sixth as much as on the earth.
Impelled by the same force, a projectile that would go ten miles on
the earth would go sixty miles on the moon. A lunar giant thirty-five
feet tall would weigh no more than an ordinary son of Adam weighs on
his greater planet. To shoot a body from the earth so that it would
not drop back again, we should have to start it with a velocity of
seven miles per second; a mile and a half per second would serve on
the moon. It is by no means difficult to believe, then, that a lunar
volcano might form a crater ring eight or ten times broader than the
greatest to be found on the earth, especially when we reflect that in
addition to the relatively slight force of gravity, the materials of
the lunar crust are probably lighter than those of our terrestrial
rocks.
For similar reasons it seems not impossible that the theory mentioned
in a former chapter -- that some of the meteorites that have fallen
upon the earth originated from the lunar volcanoes -- is well founded.
This would apply especially to the stony meteorites, for it is hardly
to be supposed that the moon, at least in its superficial parts,
contains much iron. It is surely a scene most strange that is thus
presented to the mind's eye -- that little attendant of the earth's
(the moon has only one-fiftieth of the volume, and only one-eightieth
of the mass of the earth) firing great stones back at its parent
planet! And what can have been the cause of this furious outbreak of
volcanic forces on the moon? Evidently it was but a passing stage in
its history; it had enjoyed more quiet times before. As it cooled down
from the plastic state in which it parted from the earth, it became
incrusted after the normal manner of a planet, and then oceans were
formed, its atmosphere being sufficiently dense to prevent the water
from evaporating and the would-be oceans from disappearing continually
in mist. This, if any, must have been the period of life in the lunar
world. As we look upon the vestiges of that ancient world buried in
the wreck that now covers so much of its surface, it is difficult to
restrain the imagination from picturing the scenes which were once
presented there; and, in such a case, should the imagination be
fettered? We give it free rein in terrestrial life, and it rewards us
with some of our greatest intellectual pleasures. The wonderful
landscapes of the moon offer it an ideal field with just enough
half-hidden suggestions of facts to stimulate its powers.
The great plains of the Mare Imbrium and the Mare Serenitatis (the
``Sea of Showers'' and the ``Sea of Serenity''), bordered in part by
lofty mountain ranges precisely like terrestrial mountains, scalloped
along their shores with beautiful bays curving back into the adjoining
highlands, and united by a great strait passing between the nearly
abutting ends of the ``Lunar Apennines'' and the ``Lunar Caucasus,''
offer the elements of a scene of world beauty such as it would be
difficult to match upon our planet. Look at the finely modulated
bottom of the ancient sea in Mr Ritchey's exquisite photograph of the
western part of the Mare Serenitatis, where one seems to see the play
of the watery currents heaping the ocean sands in waving lines, making
shallows, bars, and deeps for the mariner to avoid or seek, and
affording a playground for the creatures of the main. What geologist
would not wish to try his hammer on those rocks with their stony pages
of fossilized history? There is in us an instinct which forbids us to
think that there was never any life there. If we could visit the moon,
there is not among us a person so prosaic and unimaginative that he
would not, the very first thing, begin to search for traces of its
inhabitants. We would look for them in the deposits on the sea
bottoms; we would examine the shores wherever the configuration seemed
favorable for harbors and the sites of maritime cities -- forgetting
that it may be a little ridiculous to ascribe to the ancient lunarians
the same ideas that have governed the development of our race; we
would search through the valleys and along the seeming courses of
vanished streams; we would explore the mountains, not the terrible
craters, but the pinnacled chains that recall our own Alps and
Rockies; seeking everywhere some vestige of the transforming presence
of intelligent life. Perhaps we should find such traces, and perhaps,
with all our searching, we should find nothing to suggest that life
had ever existed amid that universal ruin.
Look again at the border of the ``Sea of Serenity'' -- what a name for
such a scene! -- and observe how it has been rent with almost
inconceivable violence, the wall of the colossal crater Posidonius
dropping vertically upon the ancient shore and obliterating it, while
its giant neighbor, Le Monnier, opens a yawning mouth as if to swallow
the sea itself. A scene like this makes one question whether, after
all, those may not be right who have imagined that the so-called sea
bottoms are really vast plains of frozen lava which gushed up in
floods so extensive that even the mighty volcanoes were half drowned
in the fiery sea. This suggestion becomes even stronger when we turn
to another of the photographs of Mr Ritchey's wonderful series,
showing a part of the Mare Tranquilitatis (``Sea of Tranquility''!).
Notice how near the center of the picture the outline of a huge ring
with radiating ridges shows through the sea bottom; a fossil volcano
submerged in a petrified ocean! This is by no means the only instance
in which a buried world shows itself under the great lunar plains.
Yet, as the newer craters in the sea itself prove, the volcanic
activity survived this other catastrophe, or broke out again
subsequently, bringing more ruin to pile upon ruin.
Yet notwithstanding the evidence which we have just been considering
in support of the hypothesis that the ``seas'' are lava floods,
Messrs. Loewy and Puiseux, the selenographers of the Paris
Observatory, are convinced that these great plains bear characteristic
marks of the former presence of immense bodies of water. In that case
we should be forced to conclude that the later oceans of the moon lay
upon vast sheets of solidified lava; and thus the catastrophe of the
lunar world assumes a double aspect, the earliest oceans being
swallowed up in molten floods issuing from the interior, while the
lands were reduced to chaos by a universal eruption of tremendous
volcanoes; and then a period of comparative quiet followed, during
which new seas were formed, and new life perhaps began to flourish in
the lunar world, only to end in another cataclysm, which finally put a
term to the existence of the moon as a life-supporting world.
Suppose we examine two more of Mr Ritchey's illuminating photographs,
and, first, the one showing the crater Theophilus and its
surroundings. We have spoken of Theophilus before, citing the facts
that it is sixty-four miles in diameter and eighteen thousand feet
deep. It will be noticed that it has two brother giants -- Cyrillus
the nearer, and Catharina the more distant; but Theophilus is plainly
the youngest of the trio. Centuries, and perhaps thousands of years,
must have elapsed between the periods of their upheaval, for the two
older craters are partly filled with débris, while it is manifest at a
glance that when the south eastern wall of Theophilus was formed, it
broke away and destroyed a part of the more ancient ring of Cyrillus.
There is no more tremendous scene on the moon than this; viewed with a
powerful telescope, it is absolutely appalling.
The next photograph shows, if possible, a still wilder region. It is
the part of the moon lying between Tycho and the south pole. Tycho is
seen in the lower left-hand part of the picture. To the right, at the
edge of the illuminated portion of the moon, are the crater-rings,
Longomontanus and Wilhelm I, the former being the larger. Between them
are to be seen the ruins of two or three more ancient craters which,
together with portions of the walls of Wilhelm I and Longomontanus,
have been honeycombed with smaller craters. The vast crateriform
depression above the center of the picture is Clavius, an unrivaled
wonder of lunar scenery, a hundred and forty-two miles in its greatest
length, while its whole immense floor has sunk two miles below the
general surface of the moon outside the ring. The monstrous
shadow-filled cavity above Clavius toward the right is Blancanus,
whose aspect here gives a good idea of the appearance of these chasms
when only their rims are in the sunlight. But observe the
indescribable savagery of the entire scene. It looks as though the
spirit of destruction had gone mad in this spot. The mighty craters
have broken forth one after another, each rending its predecessor; and
when their work was finished, a minor but yet tremendous outbreak
occurred, and the face of the moon was gored and punctured with
thousands of smaller craters. These relatively small craters (small,
however, only in a lunar sense, for many of them would appear gigantic
on the earth) recall once more the theory of meteoric impact. It does
not seem impossible that some of them may have been formed by such an
agency.
One would not wish for our planet such a fate as that which has
overtaken the moon, but we cannot be absolutely sure that something of
the kind may not be in store for it. We really know nothing of the
ultimate causes of volcanic activity, and some have suggested that the
internal energies of the earth may be accumulating instead of dying
out, and may never yet have exhibited their utmost destructive power.
Perhaps the best assurance that we can find that the earth will escape
the catastrophe that has overtaken its satellite is to be found in the
relatively great force of its gravitation. The moon has been the
victim of its weakness; given equal forces, and the earth would be the
better able to withstand them. It is significant, in connection with
these considerations, that the little planet Mercury, which seems also
to have parted with its air and water, shows to the telescope some
indications that it is pitted with craters resembling those that have
torn to pieces the face of the moon.
Upon the whole, after studying the dreadful lunar landscapes, one
cannot feel a very enthusiastic sympathy with those who are seeking
indications of the continued existence of some kind of life on the
moon; such a world is better without inhabitants. It has met its fate;
let it go! Fortunately, it is not so near that it cannot hide its
scars and appear beautiful -- except when curiosity impels us to look
with the penetrating eyes of the astronomer.
The Great Mars Problem
Let any thoughtful person who is acquainted with the general facts of
astronomy look up at the heavens some night when they appear in their
greatest splendor, and ask himself what is the strongest impression
that they make upon his mind. He may not find it easy to frame an
answer, but when he has succeeded it will probably be to the effect
that the stars give him an impression of the universality of
intelligence; they make him feel, as the sun and the moon cannot do,
that his world is not alone; that all this was not made simply to form
a gorgeous canopy over the tents of men. If he is of a devout turn of
mind, he thinks, as he gazes into those fathomless deeps and among
those bewildering hosts, of the infinite multitude of created beings
that the Almighty has taken under his care. The narrow ideas of the
old geocentric theology, which made the earth God's especial
footstool, and man his only rational creature, fall away from him like
a veil that had obscured his vision; they are impossible in the
presence of what he sees above. Thus the natural tendency, in the
light of modern progress, is to regard the universe as everywhere
filled with life.
But science, which is responsible for this broadening of men's
thoughts concerning the universality of life, itself proceeds to set
limits. Of spiritual existences it pretends to know nothing, but as to
physical beings, it declares that it can only entertain the
supposition of their existence where it finds evidence of an
environment suited to their needs, and such environment may not
everywhere exist. Science, though repelled by the antiquated
theological conception of the supreme isolation of man among created
beings, regards with complacency the probability that there are
regions in the universe where no organic life exists, stars which
shine upon no inhabited worlds, and planets which nourish no animate
creatures. The astronomical view of the universe is that it consists
of matter in every stage of evolution: some nebulous and chaotic; some
just condensing into stars (suns) of every magnitude and order; some
shaped into finished solar bodies surrounded by dependent planets;
some forming stars that perhaps have no planets, and will have none;
some constituting suns that are already aging, and will soon lose
their radiant energy and disappear; and some aggregated into masses
that long ago became inert, cold, and rayless, and that can only be
revivified by means about which we can form conjectures, but of which
we actually know nothing.
As with the stars, so with the planets, which are the satellites of
stars. All investigations unite to tell us that the planets are not
all in the same state of development. As some are large and some
small, so some are, in an evolutionary sense, young, and some old. As
they depend upon the suns around which they revolve for their light,
heat, and other forms of radiant energy, so their condition varies
with their distance from those suns. Many may never arrive at a state
suitable for the maintenance of life upon their surfaces; some which
are not at present in such a state may attain it later; and the forms
of life themselves may vary with the peculiar environment that
different planets afford. Thus we see that we are not scientifically
justified in affirming that life is ubiquitous, although we are thus
justified in saying that it must be, in a general sense, universal. We
might liken the universe to a garden known to contain every variety of
plant. If on entering it we see no flowers, we examine the species
before us and find that they are not of those which bloom at this
particular season, or perhaps they are such as never bear flowers. Yet
we feel no doubt that we shall find flowers somewhere in the garden,
because there are species which bloom at this season, and the garden
contains all varieties.
While it is tacitly assumed that there are planets revolving around
other stars than the sun, it would be impossible for us to see them
with any telescope yet invented, and no instrument now in the
possession of astronomers could assure us of their existence; so the
only planetary system of which we have visual knowledge is our own.
Excluding the asteroids, which could not from any point of view be
considered as habitable, we have in the solar system eight planets of
various sizes and situated at various distances from the sun. Of these
eight we know that one, the earth, is inhabited. The question, then,
arises: Are there any of the others which are inhabited or habitable?
Since it is our intention to discuss the habitability of only one of
the seven to which the question applies, the rest may be dismissed in
a few words. The smallest of them, and the nearest to the sun, is
Mercury, which is regarded as uninhabitable because it has no
perceptible supply of water and air, and because, owing to the
extraordinary eccentricity of its orbit, it is subjected to excessive
and very rapid alterations in the amount of solar heat and light
poured upon its surface, such alterations being inconsistent with the
supposition that it can support living beings. Even its average
temperature is more than six and a half times that prevailing on the
earth! Another circumstance which militates against its habitability
is that, according to the results of the best telescopic studies, it
always keeps the same face toward the sun, so that one half of the
planet is perpetually exposed to the fierce solar rays, and the other
half faces the unmitigated cold of open space. Venus, the next in
distance from the sun, is almost the exact twin of the earth in size,
and many arguments may be urged in favor of its habitability, although
it is suspected of possessing the same peculiarity as Mercury, in
always keeping the same side sunward. Unfortunately its atmosphere
appears to be so dense that no permanent markings on its surface are
certainly visible, and the question of its actual condition must, for
the present, be left in abeyance. Mars, the first planet more distant
from the sun than the earth, is the special subject of this chapter,
and will be described and discussed a few lines further on. Jupiter,
Saturn, Uranus, and Neptune, the four giant planets, all more distant
than Mars, and each more distant than the other in the order named,
are all regarded as uninhabitable because none of them appears to
possess any degree of solidity. They may have solid or liquid nuclei,
but exteriorly they seem to be mere balls of cloud. Of course, one can
imagine what he pleases about the existence of creatures suited to the
physical constitution of such planets as these, but they must be
excluded from the category of habitable worlds in the ordinary sense
of the term. We go back, then, to Mars.
It will be best to begin with a description of the planet. Mars is
4230 miles in diameter; its surface is not much more than one-quarter
as extensive as that of the earth (.285). Its mean distance from the
sun is 141,500,000 miles, 48,500,000 miles greater than that of the
earth. Since radiant energy varies inversely as the square of
distance, Mars receives less than half as much solar light and heat as
the earth gets. Mars' year (period of revolution round the sun) is 687
days. Its mean density is 71 per cent of the earth's, and the force of
gravity on its surface is 38 per cent of that on the surface of the
earth; i.e., a body weighing one hundred pounds on the earth would, if
transported to Mars, weigh but thirty-eight pounds. The inclination of
its equator to the plane of its orbit differs very little from that of
the earth's equator, and its axial rotation occupies 24 hours 37
minutes. so that the length of day and night, and the extent of the
seasonal changes on Mars, are almost precisely the same as on the
earth. But owing to the greater length of its year, the seasons of
Mars, while occurring in the same order, are almost twice as long as
ours. The surface of the planet is manifestly solid, like that of our
globe, and the telescope reveals many permanent markings on it,
recalling the appearance of a globe on which geographical features
have been represented in reddish and dusky tints. Around the poles are
plainly to be seen rounded white areas, which vary in extent with the
Martian seasons, nearly vanishing in summer and extending widely in
winter. The most recent spectroscopic determinations indicate that
Mars has an atmosphere perhaps as dense as that to be found on our
loftiest mountain peaks, and there is a perceptible amount of watery
vapor in this atmosphere. The surface of the planet appears to be
remarkably level, and it has no mountain ranges. No evidences of
volcanic action have been discovered on Mars. The dusky and reddish
areas were regarded by the early observers as respectively seas and
lands, but at present it is not believed that there are any bodies of
water on the planet. There has never been much doubt expressed that
the white areas about the poles represent snow.
It will be seen from this brief description that many remarkable
resemblances exist between Mars and the earth, and there is nothing
wonderful in the fact that the question of the habitability of the
former has become one of extreme and wide-spread interest, giving rise
to the most diverse views, to many extraordinary speculations, and
sometimes to regrettably heated controversy. The first champion of the
habitability of Mars was Sir William Herschel, although even before
his time the idea had been suggested. He was convinced by the
revelations of his telescopes, continually increasing in power, that
Mars was more like the earth than any other planet. He could not
resist the testimony of the polar snows, whose suggestive conduct was
in such striking accord with what occurs upon the earth. Gradually, as
telescopes improved and observers increased in number, the principal
features of the planet were disclosed and charted, and ``areography,''
as the geography of Mars was called, took its place among the
recognized branches of astronomical study. But it was not before 1877
that a fundamentally new discovery in areography gave a truly
sensational turn to speculation about life on ``the red planet.'' In
that year Mars made one of its nearest approaches to the earth, and
was so situated in its orbit that it could be observed to great
advantage from the northern hemisphere of the earth. The celebrated
Italian astronomer, Schiaparelli, took advantage of this opportunity
to make a trigonometrical survey of the surface of Mars -- as coolly
and confidently as if he were not taking his sights across a
thirty-five-million-mile gulf of empty space -- and in the course of
this survey he was astonished to perceive that the reddish areas, then
called continents, were crossed in many directions by narrow, dusky
lines, to which he gave the suggestive name of ``canals.'' Thus a kind
of firebrand was cast into the field of astronomical speculation,
which has ever since produced disputes that have sometimes approached
the violence of political faction. At first the accuracy of
Schiaparelli's observations was contested; it required a powerful
telescope, and the most excellent ``seeing,'' to render the
enigmatical lines visible at all, and many searchers were unable to
detect them. But Schiaparelli continued his studies in the serene sky
of Italy, and produced charts of the gridironed face of Mars
containing so much astonishing detail that one had either to reject
them in toto or to confess that Schiaparelli was right. As subsequent
favorable oppositions of Mars occurred, other observers began to see
the ``canals'' and to confirm the substantial accuracy of the Italian
astronomer's work, and finally few were found who would venture to
affirm that the ``canals'' did not exist, whatever their meaning might
be.
When Schiaparelli began his observations it was generally believed, as
we have said, that the dusky areas on Mars were seas, and since
Schiaparelli thought that the ``canals'' invariably began and ended at
the shores of the ``seas,'' the appropriateness of the title given to
the lines seemed apparent. Their artificial character was immediately
assumed by many, because they were too straight and too suggestively
geometrical in their arrangement to permit the conclusion that they
were natural watercourses. A most surprising circumstance noted by
Schiaparelli was that the ``canals'' made their appearance after the
melting of the polar snow in the corresponding hemisphere had begun,
and that they grew darker, longer, and more numerous in proportion as
the polar liquidation proceeded; another very puzzling observation was
that many of them became double as the season advanced; close beside
an already existing ``canal,'' and in perfect parallelism with it,
another would gradually make its appearance. That these phenomena
actually existed and were not illusions was proved by later
observations, and today they are seen whenever Mars is favorably
situated for observation.
In the closing decade of the nineteenth century, Mr Percival Lowell
took up the work where Schiaparelli had virtually dropped it, and soon
added a great number of ``canals'' to those previously known, so that
in his charts the surface of the wonderful little planet appears
covered as with a spider's web, the dusky lines criss-crossing in
every direction, with conspicuous knots wherever a number of them come
together. Mr Lowell has demonstrated that the areas originally called
seas, and thus named on the earlier charts, are not bodies of water,
whatever else they may be. He has also found that the mysterious lines
do not, as Schiaparelli supposed, begin and end at the edges of the
dusky regions, but often continue on across them, reaching in some
cases far up into the polar regions. But Schiaparelli was right in his
observation that the appearance of the ``canals'' is synchronous with
the gradual disappearance of the polar snows, and this fact has become
the basis of the most extraordinary theory that the subject of life in
other worlds has ever given birth to.
Now, the effect of such discoveries, as we have related, depends upon
the type of mind to whose attention they are called. Many are content
to accept them as strange and inexplicable at present, and to wait for
further light upon them; others insist upon an immediate inquiry
concerning their probable nature and meaning. Such an inquiry can only
be based upon inference proceeding from analogy. Mars, say Mr Lowell
and those who are of his opinion, is manifestly a solidly incrusted
planet like the earth; it has an atmosphere, though one of great
rarity; it has water vapor, as the snows in themselves prove; it has
the alternation of day and night, and a succession of seasons closely
resembling those of the earth; its surface is suggestively divided
into regions of contrasting colors and appearance, and upon that
surface we see an immense number of lines geometrically arranged, with
a system of symmetrical intersections where the lines expand into
circular and oval areas -- and all connected with the annual melting
of the polar snows in a way which irresistibly suggests the
interference of intelligence directed to a definite end. Why, with so
many concurrent circumstances to support the hypothesis, should we not
regard Mars as an inhabited globe?
But the differences between Mars and the earth are in many ways as
striking as their resemblances. Mars is relatively small; it gets less
than half as much light and heat as we receive; its atmosphere is so
rare that it would be distressing to us, even if we could survive in
it at all; it has no lakes, rivers, or seas; its surface is an endless
prairie. and its ``canals'' are phenomena utterly unlike anything on
the earth. Yet it is precisely upon these divergences between the
earth and Mars, this repudiation of terrestrial standards, that the
theory of ``life on Mars,'' for which Mr Lowell is mainly responsible,
is based. Because Mars is smaller than the earth, we are told it must
necessarily be more advanced in planetary evolution, the underlying
cause of which is the gradual cooling and contraction of the planet's
mass. Mars has parted with its internal heat more rapidly than the
earth; consequently its waters and its atmosphere have been mostly
withdrawn by chemical combinations, but enough of both yet remain to
render life still possible on its surface. As the globe of Mars is
evolutionally older than that of the earth, so its forms of organic
life may be proportionally further advanced, and its inhabitants may
have attained a degree of cultivated intelligence much superior to
what at present exists upon the earth. Understanding the nature and
the causes of the desiccation of their planet, and possessing
engineering science and capabilities far in advance of ours, they may
be conceived to have grappled with the stupendous problem of keeping
their world in a habitable condition as long as possible. Supposing
them to have become accustomed to live in their rarefied atmosphere (a
thing not inconceivable, since men can live for a time at least in air
hardly less rare), the most pressing problem for them is that of a
water-supply, without which plant life cannot exist, while animal life
in turn depends for its existence upon vegetation. The only direction
in which they can seek water is that of the polar regions, where it is
alternately condensed into snow and released in the liquid form by the
effect of the seasonal changes. It is, then, to the annual melting of
the polar snow-fields that the Martian engineers are supposed to have
recourse in supplying the needs of their planet, and thus providing
the means of prolonging their own existence. It is imagined that they
have for this purpose constructed a stupendous system of irrigation
extending over the temperate and equatorial regions of the planet. The
``canals'' represent the lines of irrigation, but the narrow streaks
that we see are not the canals themselves, but the irrigated bands
covered by them. Their dark hue, and their gradual appearance after
the polar melting has begun, are due to the growth of vegetation
stimulated by the water. The rounded areas visible where several
``canals'' meet and cross are called by Mr Lowell ``oases.'' These are
supposed to be the principal centers of population and industry. It
must be confessed that some of them, with their complicated systems of
radiating lines, appear to answer very well to such a theory. No
attempt to explain them by analogy with natural phenomena on the earth
has proved successful.
But a great difficulty yet remains: How to explain the seemingly
miraculous powers of the supposed engineers? Here recourse is had once
more to the relative smallness of the planet. We have remarked that
the force of gravity on Mars is only thirty-eight per cent of that on
the earth. A steam-shovel driven by a certain horse-power would be
nearly three times as effective there as here. A man of our stature on
Mars would find his effective strength increased in the same
proportion. But just because of the slight force of gravity there, a
Martian might attain to the traditional stature of Goliath without
finding his own weight an encumbrance to his activity, while at the
same time his huge muscles would come into unimpeded play, enabling
him single-handed to perform labors that would be impossible to a
whole gang of terrestrial workmen. The effective powers of huge
machines would be increased in the same way; and to all this must be
added the fact that the mean density of the materials of which Mars is
composed is much less than that of the constituents of the earth.
Combining all these considerations, it becomes much less difficult to
conceive that public works might be successfully undertaken on Mars
which would be hopelessly beyond the limits of human accomplishment.
Certain other difficulties have also to be met; as, for instance, the
relative coldness of the climate of Mars. At its distance it gets
considerably less than half as much light and heat as we receive. In
addition to this, the rarity of its atmosphere would naturally be
expected to decrease the effective temperature at the planet's
surface, since an atmosphere acts somewhat like the glass cover of a
hot-house in retaining the solar heat which has penetrated it. It has
been calculated that, unless there are mitigating circumstances of
which we know nothing, the average temperature at the surface of Mars
must be far below the freezing-point of water. To this it is replied
that the possible mitigating circumstances spoken of evidently exist
in fact, because we can see that the watery vapor condenses into snow
around the poles in winter, but melts again when summer comes. The
mitigating agent may be supposed to exist in the atmosphere where the
presence of certain gases would completely alter the temperature
gradients.
It might also be objected that it is inconceivable that the Martian
engineers, however great may be their physical powers, and however
gigantic the mechanical energies under their control, could force
water in large quantities from the poles to the equator. This is an
achievement that measures up to the cosmical standard. It is admitted
by the champions of the theory that the difficulty is a formidable
one; but they call attention to the singular fact that on Mars there
can be found no chains of mountains, and it is even doubtful if ranges
of hills exist there. The entire surface of the planet appears to be
almost ``as smooth as a billiard ball,'' and even the broad regions
which were once supposed to be seas apparently lie at practically the
same level as the other parts, since the ``canals'' in many cases run
uninterruptedly across them. Lowell's idea is that these sombre areas
may be expanses of vegetation covering ground of a more or less marshy
character, for while the largest of them appear to be permanent, there
are some which vary coincidently with the variations of the canals.
As to the kind of machinery employed to force the water from the
poles, it has been conjectured that it may have taken the form of a
gigantic system of pumps and conduits; and since the Martians are
assumed to be so far in advance of us in their mastery of scientific
principles, the hypothesis will at least not be harmed by supposing
that they have learned to harness forces of nature whose very
existence in a manageable form is yet unrecognized on the earth. If we
wish to let the imagination loose, we may conjecture that they have
conquered the secret of those intra-atomic forces whose resistless
energy is beginning to become evident to us, but the possibility of
whose utilization remains a dream, the fulfillment of which nobody
dares to predict.
Such, in very brief form, is the celebrated theory of Mars as an
inhabited world. It certainly captivates the imagination, and if we
believe it to represent the facts, we cannot but watch with the
deepest sympathy this gallant struggle of an intellectual race to
preserve its planet from the effects of advancing age and death. We
may, indeed, wonder whether our own humanity, confronted by such a
calamity, could be counted on to meet the emergency with equal
stoutness of heart and inexhaustibleness of resource. Up to the
present time we certainly have shown no capacity to confront Nature
toe to toe, and to seize her by the shoulders and turn her round when
she refuses to go our way. If we could get into wireless telephonic
communication with the Martians we might learn from their own lips the
secret of their more than ``Roman recovery.''
The Riddle of the Asteroids
Between the orbits of Mars and Jupiter revolves the most remarkable
system of little bodies with which we are acquainted -- the Asteroids,
or Minor Planets. Some six hundred are now known, and they may
actually number thousands. They form virtually a ring about the sun.
The most striking general fact about them is that they occupy the
place in the sky which should be occupied, according to Bode's Law, by
a single large planet. This fact, as we shall see, has led to the
invention of one of the most extraordinary theories in astronomy --
viz., that of the explosion of a world!
Bode's Law, so-called, is only an empiric formula, but until the
discovery of Neptune it accorded so well with the distances of the
planets that astronomers were disposed to look upon it as really
representing some underlying principle of planetary distribution. They
were puzzled by the absence of a planet in the space between Mars and
Jupiter, where the ``law'' demanded that there should be one, and an
association of astronomers was formed to search for it. There was a
decided sensation when, in 1801, Piazzi, of Palermo, announced that he
had found a little planet which apparently occupied the place in the
system which belonged to the missing body. He named it Ceres, and it
was the first of the Asteroids. The next year Olbers, of Bremen, while
looking for Ceres with his telescope, stumbled upon another small
planet which he named Pallas. Immediately he was inspired with the
idea that these two planets were fragments of a larger one which had
formerly occupied the vacant place in the planetary ranks, and he
predicted that others would be found by searching in the neighborhood
of the intersection of the orbits of the two already discovered. This
bold prediction was brilliantly fulfilled by the finding of two more
-- Juno in 1804, and Vesta in 1807. Olbers would seem to have been led
to the invention of his hypothesis of a planetary explosion by the
faith which astronomers at that time had in Bode's Law. They appear to
have thought that several planets revolving in the gap where the
``law'' called for but one could only be accounted for upon the theory
that the original one had been broken up to form the several.
Gravitation demanded that the remnants of a planet blown to pieces, no
matter how their orbits might otherwise differ, should all return at
stated periods to the point where the explosion had occurred; hence
Olbers' prediction that any asteroids that might subsequently be
discovered would be found to have a common point of orbital
intersection. And curiously enough all of the first asteroids found
practically answered to this requirement. Olbers' theory seemed to be
established.
After the first four, no more asteroids were found until 1845, when
one was discovered; then, in 1847, three more were added to the list;
and after that searchers began to pick them up with such rapidity that
by the close of the century hundreds were known, and it had become
almost impossible to keep track of them. The first four are by far the
largest members of the group, but their actual sizes remained unknown
until less than twenty years ago. It was long supposed that Vesta was
the largest, because it shines more brightly than any of the others;
but finally, in 1895, Barnard, with the Lick telescope, definitely
measured their diameters, and proved to everybody's surprise that
Ceres is really the chief, and Vesta only the third in rank. His
measures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta,
239 miles; and Juno, 120 miles. They differ greatly in the reflective
power of their surfaces, a fact of much significance in connection
with the question of their origin. Vesta is, surface for surface,
rather more than three times as brilliant as Ceres, whence the
original mistake about its magnitude.
Nowadays new asteroids are found frequently by photography, but
physically they are most insignificant bodies, their average diameter
probably not exceeding twenty miles, and some are believed not to
exceed ten. On a planet only ten miles in diameter, assuming the same
mean density as the earth's, which is undoubtedly too much, the force
of gravity would be so slight that an average man would not weigh more
than three ounces, and could jump off into space whenever he liked.
Although the asteroids all revolve around the sun in the same
direction as that pursued by the major planets, their orbits are
inclined at a great variety of angles to the general plane of the
planetary system, and some of them are very eccentric -- almost as
much so as the orbits of many of the periodic comets. It has even been
conjectured that the two tiny moons of Mars and the four smaller
satellites of Jupiter may be asteroids gone astray and captured by
those planets. Two of the asteroids are exceedingly remarkable for the
shapes and positions of their orbits; these are Eros, discovered in
1898, and T. G., 1906, found eight years later. The latter has a mean
distance from the sun slightly greater than that of Jupiter, while the
mean distance of Eros is less than that of Mars. The orbit of Eros is
so eccentric that at times it approaches within 15,000,000 miles of
the earth, nearer than any other regular member of the solar system
except the moon, thus affording an unrivaled means of measuring the
solar parallax. But for our present purpose the chief interest of Eros
lies in its extraordinary changes of light.
These changes, although irregular, have been observed and photographed
many times, and there seems to be no doubt of their reality. Their
significance consists in their possible connection with the form of
the little planet, whose diameter is generally estimated at not more
than twenty miles. Von Oppolzer found, in 1901, that Eros lost
three-fourths of its brilliancy once in every two hours and
thirty-eight minutes. Other observers have found slightly different
periods of variability, but none as long as three hours. The most
interesting interpretation that has been offered of this phenomenon is
that it is due to a great irregularity of figure, recalling at once
Olbers' hypothesis. According to some, Eros may be double, the two
bodies composing it revolving around each other at very close
quarters; but a more striking, and it may be said probable, suggestion
is that Eros has a form not unlike that of a dumb-bell, or hour-glass,
turning rapidly end over end so that the area of illuminated surface
presented to our eyes continually changes, reaching at certain times a
minimum when the amount of light that it reflects toward the earth is
reduced to a quarter of its maximum value. Various other bizarre
shapes have been ascribed to Eros, such, for instance, as that of a
flat stone revolving about one of its longer axes, so that sometimes
we see its face and sometimes its edge.
All of these explanations proceed upon the assumption that Eros cannot
have a simple globular figure like that of a typical planet, a figure
which is prescribed by the law of gravitation, but that its shape is
what may be called accidental; in a word, it is a fragment, for it
seems impossible to believe that a body formed in interplanetary
space, either through nebular condensation or through the aggregation
of particles drawn together by their mutual attractions, should not be
practically spherical in shape. Nor is Eros the only asteroid that
gives evidence by variations of brilliancy that there is something
abnormal in its constitution; several others present the same
phenomenon in varying degrees. Even Vesta was regarded by Olbers as
sufficiently variable in its light to warrant the conclusion that it
was an angular mass instead of a globe. Some of the smaller ones show
very notable variations, and all in short periods, of three or four
hours, suggesting that in turning about one of their axes they present
a surface of variable extent toward the sun and the earth.
The theory which some have preferred -- that the variability of light
is due to the differences of reflective power on different parts of
the surface -- would, if accepted, be hardly less suggestive of the
origin of these little bodies by the breaking up of a larger one,
because the most natural explanation of such differences would seem to
be that they arose from variations in the roughness or smoothness of
the reflecting surface, which would be characteristic of fragmentary
bodies. In the case of a large planet alternating expanses of land and
water, or of vegetation and desert, would produce a notable variation
in the amount of reflection, but on bodies of the size of the
asteroids neither water nor vegetation could exist, and an atmosphere
would be equally impossible.
One of the strongest objections to Olbers' hypothesis is that only a
few of the first asteroids discovered travel in orbits which
measurably satisfy the requirement that they should all intersect at
the point where the explosion occurred. To this it was at first
replied that the perturbations of the asteroidal orbits, by the
attractions of the major planets, would soon displace them in such a
manner that they would cease to intersect. One of the first
investigations undertaken by the late Prof. Simon Newcomb was directed
to the solution of this question, and he arrived at the conclusion
that the planetary perturbations could not explain the actual
situation of the asteroidal orbits. But afterward it was pointed out
that the difficulty could be avoided by supposing that not one but a
series of explosions had produced the asteroids as they now are. After
the primary disruption the fragments themselves, according to this
suggestion, may have exploded, and then the resulting orbits would be
as ``tangled'' as the heart could wish. This has so far rehabilitated
the explosion theory that it has never been entirely abandoned, and
the evidence which we have just cited of the probably abnormal shapes
of Eros and other asteroids has lately given it renewed life. It is a
subject that needs a thorough rediscussion.
We must not fail to mention, however, that there is a rival hypothesis
which commends itself to many astronomers -- viz., that the asteroids
were formed out of a relatively scant ring of matter, situated between
Mars and Jupiter and resembling in composition the immensely more
massive rings from which, according to Laplace's hypothesis, the
planets were born. It is held by the supporters of this theory that
the attraction of the giant Jupiter was sufficient to prevent the
small, nebulous ring that gave birth to the asteroids from condensing
like the others into a single planet.
But if we accept the explosion theory, with its corollary that minor
explosions followed the principal one, we have still an unanswered
question before us: What caused the explosions? The idea of a world
blowing up is too Titanic to be shocking; it rather amuses the
imagination than seriously impresses it; in a word, it seems
essentially chimerical. We can by no appeal to experience form a
mental picture of such an occurrence. Even the moon did not blow up
when it was wrecked by volcanoes. The explosive nebulæ and new stars
are far away in space, and suggest no connection with such a
catastrophe as the bursting of a planet into hundreds of pieces. We
cannot conceive of a great globe thousands of miles in diameter
resembling a pellet of gunpowder only awaiting the touch of a match to
cause its sudden disruption. Somehow the thought of human agency
obtrudes itself in connection with the word ``explosion,'' and we
smile at the idea that giant powder or nitro-glycerine could blow up a
planet. Yet it would only need enough of them to do it.
After all, we may deceive ourselves in thinking, as we are apt to do,
that explosive energies lock themselves up only in small masses of
matter. There are many causes producing explosions in nature, every
volcanic eruption manifests the activity of some of them. Think of the
giant power of confined steam; if enough steam could be suddenly
generated in the center of the earth by a downpour of all the waters
of the oceans, what might not the consequences be for our globe? In a
smaller globe, and it has never been estimated that the original
asteroid was even as large as the moon, such a catastrophe would,
perhaps, be more easily conceivable; but since we are compelled in
this case to assume that there was a series of successive explosions,
steam would hardly answer the purpose; it would be more reasonable to
suppose that the cause of the explosion was some kind of chemical
reaction, or something affecting the atoms composing the exploding
body. Here Dr Gustav Le Bon comes to our aid with a most startling
suggestion, based on his theory of the dissipation of intra-atomic
energy. It will be best to quote him at some length from his book on
The Evolution of Forces.
``It does not seem at first sight,'' says Doctor Le Bon,
very comprehensible that worlds which appear more and more stable
as they cool could become so unstable as to afterward dissociate
entirely. To explain this phenomenon, we will inquire whether
astronomical observations do not allow us to witness this
dissociation.
We know that the stability of a body in motion, such as a top or a
bicycle, ceases to be possible when its velocity of rotation
descends below a certain limit. Once this limit is reached it loses
its stability and falls to the ground. Prof. J. J. Thomson even
interprets radio-activity in this manner, and points out that when
the speed of the elements composing the atoms descends below a
certain limit they become unstable and tend to lose their
equilibria. There would result from this a commencement of
dissociation, with diminution of their potential energy and a
corresponding increase of their kinetic energy sufficient to launch
into space the products of intra-atomic disintegration.
It must not be forgotten that the atom being an enormous reservoir
of energy is by this very fact comparable with explosive bodies.
These last remain inert so long as their internal equilibria are
undisturbed. So soon as some cause or other modifies these, they
explode and smash everything around them after being themselves
broken to pieces.
Atoms, therefore, which grow old in consequence of the diminution
of a part of their intra-atomic energy gradually lose their
stability. A moment, then, arrives when this stability is so weak
that the matter disappears by a sort of explosion more or less
rapid. The bodies of the radium group offer an image of this
phenomenon -- a rather faint image, however, because the atoms of
this body have only reached a period of instability when the
dissociation is rather slow. It probably precedes another and more
rapid period of dissociation capable of producing their final
explosion. Bodies such as radium, thorium, etc., represent, no
doubt, a state of old age at which all bodies must some day arrive,
and which they already begin to manifest in our universe, since all
matter is slightly radio-active. It would suffice for the
dissociation to be fairly general and fairly rapid for an explosion
to occur in a world where it was manifested.
These theoretical considerations find a solid support in the sudden
appearances and disappearances of stars. The explosions of a world
which produce them reveal to us, perhaps, how the universes perish
when they become old.
As astronomical observations show the relative frequency of these
rapid destructions, we may ask ourselves whether the end of a
universe by a sudden explosion after a long period of old age does
not represent its most general ending.
Here, perhaps, it will be well to stop, since, entrancing as the
subject may be, we know very little about it, and Doctor Le Bon's
theory affords a limitless field for the reader's imagination.
_________________________________________________________________
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