_SPACE_
Although astronomy tells us in clearest words of the vast depths of
space which surround our earth on all sides, we are not thereby enabled
to realize their enormous extension. It is not merely that the unknown
depths beyond the range of our most powerful telescopes are
inconceivable, but that the parts of space which we can examine are on
too large a scale for us to conceive their real dimensions. It is hardly
going too far to say that our powers of actual conception are limited to
the extent of space over which the eye _seems_ to range in the daytime.
Of course in the daytime, at least in clear weather, there is one
direction in which the eyesight ranges over a distance of many millions
of miles,--namely, where we see the sun. But the sense of sight is not
cognisant of that enormous distance, and simply presents the sun to us
as a bright disc in the sky, or perhaps rather nearer to us than the
sky. Even the distance of the sky itself is under-estimated. A portion
of the light we receive from the sky on a clear day comes from parts of
the atmosphere distant more than thirty or forty miles from us; but the
eye does not recognise the fact. The blue sky seems a little farther off
than the clouds, but not much; the light clouds of summer seem a little
but not much farther off than the heavier clouds of a winter sky; a
cloud-covered winter sky seems a little farther off than heavy
rain-clouds. The actual varieties of distance among clouds of various
kinds are not much more clearly discerned than the actual varieties of
distance among the heavenly bodies. The estimate formed of the distance
of a cloud-covered sky overhead probably amounts to little more than a
mile, and it is very doubtful whether the mind presents the remotest
depths of a blue sky overhead at more than two miles. Towards the
horizon the distance seems greater, and probably on a cloudy day the sky
near the horizon is unconsciously regarded as at a distance of about
five miles, while blue sky near the horizon may be regarded as lying at
a distance of six or seven miles, the arch of a blue sky seeming to be
far more deeply curved than that of a cloud-covered sky.
It is to distances such as these that the mind unconsciously refers the
celestial bodies. We know that the moon is about 2,000 miles in
diameter, but the mind refuses to present her to us as other than a
round disc much smaller than those other objects in sight which occupy a
much larger portion of the field of vision. The sun cannot be conceived
to exceed the moon enormously in size, seeing that he appears no larger;
and all the multitude of stars are judged by the sight to be mere bright
points of light in reality as they appear to be.
How, then, can we hope to appreciate the vastness of space whereof
astronomy tells us? To the student of science attempting to conceive the
immensities of whose existence he is assured, the same lesson might be
taught in parable which the child of St. Augustine's vision taught the
Numidian theologian. As reasonably might an infant hope to pour the
waters of ocean into a hollow, scooped with his tiny fingers in the
sand, as man to picture in his narrow mind the length and breadth and
depth of the abysses of space in which our earth is lost.
Yet, as a picture of a great mansion may be so drawn on a small scrap of
paper as to convey just ideas of its proportions, so may the great
truths which astronomy has taught us about the depths of space be so
presented that just conceptions may be formed of the proportions of at
least those parts of the universe which lie within the range of
scientific vision, though it would be hopeless to attempt to conceive
their real dimensions.
Thus, when we learn that a globe as large as our earth, suspended beside
the moon, would seem to have a diameter exceeding hers nearly four
times, so that the globe would cover a space in the heavens about
thirteen times as large as the moon covers, we form a just conception of
the size of the moon as compared with the earth, though the mind cannot
conceive such a body as the moon or the earth really is. When, in turn,
we are told that if a globe as large as the earth, but glowing as
brightly as the sun, were set beside the sun, it would look a mere point
of light, we not only learn to picture rightly to ourselves how largely
the sun exceeds the earth, but also how enormous must be the real
distance of the sun.
Another step leads us to a standpoint whence we can form a correct
estimate of the vast distance of the fixed stars; for we learn that so
enormous is the distance of even the nearest fixed star, that the
tremendous space separating the earth from that star sinks in turn into
the merest point, insomuch that if a globe as bright as the sun had the
earth's orbit as a close fitting girdle, then this glorious orb (with a
diameter of some 184,000,000 of miles) would look very much smaller than
such a globe as our earth would look at the sun's distance--would, in
fact, occupy but about one-fortieth part of the space in the sky which
she, though she would then look a mere point, would occupy if viewed
from that distance.
But there is a way of viewing the immensities of space which, though not
aiding us indeed to conceive them, enables the mind to picture their
proportions better than any other. The dimensions of the earth's path
around the sun sink into insignificance beside those of the outermost
planets; but these in their turn dwindle into nothingness beside those
of some among the comets. From the paths of these comets, if only
sentient and reasoning beings could trace out in a comet's company those
mighty orbits, and could have for the duration of their existence not
the brief span of time which measures the longest human life, but many
circuits of their comet home around the same ruling orb (as we live
during many circuits of our globe around the sun), the dimensions of the
star-depths, which even to scientific insight are all but immeasurable,
would be directly discernible. Not only would the proportions of that
mighty system be perceived, whose fruits and blossoms are suns and
worlds, but even the gradually changing arrangement of its parts could
be discerned.
Some comets, indeed, as I pointed out in an essay on comets several
months ago (see Expanse of Heaven, p. 149), do not travel around the
sun, but flit from sun to sun on journeys lasting millions of years,
paying each sun but a single visit. A being inhabiting such a comet,
and having these interstellar journeys as the years of his existence, so
that he could live through many of them, would have a wonderful insight
into the economy of the stellar system. If his powers of conception as
far exceeded ours as the range of his travels and the duration of his
existence, he would be able to recognise the proportions of a large part
of the stellar universe as clearly as we recognise the proportions of
the solar system.
But leaving these wonderful wanderers, whose journeys are as far beyond
our powers of conception as the immensity of the regions of star-strewn
space, we may find, among the comets belonging to the sun's domain,
bodies whose range of travel would give their inhabitants far clearer
views of the architecture of the heavens than even the profoundest
terrestrial astronomer can possibly obtain.
Such a comet as Halley's (fig. 3) for instance, though one of
comparatively limited range in space, yet travels so far from the sun
that, from the extreme part of its path, it sees the stars displaced
nearly twenty times as much (owing to its own change of position) as
they are from the earth on opposite sides of her comparatively narrow
orbit. And the length of this comet's year, if it indicated the length
of the lives of all creatures travelling along with it, would suggest a
power of patiently watching the progress of changes lasting not a few of
our years only, but for centuries. Seventy-five or seventy-six years
elapse between each return of this comet to the sun's neighbourhood, and
one who should have lived during sixty or seventy circuits of this body
around its mighty orbit would have been able to watch the rush of stars,
with their velocities of many miles per second, until visible
displacements had taken place in their positions.
[Illustration: Fig. 3.--Halley's Comet of 1835.]
This, however, is as nothing compared with the mighty range in space and
the enormous period of the orbit of the great comet of the year 1811
(fig. 4). This comet is, on the whole, the most remarkable ever known.
It was visible for nearly seventeen months, and though it did not
approach the sun within 100,000,000 miles, and was therefore not subject
to that violence of action which has caused enormous tails to be thrown
out from comets which have come within a few million miles of him, or
even within less than a quarter of his own diameter, it flourished forth
a tail 120,000,000 of miles in length. Its orbit has, according to the
calculations of the astronomer Argelander, a space exceeding the earth's
distance from the sun 211 times, and thus surpassing even the mighty
distance of Neptune fully seven times. It occupies in circuiting this
mighty path no less than 3065 of our years (with a possible error either
way of about forty-three years). So that, according to Bible chronology,
this comet's last appearance probably occurred during the rule of the
judge Tola, son of Puah, son of Dodo, over the children of Israel,
though it may have occurred during the rule of his predecessor
Abimelech, or during that of his successor Jair.[1] During one half of
the enormous interval between that time and 1811 the comet was rushing
outwards into space, reaching the remotest part of its path somewhere
about the year 278 (A.D.), and from that time to 1811 it was on its
return journey. It is strange to think, however, that though the
remotest part of its path lay 211 times farther from the sun than the
earth's orbit, yet even this mighty path, requiring more than 3000 years
for a single circuit, cannot be said to have carried the comet into the
star-depths. If the earth were to shift its position by the same
enormous amount the nearest fixed star would have its apparent position
changed only by about an eighth part of the apparent diameter of the sun
or moon, or by about one-quarter of the distance separating the middle
star of the Bear's tail from its close companion.
[Illustration: Fig. 4.--Comet of 1811.]
But this fact of itself is most strikingly suggestive of the vast
distance of the stars. For consider what it means. Imagine the middle
star of the Bear's tail to be the really nearest of all the stars
instead of lying probably twenty or thirty times farther away. Conceive
a comet belonging to that sun after making its nearest approach to it to
travel away upon an orbit requiring 3000 years for each circuit. _Then_
(supposing that star equal to our sun in mass), the comet, though
rushing away from its sun with inconceivable velocity during 1500 years,
would, at the end of that vast period, seem to be no farther away than
one-fourth of the distance separating the sun from its near companion.
Look at the middle star of the Bear's tail on any clear night, and on
its small satellite, remembering this fact, and the awful immensity of
the star depths are strongly impressed upon the mind. But the observer
must not fail to remember that the star really is many times more remote
than we have here for a moment supposed, and that such a comet's range
of travel would be proportionately reduced. Moreover, many among the
stars are, doubtless, hundreds, even thousands, of times still farther
away.
[Illustration: Fig. 5--Six-tailed Comet of 1744.]
Let us turn lastly to the amazing comet of the year 1744, pictured, at
the time, as shown in fig. 5 (though probably the drawing is greatly
exaggerated). We find that though it had the longest period of any which
has ever been assigned to a comet as the result of actual mathematical
calculation, yet its range in space would scarcely suffice to change the
position of the stars in such sort that the aspect of the familiar
constellations would be materially altered. Euler, the eminent
mathematician, calculated for this comet a period of 122,683 years,
which would correspond, I find, to a distance of recession equal to 2469
times the distance of the earth from the sun, or about eighty times the
distance of Neptune. Yet this is but little more than twelve times the
greatest distance of the comet of 1811. Probably the actual range of
such an orbit from the middle star of the Bear's tail would be equal in
appearance to the range described above on the supposition that the star
is no farther from us than the nearest known star (Alpha Centauri). That
is, such a comet, if it could be seen and watched during a period of
about 122,000 years, would seem to recede from the star to a distance
equal to about one-fourth the space separating it from its close
companion, and then to return to the point of nearest approach to its
ruling sun.
Such are the immensities of star-strewn space! The journey of a comet
receding from the sun with inconceivable velocity during hundreds of
thousands of years carries it but so small a distance from him compared
with the distance of the nearest star as scarcely to change the
appearance of the celestial landscape; and yet the distances separating
the sun from the nearest of his fellow suns are but as hair-breadths to
leagues when compared with the proportions of the scheme of suns to
which he belongs. These distances, though so mighty that by comparison
with them the inconceivable dimensions of our own earth sink into utter
nothingness, do not bring us even to the threshold of the outermost
court of that region of space to which the scrutiny of our telescopes
extends. Yet the whole of that region is but an atom in the infinity of
space.