The planets are all massive globes, more or less flattened at the
Poles; but now we have to talk about a multitude of objects of the
most irregular shapes, and of the most flimsy description. We call
them _comets_, and they exist in such numbers that an old astronomer
has said “there were more comets in the sky than fishes in the sea,”
though I think we cannot quite believe him. There is also another
wide difference between planets and comets: planets move round in
nearly circular ellipses, and not only do we know where a planet is
to-night, but we know where it was a month ago, or a hundred years
ago, or where it will be in a hundred years or a thousand years to
come. All such movements are conducted with conspicuous regularity
and order; but now we are to speak of bodies which generally come in
upon us in the most uncertain and irregular fashion. They visit us we
hardly know whence, except that it is from outer space, and they are
adorned in a glittering raiment, almost spiritual in its texture.
They are always changing their appearance in a baffling, but still
very fascinating manner. If an artist tries to draw a comet, he will
have hardly finished his picture of it in one charming robe before he
finds it arrayed in another. The astronomer has also his complaints to
make against the comets. I have told you how thoroughly we can rely on
the movements of the planets, but comets often play sad pranks with
our calculations. They sometimes take the astronomers by surprise, and
blaze out with their long tails just when we do not expect them. Then
by way of compensation they frequently disappoint us by not appearing
when they have been most anxiously looked for.
After a voyage through space the comet at length begins to draw in
towards the central parts of our system, and as it approaches the sun,
its pace becomes gradually greater and greater; in fact, as the body
sweeps round the sun the speed is sometimes 20,000 times faster than
that of an express train. It is sometimes more than 1000 times as fast
as the swiftest of rifle bullets, occasionally attaining the rate of
200 miles a second. The closer the comet goes to the sun, the faster
it moves; and a case has been known in which a comet, after coming in
for an incalculable duration of time towards the sun, has acquired a
speed so tremendous, that in two hours it has whirled round the sun and
has commenced to return to the depths of outer space. This terrific
outburst of speed does not last long. A pace which near the sun is
20,000 times that of our express trains diminishes to 10,000 times, to
fifty times, to ten times that pace; while in the outermost part of
its path the comet seems to creep along so slowly that we might think
it had been fatigued by its previous exertions.
[Illustration: FIG. 72.--How the Comet’s Tail is disposed.]
We have so often seen a stream of sparks stretching out along the track
of a sky-rocket, that we might naturally suppose the tail of a comet
streamed out along its path in a somewhat similar manner. This would
be quite wrong. You see from Fig. 72 that the tail does not lie along
the comet’s path, but is always directed outwards from the sun. If you
will draw a line from the sun to the head of the comet and follow the
direction of the line, it shows the way in which the tail is arranged.
You will also notice how the tail of the comet seems to grow in length
as it approaches the sun. When the comet is first seen, the tail is
often a very insignificant affair, but it shoots out with enormous
rapidity until it becomes many millions of miles long by the time the
comet is whirling round the sun. Those glories soon begin to wane as
the comet flies outward; the tail gradually vanishes, and the wanderer
retreats again to the depths of space in the same undecorated condition
as that in which it first approached.
When a comet appears, it is always a matter of interest to see whether
it is an entirely new object, or whether it may not be only another
return of a comet which has paid us one or more previous visits. The
question then arises as to how they are to be identified. Here we see a
wide contrast between unsubstantial bodies like comets and the weighty
and stately planets. Sketches of the various planets or of the face of
the sun, though they might show slight differences from time to time,
are still always sufficiently characteristic, just as a photographic
portrait will identify the individual, even though the lapse of years
will bring some changes in his appearance. But the drawing of a comet
is almost useless for identification. You might as well try to identify
a cloud or a puff of smoke by making a picture of it. Make a drawing of
a comet at one appearance, and sketch particularly the ample tail with
which it is provided. The next time the comet comes round it may very
possibly have two tails, or possibly no tail at all. We are therefore
unable to place any reliance on the comet’s personal appearance in our
efforts to identify it. The highway which it follows through the sky
affords the only means of recognition; for the comet, if undisturbed
by other objects, will never change its actual orbit. But even this
method of identification often fails, for it not unfrequently happens
that during its erratic movements the comet gets into fearful trouble
with other heavenly bodies. In such cases the poor comet is sometimes
driven so completely out of its road that it has to make for itself
an entirely new path, and our efforts to identify it are plunged in
confusion. It has happened that a second comet or even a third will be
found in nearly the same track, but whether these are wholly different,
or whether they are merely parts of the same original object, it is
often impossible to determine.
The great majority of comets are only to be seen with a telescope, and
hardly a year passes without the detection of at least a few of these
faint objects. The number of really brilliant comets that can be seen
in a lifetime could, however, be counted on the fingers.