An asteroid is like a tiny star, and in fact the two bodies are very
often mistaken. If we could get close to the objects, we should see a
wide difference between them. We should find the asteroid to be a dark
planet like our earth, lighted only by the rays from the sun. The star,
small and faint though it may seem, is itself a bright sun, at such a
vast distance that it is only visible as a small point. The star is
millions of times as far from us as the planet, and utterly different
in every respect.
It is a curious fact that the planets should happen to resemble the
stars so closely. We can find an analogous fact in quite another part
of nature. In visiting a good entomological collection, you will be
shown some of the wonderful leaf-like insects. These creatures have
wings, exactly formed to imitate leaves of trees, with the stalks and
veins completely represented. When one of these insects lies at rest,
with its wings folded, among a number of leaves, it would be almost
impossible to penetrate the disguise. This mimicry is no doubt an
ingenious artifice to deceive the birds or other enemies that want to
eat the insect. There is, however, one test which the cunning bird
could apply: the leaves do not move about of their own accord, but the
leaf-insects do. If therefore the bird will only have the patience
to wait, he will see a pair of the seeming leaves move, and then the
deception will be to him a deception no longer, and he will gobble up
the poor insect.
In our attempts to discover the planets we experience just the same
difficulties as the insect-eating bird. Wide as is the true difference
between a planet and a star, there is yet such a seeming resemblance
between them that we are often puzzled to know which is which. The
planets imitate the stars so successfully, that when one of them
is presented to us among myriads of stars it is impossible for us
to detect the planet by its appearance. But we can be cunning--we
can steadily watch, and the moment we find one of these star-like
points beginning to creep about we can pounce upon it. We know by its
movements that it is only disguised as a star, but that it is really
one of the planets.
It is not always easy to discover the asteroids even by this principle,
for unfortunately these bodies move very slowly. If you have a planet
in the field of view, it will creep along so gradually that an hour or
more must have elapsed before it has shifted its position with respect
to the neighboring stars to any appreciable extent. The search for such
little planets is therefore a tedious one, but there are two methods of
conducting it: the new one, which has only recently come into use, and
the old one. I shall speak of the old one first.
Although the body’s motion is so slow, yet when sufficient time is
allowed, the planet will not only move away from the stars close by,
but will even journey round the entire heavens. The surest way of
making the discovery is to study a small part of the heavens now and
to examine the same locality again months or years afterwards. Memory
will not suffice for this purpose. No one could recollect all the stars
he saw with sufficient distinctness to be confident that the field of
the telescope on the second occasion contained either more or fewer
stars than it did on the first. The only way of doing this work is to
draw a map of the stars very carefully. This is a tedious business,
for the stars are so numerous that even in a small part of the heavens
there will be many thousands of stars visible in the telescope. All of
these will have to be entered faithfully in their true places on the
map. When this has been done the map must be laid aside for a season,
and then it is brought out again and compared with the sky. No doubt
the great majority of the objects will be found just as they were
before. These are the stars, the distant suns, and our concern is not
at present with them. Sometimes it will happen that an object marked
on the first map has left a vacant place on the second. This, however,
does not help us much, for, whatever the object was, it has vanished
into obscurity, and a new planet could hardly be discovered in this
way. But sometimes it will happen that there is a small point of light
seen in the second map which has no corresponding point in the first.
Then, indeed, the expectation of the astronomer is aroused; he may be
on the brink of a discovery. Of course he watches accurately the little
stranger. It might be some star that had been accidentally overlooked
when forming the map, or it might possibly be a star that has become
bright in the interval. But here is a ready test: is the body moving?
He looks at it very carefully, and notes its position with respect to
the adjacent stars. In an hour or two his suspicions may be confirmed;
if the object be in motion, then it is really a planet. A few further
observations, made on subsequent days, will show the path of the body.
And the astronomer has only to assure himself that the object is not
one of the planets that have been already found before he announces his
discovery.
The new method of searching for small planets, which has only come into
use in recent years, is a very beautiful one, and renders the process
of making such discoveries much more easy than the older method which I
have just described.
We can take photographs of the heavenly bodies by adjusting a sensitive
plate in the telescope so that the images of the objects we desire to
see shall fall upon it. The method will apply to very small stars, if
by excellent clockwork and careful guiding we can keep the telescope
constantly pointing to the same spot until the stars have had time to
imprint their little images. Thus we obtain a map of the heavens, made
in a thoroughly accurate manner. Indeed, the delicacy of photography
for this purpose is so great, that the plates show many stars which
cannot be seen with even the greatest of telescopes. Suppose that
a little planet happened to lie among the stars which are being
photographed. All the time that the plate is being exposed the wanderer
is, of course, creeping along, and after an hour (exposures even longer
are often used), it may have moved through a distance sufficient to
ensure its detection. The plate will, therefore, show the stars as
points, but the planet will betray its presence by producing a streak.
The asteroids now known number between 400 and 500. Out of this host
a few afford some information to the astronomer, but the majority of
them are objects possessing individually only the slightest interest.
No small planet is worth looking at as a telescopic picture. We should
consider that asteroid to be a large one which possessed a surface
altogether as great as England or France. Many of these planets have a
superficial extent not so large as some of our great counties. A globe
which was just big enough to be covered by Yorkshire--if you could
imagine that large county neatly folded round it--would make a very
respectable minor planet.
We know hardly anything of the nature of these small worlds, but it is
certain that any living beings they could support must have a totally
different nature from the creatures that we know on this earth. We can
easily prove this by making a calculation. I shall suppose a small
planet one hundred miles wide, its diameter being, therefore, the
one-eightieth part of the diameter of the earth. If we were landed
on such a globe, we should be far more puzzled by the extraordinary
lightness of everything than we should be in the similar case of the
moon to which I referred (p. 124). If we suppose the planet to be
constructed of materials which had the same density as those of which
the earth is made, then every weight would be reduced to the eightieth
part of what it is here.
There would be one curious consequence of residence on such a globe.
We have heard of attempts to make flying machines, or to provide a
man with wings by which he shall soar aloft like the birds. All such
contrivances have hitherto failed. It may be possible to make a pair of
wings by which a man can fly down, but it is quite another matter when
he tries to fly up again. Suppose, however, we were living on a small
planet, it would be perfectly easy to fly, for as our bodies would only
seem to weigh a couple of pounds, we ought to be able to flap a pair of
wings strong enough to overcome so trivial a force. I should, however,
add that this is on the supposition that the atmosphere has the same
density as our own.
Life on these small planets would indeed be extraordinary. Let us take,
for example, Flora, and see how a game of lawn tennis on that body
would be managed. The very slightest blow of the racket would drive the
ball a prodigious distance before it could touch the ground; indeed,
unless the courts were about half a mile long, it would be impossible
to serve any ball that was not a fault. Nor is there any great exertion
necessary for playing lawn tennis on Flora, even though the courts are
several hundred acres in extent. As a young lady ran to meet the ball
and return it, each of her steps might cover a hundred yards or so
without extra effort; and should she have the misfortune to get a fall,
her descent to the ground would be as gentle as if she was seeking
repose on a bed of the softest swan’s-down.
These little planets cluster together in a certain part of our
system. Inside are the four inner planets, of which we have already
spoken; outside are the four outer planets, of which we have soon to
speak. Between these two groups there was a vacant space. It seemed
unreasonable that where there was room for planets, planets should
not be found. Accordingly the search was made, and these objects
were discovered. Even at the present day, more and more are being
constantly added to the list.
Up till quite recently all the small planets which had been discovered
confined themselves to the space lying between the paths of the
major planets Mars and Jupiter. This invariable rule was, however,
departed from in the case of one of these bodies which was discovered
in August, 1898. This little body, which was known for some time by
the provisional appellation of D Q, and which has now been definitely
christened Eros, is an exception to this rule. It travels at an average
distance from the sun actually less than that of Mars, and at the
nearest point can come within 15,000,000 miles of the earth.
We occasionally get information from these little bodies; for in their
revolutions through the solar system, they sometimes pick up scraps of
useful knowledge, which we can elicit from them by careful examination.
For example, one of the most important problems in the whole of
astronomy is to determine the sun’s distance. I have already mentioned
one of the ways of doing this, which is given by the transit of Venus.
Astronomers never like to rely on a single method; we are therefore
glad to discover any other means of solving the same problem. This
it is which the little planets will sometimes do for us. Juno on one
occasion approached very close to the earth, and astronomers in various
parts of the globe observed her at the same time. When they compared
their observations they measured the sun’s distance. But I am not going
to trouble you now with a matter so difficult. Suffice it to say,
that for this, as for all similar investigations, the observers were
constrained to use the very same principle as that which we illustrated
in Fig. 5.
Let me rather close this lecture with the remark that we have here been
considering only the lesser members of the great family which circulate
round the sun, and that we shall speak in our next lecture of the giant
members of our system.