ed. Hence this body is
with propriety called a _planet_, or a wanderer, because it is
incessantly moving from one part of the starry heavens to another. By
similar comparisons it can be shown that the other bodies we have
mentioned--Venus and Mercury, Saturn and Mars--are also wanderers, and
belong to that group of heavenly bodies known as planets. Here, then, we
have the simple criterion by which the earth's neighbours are readily to
be discriminated from the stars. Each of the bodies near the earth is a
planet, or a wanderer, and the mere fact that a body is a wanderer is
alone sufficient to prove it to be one of the class which we are now
studying.

Provided with this test, we can at once make an addition to our list of
neighbours. Amid the myriad orbs which the telescope reveals, we
occasionally find one which is a wanderer. Two other mighty planets,
known as Uranus and Neptune, must thus be added to the five already
mentioned, making in all a group of seven great planets. A vastly
greater number may also be reckoned when we admit to our view bodies
which not only seem to be minute telescopic objects, but really are
small globes when compared with the mighty bulk of our earth. These
lesser planets, to the number of more than four hundred, are also among
the earth's neighbours.

We should remark that another class of heavenly bodies widely differing
from the planets must also be included in our system. These are the
comets, and, indeed, it may happen that one of these erratic bodies will
sometimes draw nearer to the earth than even the closest approach ever
made by a planet. These mysterious visitors will necessarily engage a
good deal of our attention later on. For the present we confine our
attention to those more substantial globes, whether large or small,
which are always termed planets.

Imagine for a moment that some opaque covering could be clasped around
our sun so that all his beams were extinguished. That our earth would be
plunged into the darkness of midnight is of course an obvious
consequence. A moment's consideration will show that the moon, shining
as it does by the reflected rays of the sun, would become totally
invisible. But would this extinction of the sunlight have any other
effect? Would it influence the countless brilliant points that stud the
heavens at midnight? Such an obscuration of the sun would indeed produce
a remarkable effect on the sky at night, which a little attention would
disc