ze as well as in splendour, but plunged into the depths of space to
such an appalling distance that his enfeebled rays, when they reach the
earth, give us the impression, not of a mighty sun, but only of a
brilliant star.

The principle of selection, by which the earth's neighbours can be
discriminated, will be explained presently; in the meantime, it will be
sufficient to observe that our list is to be augmented first by the
addition of the unique object known as Saturn, though its brightness is
far surpassed by that of Sirius, as well as by a few other stars. Then
we add Mars, an object which occasionally approaches so close to the
earth that it shines with a fiery radiance which would hardly prepare
us for the truth that this planet is intrinsically one of the smallest
of the celestial bodies. Besides the objects we have mentioned, the
ancient astronomers had detected a fifth, known as Mercury--a planet
which is usually invisible amid the light surrounding the sun. Mercury,
however, occasionally wanders far enough from our luminary to be seen
before sunrise or after sunset. These five--Mercury, Venus, Mars,
Jupiter, and Saturn--comprised the planets known from remote antiquity.

We can, however, now extend the list somewhat further by adding to it
the telescopic objects which have in modern times been found to be among
our neighbours. Here we must no longer postpone the introduction of the
criterion by which we can detect whether a body is near the earth or
not. The brighter planets can be recognised by the steady radiance of
their light as contrasted with the incessant twinkling of the stars. A
little attention devoted to any of the bodies we have named will,
however, point out a more definite contrast between the planets and the
stars.

Observe, for instance, Jupiter, on any clear night when the heavens can
be well seen, and note his position with regard to the constellations in
his neighbourhood--how he is to the right of this star, or to the left
of that; directly between this pair, or directly pointed to by that. We
then mark down the place of Jupiter on a celestial map, or we make a
sketch of the stars in the neighbourhood showing the position of the
planet. After a month or two, when the observations are repeated, the
place of Jupiter is to be compared again with those stars by which it
was defined. It will be found that, while the stars have preserved their
relative positions, the place of Jupiter has chang