the seas and lands of our earth, as in a
badly polished mirror.

[16] Mare Imbrium, Sinus Iridum, Lacus Somniorum.

[17] The lunar craters have, as a rule, received their names from
celebrated persons, usually men of science. This system of nomenclature
was originated by Riccioli, in 1651.

CHAPTER XVII

THE SUPERIOR PLANETS

Having, in a previous chapter, noted the various aspects which an
inferior planet presents to our view, in consequence of its orbit being
nearer to the sun than the orbit of the earth, it will be well here to
consider in the same way the case of a superior planet, and to mark
carefully the difference.

To begin with, it should be quite evident that we cannot ever have a
transit of a superior planet. The orbit of such a body being entirely
_outside_ that of the earth, the body itself can, of course, never pass
between us and the sun.

A superior planet will be at its greatest distance from us when on the
far side of the sun. It is said then to be in _conjunction_. As it comes
round in its orbit it eventually passes, so to speak, at the _back_ of
us. It is then at its nearest, or in _opposition_, as this is
technically termed, and therefore in the most favourable position for
telescopic observation of its surface. Being, besides, seen by us at
that time in the direction of the heavens exactly opposite to where the
sun is, it will thus at midnight be high up in the south side of the
sky, a further advantage to the observer.

Last of all, a superior planet cannot show crescent shapes like an
interior; for whether it be on the far side of the sun, or behind us,
or again to our right or left, the sunlight must needs appear to fall
more or less full upon its face.

THE PLANETOID EROS

The nearest to us of the superior planets is the tiny body, Eros, which,
as has been already stated, was discovered so late as the year 1898. In
point of view, however, of its small size, it can hardly be considered
as a true planet, and the name "planetoid" seems much more appropriate
to it.

Eros was not discovered, like Uranus, in the course of telescopic
examination of the heavens, nor yet, like Neptune, as the direct result
of difficult calculations, but was revealed by the impress of its light
upon a photographic plate, which had been exposed for some length of
time to the starry sky. Since many of the more recent additions to the
asteroids have been discovered in the same manner, we shall hav