Around the Sun as a centre, certain bodies, called planets, revolve, at greater or less distances: they may be divided into two groups, (i.) The inferior planets, comprising Vulcan, Mercury, and Venus; and (ii.) the superior planets, including Mars, the Minor Planets, Jupiter, Saturn, Uranus and Neptune, the Earth being the boundary between the two. If viewed from the Sun, all the planets, inferior and superior, would appear to spectators placed on the Sun, to revolve round that luminary in the order of the zodiacal signs; such, however, is not the case when the motions of the planets are watched from one of their number itself in motion; it consequently happens that the apparent motions of the inferior planets on the one hand, and the superior on the other, differ considerably. The former are never seen in those parts of the heavens in opposition to the Sun, but are sometimes to the east and sometimes to the west of it. Twice in every revolution an inferior planet is in conjunction with the Sun (Fig. I); in inferior conjunction, when it comes between the Earth and the Sun, and in superior conjunction when the Sun intervenes between the Earth and the planet. When it attains its greatest distance from the Sun, east or west, it is said to be at its greatest elongation—east or west, as the case may be.