If the movements of an inferior planet are followed sufficiently long by the aid of a star map, it will be seen that sometimes it appears to be proceeding in a forward direction through the signs of the zodiac; then for a while it will seem to stand still; then at another time it will apparently go backwards, or possess a retrograde motion. All these peculiarities have their originating cause in the motion of the Earth itself, for the absolute movement of the planet never varies, being always in the same direction, that is, forwards in the order of the signs. Turning now to the superior planets, we have to face an altogether different succession of circumstances. A superior planet is not, as it were, chained to the Sun so as to be unable to escape beyond the limits of morning or evening twilight; it may have any angular distance from the Sun up to 180°, reaching which point it approaches the Sun on the opposite side, step by step, until it again comes into conjunction with the Sun. As applied to a superior planet, the term “conjunction” means the absolute moment when the Earth and the Sun and the planet are in the same straight line, the Sun being in the middle. In such a case, to us on the Earth the planet is lost in the Sun’s rays, whilst to a spectator on the planet the Earth would appear similarly lost in the Sun’s rays, as the Earth would be at that stage of her orbit which we, speaking of _our_ inferior planets, call superior conjunction. For a clear comprehension of all the various matters which we have just been speaking of, a careful study of diagrams of a geometrical character, or better still, of models, would be necessary. Something must now be said about the phases of the planets. Mercury and Venus, in regard to their orbital motions, stand very much on the same footing with respect to the inhabitants of the Earth as the Moon does, and accordingly both those planets in their periodical circuits round the Sun exhibit the same succession of phases as the Moon does. In the case, however, of the superior planets things are otherwise. Two only of them, Mars and Jupiter, are sufficiently near the Earth to exhibit any phase at all. When they are in quadrature (_i. e._, 90° from the Sun on either side) there is a slight loss of light to be noticed along one limb. In other words, the disc of each ceases for a short time, and to a slight extent, to be truly circular; it becomes what is known as “gibbous.” This occasional feature of Mars may be fairly conspicuous, or, at least, noticeable; but in the case of Jupiter it will be less obvious unless a telescope of some size is employed.