Retrograde

Retrograde, in Astronomy, a term applied to the motion (real or apparent) of a celestial body when that is opposite in direction to the yearly course of the sun from west to east. The superior Planets (q.v.) retrograde when in opposition (see CONJUNCTION). As their motion is then nearly parallel to the earth's, they, moving more slowly than it, appear to fall behind for a time. This period of retrogradation is of course longer for the planets whose motion is slower, and less for those whose speed more nearly approaches that of the earth. The inferior planets, which move faster than the earth, retrograde when in inferior conjunction. Their course being then nearly parallel to the earth's, they gain upon it, and appear to pass the sun from east to west. Thus Venus, when nearing the end of her appearance as an evening star, descends each night nearer to the western horizon, until so near the sun as to be lost in his rays. Passing then to his west side, the planet reappears as a morning star.