RE TROGRADE. This is a term applied to the motion of the planets and comets among the fixed stars when they appear to move in the reverse order of the signs of the zodiac (q.v.). All the planets move in the same direction round the sun, and therefore their retrograde motions must be due to their motion relative to the earth. In the case of comets, however, we have instances of motion about the sun in the opposite direction to that of the planets, and in Such orbits the motion (referred now to the sun, not to the earth) is said to be retrograde.
In the case of the planets, which is thus the only one we need consider, let S be the sun, and let the two circles represent the orbits of two planets. First, let the planets he, as at P and Q, toward the same side of the sun. The inferior planet has of course the greater velocity; and therefore, if p and q represent their after the lapse of a given time (second. hour, day, etc), Pp is is greater than Qq, and therefore the direction of the line pq (in which one is seen from the other) has totaled in the apposite direction to that in which either planet revolves about the sun. when a
superior planet is in opposition (i.e., ii Q be Jupiter, and P the earth), it appwrs to Move backward among the stars. When an inferior planet is between the earth and sun (i.e., it' Q be the earth, and I' 'Venus), it appears to move backward a:so. If the planets be on opposite sides of the sun, as at P and it ii, the figure, let p and r be their positions after a given time; then pr has turned from the direction PR in the direction in which the planets revolve about the sun. Hence any planet, superior or inferior, appears to move directly when the sun is between it and the earth. Between these two opposite cases, there must, of course. be points at which the apparent motion is neither retrograde nor direct—then the planet is said to be stationary. This case occurs whenever, for an instant, the lines PQ and pg are parallel: that is, when the two planets are moving with equal velocities transverse to the line joining them, these velocities being parallel, and toward the same side of the joining line.