Retrograde

Retrograde. The planets appear to move in an extremely irregular way among the fixed stars.

Sometimes their motion is in the same direction as that of the sun, in which case it is said to be direct, while at other times it moves in the opposite direction and is said to be retrograde. These irregularities are due to the fact that the earth is moving, but to the observer it seems as though all the motion is due to the planet. Imagine a planet, P, to be viewed from the earth, E, and to remain at rest while the earth moves // a short distance, E E'. s It will then appear the same as though the observer were still at E, but the planet had moved to p', the line E p' being parallel to B' p. The direction of motion of the planet will therefore appear to be the same as that of the hands of the clock, but the sun appears to move contra-clockwise among the stars ; hence the planet in this case The planet, however, and so the final result has a retrograde motion does not remain at rest, depends on the sum of the effects produced by the motion of both earth and planet.