The apparent backward movement of a planet is called

A. eccentricity.

B. heliocentric motion.

C. Kepler's first law of motion.

D. retrograde motion.

The answer is D. Most of the rotational and orbital motions in the solar system are in the same "eastward" direction. Motions in this direction are referred to as direct motions while motions in the opposite direction are referred to as retrograde. Retrograde motion is the apparent backward motion of a planet caused by its being lapped by another planet or vice versa. Both planet move in a direct eastward motion around the sun, but the planet with the inside (smaller) orbit moves faster than the planet on the outside (larger orbit), and when it passes the slower moving planet, each sees the other one as apparently moving backward relative to its motion around the sky.