Answer:

The most commonly discussed "retrograde" motion is the apparent backward motion of a planet caused by its being lapped by another planet, or vice-versa. Both planets 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 backwards relative to its usual motion around the sky. In this "retrograde" motion, neither planet is actually moving backwards; it only appears that way during the time that one laps the other.