Backward (westward) motion of the planet among the stars, resulting from the fact that it is viewed from the moving earth. It is the motion that a planet has when its right ascension is decreasing. Each year, when Earth passes a superior planet, such as Mars, the planet appears to move backward for a short time. It is the same effect you see when two trains are running in the same direction on side by side tracks. The slower train appears to be moving backward.