Retrograde Motion Diagram. Under normal circumstances the planet appears to move to the east (upwards in the diagram), but when it is being overtaken by the earth our faster motion makes mars appear to be going backwards (downwards in the diagram). Retrograde motion, in astronomy, actual or apparent motion of a body in a direction opposite to that of the (direct) motions of most members of the solar system or of other astronomical systems with a preferred direction of motion.
When its motion brings it inside the deferent circle, the planet undergoes retrograde motion. Diagram of the copernican solar system (13kb gif) in each, much of the detail of both systems has been stripped away to show the basic arrangement of planets and orbits.