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Diagram of retrograde motion

Retrograde motion is a line-of-sight effect caused by the relative motion of the Earth and other planets. In this example, Earth and Mars are both following their orbits in the same direction, but because Earth travels more rapidly along its orbital path, Mars seems for a while to move backwards in the sky.

An effect whereby planets (and other orbiting bodies within the Solar System) display small 'loops' in their motion through the sky, travelling briefly backwards in their course. A source of much puzzlement to the ancients, we now know that this is due to the relative orbital motion of the Earth.