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Retrogradation refers to the motion of the outer planets (Mars, Jupiter, Saturn and so forth) when they move from east to west relative to the background stars (most of the time the planets move from west to east).
Retrogradation was quite a puzzle for ancient astronomers who used the geocentric model. To deal with this, Ptolemy invented the deferent and epicycle. In the heliocentric model, retrogradation is a natural result of the combined motion of the Earth and the planet observed.
See also: Prograde and retrograde motion
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