Retrograde motion is the apparent backward movement of a planet against the background of stars. This phenomenon occurs when Earth, which moves faster in its orbit, overtakes and passes an outer planet.
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Retrograde motion was a significant puzzle for ancient astronomers who believed in geocentric models.
It typically affects the outer planets like Mars, Jupiter, and Saturn.
The phenomenon is best explained by the heliocentric model proposed by Copernicus.
Retrograde motion occurs periodically as Earth catches up to and passes these slower-moving outer planets.
The term 'retrograde' means 'moving backward,' but this motion is only apparent from Earth's perspective.
A small circle along which a planet moves, whose center describes a larger circle (deferent) around Earth, used in Ptolemaic system to explain retrograde motion.