An epicycle is a geometric concept used in ancient and medieval astronomy to explain the retrograde motion of celestial objects, such as planets.
It is a small circle that moves on the circumference of a larger circle, called a deferent, with its own center.
The planet is placed at the center of the epicycle and appears to move backward in the sky as the epicycle rotates around the deferent. The epicycle and deferent system were proposed by Hipparchus, a Greek astronomer, as a way to explain the retrograde motion of planets and their changing brightness as they moved closer to and farther from the Earth. The epicycle and deferent system were later refined by Claudius Ptolemy, who added equants to the model in order to better match observations of planetary motion. Despite its limitations, the epicycle and deferent system were widely accepted as the correct explanation of retrograde motion for many centuries.
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