A circle whose center moves along the circumference of another circle, used in ancient astronomy to explain planetary motions.
From Greek 'epi' (upon) + 'kyklos' (circle). Ptolemy and other ancient astronomers invented this concept around the 2nd century CE to account for the apparent retrograde motion of planets without accepting a sun-centered solar system.
Before we knew planets orbited the sun in ellipses, astronomers thought they moved in circles-on-circles—like a wheel rolling on another wheel—which actually worked surprisingly well for predictions, even though it was fundamentally wrong!
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