1911 Encyclopædia Britannica/Epicycle

EPICYCLE (Gr. ἐπί, upon, and κύκλος, circle), in ancient astronomy, a small circle the centre of which describes a larger one. It was especially used to represent geometrically the periodic apparent retrograde motion of the outer planets, Mars, Jupiter and Saturn, which we now know to be due to the annual revolution of the earth around the sun, but which in the Ptolemaic astronomy were taken to be real.