The epicycle in Ptolemy's system
Centro Studi per il Restauro degli Orologi Antichi, ISIS "Leonardo da Vinci", Florence

The planets, seen from the Earth, move forward in the sky, stop and retrace their trajectory, then resume their journey. To reconcile this optical impression with the principle of uniform planetary motion on circular orbits, Ptolemy resorted to a geometrical model, introduced by Apollonius of Perga (3rd century B.C.), known as the epicycle. As this model portraying Jupiter and the Sun's movement around the Earth shows, the planet moves at a uniform speed on a lesser circle (the epicycle) which is simultaneously pulled along a greater circumference with the Earth at its centre. This explains why, when seen from the Earth, Jupiter moves forward in the sky, stops and then moves back.