The Copernican explanation of retrograde motion for (a) superior planets and (b) inferior planets. In each diagram the earth moves steadily on its orbit from E1 to E7 and the planet moves from P1 to P7. Simultaneously the planet's apparent position against the stellar sphere shifts eastward from 1 to 7, but as the two planets pass there is a brief westward retrogression from 3 to 5.