1. In Ptolemy’s model, the epicycle moving along the deferent causes retrograde motion (here of Mars)

2. In the Copernican model, retrograde motion arises naturally from the dual motion of Earth and Mars. One key is that more distant planets move more slowly.

3. The Copernican system’s and Ptolemaic system’s explanations of retrograde motion are mathematically equivalent. But the physical causes are very different.

4. Retrograde motion for Venus (right) works the same way, only the roles of the planets are reversed.