Retrograde Motion and Varying Brightness

The Copernican system banished the idea that the Earth was the center of the Solar System. This led immediately to a simple explanation of both the varying brightness of the planets and retrograde motion, as illustrated in the following animation for a superior planet.

Retrograde motion and varying brightness could now be explained in a different way than in the Ptolemaic model:

1. The planets in the Copernican system naturally vary in brightness because they are not always the same distance from the Earth.
2. The retrograde motion could be explained in terms of geometry and a faster motion for planets with smaller orbits.

A similar construction can be made to illustrate retrograde motion for a planet inside the orbit of the Earth. Animation: motion of the Sun through the zodiac in the Copernican model.