Retrograde Motion and Varying Brightness of the Planets

The Copernican system by banishing the idea that the Earth was the center of the Solar System, immediately led to a simple explanation of both the varying brightness of the planets and retrograde motion:

  1. The planets in such a 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, as illustrated in the following animation.
Retrograde motion in the Copernican System
A similar construction can be made to illustrate retrograde motion for a planet inside the orbit of the Earth.