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.
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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.