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© Copyright 2002, Jim Loy
On the left (below), we have the Sun and two planets; the blue one represents the Earth, and the red one represents Mars. None of this is drawn to scale. In the center, I have shown Mars' retrograde motion, the way it appears to go backward in the sky. This happens because the faster Earth passes the slower outer planet (in the left part of the diagram). And on the right, we have Ptolemy's attempt to explain retrograde motion with a fixed Earth, using epicycles. Both ideas (Copernicus' Sun-centered system, and Ptolemy's Earth-centered system) seem to work well. But the Sun-centered system is the true situation:
The above Java animation was created with Cinderella (a geometry program).
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