This model, which was developed by Plato and furthered by Ptolemy, was created to explain the observed retrograde motion of the planets. Retrograde motion is when a planet appears to move backward in the sky. This motion really occurs because both the planet and the Earth are orbitting the sun. The apparent backwards movement of the planet occurs as we pass the planet in our orbit.
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These pages designed by Erin McNally (CU'00)... Last modified 18Apr00