So, clearly, a better model was needed. This model worked well as far as retrograde motion was concerned, since when the planet is in the lower part of the epicycle it is moving in the opposite direction as viewed from the earth. But, as mentioned above, there were subtler oddities that needed to be taken into account, too, and Ptolemy was forced to introduce more and more complications into his model to attempt to explain them. The model grew rather complicated and cumbersome to use, but still reproduced the actual motions well enough that it remained in use for over 1400 years. Around the year 1540, Copernicus offered a different kind of model, a heliocentric one, where the center was occupied no longer by the Earth, but by the Sun.
Retrograde motions arise from the combined motions of the planets and the Earth.
Copernicus reasoned that planets further away from the sun would move more slowly than closer ones. As a faster planet overtakes the Earth in their rotations, its motion against the stars, as seen from the Earth, reverses. This corresponds to the first reversal of apparent motion. The same explanation works for the outer planets, too, only now it is the Earth that overtakes the other planet. All right reserved.