One of the major problems with the Ptolomeic theory of the solar system was explaining the observed westward drift of the planet Mars. Normally the superior planets change their positions as the months go by in an eastward movement with respect to the background stars. Mars follows this easterly motion most of the time, but sometimes slows down, reverses direction and actually drifts westward, then regains its position and moves eastward again.

This apparent RETROGRADE motion is an illusion caused by the fact that the Earth travels around the Sun faster than Mars and in a smaller orbit, so that the Earth overtakes and passes Mars.  The applet below demonstrates this illusion.   The upper part of the display shows how the apparent position of Mars relative to the background stars changes.  Notice that the tick marks around the Earth’s orbit indicate time in months. Mars does not come to opposition every year.