Retrograde Motion

Retrograde motion has played an important role in the history of astronomy. When devising his model of the solar system, Ptolemy, in order to account for this motion, was forced to add awkward epicycles to his otherwise simple model. Later, Copernicus used retrograde motion as crucial evidence for a heliocentric, rather than geocentric, solar system. The above image is a composite of numerous images taken of Mars' motion across the sky, taken about one week apart each. The bottom right picture was taken in July 2005, and the above left image in February 2006.