Retrograde loops in a Copernican Universe
. A Sun-centered model of the solar system explains the looping path of Mars in terms of the relative speeds of the Earth and Mars. The Earth travels around the Sun more rapidly than Mars does. As Earth overtakes and passes the slower moving planet (points 2 to 4), Mars appears to move backward (points B to D) for a few months.
Copyright 2010, Professor Kenneth R. Lang, Tufts University