An
animated image showing the
apparent retrograde motion of Mars in 2003 as seen from
Earth. All the true planets appear to periodically switch direction as they cross the sky. Because Earth completes its
orbit in a shorter period of time than the planets outside its orbit, we periodically overtake them, like a faster car on a multi-lane highway. When this occurs, the planet will first appear to stop its eastward drift, and then drift back toward the west. Then, as Earth swings past the planet in its orbit, it appears to resume its normal motion west to east.
Image credit: Seav
