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Retrograde motion is the apparent motion backwards in the sky as viewed from the Earth .
Normal motion of planets in the sky is from west to east, but sometimes, outer planets seem to slow to a stop, reverse direction for a time, and then resume their original direction. In the above applet, the Earth is the blue planet closest to the Sun, and Mars is the red planet furthest from the Sun. The line drawn in between the Earth and Mars is the line that humans look along in order to see Mars . This line is called the line of sight to Mars . The line of sight continues after Mars , and points to the constellation in which Mars is located.