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This applet demonstrates the retrograde motion of Mars. The left frame shows the actual motion of the Earth and Mars. The right frame shows the motion of Mars as seen from Earth.
Although the orbital motion of the objects in the sky is relatively simple and regular as observed from some fixed point in space, planetary motion appears uneven and somewhat complex when observed from the moving Earth. As seen from the Earth, the Sun, Moon, and planets all appear to move along the ecliptic. Unlike the Sun and Moon, however, the planets don't always make steady progress along the ecliptic. They usually move in the same direction as the Sun (i.e., eastward), but from time to time they seem to slow down, stop, and reverse direction (i.e., move westward). This apparent reversal is called retrograde motion.
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