Watch the friend relative to some distant trees. After ten seconds, start running faster than your friend in the same direction. Have a friend stand 50 yd (46 m) away and begin jogging in the direction shown. One can see retrograde motion with the following experiment (Figure 2). Inner planets exhibit retrograde motion as well, as they catch up with and pass Earth, moving between it and the sun. This changed when Nikolaus Copernicus (1473 –1543) argued that Earth orbits the sun like all the other planets, providing a more natural explanation for retrograde motion. For such a system, the planet indeed had to be going backwards, because Earth was stationary. 2nd century AD), who believed that Earth was at the center of the universe. Retrograde motion of the planets confounded early astronomers such as Ptolemy (c. The changing line of sight from Earth to the planet makes it appear that the planet has stopped and begun to move backwards, though it is still moving in its original direction. This is an optical illusion produced as Earth, which orbits the sun faster than any of the outer planets, catches up and passes them in its orbit (Figure 1). However, if one carefully charts an outer planet ’s motion for several months one will notice it appears to stop, reverses direction (goes from east to west) for a few weeks, then stops again and resumes its former west-to-east motion.
The planets generally appear to move from west to east, as seen from Earth and relative to the stars.
All the visible planets farther from the sun than Earth (Mars, Jupiter, Saturn, and, for the eagle-eyed, Uranus) show retrograde motion, or what is sometimes also called retrogression. Retrograde motion means moving backward, and, in astronomy, describes the loop, or Z-shaped, path that planets farther from the sun than the Earth appear to trace in the sky over the course of a few months.
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