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Retrograde Motion

Retrograde motion means "moving backward," and describes the loop or Z-shaped path that planets farther from the Sun than Earth appear to trace in the sky over Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 2. Illustration by Hans & Cassidy. Courtesy of Gale Group. the course of a few months. All the visible planets farther from the Sun than Earth (Mars, Jupiter, Saturn, and, for the eagle-eyed, Uranus) show retrograde motion. Planets appear to move from west to east relative to the stars, but if you carefully chart an outer planet's motion for several months you will notice it appear to stop, reverse direction for a few weeks, then stop again and resume its former west-to-east motion.

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. 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. Retrograde motion of the planets confounded early astronomers such as Ptolemy (c. 2nd century A.D.), who believed that Earth was at the center of the Universe. For such a system the planet indeed had to be going backwards, because the Earth was stationary. 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. Inner planets exhibit retrograde motion as well, as they catch up with and pass Earth, moving between it and the Sun.

You can see retrograde motion for yourself if you do this experiment. Have a friend stand 50 yards away and begin jogging in the direction shown. After ten seconds, start running faster than your friend in the same direction. Watch your friend relative to some distant trees. As you catch up, your friend will appear to stop relative to the trees, move backwards, and then move forward again. Just like the planets, your friend is always going in the same direction, but relative to the trees the situation looks quite different! Because the effect described above is an optical illusion, it is sometimes called apparent retrograde motion. This distinguishes it from true retrograde motion, which is the revolution or rotation of an object in the solar system in a clockwise direction as seen from the north pole (i.e., looking "down" on the solar system). All the planets orbit the Sun in a counterclockwise direction as seen from the north pole, and this motion is called prograde. However, some of the satellites of the planets (such as Phoebe, a satellite of Saturn, and Triton, the largest satellite of Neptune) orbit in a retrograde direction. And while Earth rotates about its axis in a prograde sense, Venus, Uranus, and Pluto exhibit retrograde rotation.

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