Retrograde Motion

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. 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. The planets generally appear to move from west to east, as seen from Earth and relative to the stars. 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.

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). 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 AD), who believed that Earth was at the center of the universe. For such a system, the planet indeed had to be going backwards, because 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.

One can see retrograde motion with the following experiment (Figure 2). Have a friend stand 50 yd (46 m) away and begin jogging in the direction shown. After ten seconds, start running faster than your friend in the same direction. Watch the friend relative to some distant trees. As one catches up, the friend will appear to stop relative to the trees, move backwards, and then move forward again. Just like the planets, the 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 dwarf planet Pluto exhibit retrograde rotation.