How many planets have retrograde motion

PHASE —. Tonight's Sky — Change location. US state, Canadian province, or country. Tonight's Sky — Select location. Tonight's Sky — Enter coordinates. UTC Offset:. Picture of the Day Image Galleries. Watch : Mining the Moon for rocket fuel. Queen guitarist Brian May and David Eicher launch new astronomy book. Last chance to join our Costa Rica Star Party!

Learn about the Moon in a great new book New book chronicles the space program. Dave's Universe Year of Pluto. Groups Why Join? Astronomy Day. The Complete Star Atlas. Do other planets appear to do this also? When astronomers talk about retrograde motion, generally they refer to the apparent motion outer planets make as Earth passes them while orbiting the Sun.

Most of the time, all the outer planets appear to move eastward through our sky. During retrograde motion, however, each appears to reverse direction and head westward. Note that this is an apparent motion. See more Encyclopedia articles on: Astronomy: General. Enter your search terms:. The most common direction of motion in the solar system, both for orbital revolution and axial rotation, is from west to east counterclockwise as seen from the north celestial pole ; revolution or rotation in the opposite direction is actual retrograde motion.

Bodies in the solar system with real retrograde orbits include certain moons of the outer planets, and some asteroids and comets. All the planets exhibit apparent retrograde motion when they are nearest the earth; i.

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. For such a system the planet indeed had to be going backwards, because the Earth was stationary.

This changed when Nikolaus Copernicus 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.