retrograde motion

retrograde motion, in astronomy, real or apparent movement of a planet, dwarf planet, moon, asteroid, or comet from east to west relative to the fixed stars. 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. With the exception of the rotation of Venus, there is no real retrograde motion among the planets, although the plane in which Uranus rotates and its five satellites revolve is tilted slightly more than 90° to the plane of the ecliptic, so that these motions are technically retrograde. All the planets exhibit apparent retrograde motion when they are nearest the earth; i.e., they appear to move backward (east to west) against the background of stars. The superior planets, whose orbits lie outside that of the earth, appear to move backward at opposition, because the earth is overtaking and passing them. (Of any two planets, the one closer to the sun has the greater orbital speed.) As a consequence, a superior planet's progress through the zodiac is interrupted by annual loops or switchbacks. The effect is similar to passing an automobile on a highway; observers in the faster car see the slower car apparently moving backwards as they overtake it. Mercury and Venus, the inferior planets, exhibit apparent retrograde motion when at inferior conjunction. They are then passing between the earth and the sun, overtaking the earth, and thus seem to move east to west, relative to both the sun and the background stars. In the geocentric Ptolemaic system, the retrograde motion of the planets was explained, using epicycles, as real retrograde motion; the modern heliocentric theory satisfactorily explains these motions as apparent, due to the relative speeds of the planets in their orbits about the sun.

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