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 polecelestial pole,
one of the two points at which the earth's axis of rotation intersects the celestial sphere. The celestial pole is important as a reference point in the equatorial coordinate system; the celestial meridian passes through it, as do the hour circles of the stars.
..... Click the link for more information. ); 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 eclipticecliptic
, the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic). Because of the earth's yearly revolution around the sun, the sun appears to move in an annual journey through the heavens with the ecliptic as its
..... Click the link for more information. , 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 oppositionopposition,
in astronomy, alignment of two celestial bodies on opposite sides of the sky as viewed from earth. Opposition of the moon or planets is often determined in reference to the sun.
..... Click the link for more information. , 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 conjunctionconjunction,
in astronomy, alignment of two celestial bodies as seen from the earth. Conjunction of the moon and the planets is often determined by reference to the sun.
..... Click the link for more information. . 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 systemPtolemaic system
, historically the most influential of the geocentric cosmological theories, i.e., theories that placed the earth motionless at the center of the universe with all celestial bodies revolving around it (see cosmology).
..... Click the link for more information. , 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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retrograde motion(ret -rŏ-grayd) See direct motion.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
retrograde motion[′re·trə‚grād ′mō·shən]
An apparent backward motion of a planet among the stars resulting from the observation of the planet from the planet earth which is also revolving about the sun at a different velocity. Also known as retrogression.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.