)
Direct and retrograde motion of superior planet
direct motion
(prograde motion) 1. The apparent west to east motion of a planet or other object as seen from Earth against the background of stars. Retrograde motion is an apparent motion from east to west. When a superior planet near opposition is overtaken by the Earth, moving with higher relative velocity, its normal direct motion seems to become temporarily retrograde and it appears to undergo a loop or zigzag in the sky (see illustration); the turning points between these motions, when the planet appears motionless in the sky, are known as stationary points.
2. The anticlockwise orbital motion, as seen from the north pole of the ecliptic, of a planet or comet around the Sun or of a satellite around its primary. The body is then said to move in a direct orbit or prograde orbit. A clockwise orbital motion is retrograde. All the planets move in direct orbits.
3. The anticlockwise rotation of a planet, as seen from its north pole. Venus, Uranus, and Pluto have a clockwise retrograde rotation.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
direct motion
[də¦rekt ′mō·shən] (astronomy)
Eastward, or counterclockwise, motion of a planet or other object as seen from the North Pole (motion in the direction of increasing right ascension).
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.