Direct (Prograde) and Retrograde Motion

 

Definitions:

1. Direct (Prograde) Motion -                          “The apparent motion from west to east (looking / viewing to the south – apparent right to left) motion of a planet or other object as seen from Earth against the background of stars.”
2. Retrograde Motion –                                    “The apparent motion from east to west (looking / viewing to the south – apparent left to right) motion of a planet or other object as seen from Earth against the background of stars.”

 

“When a superior planet (Mars, Jupiter, Saturn, Uranus, etc.) orbits the Sun at distances greater than that of the Earth and is overtaken by the Earth which is moving with higher relative velocity, the ‘Superior’ planet’s normal direct motion seems to stop and then become temporarily retrograde and it appears to undergo a loop or zigzag in the sky; the turning points between these motions, when the planet appears motionless in the sky, are known as stationary points.”

The definitions above are adapted from Daintith, John and William Gould. The Facts on File Dictionary of Astronomy (Fifth Edition). New York: Market House Books Ltd, © 2006; pages 123 and 472.

The following 9 sequential diagrams should help you to better ‘picture in your mind’ what is Direct and Retrograde motion of a ‘Superior’ planet