Direct (Prograde) and Retrograde Motion
- 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.”
- 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
About the Astronomer
Warren’s interest in astronomy and the night sky began when he was enrolled in the U.S. Air Force Navigators school. He was required to learn how to find and use the 88 constellations and the 59 navigation stars in order to navigate an airplane anywhere in the world. Later he became a navigator instructor and taught advanced celestial navigation classes. After retiring from the Air Force he has kept up his astronomical interest and used his knowledge to present “Star Parties” on cruise ships, camps, schools, churches, community social gatherings and other venues. He joined the Mayborn Science Theater nine years ago. His personal nickname is “The Naked-Eye Astronomer” and motto is “Have Laser, Will Travel.”