Retrograde Motion of Planets and Planetoids

Definition of Terms

Prograde Motion
Regular average yearly motion of planet through the stars = movement west to east
Retrograde Motion
Reversed motion through the stars relative to average yearly motion = movement east to west
That position in the sky opposite to the Sun ie. 180o from the Sun.

The animation shows three different views of the motion of the Earth and a planet around the Sun

[Click on Reload to run the animation again.]

Helocentric View from the North Ecliptic Pole [upper left]
This views shows the motion of the Earth and a planet relative to the Sun - Both move anti-clockwize as seen from the north. The Earth moves on the inner orbit. This animation draws a line from the sun to the planet to show the helocentric longitude p lus the line of sight to the planet from the Earth. Only the line of sight remains with circles to show the former positions as the animation proceeds.
Geocentric view from the North [upper right]
This view show the relative angle of view and the relatitve distance to the planet keeping the Earth fixed.
Motion of the Planet in the sky as seen from the Earth [lower right]
This view show the projection of the image of the planet motion on the sky and illustrates the retrograde motion near opposition
Position angle [number to lower left]
The angle of the planet from opposition (negative angle is to the west of opposition) When this angle is zero the planet is at oppostion (with the Sun).

Created by L.Bogan - Mar 1998
Physics Nova Scotia