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
- Opposition
- 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