What is meant by retrograde motion?

Answer 1

Consider being on a merry-go-round. As you come around you see a friend walking perpendicular to your line of sight. It is easy to see his direction of motion. However, as you curve around he seems to move backwards against the background objects. As he disappears behind you, you can again see what direction he really is moving. So it is with planets.

If there is a planet, say, Mars in a 12:00 position and you are on Earth at the 3:00 position (you're looking down at the solar system, from above the north pole of the sun) the outer planet will move against the background stars in a right-to-left direction. However, as you near it and pass it your orbital speed makes Mars seem to stop, go backwards, and then start back up again. That reversal is retrograde motion. In the sky, of course your orbital speed makes it move through the sky relative to the sun from east to west (but you still see the reversal against the background stars). It happens close to opposition, when the planet is opposite the sun.

If you draw a simple diagram, looking down on the solar system, with two planets and have the one closest to the sun move more than the further one, you can draw lines of sight that show how it works.

Answer 2

For example, "If observed from one night to the next....a planet appears to move from West to East against the background stars most of the time. Occasionally, however, the planet's motion will appear to reverse direction, and the planet will, for a short time, move from East to West against the background constellations. This reversal is known as retrograde motion...."

Answer 3

i think we learned last year that it is when something is going so fast it appears to be moving backwards. not 100 percent sure tho