There is an excellent classroom activity for illustrating why a planet seems to go backwards against the background stars as Earth overtakes it in orbit.

Set up model as follows:

1. Sun is in center of room.
   
2. Earth orbit is created about meter or so radius, with 6 masking tape "X" positions marked for the first six months of the year
   
3. Mars orbit is created about 2 meters radius, with 12 masking tape "X" positions marked in approximately correct places in Mars orbit, corresponding to the 12 Earth months, keeping in mind that Mars orbital period is nearly 2 Earth years long. Arrange marks so that if you were to draw a line through the January position in the Earth orbit to the corresponding January position in Mars orbit, the line will hit the chalkboard or whiteboard.
   
4. Chalkboard or whiteboard represents the background stars: put some constellation patterns up on the board.
   
5. Have one volunteer stand at January in Earth orbit and a second volunteer at corresponding January position of Mars in its orbit.
   
6. A third volunteer goes to the chalkboard and follows instructions of the Earth volunteer, who tells where to place a mark on the chalkboard that lines up with Earth-Mars, to represent where Mars appears in the sky (chalkboard) from the viewpoint of Earth. Mark on chalkboard is labeled January.
   
7. New volunteers repeat steps 5 and 6 for the February positions in Earth and Mars orbits.
   
8. Repeat step 7 for March, April, May, June positions.
   
9. Discuss results--ask students if they can now explain what "retrograde motion" means? Have them describe how it happens.

You can generate your own, much better animations of planet movements using planetarium programs such as Voyager (Carina Software --http://www.carinasoft.com) or Starry Night (http://www.starrynight.com). See also http://seds.org/billa/astrosoftware.html.