Go to a running track with a partner. Run slowly around the track while your partner runs quickly around the track (or vice versa). Carefully watch the background against which you see your partner as you both run. Have your partner observe your motion against the background as well. Raise your hand whenever you see your partner move in a retrograde direction against the background. Have your partner raise his or her hand whenever he or she sees you moving retrograde. Compare the times at which you and your partner see each other in retrograde motion.
Divide your group into two subgroups. Choose one group to be "heliocentrists" and the other group "geocentrists." After a few minutes of preparation, have a short debate about which group can best explain the daily motion of the sky, the annual motion of the Sun, and the retrograde motions of the planets. Discuss what might constitute firm evidence that one theory is better than the other.
