The motions of a Sun and two planets as seen in each of their coordinate systems.[br][br]You can animate the motion by clicking on the lower left hand corner of the screen.[br][br]You can show the trajectories by checking the trajectories checkbox.[br][br]You can show the distances between the bodies by checking the distance checkbox.[br][br]Does this help you understand the retrograde motion of Mars?[br][br]Why does the triangle of distances look the same in all three coordinate systems?[br][br][color=#ff0000][i][b]What questions could/would you ask your students based on this applet?[/b][/i][/color]