Two Planets & A Sun - A Digital Triptych

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