... A discussion of retrograde motion of planets helps the students understand the logic underlying the ideas of both Ptolemy and Copernicus. ... This lesson plan supplements: "The Discovery of the Solar System," section #9c: on disk Ssolsys.htm , on the web . ... Learn about the system proposed by Copernicus, and the way it explained retrograde motion: all planets moved in circles around the Sun, but the ones closer to the Sun always moved faster. ... Did this predict correct planetary motions? ...