A Backward Planet — Retrograde Motion
From the time of Aristotle (384-322 B.C.) until the 1500s, mankind tried to come up with a model explaining the motions of the planets. Although the heliocentric theory seems so plausible to today's students, they often cannot easily use evidence to prove it. Ask them what they believe, and none will believe in a geocentric solar system. But ask them to prove it with scientific evidence, and suddenly you see many blank stares!
We shouldn't be so surprised. For several hundred years, astronomers watched the heavens, plotted the courses of planets, and still struggled to come up with a model to explain the motion of the planets. One of the most interesting phenomenon was the apparent retrograde motion of planets like Mars and Jupiter. They seemed to slow down and begin to move backwards in space for a few months, and then slow down and begin to move forward again. Trying to explain this proved difficult because scientists were thinking from only one perspective — Earth!
Using a computer planetarium program, students can simulate and observe in minutes what took scientists decades to see.
This short project makes use of the simulation of star and planet movement that can be modeled by planetarium programs. By observing both from Earth and from above the solar system in a "hover" mode, students can usually construct the reasons for retrograde motion — something the greatest minds of early astronomy could not do! In the process, students come away with a true conceptual understanding of the motion of planets.