The moons of Jupiter.

The idea behind this proof is aptly illustrated by the moons of Jupiter. Their discovery by Galileo was an epochal event in astronomy, because it proved that not everything in the universe had to revolve around the earth as had been believed. Galileo's telescope was of poor quality by modern standards, but the figure below shows a simulation of how Jupiter and its moons might appear at intervals of three hours through a large presentday instrument. Because we see the moons' circular orbits edgeon, they appear to perform sinusoidal vibrations. Over this time period, the innermost moon, Io, completes half a cycle.