While Tycho Brahe was convinced that the Earth did not move, his assistant, Johannes Kepler, through his mathematical manipulations of Tycho's data, discovered otherwise. Though Tycho would have been upset with the ultimate results, Kepler gave us three laws of planetary motion: Here is an animation that allows us to study each of the three laws. • Planets orbit the Sun in elliptical orbits with the Sun at one focus of this ellipse (the orbits are not round!) • Planets sweep out equal areas of an orbit in equal intervals of time (planets change speed in orbit!) • The cube of the semi-major axis of a planet's orbit is proportional to the square of its period . (beautifully simple math!) Here is Carl Sagan to explain the significance of Kepler's accomplishment. These laws did a wonderful job of explaining exactly how the planets orbit the Sun if not why. You might enjoy watching this somewhat amusing YouTube presentation explaning Kepler's laws! With Kepler’s work, we finally had a clear and correct description of the system of planets orbiting the Sun.