Kepler was working at the same time as Galileo and they often wrote to each other. Kepler was able to use information on the position of stars to work out how the planets moved. He found out that the planets’ orbits are not perfect circles but elliptical (egg shaped).As planets move closer to the Sun they speed up, and as they move away they slow down. The more distant a planet is from the Sun the slower it moves. These three things meant that Ptolemy’s system now had several flaws but supported the theories of Copernicus and Galileo.

What was it that kept the planets in their orbits? Kepler thought it had something to do with the Sun.