German mathematician Johannes Kepler is one of the most important figures in the history of astronomy. Due to his studies of Mars, Kepler realized that, while Copernicus was right in saying the planets orbited the sun, he was wrong to believe their orbits were circular. Instead, Kepler said, the planets' orbits were elliptical. Once he looked at the solar system with this new piece of information, he was able to formulate his three laws of planetary motion. The third of these was harmonics law. Harmonics law provides a tool for calculating the average radius of a planet's orbit using the time it takes to move around the Sun. Kepler was considered the founder of celestial mechanics, and he published harmonics law in his five-volume Harmonice Mundi (Harmony of the Worlds, 1619).