Johannes Kepler was born in1571 in Weil der Stadt. There he was introduced to the ideas of Copernicus and delighted in them. In 1596, while a mathematics teacher in Graz, he wrote the first outspoken defense of the Copernican system, the Mysterium Cosmographicum. Kepler's Laws of Planetary Motion The First Law: Planets move in ellipses with the Sun at one focus. The Second Law: The radius vector describes equal areas in equal times. The Third Law: The squares of the periodic times are to each other as the cubes of the mean distances.