Kepler's monument. Monument to the German astronomer Johannes Kepler (1571-1630), in Weil der Stadt, Germany, where he was born. Kepler devised the 3 fundamental laws of planetary motion (Kepler's laws). The first law states that the planets orbit the Sun in elliptical paths, with the Sun at one focus of the ellipse. The second law states that the closer a planet comes to the Sun, the faster it moves. The third law states that the ratio of the cube of a planet's mean distance from the Sun to the square of its orbital period is a constant. Newton used these ideas to formulate his theory of gravity.