Johannes Kepler

Johannes Kepler was a German mathematician and astronomer who discovered that planets move in elliptical orbits — one of the most important breakthroughs in the history of science. Born in Weil der Stadt in the Duchy of Württemberg, Kepler studied theology at the University of Tübingen, where he encountered Copernican astronomy. After working as Tycho Brahe's assistant in Prague, he inherited Tycho's vast collection of observations upon the latter's death in 1601. Through years of painstaking calculation, particularly on the orbit of Mars, Kepler formulated his three laws of planetary motion: planets move in ellipses with the Sun at one focus; a planet sweeps equal areas in equal times; and the square of a planet's orbital period is proportional to the cube of its semi-major axis. These laws demolished the 2,000-year-old assumption of circular orbits and later provided the empirical foundation for Newton's law of universal gravitation.

First Law: Planets orbit the Sun in ellipses, not circles. Second Law: A line from the Sun to a planet sweeps equal areas in equal times (planets move faster near the Sun). Third Law: The square of orbital period is proportional to the cube of semi-major axis (P² = a³). Published the Rudolphine Tables, the most accurate astronomical tables of the era. Laid the empirical foundation for Newton's theory of gravity.