Johann Kepler was born near Stuttgart in 1571 and educated at the University of Tübingen with the original intention of becoming a Lutheran minister. His deep interest in astronomy led him to change his plans, and in 1594, when in his early twenties, he accepted a iectureship at the University of Grätz in Austria. In 1599, he became assistant to the famous but quarrelsome Danish-Swedish astronomer Tycho Brahe, who had moved to Prague as court astronomer to Kaiser Rudolph II. Shortly after, in 1601, Brahe suddenly died and Kepler inherited both his master’s position and his vast and vary accurate collection of astronomical data on the motion of the planets. With extraordinary pertinacity and after much computational labor, trial and error, and false solutions, Kepler was able finally to formulate in 1609 his first two laws, and then ten years later in 1619, his third law, of planetary motion.
These laws of planetary motion are landmarks in the history of astronomy and mathematics, for in the affort to justify them Isaac Newton was led to create modern celestial mechanics. The three laws are
I. The planets move about the sun in elliptical orbits with the sun at one focus.
II. The radius vector joining a planet to the sun sweeps over equal areas in equal intervals of time.
III. The square of the time of one complete revolution of a planet about its orbit is proportional to the cube of the orbit's semimajor axis.