Austrian mathematician and astronomer who got himself taken on as an assistant to Brahe in order to get access to his planetary tables. Kepler had been trained as a Platonist and Neopythagorean, and was given to rather mystical views, as exemplified in his work Mysterium Cosmographicum. Nevertheless, Kepler was also a confirmed Copernican. In fact, he wanted to use Tycho's data to prove the validity of the Copernican theory. He analyzed the vast amount of data upon Brahe's death. From this data, he prepared new planetary tables (called the Rudolphine Tables). At first, he determined the shape of planetary orbits to be ovoid, but rejected this result for aesthetic reasons. Going back over his calculations, he found and corrected an error. The new shape turned out to be an ellipse, which fit well into Kepler's Pythagorean views on nature.

Kepler tried all sorts of mystical notions to describe planetary orbits, using the Platonic solids and musical analogies. Spread out through his voluminous calculations in Astronomia Nova, however, were three gems: Kepler's laws of planetary motion. For the formulation of these laws, Kepler is considered the founder of physical astronomy. The first law states that the planets move in elliptical orbits with the Sun at one focus. The second law states that the planets sweep out equal areas in equal times (which is equivalent to the statement of conservation of angular momentum. ) The third law states that the period squared in proportional to the semimajor axis cubed. Kepler believed that the planets were kept in their orbits by a "anima motrix" (motive soul), but later modified it to "vis motrix" (life force). He also studied optics as an aspect of astronomy in Astronomiae Pars Optica (1604), and developed the concept of a ray.

Brahe, Copernicus

Additional biographies: MacTutor (St. Andrews), Firenze, Bonn