Kepler, Johannes

German mathematician and astronomer. He formulated what are now called Kepler's laws of planetary motion. Kepler's laws are the basis of our understanding of the Solar System, and such scientists as Isaac Newton built on his ideas.

Kepler was one of the first advocates of Sun-centred cosmology, as put forward by Copernicus. Unlike Copernicus and Galileo, Kepler rejected the Greek and medieval belief that orbits must be circular in order to maintain the fabric of the cosmos in a state of perfection.

Early work
Kepler also produced a calendar of predictions for the year 1595 which proved uncanny in its accuracy. In 1596, he published his Prodromus dissertationum cosmographicarum seu mysterium cosmographicum in which he demonstrated that the five Platonic solids (the only five regular polyhedrons) could be fitted alternately inside a series of spheres to form a ‘nest’. The nest described quite accurately (within 5%) the distances of the planets from the Sun. Kepler regarded this discovery as a divine inspiration that revealed the secret of the Universe. Written in accordance with Copernican theories, it brought Kepler to the attention of all European astronomers.

In 1601 Kepler was bequeathed all of Tycho Brahe's data on planetary motion. He had already made a bet that, given Tycho's unfinished tables, he could find an accurate planetary orbit within a week. It was five years before Kepler obtained his first planetary orbit, that of Mars. His analysis of these data led to the discovery of his three laws. In 1604 his attention was diverted from the planets by his observation of the appearance of a new star, ‘Kepler's nova’. Kepler had observed the first supernova visible since the one discovered by Brahe in 1572.

Kepler's laws
Kepler's first two laws of planetary motion were published in Astronomia Nova (1609). The first law stated that planets travel in elliptical rather than circular, or epicyclic, orbits and that the Sun occupies one of the two foci of the ellipses. The second law established the Sun as the main force governing the orbits of the planets. It stated that the line joining the Sun and a planet traverses equal areas of space in equal periods of time, so that the planets move more quickly when they are nearer the Sun. He also suggested that the Sun itself rotates, a theory that was confirmed using Galileo's observations of sunspots, and he postulated that this established some sort of ‘magnetic’ interaction between the planets and the Sun, driving them in orbit. This idea, although incorrect, was an important precursor of Newton's gravitational theory.

Kepler's third law was published in De Harmonices Mundi. It described in precise mathematical language the link between the distances of the planets from the Sun and their velocities – specifically, that the orbital velocity of a planet is inversely proportional to its distance from the Sun.

Rudolphine Tables and other work
Kepler finally completed and published the Rudolphine Tables (1627) based on Brahe's observations. These were the first modern astronomical tables, enabling astronomers to calculate the positions of the planets at any time in the past, present, or future. The publication also included other vital information, such as a map of the world, a catalogue of stars, and the latest aid to computation, logarithms.