Kepler, Johannes (1571–1630)
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.
Other works Kepler's second book on optics, the Dioptrice, was published in 1611. The Epitome, intended as an introduction to Copernican astronomy, was an effective summary of Kepler's life's work in theoretical astronomy. It was a long treatise of seven books, published over a period of four years, and it had more impact than any other astronomical text of the mid-17th century. Kepler wrote the first science fiction story, Somnium, describing a man who travelled to the Moon. It was published in 1631, a year after his death. He had published 33 works and left 22 volumes of manuscript and much correspondence. |
Life Kepler was born in Weil der Stadt in Baden-Württemberg, and studied at Tübingen. He originally intended to prepare for the ministry of the Lutheran Church, but instead became lecturer in astronomy at the University of Graz in about 1594. As a Lutheran Protestant, he was expelled twice from Graz; then from Prague in 1612; then from Linz, Austria, from where he moved to Ulm. His Prodromus dissertationum cosmographicarum seu mysterium cosmographicum won him the friendship of Brahe and Galileo. |
| Kepler became assistant to Brahe in 1600, and succeeded him in 1601 as mathematician to Holy Roman Emperor Rudolph II. Rudolph was more interested in astrology than in astronomy, and Kepler was of great value to him and to his successor, Matthias, because of his knowledge of the casting of nativities. Kepler himself believed in the subject, and one of the publications that brought him fame during his lifetime was an astrological one, De fundamentis astrologiae (1602). |
| After Rudolph's death, Kepler remained in favour and continued his astronomical research. He was invited to England after he dedicated Harmonices mundi to James I. Although in constant poverty at a court that rarely paid him a salary, Kepler refused the invitation. In about 1629, he became astrologer to Wallenstein. |
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