Kepler Johannes (1571–1630), German mathematical astronomer, speculative metaphysician, and natural philosopher. He was born in Weil der Stadt, near Stuttgart. He studied astronomy with Michael Maestlin at the University of Tübingen, and then began the regular course of theological studies that prepared him to become a Lutheran pastor. Shortly before completing these studies he accepted the post of mathematician at Graz. ‘Mathematics’ was still construed as including astronomy and astrology. There he published the Mysterium cosmographicum (1596), the first mjaor astronomical work to utilize the Copernican system since Copernicus’s own De revolutionibus half a century before. The Copernican shift of the sun to the center allowed Kepler to propose an explanation for the spacing of the planets (the Creator inscribed the successive planetary orbits in the five regular polyhedra) and for their motions (a sun-centered driving force diminishing with distance from the sun). In this way, he could claim to have overcome the traditional prohibition against the mathematical astronomer’s claiming reality for the motion he postulates. Ability to explain had always been the mark of the philosopher. Kepler, a staunch Lutheran, was forced to leave Catholic Graz as bitter religious and political disputes engulfed much of northern Europe. He took refuge in the imperial capital, Prague, where Tycho Brahe, the greatest observational astronomer of the day, had established an observatory. Tycho asked Kepler to compose a defense of Tycho’s astronomy against a critic, Nicolaus Ursus, who had charged that it was ‘mere hypothesis.’ The resulting Apologia (1600) remained unpublished; it contains a perceptive analysis of the nature of astronomical hypothesis. Merely saving the phenomena, Kepler argues, is in general not sufficient to separate two mathematical systems like those of Ptolemy and Copernicus. Other more properly explanatory ‘physical’ criteria will be needed.
Kepler was allowed to begin work on the orbit of Mars, using the mass of data Tycho had accumulated. But shortly afterward, Tycho died suddenly (1601). Kepler succeeded to Tycho’s post as Imperial Mathematician; more important, he was entrusted with Tycho’s precious data. Years of labor led to the publication of the Astronomia nova (1609), which announced the discovery of the elliptical orbit of Mars. One distinctive feature of Kepler’s long quest for the true shape of the orbit was his emphasis on finding a possible physical evaluation for any planetary motion he postulated before concluding that it was the true motion. Making the sun’s force magnetic allowed him to suppose that its effect on the earth would vary as the earth’s magnetic axis altered its orientation to the sun, thus perhaps explaining the varying distances and speeds of the earth in its elliptical orbit. The full title of his book makes his ambition clear: A New Astronomy Based on Causes, or A Physics of the Sky.
Trouble in Prague once more forced Kepler to move. He eventually found a place in Linz (1612), where he continued his exploration of cosmic harmonies, drawing on theology and philosophy as well as on music and mathematics. The Harmonia mundi (1618) was his favorite among his books: ‘It can wait a century for a reader, as God himself has waited six thousand years for a witness.’ The discovery of what later became known as his third law, relating the periodic times of any two planets as the ratio of the firm his long-standing conviction that the universe is fashioned according to ideal harmonic relationships. In the Epitome astronomiae Copernicanae (1612), he continued his search for causes ‘either natural or archetypal,’ not only for the planetary motions, but for such details as the size of the sun and the densities of the planets. He was more convinced than ever that a physics of the heavens had to rest upon its ability to explain (and not just to predict) the peculiarities of the planetary and lunar motions. What prevented him from moving even further than he did toward a new physics was that he had not grasped what later came to be called the principle of inertia. Thus he was compelled to postulate not only an attractive force between planet and sun but also a second force to urge the planet onward. It was Newton who showed that the second force is unnecessary, and who finally constructed the ‘physics of the sky’ that had been Kepler’s ambition. But he could not have done it without Kepler’s notion of a quantifiable force operating between planet and sun, an unorthodox notion shaped in the first place by an imagination steeped in Neoplatonic metaphysics and the theology of the Holy Spirit. See also NEWTON. E.M.