Kepler, or KEPLER, JOHANN, one of the very greatest astronomers, was born at Weil der Stadt, a village in Württemberg, 10 miles from Stuttgart, 27th December 1571. He was left to his own resources when a mere child, his education depending on his admission into the convent of Maulbronn. He afterwards studied at the university of Tübingen, applying himself chiefly to mathematics and astronomy. In 1593 he was appointed professor of Mathematics at Gratz, and about 1596 commenced a correspondence with Tycho Brahé (q.v.), which resulted in his going to Prague in 1599 to aid Tycho in his work. Tycho obtained for him a government appointment, but the salary was not paid, and Kepler lived for eleven years there in great poverty. He then obtained a mathematical appointment at Linz, and fifteen years afterwards was removed to the university of Rostock, poverty still pursuing him. He died shortly afterwards at Ratisbon, 15th November 1630.
In character he was intensely enthusiastic, imaginative, laborious, and persevering, all qualities fitting him for the great task of transforming astronomy from a merely formal into a true physical science. Though Copernicus (q.v.) had transferred the centre of the planets' movements to the sun, these were still considered as compounded of various circles, the only curve thought fit for celestial bodies to pursue. No cause was assigned for their movements, and no unity observed among them, except in the one fact of the sun being their centre. Kepler says, 'I brooded with the whole energy of my mind' on this subject, asking 'why they are not other than they are,' the number, the size, and the motion of the orbits.' In fact he had first to determine what the orbits were before answering some of these questions. But one question lay open before him. The periods of the planets were fairly well known, so were their proportionate distances from the sun. Was there any invariable relation between these? In his Mysterium, published in 1596, he triumphantly proclaims that five kinds of regular polyhedral bodies govern the five planetary orbits. Yet after publication he still continued to 'brood,' becoming at length convinced that this theory was only an error, until after twenty-two years of patient study and numberless speculative failures, he was able at last to announce (in his Harmonice Mundi, 1619) that the 'square of a planet's periodic time is proportional to the cube of its mean distance from the sun.' This rule is known as Kepler's Third Law. He saw clearly enough that it implies that the planets are moved by a force greater near the sun, and lessening with distance, but he did not grasp, as Newton after him did, the truth that this is an attractive force constantly acting towards the sun, nor could he therefore guess the law of its action. Finding the theory of epicycles unable to bear the strain of Tycho Brahé's accurate observations, especially in the case of the planet Mars, he endeavoured to find a law for the planet's movements which would be simple and satisfactory. After enormous labour, and by a process of trial and error, he found that (1) the planet's orbit was an ellipse, of which the sun is in one focus, and (2) that, as the planet describes its orbit, its radius vector traverses equal areas in equal times. These rules (published in 1609 in his work on The Motions of Mars) are known as Kepler's First and Second Laws respectively. These laws formed the groundwork of Newton's discoveries, and are the starting-point of modern astronomy. Besides, we owe to Kepler many discoveries in optics, general physics, and geometry. A collected edition of his works was published by Frisch (1858-71).
For further information, see Brewster's Lives of Galileo, Tycho Brahé, and Kepler (1841); Reitlinger, Neumann, and Gruner, Johannes Kepler (1868); and Whewell's Hist. of Inductive Sciences (vol. i.).