Contemporary with Tycho Brahé and Galileo, and to some extent the associate and successor of the former, was John Kepler, one of the most eminent astronomers who have appeared in any age, and to whom the science is indebted for much of its present perfection. He was born December 27, 1571, at Wiel in Wurtemberg, and was descended of a noble but reduced family. His father, originally an officer of distinction in the army of Wurtemberg, was, at the time of young Kepler's birth, in the humble capacity of a small inn-keeper; and thus, as is too often the case with genius, our philosopher had to struggle into fame through poverty and the vicissitudes of his father's fortune. Poor, unbefriended, of a weakly constitution, and one of the most diminutive of children, Kepler received the rudiments of knowledge at the Monastic school of Maulbrunn, where he gave early indications of talent, and of that irrepressible spirit which, amid the severest obstruction, was never diverted from the main object of its pursuit. After his father's death, which took place in his eighteenth year, he left Maulbrunn, arid succeeded in entering the college of Tubingen. Here he completed the course of study then prescribed - first philosophy and mathematics, and then theology taking the degree of Bachelor in the year 1588, and that of Master of Philosophy in the year 1591. Of apt inquiring powers as a divine, and of more than average eloquence as a preacher, Kepler could now have readily succeeded in the church; but mathematics and the exact sciences were his favorite themes; and it may be fairly questioned if ever he turned a single thought to the clerical profession, beyond what the curriculum of the university compelled. In 1593-4, his reputation as a geometrician had so increased, that he was invited to fill the mathematical chair in the university of Gratz, in Styria. Here he pursued his astronomical studies with the most commendable zeal, devoting himself especially to the investigation of the physical causes of the motion of the celestial bodies.

Shortly after his installment, he married a lady descended from a noble family, and was beginning to enjoy that domestic happiness and studious quiet so congenial to his wishes, when persecution on account of his religion compelled him to leave Gratz, to which, however, he was afterwards recalled by the states of Styria. Meanwhile Tycho Brahé, who had come to Germany, and was comfortably settled under the munificent patronage of Rodolphus, fixed upon Kepler as a suitable assistant, and soon induced him, by urgent letters and flattering promises, to accept of the situation. Compelled in a great measure by the unsettled state of affairs in Austria, Kepler speedily repaired to Prague, and applied himself, in conjunction with Tycho, to the completion of the Rodolphine Tables, which were first published at Ulm in 1626. At Tycho's recommendation, he was established at that place; but as his office and science did not afford him a subsistence, he studied medicine, in order to gain a livelihood by its practice. The emperor had assigned him a salary, but in the period of trouble which preceded the Thirty Years' War, it was not paid. Even when he was appointed imperial mathematician by Matthias, Rodolphus' successor, his hopes of recovering his arrears were disappointed. Fresh controversies with the clergy, and the disturbed state of the country, made his situation very uncomfortable: he therefore left Lintz, repaired to Ratisbon, declined an invitation to England, was confirmed by the succeeding emperor, Ferdinand, in the office of imperial mathematician, and afterwards went to Ulm to superintend the printing of the Rodolphine Tables. In 1627 he return ed to Prague, and received from the emperor six thousand guilders. He finally became a professor at Rostock, on the recommendation of Albert, duke of Wallenstein, but did not receive the promised compensation. In 1630 he went, by permission of the emperor, to Ratisbon, to claim payment of the arrears of his pension; but he was there seized with a violent fever, supposed to have been brought upon him by too hard riding; and to this he fell a victim in the month of November, in the fifty-ninth year of his age. In 1808, a monument, consisting of a Doric temple enshrining his bust, was erected to his memory in Ratisbon by Charles Theodore Von Dalberg.

Kepler is represented by his biographers as a man of small stature, thin, of a weak constitution, and defective sight; but of somewhat gay and sportive manners. He was attached to his science with the most fervent enthusiasm; he sought after truth with eagerness, but forgot, in the search, the maxims of worldly prudence. To him were allotted but a scanty share of what are commonly esteemed the pleasures of life; but he endured all calamities with firmness, being consoled by the higher enjoyments which science never fails to impart to her true and cordial votaries. As an astronomer,' says Lalande, he is as famous in astronomy for the sagacious application which he made of Tycho's numerous observations (for he was not himself an observer), as the Danish philosopher for the collection of such vast materials.' To him, says another authority, the world is indebted for the discovery of the true figure of the planetary orbits, and the proportions of the motions of the solar system. Like the disciples of Pythagoras and Plato, Kepler was seized with a peculiar passion for finding analogies and harmonies in nature; and though this led him to the adoption of strange and ridiculous conceits, we shall readily be disposed to overlook these, when we reflect they were the means of leading him to the most important discoveries. He was the first who discovered that astronomers had been mistaken in ascribing circular orbits and uniform motions to the planets, since each of them moves in an ellipse, having one of its foci in the sun; and after a variety of fruitless efforts, he, on the 15th of May 1618, made his splendid discovery, that the squares of the periodic times of the planets are always in the same portion as the cubes of their mean distances from the sun. The sagacity of this wonderful man, and his incessant application to the study of the planetary motions, pointed out to him some of the genuine principles from which these motions originate. He considered gravity as a power that is mutual between bodies; that the earth and moon, tend toward each other, and would meet in a point so many times nearer to the earth than to the moon as the earth is greater than the moon, if their motions did not prevent it. His opinion of the tides was, that they arise from the gravitation of the waters towards the moon but his notions of the laws of motion not being accurate, he could not turn his conceptions to the best advantage. The prediction he uttered at the end of his epitome of astronomy, has been long since verified by the discoveries of Sir Isaac Newton; namely, that the determination of the true laws of gravity was reserved for the succeeding age, when the Author of Nature would be pleased to reveal these mysteries.