The German astronomer Johannes Kepler, (Dec. 27, 1571-Nov. 15, 1630) was the first strong supporter of the heliocentric theory of Copernicus and the discoverer of the three laws of planetary motion.
At the age of 24, Kepler published Cosmographic Mystery, 1596, in which he defended the Copernican theory and described his ideas on the structure of the planetary system. Influenced by the Pythagoreans, Kepler viewed the universe as being governed by geometric relationships that conform to the inscribed and circumscribed circles of the five regular polygons.
Kepler embarked on an intensive study of the true orbits of the planets. Abandoning the ancient belief that the planets must move in perfect circles, Kepler concentrated on Mars. He proved that the orbit of Mars is an ellipse, with the Sun occupying one of its two foci. This, the first of Kepler’s laws of planetary motion, appeared in New Astronomy in 1609, with the second ‘law of areas’ governing planetary velocity.
Always guided by the concept of beauty in the structure of the universe, and specifically by a theory of harmony in geometric figures, numbers, and music, Kepler, in his Harmonies of the World, 1619, announced his third law—a relationship between the orbital periods and the distances of the planets from the Sun. His belief that the Sun regulates the velocity of the planets was a milestone in scientific thought, laying the foundation for Newton’s theory of universal gravitation.
Among Kepler’s numerous scientific contributions are an influential treatise on the theory of optics (1604), a treatise on optics as applied to telescope lenses (1611), a work offering physical explanations of the appearance of a nova in 1604, and an enthusiastic acceptance of and elaboration on Galileo’s observations with a telescope (1610). His Introduction to Copernican Astronomy, 1618-21 became one of the most widely read treatises on astronomy in Europe. Kepler’s last great work, known as the Rudolphine Tables (1627), was a widely used compilation of accurate tables of planetary motion.
The posthumous Somnium (Dream, 1634), on which Kepler laboured until shortly before his death, is indicative of his fertile mind. In this work, Kepler describes a journey to the Moon and discusses the existence of lunar inhabitants. A crucial link between the thought of Copernicus and that of Newton, Kepler was an important figure in the 17th-century scientific revolution.
