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Johannes Kepler was the first to claim that the planets move in an elliptical orbit, rather than a circular one as claimed by the astronomers and physicists before him. Kepler believed that the Copernican system was true and claimed that the earth is one of the planets and that it revolves about the sun with the other planets. Unlike Copernicus and Galileo, he claimed in his book New Astronomy (1609) that this motion is not circular but elliptic. Moreover, he discovered the law of motion, according to which the planet "covers" equal area over equal periods of time. Kepler created a revolution in astronomy which paralleled Galileo's revolution in physical mechanics.
Kepler's laws of stellar motion were based on the precise measurements of Ticho Brahae, mainly of the motion of Mars. Kepler served as Brahae's assistant for a year until Brahae's death in 1601. Kepler was not satisfied with the descriptions of stellar motion offered by the accepted astronomers of his time. He searched for a reason for their movement and for the forces which motivated them. These forces were based on Aristotelian mechanics rather than on Galileo's new mechanics. These explanations are no longer accepted today.
Kepler was involved in both mathematics and optics. His researches in optics dealt with vision and with lenses. Among other things he investigated Galileo's telescope and proposed an optical theory to explain how it works. In addition to his scientific activities, Kepler was also involved in astrology, which he sought to relate to his astronomy. He prepared many astronomical tables. His astronomical researches were influenced by his belief in various mystic creeds, such as the search for harmony between all elements of the cosmos. Among other things, in his book De Harmonie Mundi, he attempted to show the existence of a relationship between the distances of the stars and musical harmonies. In the same book he presents the physical law which relates the a planet's distance from the sun to its orbital cycles around it.
Johannes Kepler was born in 1571 in Weil, in the principality of Wuettemberg in southern Germany. He studied at the University of Tuebingen with the aim of joining the clergy, but was eventually sent to study in Gretz in 1594. At this time, following his meeting with Masteline, he was already convinced that Copernicus's ideas were true. In 1600, Kepler joined Ticho Brahae in Prague, and a year later, following the death of Ticho, he was nominated to replace him as the emperor's astronomer, and later as the astronomer of Upper Austria. Kepler moved to Silesia in 1628. He died in Ratisban in Bavaria in 1630 (in southern Germany).(Johannes Kepler's picture is from - Life two Lehers, By -C. Baumgardt, . p. inner cover.)
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