Around 1605, Johannes Kepler (1571-1630) found out from the astronomical observations of Tycho Brahe (1546-1601), a Danish nobleman from Scania (Skåne then Danish), the following empirical relations:
1. The orbit of any planetary orbit is an ellipse with the sun at one focus.
2. A line joining the planet and the sun sweeps out equal areas during equal intervals of time.
3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit (the semi-major axis of the ellipse is half the major axis which is the longest possible axis of the ellipse running through both foci).
Almost a century later Isaac Newton was able to derive Kepler's laws from his own laws of motion and the law of gravitation with the help of Euclidean geometry.
The "divine" perfect circle of planetary motion was crushed.
Kepler was born in the Free Imperial City of Weil der Stadt, now close to Stuttgart. Childhood small pox left him weak and with poor vision so that he could not perform astronomical observations himself although introduced to astronomy early and with a profound love for it. Kepler began as a theology student at the University of Tübingen but was becoming an excellent mathematician but also a skillful astrologer casting horoscopes for his friends. Carl Sagan has called him the first astrophysicist and the last scientific astrologer. At the time both the disciplines of astrology and astronomy was in existence.
In a student disputation he defended heliocentrism both from a theoretic but also from a theological perspective, maintaining that the sun was the principal source of motive power in the universe. Despite his interest in becoming a protestant minister, at the age of 23, two years earlier than Galileo, he became a professor of mathematics and astronomy at the School of Graz (later University of Graz). He actually later helped Galileo with observational support of the heliocentric theory when he was in trouble with the Catholic Church.
In 1600 Kepler and his family was banished from Graz as a result of the counter reformation. They refused to convert to Catholicism. He then returned to Tycho Brahe in Prague where he had had an earlier not so successful stint of collaboration. Brahe had been secretive with his data. Brahe was one of the last astronomers that worked without the telescope. When Brahe died in 1601, Kepler replaced him as the imperial mathematician with the mission of completing his unfinished work. Kepler had now access to the extensive observations of Brahe and his group and the following 11 years were the most productive in his life.
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