Kepler's great contribution was in his discovery of the laws of planetary motion, which broke from 2,000 year-old tradition and represented a large step towards a more complete understanding of our universe. Openly endorsing Copernicus's theory of a solar system and of Earth revolving around the sun, Kepler added to the breakthroughs of his predecessor, making critical discoveries and inspiring many after him.
Johannes Kepler was born in the small town of Weil der Stadt, Germany in 1571, twenty-eight years after Copernicus published his great work, De revolutionibus orbium coelestium, in which he proposed the notion of a solar system, and thirty-eight years before Italian astronomer Galileo would popularize the telescope for astronomical observation.
Kepler earned his degrees at the University of Tübingen, his bachelor's in 1588 and his master's in 1591. At the Lutheran Academy in Graz, Kepler worked for several years as a professor and wrote his first book on astronomy but left, rather than be compelled to convert to Roman Catholicism.
Though the theory Kepler proposed in his book was later proven to be entirely false, the skill and originality of his writing and thinking was keenly examined by Tycho Brahe, perhaps the last great astronomer before the wide-spread use of the telescope. Unlike many of the other figures in these biographies, Kepler's formal education ended early, and he had great fortune in catching the attention of a great astronomer.
Kepler lived after Copernicus first asserted that Earth and the other planets revolved around the sun. But the majority in his day were still staunchly for a theory in which all of the planets revolved around a stationary Earth. In fact, one of the great influences upon Kepler's thinking was a series of talks at the University of Tübingen, in which he heard Copernicus's theory intelligently defended. Quickly, he warmed to the notion of a solar system and came to fully endorse it.
In 1601, just one year after making Kepler his assistant in his Prague observatory, Tycho died, leaving behind a wealth of observations -- renowned for their accuracy -- and leaving Kepler to inherit his position as Imperial Mathematician of the Holy Roman Empire.
The first years of Kepler's work were spent compiling numerical analysis of Tycho's observations, in the hopes of matching the results with Copernicus's theory of planetary motion. Unhappily, however, Kepler found that his mathematical calculations were far from fitting the imprecisely figured Copernican model.
During many months of additional calculations, he happened upon the most important of his discoveries: that planetary orbits were elliptical, not circular, as he and those before him had believed.
In 1609, Kepler published Astronomia nova, his great work, in which he laid forth two laws of planetary motion:
Diagram. A focus is one of two fixed points (foci) used to determine an ellipse. If you put a loop of string around the foci, you could draw the ellipse with a pencil. Original diagram by The Online Planetarium Show.
Diagram. Near the sun, the line connecting the planet to the sun sweeps over a small area quickly; conversely, it covers a larger area in a longer time. So, the area per time is a constant for each planet. Original diagram by The Online Planetarium Show
Kepler would add a third law to the first two ten years later, proposing that it takes longer for planets more distant from the sun to complete one full orbit around the sun.
Strongly convinced through his work that the planets did in fact orbit around the sun, Kepler openly endorsed the Copernican theory that the earth and all other planets were in fact orbiting the sun. Undoubtedly, Kepler's discoveries and intricate calculations were vital as a basis for Isaac Newton's later work.
For his bold assertions about orbits, for his extensive observations and copious calculations, and for his defense of the Copernicus's theory of the universe, later astronomers and scientists -- especially Newton -- were indebted to Kepler. He was the first to support Copernicus with substantial evidence, and he consequently did much to help explain the great mystery of planetary orbits.