Daniel E. Harmon 
Johannes Kepler [PDF ebook] 

Johannes Kepler was just twenty-three years old when he became a teacher of mathematics and astronomy at the university in Graz, Austria, in 1594. For the next thirty-five years, his intensive research based on the theories of Nicolaus Copernicus resulted in astonishing new ideas on the physics of the solar system. Most important was his realization that the planets move in elliptical orbits. Kepler’s laws greatly influenced the later findings of Sir Isaac Newton and other famous scientists. Kepler is considered one of the most important thinkers of the Scientific Revolution.