Johannes Kepler (1571-1630) was a German mathematician and astronomer of the late Renaissance, who discovered the laws of planetary motion. sir Isaac Newton later used these laws as the basis of his Law of Universal Gravitation.
Kepler spent his entire life trying to answer this question: How do the bodies in the solar system maintain their positions? His firm belief in the order of the universe drove him to find a regularity in the universe. In finding that regularity, Kepler laid the foundation for scientific astronomy.
Johannes Kepler was born in Württem-berg, Germany, and attended the University of Tübingen. At the age of twenty-three he became professor of astronomy at the University of Graz. Never a strong man, he suffered from the effects of a premature birth, and a severe case of smallpox at the age of four which left him almost blind.
At the University of Tübingen, Kepler studied the writings of Copernicus, and later at the University of Graz he wrote a book on how the heavenly bodies are kept in position in the solar system. This work came to the attention of Tycho Brahe, the famous Danish astronomer, who secured an appointment for Kepler as his assistant.
He used the theory developed by Copernicus that the Sun is the center of the universe, a theory which was accepted at that time. However, according to this theory, orbits of the planets were circles. As Kepler studied Brahe's observation of the planet Mars, he realized that it travelled around the Sun in an ellipse, with the Sun at one focus of the ellipse, which the planet's orbit forms. This was the first of Kepler's three great laws of planetary motion. Next he proved that if a line could be drawn from the center of a planet to the center of the Sun, as the planet revolved in its orbit, the line would sweep the same area in a given time. He also proved that the motion of the planet was fastest when it was nearest the Sun, and slowest when it was farthest away. This was his second law.
Finally, using Brahe's data, Kepler announced his third law: The squares of the periods of revolution are proportional to the cubes of their average distances from the Sun.
