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LawsOfKepler

The astronomer Johannes Kepler's main contribution to astronomy was his three laws of planetary motion.  They predate Isaac Newton and provide the foundation for his work, and can be derived from his laws of motion and gravity.

Kepler's First Law:  Planets move in ellipses, with the sun located on one focus of the ellipse.
Kepler's Second Law: A line between a planet and the sun will sweep out equal areas over equals times. 
Kepler's Third Law:  The square of a planet's radius of orbit is proportional to the cube of the time it takes to orbit the sun.