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Kepler built on [[Copernicus]]'s work and discovered that planets orbited the sun in elliptical rather than perfectly circular orbits. This discovery accounted for the apparent [[retrograde motion]] of the planet [[Mars]].

Kepler built on [[Copernicus]]'s work and discovered that planets orbited the sun in elliptical rather than perfectly circular orbits. This discovery accounted for the apparent [[retrograde motion]] of the planet [[Mars]].

Kepler overcame an enormous obstacle to become a great scientist, as he suffered from an attack of [[smallpox]] when he was only four years old.  The disease left him with poor eyesight and crippled hands for the rest of his life.  However, Kepler went on to propose three laws of planetary motion, known as Kepler's Laws, which later helped Newton formulate the theory of [[gravity]].  One of these laws described how the planets have elliptical orbits with the sun at one focus of each [[ellipse]].

Kepler overcame an enormous obstacle to become a great scientist, as he suffered from an attack of [[smallpox]] when he was only four years old.  The disease left him with poor eyesight and crippled hands for the rest of his life.  However, Kepler went on to propose three laws of planetary motion, known as [[Kepler's laws of planetary motion|Kepler's Laws]], which later helped Newton formulate the theory of [[gravity]].  One of these laws described how the planets have elliptical orbits with the sun at one focus of each [[ellipse]].

Kepler wrote in book five of his work, "The Harmonies of the World,":

Kepler wrote in book five of his work, "The Harmonies of the World,":