Picture of Johan Kepler
The Bettman Archive. From engraving by MacKenzie
Johan Kepler was a German astronomer who lived between 1571-1630. He introduced three important laws of planetary motion and helped the Copernican model of the solar system gain general acceptance.
An original problem with Copernicus' heliocentric theory was its inability to explain the periodic retrograde (apparent backward motion) of planet Mars. Kepler inherited Tycho Brahe's observational data on Mars following Brahe's death and showed that Mars' orbit was really elliptical, and thus may occasionally appear to retrograde. In fact, all planetary orbits are elliptical. This new revelation contradicted the age old belief that heavenly bodies traced out perfect circles.
During his life, Kepler also cast horoscopes and wrote science fiction novels.
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