Kepler, Johannes (1571-1630), a German astronomer and mathematician, discovered three laws of planetary motion.  The English scientist Sir Isaac Newton later used Kepler's three laws to arrive at the principle of universal gravitation.  Kepler's laws are:  

(1) Every planet follows an oval-shaped path, or orbit, around the sun, called an ellipse.  The sun is located at one focus of the elliptical orbit. 

(2) An imaginary line from the centre of the sun to the centre of a planet sweeps out the same area in a given time.  This means that planets move faster when they are closer to the sun. 

(3) The time taken by a planet to make one complete trip around the sun is its period.  The squares of the periods of two planets are proportional to the cubes of their mean distances from the sun. 

Kepler was born in Weil (near Stuttgart), Germany, and graduated from the University of Tubingen.  He accepted an offer to teach at the Lutheran school in Graz, Austria.  But he left Graz rather than undergo compulsory conversion to Roman Catholicism.  While he was seeking another post, he formed an association with Tycho Brahe, which shaped the rest of his life.  Brahe, the greatest astronomical observer before the introduction of the telescope, needed an assistant, and Kepler joined him at his observatory in Prague, in what is now the Czech Republic.  After Brahe died, Rudolf II, the Holy Roman emperor, appointed Kepler to be Brahe's successor as imperial mathematician. 

Kepler made his most significant discoveries trying to find an orbit to fit all Brahe's observations of the planet Mars.  Earlier astronomers thought a planet's orbit was a circle or a combination of circles.  However, Kepler could not find a circular arrangement to agree with Brahe's observations.  He realized that the orbit could not be circular and resorted to an ellipse in his calculations.  The ellipse worked, and Kepler destroyed a belief that was more than 2,000 years old. 

Kepler was the first astronomer openly to uphold Copernicus' theories.  He also made important contributions to the science of optics.  For example, he helped explain how lenses work.  In addition, he showed that the eye functions like a camera when images are projected through the eye's lens onto the retina

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