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He invented the armillary sphere. It was a mechanical device used by early astronomers to demonstrate and predict the apparent motions of the stars in the sky.
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He compiled a star catalog and the earliest surviving table of a trigonometric function. This established mathematically that an object and its mirror image must make equal angles to a mirror.
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Earth and other planets revolve around the sun, or the Heliocentric Theory of the universe. Very slow changes in the direction of this axis account for the precession of the equinoxes.
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He first used the telescope to make extensive astronomical observations. He provided the crucial data for later astronomers like Kepler to construct our present model of the solar system.
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Kepler discovered that the orbit of Mars was an ellipse. This led to the discovery that the Earth and planets travel about the sun in elliptical orbits.
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He came up with the formulation of three Laws of Motion. Newton showed that astronomical phenomena can be explained using only basic physics and mathematics.
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He created the theory of relativity. This lead to an explanatin of the origin of the universe.