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Published his "Almagest". This work was important because it was the dominant model of the universe for over a thousand years and provided a framework for understanding the motions of the planets. (In the 2nd century)
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Measured the circumference of the Earth. This discovery was important because it was the first time anyone had accurately measured the size of the Earth.
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Published "De Revolutionibus Orbium Coelestium". This work was important because it challenged the geocentric model of the universe that had been accepted for centuries and proposed the heliocentric model.
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Discovered a new star in the constellation Cassiopeia. The discovery was important because it contradicted the Aristotelian Idea that the universe was perfect and unchanging.
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Published his Astronomia Nova. This work was important because it provided a mathematical basis for Copernicus's heliocentric model of the universe.
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Published his "Philosophiae Naturalis Principia Mathematica". This work was important because it explained how objects move in the universe and provided a foundation for modern physics.
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Published his theory of general relativity. This theory was important because it revolutionized the way we think about gravity and space-time.