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He used the Sun to measure the size of the Earth and noted that it was round.
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Set up a model of the solar system where the Sun, stars and planets revolved around the Earth. It was called the Ptolemaic system.
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He discovered that our solar system was heliocentric. That means that the Sun was in the center.
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He measured one minute to arc. This was important because it gave his own picture of the universe. That the Sun revolved around the Earth.
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He found the 3 laws of planetary motion. These were important because this is what Newton used to discover universal gravitation.
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He discovered gravity because an apple fell on his head.
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He discovered that the laws of physics are the same throughout the universe. That the speed of light is constant.