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He used the sun to measure the size of round Earth which was only 211 miles off. This was a big deal because most of the people of that time believed the Earth was flat.
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Ptolemy made a model of the universe with Earth at the center, and the planets moving in epicycles. This was incorrect, but the model was accepted for centuries.
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Copernicus made the heliocentric model of the universe popular during the Renaissance and changed the way many scientists viewed the universe.
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He measured the planet's positions down to within a minute of an arc. This was important because the telescope had not been invented yet, he created his picture of the sun.
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He set up the three laws of planetary motion which was the first time someone stated that the planets moved because of a force from the sun.
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Newton defined the laws of gravity which helped scientists understand how the moon revolves around the Earth and how the other planets move around the sun.
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Einstein explained the photoelectric effect, which states that light is a stream of particles known as photons rather than a single wave of light. This helped explained a lot of scientist's curious results.