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No one had any idea how big the Earth was but using sun rays and 2 measuring points Eratosthenes figured out the distance. This was a brand new incredible discovery.
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He unveiled his geocentric system which was backed by math. Which detailed how things moved around the Earth which was the first discovery.
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He discovered that the sun was the center of all the planets moving around it which he considered circular like. This proved the current thinking of everything revolving around the Earth wrong.
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He studied the stars and discovered that a new star was farther than the moon. At the time people thought nothing was beyond the moon so this was very important.
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He discovered the laws which were new information to the world. The first was that planets move in ellipses with the sun at one focus.
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Revealed his main laws of motion which included the three main keys to gravity. This was new information and that is why this is such an important discovery.
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Basically completely created new theories and solutions to how gravity and the Earth worked. Before the ideas were incorrect so his discovery was important. He mainly discussed how gravity and other things worked.