It seems preposterous to us today that people once thought that the Earth was flat. Who could have possibly thought of our planet as a giant disk with the stars and heavens above, and boulders, tree roots, and other things below? But this was the dominant view of Earth in much of the world before the 2nd century BCE, though the details differed from culture to culture. And it was not explorers who sailed around the world that finally laid the idea to rest, but an accumulation of evidence long before this. Greek philosophers referred to a spherical Earth as early as the 6th century BCE. They observed that the moon appeared to be a sphere and therefore inferred that Earth might also be spherical. Two hundred years later, in the 4th century BCE, the Greek philosopher Aristotle observed that the shadow of the Earth on the Moon during a lunar eclipse is always curved, thus providing some of the first evidence that Earth is spherical. In the 3rd century BCE, the mathematician Eratosthenes observed that at noon on the summer solstice in the ancient Egyptian city of Syene, the sun was directly overhead as objects did not cast a shadow. Eratosthenes was from Alexandria, Egypt, some 500 miles to the north, and he knew that a tall tower cast a shadow in that city at the same time on the summer solstice. Using these observations and measurements of shadow length and distance, he inferred that the surface of the Earth is curved and he calculated a remarkably accurate estimate of the circumference of the planet (Figure 1). Some years later, the Greek geographer Strabo added to this evidence when he observed that sailors saw distant objects move downward on the horizon and disappear as they sailed away from them. He proposed that this was because Earth was curved and those sailors were not simply moving further away from the objects but also curving around the planet as they sailed.
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