|MadSci Network: Science History|
Nobody is sure who first deduced that the world is round. It is most likely
that it was done by observing the Earth's shadow on the Moon during a lunar
eclipse. Aristotle (384-322 BC) said that it was common knowledge, at least
among the learned, so it's been known for at least 2,500 years.
What he did was use the information that, at noon on the Summer Solstice, the sun shone to the bottom of a well in Syene (now called Aswan), Egypt. This meant that the sun was making a 90° angle to the ground on that day, in Syene.
Eratosthenes then measured the angle of sunlight to the ground in Alexandria, Egypt, at noon on the same day. He used that angle to calculate what fraction of the Earth's circumference (which is 360°) was between Alexandria and Syene. Since he knew the distance to Syene, and that it was exactly south of Alexandria, he was able to calculate the Earth's circumference.
Actually, he didn't have the distance quite right--and Syene isn't exactly south of Alexandria, either.This was the first reasonable accurate measurement of the Earth's circumference, and it came to about 40,000 km (25,000 miles). We now know that this is an overestimate, but not by too much.
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