MadSci Network: Science History
Query:

Re: Who discovered the world was round and what year was it?

Date: Thu Jan 31 13:54:41 2002
Posted By: Dan Berger, MadSci Administrator
Area of science: Science History
ID: 1011035598.Sh
Message:

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.

We do know that Eratosthenes of Cyrene was the first to accurately estimate the Earth's diameter, around 220 BC. How he did it is detailed at Astronomy Online.

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.

Dan Berger
MadSci Administrator



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