By this means he found that the circumference of the earth was , stadia, according to Cleomedes, but Strabo says that it was , stadia. A discrepancy arises when we come to consider the length of the stadium. The Greek stadium was ft. If we accept the account of Strabo that Eratosthenes measured the circumference of the earth as , stadia, the circumference would be 24, miles, and the polar diameter 7, miles-only 50 miles short of the true polar diameter. This is considered to be one of the first great triumphs of scientific calculation.
Reprints and Permissions. Eratosthenes and the Circumference of the Earth. Nature , Download citation. Eratosthenes reasoned that the difference in the angle of incoming sunlight was due to the curvature of Earth's surface, and so by measuring this angle, he related the distance between Alexandria and Syene to the total dimension of the globe. On the day the Sun shone on the bottom of the wells in Syene, Eratosthenes measured the Sun's position in the sky over Alexandria. It was seven degrees away from the zenith, meaning Syene must be seven degrees away from Alexandria as measured on the circle that is Earth's circumference.
That man was Eratosthenes. A Greek mathematician and the head of the library at Alexandria. Eratosthenes had heard that in Syene, a city south of Alexandria, no vertical shadows were cast at noon on the summer solstice. The sun was directly overhead. He wondered if this were also true in Alexandria. So, on June 21 he planted a stick directly in the ground and waited to see if a shadow would be cast at noon.
It turns out there was one. And it measured about 7 degrees. Now, if the sun's rays are coming in at the same angle at the same time of day, and a stick in Alexandria is casting a shadow while a stick in Syene is not, it must mean that the Earth's surface is curved.
And Eratosthenes probably already knew that. The idea of a spherical Earth was floated around by Pythagoras around BC and validated by Aristotle a couple centuries later.
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