The geographer Eratosthenes, who lived in Alexandria in 275-195. BC, calculated the size of the Earth. He made his wonderful discovery ... in a well. Eratosthenes noticed that during the summer solstice, the sun's rays illuminate the bottom of the deepest wells of the city of Siena (this is what the current Aswan was called then). This means that the sun here these days is exactly at its zenith. And in Alexandria, which lies to the north, the bottom of deep wells remains even a shadow. Casting a shadow here and a small peg, fixed inside the hollow hemisphere. The Egyptians called such a device "skafe", it replaced the clock with it. Eratosthenes reasoned that the ratio of the length of this shadow of the peg and the great circumference of the scafe should be equal to the ratio of the arc of the meridian between Siena and Alexandria to the entire circumference of the Earth (he believed that both cities lie on the same meridian, although this is not entirely accurate).
As a result of long calculations, Eratosthenes received the length of the great circle of the Earth, equal to 39 690 kilometers. Amazing precision with the primitive instruments he used!