Eratosthenes
Eratosthenes put a stick on Syene southern Egypt and notices on summer solstice, that stick casts no shadow as the sun is directly over the stick. But on the same day another stick on Alexandria cast a shadow. That was not supposed to happen if the earth's surface is flat. But that will happen if earth is round.
Then he measured the length of the shadow and calculated the angle of the sun in the sky. The angle is 1/50 of complete circle. As he knew the distance between two sticks, he multiplied the distance with 50 and got the length of circumference of the Earth 24,662 miles or 39690 km. The accurate measurement calculated today is 24901 miles or 40074 km, which is very close to Eratosthenes's calculation.
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