The Greek mathematician Eratosthenes (a 225 v. Chr.) was librarian in the famous great library at Alexandria in Egypt. He is the first, of which is known, that he measured the size of the earth. He applied the mathematics to two observations:
In Aswan, near the first cataract of the Nile, you could see the reflection of the sun in a deep well, when on a certain day of the year the sun was at its highest point and cast no shadow.
At the same time cast the sun 800 km north, in Alexandria, a shadow of 7½°.
Eratosthenes now used two geometric theorems, which Greek mathematicians had developed earlier. First, you knew, that opposite angles are equal to each other, second, one had proved, that every straight line, which intersects two parallels, forms the same angles with both parallels.
Besides, Eratosthenes knew, that a circle has 360°. He also knew from his measurements, that TU0 den 800 km from Aswan to Alexandria. Da 7½ nun 48mal in 360 (the number of degrees of a full circle) is included, he multiplied 48 With 800. So he calculated the circumference of the earth 38400 km. Today's precision instruments are used to calculate the circumference of the earth's equator 40076,5 km.