by jamming a stick
into the ground.
Eratosthenes was this ancient greek guy who lived around the late 200s BC. He did a lot of work with Geography.
His claim to fame is that at one point, using nothing but simple geometric theorems and a stick, he managed to find a fairly accurate measurement of the circumference of the earth.
He got the idea when he was in Alexandria, where he heard of a deep well in Syrene (a town in southern Egypt) that for one day a year, at noon, was completely filled by light, with no shadows. He interpreted this correctly to mean that at that moment on that day, the sun was directly over the Syrene well.
So, when the day the Syrene well would be shadowless came, Eratosthenes (in Alexandria) went outside, stuck a stick straight vertically into the ground, and took the following logic:
The correct answer would be somewhere closer to 40000 km (the earth is not, however, a perfect sphere
, so it varies a tiny bit) but still-- considering the rediculous impreciseness
of the numbers he was working with (eye-measurement of the shadow of a pole, distance between two cities measured based on how long it takes to walk between them
)-- that's pretty damn impressive.
numbers from http://youth.net/eratosthenes/welcome.html