It started with Eratosthenes who realised that there was a particular well that the sun illuminated all the way to the bottom on a particular day. He knew that on this particular day, another well of another town did not have the sun directly above it.

He then realised that if he placed a few sticks on the ground and measured the difference in shadow angle, he could work out the circumference of the world.

For a much better explanation :-) check out this link http://www.dummies.com/education/math/geometry/how-to-determine-the-earths-circumference/

Flat earthers will tell you that you can get the same effect with two sticks on a flat plane; what they don't tell you is that you can't recreate the same thing with 3 sticks, and thus the theory breaks down again.

Cg