Eratosthenes, a Greek geographer (about 276 to 194 B.C.), made a surprisingly accurate estimate of the Earth's circumference. In the great library in Alexandria he read that a deep vertical well near Syene, in southern Egypt, was entirely lit up by the sun at noon once a year. Eratosthenes reasoned that at this time the sun must be directly overhead, with its rays shining directly into the well. In Alexandria, almost due north of Syene, he knew that the sun was not directly overhead at noon on the same day because a vertical object cast a shadow. Eratosthenes could now measure the circumference of the Earth (sorry Columbus) by making two assumptions - that the Earth is round and that the sun's rays are essentially parallel.
I'm going to have to google more about that one!
(Working from memory here.)