In the 3rd century BCE, Eratosthenes calculated the circumference of the Earth to high precision.
This is a short segment from the series Cosmos by the late Carl Sagan. The most poetic popularizer of science in the 20th century.
If you have never seen Cosmos, I strongly recommend it. It's full of great stories like this one. It is available on DVD, in 7 subtitle languages. Purchase links are below.
Spanish version are available from DVDGO
It is also available in Portuguese from the Brazilian web site, Loja Abril: