Video: How Eratosthenes calculated Earth's circumference
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.
More Astronomy Videos
Dark Energy Rules the Universe: Eric Linder (Nov 2008)
First Pictures of a Black Hole
Kim Griest: The Mystery of Empty Space
Target earth: the asteroid impact history of Australia, with Dr Andrew Glikson
Dark Energy, or Worse: Was Einstein Wrong?