cosmos

A short clip from Carl Sagan's Cosmos in which he explains how Eratosthenes measured the circumference of the Earth using shadows in two Egyptian cities. It is a clear, visual account of an ancient scientific feat.

An episode of Journey to the Microcosmos, hosted by Hank Green, about diatoms, single-celled algae with glass shells. It covers their role in producing a large share of Earth's oxygen and their uses in beer filtering, paint and kitty litter.