Eratosthenes’ calculation.
(Science Week – Day 5!)
Today, a classic scene with Carl Sagan, storytelling in his seldom-matched starry-eyed yet down-to-Earth manner.
Simple as anything, but such a stark example of a critical mindset: once you understand how someone could really, truly work their way from everyday observations to the model of a round Earth, you understand what kind of substantiation to request for almost any claim about the world.