Carl Sagan - Cosmos - Eratosthenes (2009)

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Carl Sagan - Cosmos - Eratosthenes (2009)

In the very first episode of Cosmos, Sagan addressed the flat-earthers, indirectly, by explaining how Eratosthenes (276-194 BC), a Libyan-Greek scholar and chief librarian at the Library of Alexandria, discovered over 2000 years ago that the earth is a sphere. Given the geographer, mathematician, poet, historian, and astronomer’s incredible list of accomplishments—a system of latitude and longitude, a map of the world, a system for finding prime numbers—this may not even rank as his highest achievement. In the Cosmos clip above, Sagan explains Eratosthenes’ scientific method: he made observations of how shadows change length given the position of the sun in the sky. Estimating the distance between the cities of Syene and Alexandria, he was then able to mathematically calculate the circumference of the earth, as Cynthia Stokes Brown explains at Khan Academy. Although “several sources of error crept into Eratosthenes’ calculations and our interpretation of them,” he nonetheless succeeded almost perfectly. His estimation: 250,000 stadia, or 25,000 miles. The actual circumference: 24,860 miles (40.008 kilometers). No, of course the Earth isn’t flat. But Sagan’s lesson on how one scientist from antiquity came to know that isn’t an exercise in debunking. It’s a journey into the movement of the solar system, into ancient scientific history, and most importantly, perhaps, into the scientific method, which does not rely on hearsay from “global elites” or shadowy figures, but on the tools of observation, inference, reasoning, and math. Professional scientists are not without their biases and conflicts of interest, but the most fundamental intellectual tools they use are available to everyone on Earth.