Eratosthenes by Sagan

Amicus Plato, sed magis amica veritas.

Miguel de Cervantes, Don Quichotte (1605).
How did we find out that Earth is a sphere? 

How in the same moment a stick in Syene (Assouan, nowadays) casts no shadow and a stick in Alexandria, 800 kilometers to the North, would cast a very substantial shadow? Carl Sagan explained how Eratosthenes provided a way to measure Earth’s circumference more than 2000 years ago… The Earth is not flat.

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