HINTS： 

Eratosthenes 

Alexandria

June the 21st. 

It was an observation that someone else might easily have ignored, sticks, shadows, reflections in wells, position of the sun, simple, everyday matters. Of what possible importance might they be? But Eratosthenes was a scientist and his contemplation of these homely matters changed the world, in a way, made the world. Because Eratosthenes had the presence of mind to experiment to actually ask whether back here, near Alexandria, a stick cast a shadow near noon on June the 21st. And it turns out, sticks do.

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