That man was Eratosthenes. A Greek mathematician and the head of the library at Alexandria. Eratosthenes had heard that in Syene, a city south of Alexandria, no vertical shadows were cast at noon on the summer solstice. The sun was directly overhead.

It turns out there was actually one. And it measured about 7 degrees. Now, if the sun's rays are coming in at the same angle at the same time of day, and a stick in Alexandria is casting a shadow while a stick in Syene is not, it must mean that the Earth's surface is curved.

And Eratosthenes probably already knew that. The idea of a spherical Earth was floated around by Pythagoras around 500 BC and validated by Aristotle a couple centuries later.

Eratosthenes hired a man to pace the distance between the two cities and learned they were 5,000 stadia apart, which is about 800 kilometers. He could then use simple proportions to find the Earth's circumference — 7.2 degrees is 1/50 of 360 degrees, so 800 times 50 equals 40,000 kilometers. And just like that, a man 2200 years ago found the circumference of our entire planet with just a stick and his brain.

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