History

How a stick and a well measured the whole Earth: Eratosthenes

Dark diagram of the Earth with parallel sunrays, a stick at the top and a second stick at an angle along the surface, titled One stick, one well, the whole Earth
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Around 240 BC, the librarian Eratosthenes measured the size of the Earth using a well, a stick and a shadow. At noon on the summer solstice the sun shone straight down a well in Syene, yet a stick in Alexandria cast a shadow of about 7.2 degrees, one fiftieth of a circle.

Drag a city round the globe to see how the shadow angle grows with distance, then choose the length of a stadion to see how close his answer of 250,000 stadia comes to the real 40,075 km.

Deep dive · 4 min
Deep dive · 4 min

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