How a stick's shadow measured the whole Earth: Eratosthenes, a well and 5,000 stadia
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Two sticks, one shadow and one distance were enough for Eratosthenes to size the whole planet around 240 BC. On the summer solstice the noon Sun shone straight down a well at Syene, yet a stick at Alexandria still threw a shadow, and that shadow was one fiftieth of a circle.
Drag a stick around a globe to see why a shadow angle is a slice of the Earth, then turn it into a circumference and test how close it lands, depending on which Greek stadion he meant.
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