Eratosthenes Measures the Earth

Comparing noon shadows in two Egyptian cities to compute the circumference of the entire Earth.

At summer solstice the Sun lit the bottom of a well at Syene while casting a 7.2° shadow at Alexandria.
That angle is one-fiftieth of a circle, so fifty times the city distance gave the circumference.
His answer of 250,000 stadia is within a few to fifteen percent of the modern value, depending on the stadion used.