SUNLIGHT 7.2° 7.2° 5,000 stadia Alexandria Syene
EGYPT · c. 240 BC
Eratosthenes measures the Earth
I.
At noon on the solstice, the Sun shines straight down a well in Syene.
II.
In Alexandria, an obelisk casts a shadow of 7.2°.
III.
7.2° is 1/50 of a full circle.
IV.
50 × 5,000 stadia = 250,000 stadia — close to the true 40,000 km.
Angles exaggerated for clarity