Math & science · c. 240 BCE

Earth's circumference measured

Shadow angles in two Egyptian cities give the size of the round Earth.

Eratosthenes compared the Sun's angle at Syene and Alexandria, 7.2 degrees apart, to estimate a circumference of 250,000 stadia, close to the true 40,000 km. It was the first measurement of a planet.

See it in the living tree Wikipedia

Arcs above join it to what it stood on, arcs below to what it led to. The bar places this stretch on the tree's whole timeline.

What it built on 1

Euclid's Elements 300 BCE

What it led to 1

Metric system 1795