Eratosthenes
Notes
Eratosthenes of Cyrene (c. 276–194 BC), Greek polymath and chief librarian at Alexandria. Around 240 BC he measured the circumference of the Earth from the difference in the Sun's angle at noon between Syene and Alexandria and the distance between them, and came within a few per cent of the modern value. The Earth's shape was therefore not merely known but measured two millennia before anyone pretended otherwise. See Flat Earth.
Wikipedia: https://en.wikipedia.org/wiki/Eratosthenes