Eratosthenes

Greek mathematician and chief librarian of the Library of Alexandria - the largest library of the ancient world.

Around 240 BCE he measured the size of the Earth with a stick, a well, and some geometry. On the summer solstice the Sun shone straight down a well in Syene (modern Aswan) while in Alexandria, due north, a stick cast a shadow of about 7.2° - 1/50 of a circle. If the 5,000-stadia distance between the cities is 1/50 of the way around, the whole circumference is 250,000 stadia: about 39,000-40,000 km, within ~2% of the modern 40,075 km. No telescope, no spacecraft - one angle, one distance.

Built on the Greek tradition that the Earth is a sphere (the curved shadow it puts on the Moon during eclipses). Enabled the entire science of geodesy - and a unit still in daily use: the nautical mile is exactly one minute of latitude, the same angular step Eratosthenes used.