Eratosthenes (about 273BC-about 192BC)
- Astronomer born in north Africa, who lived mainly in Egypt where he produced a fairly accurate estimate of the Earth’s diameter, accurate to within about 20 per cent. He did this by noticing that the Sun is exactly overhead at Aswan (Syene) in Egypt at the summer solstice, but at a measurable angle from the vertical at Alexandria, due north. The known distance between the two allows the total size of the Earth to be determined provided Eratosthenes assumed that the Sun is infinitely distant and the Earth round, both assumptions being correct for this purpose. He also measured the obliquity of the ecliptic to the equator.