[shotgun]

Who discovered the earth was round?

[Brookhaven]

Eratosthenes determined the Earth's circumference in 240 BC.

http://www.windows2universe.org/citizen_science/myw/w2u_eratosthenes_calc_earth_size.html

[Arrian]

The spherical construct of Earth was argued by Thales w/o formal proof around 600 BC. Later Anaximander extended the concept of sphericality to all Planets. Both Aristotle and Galileo, among others, supported the concept. But realistically, the formal proof belongs to the the Portuguese Seafarers, such as Magellan, who circumnavigated the globe.