The idea that the Earth is spherical originated in Ancient Greece, and the oldest reliable sources credit Pythagoras from the 6th century BCE. For seafaring people who navigated by the stars, though, the proof of a spherical world was abundant throughout antiquity.
As travellers sailed south, they saw constellations rising higher above the horizon. During a lunar eclipse, they could trace the circular shape of the Earth’s shadow on the Moon.
Not to mention ships returning to harbour: how else to explain why the tip of the mast was always the first bit to appear on the horizon?
The first person to prove a spherical Earth was Portuguese explorer Ferdinand Magellan (left), whose expedition circumnavigated the globe in 1522, though sadly Magellan died before reaching the end.