The only six-letter name that comes to mind is Kepler, but by that time, the spherical earth was pretty much a given. Most of his work was concerned with planetary motion, cleverly deducing the orbit of mars and helping cement heliocentric theories.
The identity of the first person to consider the earth round is pretty much unknown, since it is not unreasonable to assume that such thought predated even the greeks. Aristotle was the first to go to some lengths to prove his assertion that the earth was spherical, when he pointed out the appearance of southern constellations in relation to the horizon, or that ships disappeared from view because of the earth's curvature.
Many general trivia questions are framed in such a way as to dismiss pre-rennaisance developments or discoveries made by nonwestern scientists (the tremendous leaps made by arab mathematicians around the turn of the first millenium, for instance).
It's easy enough to prove that the world is spherical, at least to yourself. Believing that it is flat requires many leaps of faith, and a determination to ignore any evidence that disputes your hypothesis. So, congratulations to the flat-earthers, they probably go to great lengths to sustain their beliefs.
I wonder if any of them ever considered shelling out twenty million dollars to go into orbit, the struggling Russian space agency would be quite happy to take up on the offer.