Per our conversation in L02 today, a history of the people who knew the earth was round from Today I Found Out: "People in Columbus' Time Did Not Think the World Was Flat." (I should apologize, though, for overstating my point in class today. Some people before the 6th century BCE did think the earth was a round, flat disk. So, it isn't that no one thought the earth was flat...it's just that humans have know it was round for a really, really long time.)
Get this, though. Pythagoras and Aristotle and a bunch of people without spiffy telescopes or satellite images knew the world was a sphere, but there are still people TODAY who think the world is actually flat because there's a horizon. Thank goodness Neil DeGrasse Tyson is around to set things straight.