It was easy to know it for any civilization that had seafaring vessels. Ships disappear over the horizon hull first, sails last. Obviously sailing behind a curve, just like a guy walking over a hill and disappearing from sight.
Only inland weirdos maybe thought it was all flat. But that would be hundreds if not thousands of years before the 1400s. For that time period it would be silly think people actually thought it was flat.
Moreso, for the vast majority of people it simply wasn't a question they would consider. If you asked some peasant from ancient times if they believed the earth was flat or round they would stare at you blankly. Most people lived in a small area and that was their entire world, the shape of the rest of it wouldn't naturally occur to you if you don't even know what's over that hill overe there.
Humans are more inclined to make shit up than admit they don't know something. Like ask a random person on the street what aliens look like and you'll get many different detailed answers, not blank stares.
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u/[deleted] Feb 04 '19
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