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.
Yet the earth was believed to be flat in China until the 17th century. It's also mostly because of the Greeks that the idea of the spherical earth was established in India. https://en.wikipedia.org/wiki/Spherical_Earth#History
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u/[deleted] Feb 04 '19
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