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M-T - "anyone who can see a horizon, especially at sea knows the world is round".
I would argue that if you know the world is round, then seeing a horizon enables you to understand that concept.
Simply seeing a horizon without the knowledge that the earth was not in fact flat would simply underline your received knowledge that the earth was flat, and what you could see was the edge of it.
It is simply not logical to assume that previous to proof of a journey from point A back to point A without 'falling off', anyone would think anything other than the obvious - backed up by their own eyesight - you can see the edge of the world out there - it may be a horizon to us, but not to them.