Question

how do people in ancient times know that the earth was round ?

Answer 1

There was different explanations they did not exactly knew that the earth was round some said that earth was flat

Hope this helps

Answer 2

Early Greek philosophers alluded to a spherical Earth, though with some ambiguity. Pythagoras (6th century BC) was among those said to have originated the idea, but this might reflect the ancient Greek practice of ascribing every discovery to one or another of their ancient wise men.