I beg to differ, we've known no such thing. The idea that the earth is a ball is an extreme view because what you are saying is everything can be the right way up and upside down at the same time although we know that to be patently silly. Your only bit of evidence for this nonsense, it seems, is a picture you were told was taken from space. No one can show us how a body of water can bend outside of a container and therefore according to the scientific method means it's not demonstrable (a necessary criteria in the natural sciences, you'll agree). Your view is non-scientific and until you can demonstrate how to get a body of water to bend and stick to a ball is-at best-hypothetical. It's not enough to appeal to authority for your beliefs, you really do need to think about what you're being asked to believe. Then you might understand why the ancients took the view they did.
In my comment, if you were paying attention, you would have noticed I never mentioned the word 'belief'. The word I used, was 'respect' which is somewhat different. I don't think you are really trying to understand why ancient peoples believed the earth was flat simply because you feel those beliefs can be safely dismissed because you already believe you know the truth although you are only taking someone else's word for it. In other words your view on the subject is only a belief and until you can demonstrate what you claim, always will be.
