// ... prior to the first circumnavigation, sailors believed the earth was flat, and they would fall off it. //
Like I said above, the flaw here is stating a niche belief as universal. Very few sailors, if any, believed that the world was flat in the 1500s. It's simply a nonsense to report this as fact. Ditto the following:
// The first recorded circumnavigation was in 1519 - and there are plenty of maps drawn before then showing the earth as flat and square. //
Again, that's taking too literally what's on a page. The debate in the 1400s in particular was over the size of the Earth, not its shape. For example, the mappa mundi may look flat, but in fact they are (a) not meant to be literal maps, and (b) were still based on the notion of an Earth being spherical.
You're mistaken, then, in implying that the belief of a Flat Earth was prevalent until its refutation via circumnavigation.