For the umpteenth time, I'm reading in some book (no intention to pillory the authors here) that, say, 500 years ago people believed in a flat Earth, since, in the author's opinion, there is no apparent evidence of it being spherical. This is of course false, especially speaking about 500 years ago, when people built globes, circumnavigated Earth and so on. But it was, more or less, common-ish knowledge well before that: Dante describes how he and Virgil rotated their posture around the center of Earth, Aristotle wrote about it, ancient Greeks in general observed masts before hulls and measured the radius of Earth, and so on.
My question is: assuming that that author referred not to humanity as a whole but to the “average” person, would they be right? Is it possible to estimate which fraction of humanity knew about Earth being spherical in different ages?