This is one that's bothered me for a while. I've posted it before, and didn't get an answer... So I was prepared to leave it, but this burning question got into my dream last night...
It's basically, when did the majority of people know that the Earth wasn't flat?
I remember in school being told that Christopher Columbus proved that the Earth was round by circumnavigating the globe. But I have been told repeatedly ever since that that's not true: eveyone has pretty much always known that the Earth was round. But that always makes me wonder "who's, everyone?" and "really, always?"
For instance, I remember hearing that the ancient Greeks knew that the Earth was round, because when they watched a ship approaching, they saw the sails rising over the horizon first. But what would the average ancient Grecian have said if you'd asked him the shape of the world.
Or in the 15th century when Christopher Columbus famously didn't prove that the Earth wasn't flat, did the average Spanish peasant really know that the Earth was round.
When did it become that average people had that level of scientific literacy?
The reason you didn't get an answer is that demographic studies of that era is very difficult and there is a high degree of uncertainty, even for well-studied regions like Western and Central Europe. So if you want a global survey, you'll have to accept a very very high degree of uncertainty. And if on top of it you want a comparative study of what is "common knowledge", that makes it exponentially more difficult because most people were illiterate. This is not to say they were dumb, but rather that it becomes even more complicated to assess statements such as "when did xxxx become common knowledge" or "what did the average people of yyyyy think of zzzzzz".
I'm glad that you already know about the Flat Earth Myth. Have you read any of the other discussions re: the Flat Earth Myth?