I've looked around a bit (the history today article is behind a paywall now sadly, I remember it being wuite thorough) and most text i can find tends to phrase pretty explicitly '[the knowledge of the round earth was very widespread] among educated people' or in a smiliar vain. Does this mean that common folk believed in the flat earth, and are there any estimates on how big the share of people believing in a flat vs round earth (or maybe vs something else as well) was?
In a smiliar vain, how common was it for such knowledge to be passed from for educated people to common folk? I am aware that it's probably not very useful to generalize for all of european people in medival times, but are there notable trends on how much 'scientific' information was passing from educated circles to lower ones? Should I imagine knowledge to be rather elitist in medival europe? Again, are there notable trends regarding time and/or place?
I've written about the understanding of a spherical earth a few times before:
https://www.reddit.com/r/AskHistorians/comments/3t85dc/how_did_the_myth_of_the_flat_earth_become_so/
Caveat emptor, that bottom thread isn't super great but gets to the issue of skepticism over the argument.
Anyhow, it's probably the case that folks who were uneducated were familiar with the concept in general. To quote from that top answer:
- the Elucidarium written by Honorius Augustodunensis around 1120 refers to the Earth as a sphere. The Elucidarium was a manual for the instruction of clergy, and was translated into multiple languages. (I'm copy-pasting these from Wiki because I don't feel like retyping them: "Middle English, Old French, Middle High German, Old Russian, Middle Dutch, Old Norse, Icelandic, Spanish, and several Italian dialects.") The lesser clergy were those most likely to preach and minister directly to commoners, of course.
- We have surviving sermons from Bertold von Regensburg, a German cleric, that refer to the Earth as spherical (not in the sense that he's revealing a great mystery of the Earth, but as an analogy that his hearers would be familiar with). His sermons were in the vernacular German dialect of his time period (mid-13th century), so there's evidence that he was using that metaphor in the local language that the local people would be aware of.
The question that early explorers faced, of course, wasn't so much whether the world was a sphere as it was how big it was -- Columbus in particular overestimated the size of Eurasia and underestimated the Earth's circumference. I wrote about that some here.
A fairly accessible source on this (easy to read and any university library should have it) is Ronald H. Fritze's New Worlds: The Great Voyages of Discovery, 1400-1600.