[jcranmer]

As a general rule of thumb, "popular common knowledge" of the Medieval and Early Modern periods tends to be the complete opposite of the truth often enough that anyone who cites to popular knowledge for this period is very suspect in their use of sources.

It really doesn't help when you start your citation to this by comparing it to flat earth--no educated person in the Middle Ages would believe it to be flat; even the standard textbooks of the day (e.g., Ptolemy, Aristotle) would give you several proofs of its spherical nature. It's even mentioned on Wikipedia's List of Common Misconceptions, which is a clear sign you should purge it from your list of comebacks.