It would appear that you are the victim of another myth.  While different cultures came to the realization of the spherical nature of the earth at varied times, that fact was  well understood by educated people in 15th century Europe (before the Renaissance and the beginning of modern science). The ancient Greeks first wrote about it in 431 BC and by around 240 BC the Hellenistic astronomer Eratosthenes estimated the Earths circumference, using trigonometric principles, within a margin of error of less than 15%.