Patrice Scattolin said:
Eratosthene measured the earth diameter somewhere around 200BC https://en.wikipedia.org/wiki/Eratosthenes#Measurement_of_the_Earth's_circumference
A better question to ask is when circular earth became dominant belief. That, I am not aware of good documentation. That said, Columbus knew the earth to be round so it wasn't super obscure knowledge.
My understanding is that nobody seriously disputed that. Rather, while the majority view agreed with Eratosthene's measurement, Columbus adhered to a minority that believed the diameter was rather smaller, meaning the distance from the west European coast to Japan was only about 4000 miles. As it turned out, Eratosthene was correct to within about 1%. -- Clive D.W. Feather | If you lie to the compiler, Email: clive@davros.org | it will get its revenge. Web: http://www.davros.org | - Henry Spencer Mobile: +44 7973 377646