Greek mathematician Eratosthenes calculated the circumference of the Earth very accurately around 240 B.C. His figure of 25,000 miles was off about 100 miles from the modern calculation. There’s no question that educated ancients knew the Earth is a sphere. Eratosthenes was head Librarian at the great Library of Alexandria. A great deal of knowledge was lost when it was burned but Eratosthenes’ measurement was preserved and widely known.
The calculations of the Earth’s size by Eratosthenes and and Ptolemy differed considerably, with Ptolemy thinking it was something like 1/3 smaller.
The actual size calculated by either is debatable, since we aren’t sure of the exact length of the unit of measurement, the stadion, they used. There were several different stadions in use at the time.
Eratosthenes is highly accurate only if you make all possible assumptions in his favor, and the accuracy may be partly a matter of luck. But it was still an amazing feat of mental gymnastics, given the highly limited tech he had to work with.
Columbus believed Japan was only about 5000 kilometers from Europe, or claimed he did to get his financing, using figures closer to those of Ptolemy. The actual distance is about 20,000 kilometers.
IOW, Chris claimed Japan was about where the eastern Caribbean is. Had the Americas not been there, he and all his crew would have died long before they reached Asia. The Spanish government skeptics, using Eratosthenes figures, were actually correct.