Eratosthenes had proved the Earth was round and calculated it's circumference to within 1% of it's actual circumference(he calculated 1°, i.e. 1/360, of circumference to be 59.5 nautical(6076 ft) miles) in ~200BC and his writings were well known among educated people at the time(And since universities were still primarily religious and geared towards theology, this included the Church leadership). Columbus actually massively underestimated how far west Asia due to 2 mistakes:

1. He preferred(because it was newer) the circumference calculated by the Persian geographer Alfraganus, who had calculated 1° to be 56.67 miles. However, he was using Arabic miles (7091 ft), which Columbus didn't know about, so he assumed that those were Roman(4856 ft) miles and thought that the Earth's circumference was 3/4 of it's actual size.
2. He "interpreted" Marco Polo to mean that Asia stretched 8000 miles further east than it actually did.

In combination, this made sailing West to Asia(without the Americas in the way) a workable proposition from a 101% suicide mission. After been repeatedly told that he's an idiot by people in the know, Columbus famously convinced queen Isabella of his idea and the rest, as they say, is history.

That's why, to his last breath, he refused to believe he had discovered a new continent, because that would make him a lucky fool and the continent is named America and not Columbia.

Eratosthenes had proved the Earth was round and calculated its circumference to within 1% of its actual circumference  (he calculated 1°, i.e. 1/360, of circumference to be 59.5 nautical  (6076 ft) miles) in ~200BC and his writings were well known among educated people at the time  (And since universities were still primarily religious and geared towards theology, this included the Church leadership). Columbus actually massively underestimated how far west Asia due to 2 mistakes:

1. He preferred  (because it was newer) the circumference calculated by the Persian geographer Alfraganus, who had calculated 1° to be 56.67 miles. However, he was using Arabic miles (7091 ft), which Columbus didn't know about, so he assumed that those were Roman  (4856 ft) miles and thought that the Earth's circumference was 3/4 of its actual size.
2. He "interpreted" Marco Polo to mean that Asia stretched 8000 miles further east than it actually did.

In combination, this made sailing West to Asia  (without the Americas in the way) a workable proposition from a 101% suicide mission. After been repeatedly told that he's an idiot by people in the know, Columbus famously convinced queen Isabella of his idea and the rest, as they say, is history.

That's why, to his last breath, he refused to believe he had discovered a new continent, because that would make him a lucky fool and the continent is named America and not Columbia.