How could getting the circumference right tell you there is a continent in between? Maybe he wouldnt have done it if he knew how far it was, but the land trip was dangerous so who knows.
Well, at the time of Columbus' voyage, people re-discovered that the Greeks were hot as shit when it came to thinking smart and knew the circumference of the planet. Columbus, however, after reading Marco Polo's Journals and getting Eratosthenes completely wrong thought that the distance between Europe and Asia was only 3,000 miles.
So to shut him up and maybe earn some scratch, Queen Isabella decided to finance his expedition with the Santa Maria, two glorified rowboats, and a crew of criminals.
Once they got out about 3,000 miles, they struck land, and Columbus thought that proved that he wasn't wrong, even insisting that he reached the East Indies up to his death in Debtor's Prison.