As to the Earth being flat, only landlubbers ever thought that. Seafarers throughout history knew otherwise.
Actually not even landlubbers thought that - the flat earth thing is only an 18th century invention, really. A gag that went too far.
The fact of the Earth being spheroid had been known since at least Ancient Greece - Columbus (and others) merely thought it was a lot smaller than it is, and didn't anticipate there being enough room for a whole extra continent...