Wikipedia’s List of Common Misconceptions
Christopher Columbus’s efforts to obtain support for his voyages were not hampered by a European belief in a flat Earth. In fact, sailors and navigators of the time knew that the Earth is spherical, but (correctly) disagreed with Columbus’ estimates of the distance to India. If the Americas did not exist, and had Columbus continued to India (even putting aside the threat of mutiny he was under), he would have run out of supplies before reaching it at the rate he was traveling. The problem here was mainly a navigational one, the difficulty of determining longitude without an accurate clock. This problem remained until inventor John Harrison designed his first marine chronometers. The intellectual class had known that the Earth was spherical since the works of the Greek philosophers Plato and Aristotle. Eratosthenes made a very good estimate of the Earth’s diameter in the third century BC.