jesus christ, even in the old ages its a myth people thought it was flat. people had the same mental capacity as now pretty much, 
How do you know, where you there?
well for example, Eratosthenes in about 200BC calculated the size of the earth, the tilt axis of the earth, etc.
Don't forget to mention Errortosthenes' calculations also work for a flat stationary Earth with a small Sun orbiting 3,000 miles above!