All planet's moons etc are round and there is no evidence that Columbus or anyone for that matter thought the earth was flat?!They are round because at the dawn of time (big bang) cravity pulled what was "debrey" together to form the center of the Eart and so on...For a full explanation don't wicipedea it!If you realy want to know and it's not just a random question?That won't keep you up at night!Do some proper reserch and not ask opinions?Columbus actualy thought the Earth was like erm a (lamen terms) Trebor chewie mint?! Im trying to simplify it as much as possible btw.Erm a "flying saucer space ship" if that makes any sence?Just look it up! :)