I had an experience with a flat earther who was dumb obviously, because he believed that the earth was flat. Well I took him and my telescope to a far point on the coast and pointed toward a lighthouse that according to my calculations should be all below the horizon with the top of the tower perking up. Well, sure enough, that lighthouse was sticking out of the horizon like a penis it was a great sight, you can actually see the top of the water close then at distance behind the lighthouse. Even that dumb guy left there finally convinced that the earth is not flat. You can try it anywhere on earth, much easier where there is a coast, and see with your own eyes. But then again, every time i hear the argument that there are millions who believe in it, I think of that George Carlin part about stupid people, and is true, half of the people are below average...