When you look outside of the window of a plane, if you are high enough, you can see the curvature of the Earth. Having seen this first hand, I think it justifies the roundness of the Earth without relying on third-party info.
They believe that all the airplane windows are intentionally made with a special curve built into them, so it appears that the earth is round when it is not. They think that the conspiracy of NASA goes this deep.
Also, if you happen to get some really flat windows on your plane, they will tell you it has to do with our general misconception about perspective.
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Have you ever received a Nigerian email? Lots of folks get hung up on the idea that they have friends over there in Africa, friends who need their money. So, they send them money.
I knew a guy like this, once. He was absolutely hung up on some woman who he had never even seen a picture of, hung up for at least months, maybe years, sent her many $thousands, and when I saw him last, was finally finding out the hard way that he had been had. I think the embarrassment was the reason he left town... couldn't face all his buddies.
We all have blind sides. Most of us can be made to see the truth by a few friends showing us where we went wrong. A few folks just can't see their mistake... usually because they are emotionally hung up on it... like a religion.
Flat-earthers have a blind spot against NASA or globular earth for some reason. It has become like a philosophy or religion in them. The handful of them that exist suggest that it is the majority of the rest of us who have the blind spot. It is difficult setting them straight without hurting their feelings, maybe in a big way.
