Well, I guarantee you can see the curvature of the earth at 55,000'
There is that. Bit lower actually.
Ah! See, the dedicated Flat Earther will tell you that it is an optical illusion brought on by the effects of altitude, the curvature of the window or the camera lens, blah, blah. . .
However, once you've gone down this bunny hole, nothing makes sense anymore. The other thing you have to understand is the purveyors of this crap don't believe it either. However, if you buy into it, you are checking your logical thinking at the threshold.
I have a 10" Dobsonian telescope that I sometimes bring out for some stargazing. Years ago, a telcom salesman lived down the street, and he used to come up and watch me do stuff. That, in and of itself, was weird. He'd never pick up a hammer or anything, but he enjoyed watching me do home improvement.
Anyway, Steve saw me out in my driveway one night and he came up to see what I was doing. He and his wife had just joined a new church that was deeply into anti-Evolution, anti-this and anti-that. It was a clear moonless night in the winter and I was trying to see what I could see. He stood and watched me for a while, and I described things to him like the Andromeda Galaxy and such. I offered to have him take a look. He stiffened.
"No," he said. "I'm sure if I looked in there, it would be nice, but I'd see things that would make my life more complicated. I don't need complicated. I like simple answers."
