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.
There is no curvature, so don't know what exactly did you saw.
Can you provide us proof or evidence that can back up your claim?
Like this:
Commercial Airliner USA LAX to Frankfurt Germany 5,800 mile trip NO CURVATURE https://www.youtube.com/watch?v=jB5YwfUcdWI
LAX to PHX Where is the Curvaturehttps://www.youtube.com/watch?v=3Nbg00TJqJo
There is also footage from concorde, no curvature, high altitude balloons, no curvature. Actually tons of them, all without curvature.
There are a reasonable number of videos and stills that show the curvature of the earth from high altitude. Many are listed right in this thread. Why do you want a repeat?
