Video explaining how fish-eye distortion works and how to account for it:
https://youtu.be/cHhDhYeYf2QCompilation of high altitude balloon footage from various different sources, analysed at the point where distortion is eliminated as explained in the previous video:
https://youtu.be/dV99TJ9KlJQSimple repeatable experiment that allows one to photograph the curve of the horizon from a cliff overlooking the sea, take note of the straight-edges remaining straight when the image is compressed horizontally, indicating that lens distortion is not the cause of the curvature:
https://youtu.be/tOCodgq1oM8 