I have a friend that used to try to sell me the idea of a flat earth. As a terrestrial being, I had no way to fly up and verify his or my own theories, but my annoying logic would often pin him into a corner, where he would quickly change to a new talking point.

Finally I found a 'proof' that the world was spherical, in videos of sunsets in Hawaii, where the sun drops straight down behind the horizon. He admitted that such videos were the best evidence of a round earth that he'd ever seen, but then laughed that it was 'all I had'. It was all I had, but it was still more than he had offered as evidence of his flat earth. He eventually unfriended me for a while because I didn't easily accept his model. In those days, I had tried installing the magnifying lens, but it didn't solve all of the issues, so I abandoned the 'lens' theory and went back to the spinning ball. Since then I've been testing a world where we are on the inside wall of a double torus feedback device, where light, time and space are bent into what we perceive as a sphere, but it is quite abstract in my head still.

Search 'green flash' 'Hawaii' to see the videos I used as my 'only defense'. The curvature can be seen, but from our surface viewpoint it's seen as vertical curvature, NOT horizontal.