Next time you're in a jet at 36,000' on a clear day, observe the horizon is about 3-1/2° below level. You need a level reference such as a water surface or pendulum, you need to be unaccelerated so level is referenced to gravity, and it must be clear enough to actually see a land horizon. This observation checks with the trig where the diameter of a sphere is about 4000 n.m. and you are 6 n.m. above it.
Another observation you can make is that converging air traffic at a lower altitude will appear above the horizon when distant. Level traffic 1000' below when you're in the 30's will cross the horizon at just over 3 miles distance before passing under you. This is consistent with the horizon being below horizontal because you're looking down at a sphere.
Another is that climbing at sunset will make the sun rise again. This is consistent with sphere shape of earth and you peeking over the hump.
Yet another is that a tall tree or radio tower that is below you will appear above the horizon when distant. A tree on the horizon literally stands above it.
Yet another is that ships on the level sea surface disappear from the bottom-up as they sail away.
Water level check, excellent. That's an easy one anyone can do. Even phone camera apps have options to show the horizon based on the orientation sensor.
Yet another is that ships on the level sea surface disappear from the bottom-up as they sail away.
There are endless terrible YouTube videos claiming to show otherwise.
Ships do not disappear over the curve. they can be brought back into focus using a powerful lens. there are videos available showing this. Rising in height will naturally allow you to see further, but distant vision is limited by atmospheric occlusion and the resolution our eyes can see at. Part of what i find so strange about this whole argument is people typically refer to moving lights in the sky to determine the shape and movement of the ground below our feet. Shouldnt there be a better way? Perhaps measuring the physical curvature or motion instead of relying on so called distant lights?
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