Many people have asked how high above the earth one must be to see the horizon curve, but I have yet to see a mathematical answer that correlates to the dimensions of the earth. As you rise above the earth the horizon continues to seem to be a circle around you, all in the same plane, with your eyes in the center and your line of sight the radius. But at some altitude suddenly your eyes seem to be above the center and you are looking down at the circle. At that point you see the curvature. Surely that magical altitude is some multiple of the earth’s radius, no?



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