Whether at sea-level, the top of Mount Everest, or flying over a hundred thousand feet in the air, the always horizontal horizon line always rises up to meet the eye-level of the observer and remains perfectly flat. You can test for yourself on a beach or hilltop, in a large field or desert, aboard a hot-air balloon or helicopter; you will see the panoramic horizon ascend with you and remain completely level all around. If the Earth were actually a big ball, however, the horizon should sink as you ascend, not rise to your eye-level, and it would dip at each end of your periphery, not remain flat all around. Standing in a rising balloon, you would have to look downwards to the horizon; the highest point of the ball-Earth would be directly beneath you and declining on each side.
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