Whether at sea-level, the top of Mount Everest, or flying 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 right at eye level and remain completely flat all around.  If the Earth were actually a big ball, however, the horizon …