Well, its not because the Moon itself changes its size. It has to do with how our brain uses information in the eye's visual field for finding clues about the sizes of things it is seeing. When the Moon is near the horizon, there are trees, buildings and other things that are used by the brain to give us an impression of how large the Moon is. But way up in the sky, there are no familiar objects for comparison, so the brain gives up and doesn't provide us with such a sense of proportion. There are many other visual illusions that our eye/brain system fall victim to. This particular one, called by the way, the Moon Illusion, is just one of many!
Copyright 1997 Dr. Sten Odenwald
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