There seem to be many theories, people even write books about this:<p><i>"For over 100 years, research on the Moon illusion has been conducted by vision scientists who invariably have been psychologists specializing in human perception. After reviewing the many different explanations in their 2002 book The Mystery of the Moon Illusion, Ross and Plug conclude "No single theory has emerged victorious". The same conclusion is reached in the 1989 book, The Moon Illusion  edited by Hershenson, which offers about 24 chapters written by different illusion researchers."</i><p><a href="http://en.wikipedia.org/wiki/Moon_illusion" rel="nofollow">http://en.wikipedia.org/wiki/Moon_illusion</a>