moon illusion
   Also known as Moon-size illusion and horizon illusion. All three terms refer to the Moon's apparently increased size when it approaches the horizon, in comparison with its perceived size when it is in the zenith. The ratio of this apparent increase in size lies around 2, although larger ratios (referred to as 'super illusion') have also been documented. This *size illusion is not restricted to the Moon. As it is also known to occur in relation to the Sun and other heavenly bodies, the generic term for this type of *illusion is * celestial illusion. It has been suggested that the apparent increase in the size of celestial bodies approaching the horizon was known as far back as prehistoric times, and that it was registered in written form as early as the seventh century BC by the Assyrians. In his book Meteorology, the Greek philosopher Aristotle (384-322 BC) speaks of "the sun and stars which at their rising and setting appear larger than at the meridian." Since Aristotle, this phenomenon has been described and analysed by numerous authors. Until the 19th century, the * Sun illusion would seem to have attracted more interest than the Moon illusion. However, today the Moon illusion is generally considered the archetypal representative of the group of celestial illusions. Throughout history, perhaps a 100 different explanations for the mediation of celestial illusions have been put forward. Starting with Aristotle, they have long been attributed to atmospheric mechanisms (referred to as the optical or atmospheric refraction hypothesis). As a corollary, celestial illusions were for a long time designated as *physical illusions (i.e. illusions stemming from physical rather than physiological or psychological mechanisms). Another classical explanation, called the size contrast hypothesis, involves the relative proportions of distant objects (such as mountains or buildings) and celestial bodies perceived in their proximity. To this day, the mediation of the Moon illusion has not been fully explained. It has been suggested that size contrast may account for some 40% of the illusion, and that oculomotor commands, angle of regard, and body posture may attribute another 10%. An additional proportion of unknown size is attributed to conflicting spatial representations in different visual pathways (i.e. the * corollary discharge hypothesis). A competing theory suggests that the Moon's * magnification at the horizon may be largely due to * oculomotor macropsia, a macropsia illusion occurring when objects at a distance of 1 m or more are observed while the eyes remain in their resting focus (i.e. dark focus) position, meaning that they are focused at a distance of about 1 m. In short, it is still uncertain whether the Moon illusion should be regarded as a * physiological illusion, a * cognitive illusion, or possibly a combination of the two.
   References
   Aristotle (1984). Meteorology.In:The complete works of Aristotle. The revised Oxford translation. Volume 1. Edited by Barnes, J. Princeton, NJ: Princeton University Press.
   McCready, D. (1985). On size, distance and visual angle perception. Perception & Psychophysics, 37, 323-334.
   Ross, H., Plug, C. (2002). The mystery of the moon illusion. Exploring size perception. Oxford: Oxford University Press.

Dictionary of Hallucinations. . 2010.

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