CELESTIAL ILLUSION

   The term celestial illusion is indebted to the Latin noun caelum, which means heaven. It is used to denote a group of *size illusions characterized by an apparent increase in the size of celestial bodies when these are perceived above the horizon (as compared to the way they are perceived in the zenith). This illusion would seem to apply to all celestial bodies, but the most famous examples of the celestial illusion are the * Moon illusion, the * Sun illusion, and a similar, apparently nameless phenomenon pertaining to the perceived distance between the stars in constellations such as the Great Bear and Orion. Celestial illusions have been known since ancient times. They are traditionally regarded as * physical illusions, i.e. illusions based on the physical properties ofthe celestial bodies themselves, and/or the surrounding atmosphere. Although today most experts would dispute this, it is still uncertain whether celestial illusions should be regarded as * physiological illusions, * cognitive illusions, or - most probably - a combination of the two. For a more detailed account of celestial illusions, see the entry Moon illusion. The notion of celestial illusion should not be confused with the notion of *autokinetic effect, which refers, among other things, to the illusory motion of stars.

   References

   Ross, H., Plug, C. (2002). The mystery of the moon illusion. Exploring size perception. Oxford: Oxford University Press.