THE MOON ILLUSION AND THE APPARENT SIZE OF THE SUN

97th DOG Annual Meeting 1999

V277
THE MOON ILLUSION AND THE APPARENT SIZE OF THE SUN

Robert A. Crone

The moon illusion is still unexplained. The main problem is the apparent size of the moon and the sun in free space. Heraclitus sajd that the sun had the size of a human foot, which implies an apparent sun distance of about 40 m. That is also the outer limit of our (hyperbolic) stereoscopic space. According to G. ten Doesschate (Ned. Tijdschr.v Geneeskunde 74, 1930, 748), in a study of the non-linearity of Emmert's law, the apparent distance of the celestial vault in the zenith is similar. Above the horizon, thanks to depth vision from perspective, the sun and the moon can be localized at a greater distance and so are judged to be larger.

The moon illusion is therefore caused by the difference in structure of the stereoscopic and perspectival visual space. For this difference in structure Alhazen's 'flattening of the celestial vault' is an adequate metaphore.

Reguliersgracht 1, NL-1017 LJ Amsterdam

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