Moon Illusion

From Michael’s “Optical Illusions & Visual Phenomena”

The “moon illusion” lets objects near the horizon look larger than when the same object is high on the sky.

The moon in the neighbouring picture is of “aesthetically correct” size. By placing the mouse over the picture the actual size is seen (picture by Henri Daumier, after an idea by Rock 1984).

Comment. The moon illusion is usually explained through size constancy, with the additional –and necessary– assumption that the ‘default’ distance for any object is less than the horizon/skyline distance.

Sources

Rock I, Kaufman L (1962) The Moon Illusion, I: Explanation of this phenomenon was sought through the use of artificial moons seen on the sky. Science 136:953–961

Rock I, Kaufman L (1962) The Moon Illusion, II: The moon's apparent size is a function of the presence or absence of terrain. Science 136:1023–1031

Rock I (1984/1995) Perception. New York: W.H. Freeman, Scientific American Library

Kaufman L, Kaufman JH (2000) Explaining the moon illusion. PNAS 97:500–505 [PDF]

Ross HE and Plug C (2002) The mystery of the moon illusion: Exploring size perception. Oxford University Press. ISBN 0 19 850862 X

Donald E. Simanek has a nice overview of moon-illusion explanation attempts

Amazingly, when you bend over and look through your legs the moon illusion is greatly diminished, underlining the influence of context:
Higashiyama A, Adachi K (2006) Perceived size and perceived distance of targets viewed from between the legs: Evidence for proprioceptive theory. Vision Res 46:3961–3976