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The Moon Illusion

Keywords: t-test, paired t-test


Why does the moon appear to be so much larger when it is near the horizon than when it is directly overhead? This question has produced a wide variety of theories from psychologists. An important early hypothesis was put forth by Holway and Boring (1940) who suggested that the illusion was due to the fact that when the moon was on the horizon, the observer looked straight at it with eyes level, whereas when it was at its zenith, the observer had to elevate his or her eyes as well as his or her head to see it. To test this hypothesis, Kaufman and Rock (1962) devised an apparatus that allowed them to present two artificial moons, one at the horizon and one at the zenith, and to control whether the subjects elevated their eyes or kept them level to see the zenith moon. The horizon, or comparison, moon was always viewed with eyes level. Subjects were asked to adjust the variable horizon moon to match the size of the zenith moon or vice versa. For each subject the ratio of the perceived size of the horizon moon to the perceived size of the zenith moon was recorded with eyes elevated and with eyes level. A ratio of 1.00 would represent no illusion. If Holway and Boring were correct, there should be a greater illusion in the eyes-elevated condition than in the eyes-level condition.

Variable Description

Subject Subject number, 1 to 10
Elevated Perceived ratio with eyes elevated
Level Perceived ratio with eyes level


Data File (tab-delimited text)


Holway, A. H., and Boring, E. G. (1940). The moon illusion and the angle of regard. American Journal of Psychology 53, 509-516.
Kaufman, L., and Rock, I. (1962). The moon illusion I.  Science 136, 953-961.
Howell, D. C. (1999). Fundamental Statistics for the Behavioral Sciences, 4th Edition. Duxbury Press, Pacific Grove, California.




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