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Moon illusion: a new perspective

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Abstract

An illusion reported from classical times is that the horizon moon or sun is much larger than the zenith view of the same object. Explanation has been sought for this illusory magnification of the horizon object by examining the object color, position, and movements of the head or eyes, neural-muscular responses of the observer, sky and ground cues, etc. For the situation reported here, the sun or moon is viewed from an airplane at an altitude of 7 km or more with the head and eyes depressed and is seen as a very small, very red object on an unobstructed horizon. This horizon moon viewed from the air, and the zenith moon viewed from the ground, have in common the absence of a continuously textured field extending from the observer to the horizon, just those cues which prevent empty field myopia. We infer this absence of cues needed to reduce accommodation causes underestimation of the size of distant objects. The analysis, together with a review of the literature, indicates that the classical moon illusion is one of reduction in apparent size of the zenith object rather than overestimation of the size of the horizon object. Indeed, relatively proper judgments about the size of horizon objects are made by observers on the ground who have an unobstructed view to the horizon. The sun and moon really are large, but they appear that way only when viewed in a continuous environment. The moon illusion is considered here to be a part of a more general visual distortion, the toy illusion, in which large, distant objects are judged to be small when they are seen in conditions tending to produce empty field myopia. Thus, the size constancy normally recorded when ground objects are viewed from the ground is lost when these objects are viewed from an airplane at some altitude that separates the viewer from the environment. These ordinary objects are then perceived to be toys rather than full-sized familiar objects.

© 1985 Optical Society of America

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