ABSTRACT
The moon illusion is the preeminent example of the size-distance paradox. Moonwatchers almost invariably report that the moon at the horizon looks larger and closer than the moon at zenith. Many observers report similar paradoxical changes in perceived size and perceived distance when viewing the setting sun (the sun illusion) or when viewing distant mountains over an expanse of terrain. Examples of the size-distance paradox produced in the laboratory include the "S-motion" demonstration (Kilpatrick & Ittelson, 1953), accommodation-convergence microposia/macropsia (Biersdorf, 1966; Heinemann, Tulving, & Nachmias, 1959; Komoda & Ono, 1974; Leibowitz & Moore, 1966; McCready, 1965; Ono, Muter, & Mitson, 1974; Wheatstone, 1852), and the spiral aftereffect (Gates, 1934; Hershenson, 1982; Holland, 1965; Scott & Noland, 1965; Wohlgemuth, 1911). 1
The situations that produce the size-distance paradox are strikingly similar. In each instance, paradoxical perceived size-perceived distance changes appear when an object subtending a fixed visual angle is viewed in the context of a stimulus that alters the object's perceived distance. In the "S-motion" demonstration, the visual angle subtended by the laterally moving object (playing card) remains constant in the context of a visual space whose qualities of perceived relative depth are altered by the trapezoidal window. In studies of microp-
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sia/macropsia, visual angle remains constant and changes in perceived distance are induced by changes in accommodation and/or convergence. In the spiral aftereffect, the test stimulus subtends a fixed visual angle and perceived distance changes are induced in the aftereffect as part of the afteractivity of the visual system. In the moon illusion, the moon subtends a constant visual angle regardless of elevation, but the terrain and the equidistance tendency produce different perceived distances when the moon is at different elevations.
