todd@islenet.UUCP (03/29/84)
>Subject: Re: An oldie revisited... >Message-ID: <942@sdcrdcf.UUCP> >Date: Sat, 24-Mar-84 10:21:39 HST >Date-Received: Wed, 28-Mar-84 07:12:57 HST >References: <2549@fortune.UUCP> >Reply-To: leeway@sdcrdcf.UUCP (Lee Gold) >Organization: System Development Corporation, Santa Monica >THE MOON ILLUSION >quoted from ELEMENTS OF PSYCHOLOGY by Krech & Crutchfield >The smaller apparent size of the moon when seen high in the sky >is related to the upward tilt of the eyes in the head when looking upward. >To check this, lie on a tilted board so you look directly at the moon >riding high in the sky without having to raise your eyes in your sockets. >The moon will look as large as it does on the horizon. >The primary source of this (disillusioning?) information is >E. G. Boring, "The Moon Illusion," American Journal of Physics II, >1943, pp. 55-60. This explanation of the Moon Illusion was accepted for quite a long time (up until the 1950s). However, counter evidence later discounted it. There are still a number of more-or-less competing theories. The theory I subscribe to follows. Several "facts" first: 1. The moon appears larger when "on the horizon" than when it is zenith. This is the Moon illusion. 2. The sky is not a perceptually perfect hemisphere. Instead, the sky is perceived to be flattened at the top. Therefore, the top of the sky appears closer than the sky at the horizon. 3. For all practical purposes, the physical distance of the moon nearly the same at zenith and at horizon. Therefore the retinal size of the moon image is the same at both sky positions. 4. A property known as "size constancy" allows us (humans) to perceive a given object as "the same size" at different distances despite changes in the size of the retinal image. E.g., a six foot man is perceived to be six feet tall whether he is standing 10 feet away from you or 20 feet away from you. Take a look at the telephone poles lining a long straight stretch of road for another example of size constancy. Size constancy does have limits though. One situation in which it breaks down is when you are in a flying airplane looking down at the ground. Note that the objects on the ground, houses, cars, etc. appear "toylike." Now back to the explanation: Recall from 2 that the zenith sky is nearer than the horizon sky. Since the moon is perceptually coplanar with the sky plane the zenith moon is perceptually closer than the horizon moon. Recall from 3 that the retinal image of the moon in all sky positions remains the same. Finally, recall from 4 that a process called "size constancy" attempts to correct perceived size when the distance of an object changes. Given that the horizon moon appears farther than the zenith moon, a cognitive correction occurs which causes the horizon moon (which is the same retinal size as the zenith moon despite being perceptually further away) to appear larger than the zenith moon. This cognitive correction which leads to a perceptual error should not be construed as a flaw in the human visual processing system. It is, instead, an artifact of a usually useful correction factor: size constancy. For instance, a 5 foot person standing 20 feet away from you subtends the same visual angle (retinal image size in the vertical dimension) as a 7 foot person standing 28 feet away. However, you correctly perceive the 7 foot person to be "taller than" the 5 foot person. Todd Ogasawara -- University of Hawaii -- Dept. of Psychology !vortex!islenet!todd or !dual!vortex!todd