One of the more complicated problems of perception is that of the moon illusion. For most people, when they look at the moon on the horizon, it appears larger than it does when it is high in the sky. Although many explanations have been suggested for the moon illusion none have, so far, been completely acceptable to scientists. The most common explanation is the 1962 theory by Kaufman and Rock. Based on the principle of size constancy, and the apparent-distance hypothesis of Ptolemy, they propose that when we view the moon on the horizon, we compare it to the objects on the terrain, such as houses, trees, and telephone poles. Since our perception of distance tells us that the horizon is farther away than the open sky above, objects seen in relation to the horizon appear larger than those in the apparently closer open sky. However, the moon is actually the same size in both cases.
Try this: in a sheet of paper, cut a hole just large enough through which to see the moon on the horizon. Later on, when the moon is higher in the sky, look at it through the same hole. In both cases, the view of the moon will fit the same-sized hole.
For students interested in examining this phenomenon further, we suggest you visit the site of Professor Emeritus Don McCready of the University of Wisconsin-Whitewater at: http://facstaff.uww.edu/mccreadd. He has kindly contributed his comments on this illusion. His site, while lengthy, is a detailed and scientifically based explanation of the illusion.
