given to it from the eye and it apprehends a large Moon because "our intuitively perceiving understanding regards everything that is seen in a horizontal direction as being more distant and therefore as being larger than objects that are seen in a vertical direction." The brain is accustomed to seeing terrestrially-sized objects in a horizontal direction and also as they are affected by atmospheric perspective, according to Schopenhauer. If the Moon is perceived to be in the general vicinity of the other things seen in the sky, it would be expected to also recede as it approaches the horizon, which should result in a smaller retinal image. But since its retinal image is approximately the same size whether it is near the horizon or not, the brain, attempting to compensate for perspective, assume that a low Moon must be physically larger. Extensive experiments in 1962 by Kaufman and Rock showed that a crucial causative factor in the illusion is a change in the pattern of cues to distance, comparable to the Ponzo illusion. The horizon Moon is perceived to be at the end of a stretch of terrain receding into the distance, accompanied by distant trees, buildings and so forth, all of which indicate that it must be a long way away, while these cues are absent from the zenith moon. Experiments by many other researchers have found the same result; namely, when pictorial cues to a great distance are subtracted from the vista of the large-looking horizon Moon it looks smaller. When pictorial cues to an increased distance are added into the vista of the zenith Moon, it appears larger. A potential problem for the apparent distance theory has been that few people (perhaps about 5%) perceive the horizon Moon as being both larger and farther away. Indeed, most people (perhaps 90%) say the horizon Moon looks both larger and closer than the zenith Moon (Boring, 1962; Hershenson, 1982; McCready, 1965, 1986; Restle, 1970). Most of the rest say it looks larger and about the same distance away as the zenith Moon, with a few people reporting no Moon illusion at all. However, the response that the horizon Moon appears larger, but not closer than the zenith Moon could be because the viewer's logic confounds their perception; because the viewer knows that the Moon can't possibly be physically farther away, they are not consciously aware of the perception. This