<P> 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 . </P> <P> A potential problem for the apparent distance theory has been that very 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 his or her perception; because the viewer knows that the moon can't possibly be physically farther away, he or she is not consciously aware of the perception . This is reinforced by the idea that we do not consciously perceive distance and size, as spatial awareness is a subconscious, retino cortical cognition . In line with the possibility that the reported distance of the moon is due to logic, rather than perception, is the finding that these varying reports--with some reporting closer distances and others not--are likely due to response biases . Nevertheless, the apparent distance explanation is the one most often found in textbooks . </P> <P> Those advocating the apparent distance hypothesis might counter that in the Ponzo illusion, people do not perceive one line to actually be farther away than another either, or that everything on the horizon is perceived as farther away, out of the habit that this is true for every object ever seen there (average - altitude clouds are about 100 times farther away at the horizon than when overhead). </P> <P> Historically, the best - known alternative to the "apparent distance" theory has been a "relative size" theory . This states that the perceived size of an object depends not only on its retinal size, but also on the size of objects in its immediate visual environment . In the case of the Moon illusion, objects in the vicinity of the horizon moon (that is, objects on or near the horizon) exhibit a fine detail that makes the Moon appear larger, while the zenith moon is surrounded by large expanses of empty sky that make it appear smaller . </P>

Where does the moon look the biggest in the world