question: Why when the moon is rising on the horizon does it look bigger than when its high in the sky?
comment: There are two explanations for this: 1) there is a slight magnification effect produced by the Earth’s atmosphere. When you look through the atmosphere right at the horizon, the curve of the atmosphere acts like a lens to magnify things. This is also why the sun seems to widen and squash when it’s setting. It’s a marginal effect right at the horizon, and NOT responsible for most of the impression that the moon is huge close to the horizon. 2) there is an optical illusion associated with our brain’s attempt to interpret how far away what we’re seeing actually is. When the moon is high in the sky, there’s nothing around it to give our brain any context for how far away it is. But when it’s close to the horizon, it’s clearly much farther away than stuff we see at the horizon, so our brain registers size in the context of “really far away”, making the moon seem much bigger.
passages: Section::::Possible explanations.:Relative size hypothesis.
  
  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.
   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.
   If we perceive the Moon to be in the general vicinity of the other things we see in the sky, we would expect it 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, our brains, attempting to compensate for perspective, assume that a low Moon must be physically larger.
   Towering and stooping are more complex forms of atmospheric refraction than looming and sinking. While looming and sinking change the apparent elevation of an object, towering and stooping change the apparent shape of the object itself. With towering, objects appear stretched; with stooping, objects seem to be shortened. The apparent stretching and shortening of the objects are not symmetrical and depends on the thermal profile of the atmosphere. The curvature of the rays changes more rapidly in some places because the thermal profile is curved.
  
  Section::::Image example and explanation.
   Sinking is the opposite of looming. In sinking, stationary objects that are normally seen above the horizon appear to be lowered, or may even disappear below the horizon. In looming, the curvature of the rays is increasing, while sinking produces the opposite effect. In general, looming is more noticeable than sinking because objects that appear to grow stand out more than those that appear to shrink.
  
  Section::::Towering and stooping.
   BULLET::::- Distance constancy refers to the relationship between apparent distance and physical distance. An illusion example of this would be the moon - when it is near the horizon it is perceived as larger (size constancy) and/or closer to earth than when it is above our heads.
   Research indicates that 3D vision capabilities emerge and are learned jointly with the planning of movements. After a long process of learning, an internal representation of the world emerges that is well-adjusted to the perceived data coming from closer objects. The representation of distant objects near the horizon is less "adequate". In fact, it is not only the Moon that seems larger when we perceive it near the horizon. In a photo of a distant scene, all distant objects are perceived as smaller than when we observe them directly using our vision.
  
  Section::::Further reading.
   The method took advantage of the fact that the Moon is actually closest to an observer when it is at its highest point in the sky, compared to when it is on the horizon. Although it appears that the Moon is biggest when it is near the horizon, the opposite is true. This phenomenon is known as the Moon illusion. The reason for the difference in distance is that the distance from the center of the Moon to the center of the Earth is nearly constant throughout the night, but an observer on the surface of Earth is actually 1 Earth radius from the center of Earth. This offset brings them closest to the Moon when it is overhead.
   Atmospheric refraction – the bending of the light from the Moon as it passes through the Earth's atmosphere – alters the observed declination of the Moon, more so at low elevation, where the atmosphere is thicker (deeper).
  
  Not all the maxima are observable from all places in the world – the Moon may be below the horizon at a particular observing site during the maximum, and by the time it rises, it may have a lower declination than an observable maximum at some other date.
  
  Section::::2006 standstill.
   A simple way of demonstrating that the effect is an illusion is to hold a small pebble (say, wide) at arm's length () with one eye closed, positioning the pebble so that it covers (eclipses) the full Moon when high in the night sky. Then, when the seemingly very large Moon is on the horizon, the same pebble will also cover it, revealing that there has been no change in the size of the Moon, because the pebble will still cover the Moon.
   An apparent distance theory evidently was first clearly described by Cleomedes around 200 A.D. The theory proposes that the horizon Moon looks larger than the zenith Moon because it looks farther away. Ibn al-Haytham was more specific. His argument was that judging the distance of an object depends on there being an uninterrupted sequence of intervening bodies between the object and the observer; however, since there are no intervening objects between the Earth and the Moon, the perceived distance is too short and the Moon appears smaller than on the horizon. Researchers have argued that the apparent distance hypothesis is problematic scientifically because it explains perceptions as consequences of perceptions: the Moon looks farther away because it looks larger. However, there are probably complex internal processes behind this relationship.
   Section::::External links.
  
  BULLET::::- A TED-Ed video on the Moon illusion, offering many theories.
  
  BULLET::::- A vision scientist reviews and critiques Moon illusion theories (and argues for oculomotor micropsia).
  
  BULLET::::- Another careful review of Moon illusion research.
  
  BULLET::::- A physicist offers opinions about current theories
  
  BULLET::::- Summer Moon Illusion – NASA
  
  BULLET::::- "Finally! Why the Moon Looks Big at the Horizon and Smaller When Higher Up" by Don McCready, Professor Emeritus, Psychology Department, University of Wisconsin-Whitewater, Revised 10 November 2004, retrieved 31 October 2015
  
  BULLET::::- Why does the moon look so big now?
   At a major lunar standstill, the Moon's range of declination, and consequently its range of azimuth at moonrise and moonset, reaches a maximum. As a result, viewed from the middle latitudes, the Moon's altitude at upper culmination (the daily moment when the object appears to contact the observer's meridian) changes in just two weeks – from highest to lowest above the horizon due north or south, depending on the observer's hemisphere. Similarly, its azimuth at moonrise changes from northeast to southeast and at moonset from northwest to southwest. Solar eclipses at ascending node occurs in March, solar eclipses at descending node occurs in September. Lunar eclipses at descending node occurs in March, lunar eclipses at ascending node occurs in September.
   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 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.
   Moon illusion
  
  The Moon illusion is an optical illusion which causes the Moon to appear larger near the horizon than it does higher up in the sky. It has been known since ancient times and recorded by various cultures.
  
  The explanation of this illusion is still debated.
  
  Section::::Proof of illusion.
  
  The angle that the full Moon subtends at an observer's eye can be measured directly with a theodolite to show that it remains constant as the Moon rises or sinks in the sky. Photographs of the Moon at different elevations also show that its size remains the same. 
   In 1813, Schopenhauer wrote about this, that the Moon illusion is "purely intellectual or cerebral and not optical or sensuous." The brain takes the sense data that is 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.
   Occasionally, a supermoon coincides with a total lunar eclipse. The most recent occurrence of this was in January 2019, and the next will be in May 2021.
  
  Section::::Appearance.
  
  A full moon at perigee appears roughly 14% larger in diameter than at apogee. Many observers insist that the moon looks bigger to them. This is likely due to observations shortly after sunset when the moon is near the horizon and the moon illusion is at its most apparent.
   The effect is illustrated by the classic Ebbinghaus illusion, where a circle appears larger when surrounded by smaller circles, than it does when surrounded by larger circles.
  
  Section::::Possible explanations.:Angle of regard hypothesis.
   In the Book of Optics (1011–22 AD), Ibn al-Haytham argued that vision occurs in the brain, and that personal experience has an effect on what people see and how they see, and that vision and perception are subjective. Arguing against Ptolemy's refraction theory for why people perceive the Sun and Moon larger at the horizon than when they are higher in the sky, he redefined the problem in terms of perceived, rather than real, enlargement. He said that judging the distance of an object depends on there being an uninterrupted sequence of intervening bodies between the object and the observer. With the Moon, however, there are no intervening objects. Therefore, since the size of an object depends on its observed distance, which is in this case inaccurate, the Moon appears larger on the horizon. Through works by Roger Bacon, John Pecham and Witelo based on Ibn al-Haytham's explanation, the Moon illusion gradually came to be accepted as a psychological phenomenon, with Ptolemy's theory being rejected in the 17th century.
   Cognitive illusions include some which result from the unconscious misapplication of certain optical principles. For example, the Ames room, Hering, Müller-Lyer, Orbison, Ponzo, Sander, and Wundt illusions all rely on the suggestion of the appearance of distance by using converging and diverging lines, in the same way that parallel light rays (or indeed any set of parallel lines) appear to converge at a vanishing point at infinity in two-dimensionally rendered images with artistic perspective. This suggestion is also responsible for the famous moon illusion where the moon, despite having essentially the same angular size, appears much larger near the horizon than it does at zenith. This illusion so confounded Ptolemy that he incorrectly attributed it to atmospheric refraction when he described it in his treatise, "Optics".
   The first stage in the operations that transfer information from a visual target in front of an observer into its neural representation in the brain and then allow a percept to emerge, is the imaging by the eye and the processing by the neural circuits in the retina. Some components of geometrical-optical illusions can be ascribed to aberrations at that level. Even if this does not fully account for an illusion, the step is helpful because it puts elaborate mental theories in a more secure place. The moon illusion is a good example. Before invoking concepts of apparent distance and size constancy, it helps to be sure that the retinal image hasn't changed much when the moon looks larger as it descends to the horizon. 
   The Ponzo illusion is one possible explanation of the Moon illusion, with objects appearing "far away" (because they are "on" the horizon) appearing bigger than objects "overhead". However, some have argued that explaining one perception ("appears far away") in terms of another ("appears bigger") is problematic scientifically, and that there are probably complex internal processes behind these illusions.
  
  The Ponzo illusion also occurs in touch and with an auditory-to-visual sensory-substitution device. However, prior visual experience seems mandatory to perceive it as demonstrated by the fact that congenitally blind subjects are not sensitive to it.
   The size of a viewed object can be measured objectively either as an "angular size" (the visual angle that it subtends at the eye, corresponding to the proportion of the visual field that it occupies), or as "physical size" (its real size measured in, say, meters). Perceived size is only loosely related to these concepts, however. For example, if two identical, familiar objects are placed at distances of five and ten meters, respectively, then the more distant object subtends approximately half the visual angle of the nearer object, but we normally perceive that as the same size (a phenomenon referred to as "size constancy"), not as half the size. Conversely, if the more distant object did subtend the same angle as the nearer object then we would normally perceive it to be twice as big.
   If the Earth were an airless world like the Moon, the above calculations would be accurate. However, Earth has an atmosphere of air, whose density and refractive index vary considerably depending on the temperature and pressure. This makes the air refract light to varying extents, affecting the appearance of the horizon. Usually, the density of the air just above the surface of the Earth is greater than its density at greater altitudes. This makes its refractive index greater near the surface than at higher altitudes, which causes light that is travelling roughly horizontally to be refracted downward. This makes the actual distance to the horizon greater than the distance calculated with geometrical formulas. With standard atmospheric conditions, the difference is about 8%. This changes the factor of 3.57, in the metric formulas used above, to about 3.86. For instance, if an observer is standing on seashore, with eyes 1,70 m above sea level, according to the simple geometrical formulas given above the horizon should be 4,7 km away. Actually, atmospheric refraction allows the observer to see 300 metres farther, moving the true horizon 5 km away from the observer.
   Some of the most spectacular moons come during the full moon phase near sunset or sunrise. The moon on the horizon benefits from the moon illusion which makes it appear larger. The light reflected from the moon traveling through the atmosphere also colors the moon orange and/or red.
  
  Section::::Visual presentation.:Comets.
  