Difference between revisions of "Moon illusion" - New World Encyclopedia

Latest revision as of 13:13, 10 March 2023

High up in the sky, the moon appears smaller.

The Moon illusion is a visual illusion (or optical illusion) in which the Moon appears larger near the horizon than it does while higher up in the sky. This optical illusion also occurs with the Sun and star constellations. Many explanations for this illusion have been proposed, some purely physical in nature, others involving innate or learned cognitive processes. Although the final solution to the phenomenon has not been agreed upon, the moon illusion has contributed greatly to research on how images are perceived and processed. In addition, the moon illusion has inspired in many a sense of wonder and amazement at the natural world.

When the moon is close to the horizon, it appears to be much larger.

Discovery

The moon illusion is seemingly as old as history itself. References to the moon illusion can be found as early as the seventh century B.C.E., and is mentioned in early Greek and Chinese writings. Aristotle also mentioned it in 250 B.C.E.^[1]

Description

When looking at the moon while it is high overhead, it appears to be significantly smaller than when it is close to the horizon. In many instances, it is difficult to convince people that the moon has remained the same size without proving it to them. The constant size of the moon regardless of its position in the sky can be proven using a theodolite, or by the much simpler method of holding a small coin at arm's length. The coin will cover the same amount of the moon no matter where the moon is in the sky. The illusion also becomes much less convincing when the horizon moon is viewed upside down (for instance, by bending over and viewing the moon through one's legs.)

Explanation

Clouds near the horizon are typically farther away from the viewer, while those high in the sky are closer, giving the impression of a flat, or gently curved, sky surface.

It is sometimes thought that the moon appears larger near the horizon as a result of some kind of magnification effect caused by the Earth's atmosphere. This is not true, although the atmosphere does change the color of the moon. The moon does appear slightly more ovoid in shape near the horizon, due to the weak prism effect of the atmosphere, but does not change significantly in size. Any change in size is a perceived change, not an actual visual change, and therefore cannot be attributed to atmospheric conditions.

One of the oldest explanations for the moon illusion is the apparent distance theory, which was first clearly described by Cleomedes around 200 C.E. This theory proposes that we tend to perceive the sky as more or less a surface, but unlike a hemispherical surface, it does not seem to be equally distant from us at all points. When we see clouds, birds, and airplanes in the sky, those near the horizon are typically farther away from us than those overhead. If we see an airplane overhead, its image gets smaller and smaller as it nears the horizon. This results in the perception of the sky as a fairly flat surface. In other words, we perceive the sky near the horizon to be farther away than the sky overhead. This theory is usually illustrated by the well-known drawing of the "flattened sky dome." This theory is related to the Ponzo Illusion, where a shape placed higher up on converging lines appears to be larger than a shape placed lower down. Because of perspective and distance cues, we expect shapes that are further away to be smaller; because of this, shapes that are the same size appear to be larger than they actually are.

One of the problems with the apparent size theory is that, in addition to the moon looking larger, it should theoretically also look farther away. In reality, most people perceive the moon as not only being larger when it is close to the horizon, but also nearer. To explain this, some use the "relative size hypothesis," which suggests that the perceived size of an object depends partly on the size of objects in its immediate visual environment. Thus, any trees or buildings visible in the vicinity of the moon make the moon appear larger.

Another attempt to explain the moon illusion was proposed by Restle, based on the assumption that size is always judged relative to other extents in the visual field. He suggested that differences in the grain of the visual scene, such that a distant horizon has finer grain than a closer one, result in terrain of different extents. These cause the observer to judge the moon close to the horizon as relatively larger.

Trehub has postulated that we have an innate subconscious cognitive processing mechanism that causes the angular size of objects at eye level to be perceived as greater than the angular size of the same objects seen when looking upward. Such a process is advantageous as it leads us to devote more visual processing resources to nearby space, and space at eye level, but fewer resources to things seen at higher elevation. Although he suggested this is a preprogrammed biologically-based mechanism, it is also possible that such perceptions can be modified through experience and by particular sets of visual cues.

A more recent explanation regarding the moon illusion was proposed by Don McCready of the University of Wisconsin. McCready suggests that the illusion is due to oculomotor macropsia and oculomotor micropsia. Oculomotor macropsia causes objects to appear larger when they appear far away based on distance cues such as buildings or trees. Oculomotor micropsia makes objects appear smaller when we perceive them to be closer. Oculomotor micropsia exists in order to make it easier and faster to turn our heads the right amount to perceive a nearby object that might pose a threat. Oculomotor macropsia exists because it is a necessary byproduct of using oculomotor micropsia, and there is no harm done if we misperceive an object that is far away.^[2]

Thus, while numerous explanations of the moon illusion have been proposed, there is no consensus. However, it is clear that the moon illusion is a robust and universal phenomenon, and has led to much research that has proven valuable in developing our understanding of our perceptual systems.

Applications

The moon illusion, like many visual and perceptual illusions, helps scientists formulate theories on how images are perceived and processed. In addition to scientific study, the moon illusion has probably done more than any other illusion to inspire a sense of wonder and amazement at the natural world, particularly in poets and artists.

Notes

↑ Donald Simanek, "The Moon Illusion, An Unsolved Mystery" Jan 11, 2002.
↑ Lee Krystek, "Experiment in Perception: The Ponzo Illusion and the Moon" 1998. Retrieved December 4, 2007.

References
ISBN links support NWE through referral fees

Gregory, Richard L. 1997. Eye and Brain. Princeton University Press. ISBN 0691048371
Hershenson, Maurice. 1989. The Moon Illusion. Lawrence Erlbaum. ISBN 0805801219
Long, Kim. 1998. The Moon Book: Fascinating Facts About the Magnificent, Mysterious Moon. Johnson Books. ISBN 1555662307
Restle, F. 1967. "Moon Illusion Explained on the Basis of Relative Size" in Science, Vol. 167, 1092-1096.
Robinson, J. O. 1998. The Psychology of Visual Illusion. Dover Publications. ISBN 978-0486404493
Ross, HE and Plug C. 2002. The mystery of the moon illusion: Exploring size perception. Oxford University Press. ISBN 019850862X
Trehub, Arnold. 1994. The Cognitive Brain. MIT Press. ISBN 978-0262700498

External links

All links retrieved November 9, 2022.

Credits

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Moon_illusion ^history

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History of "Moon illusion"

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[1] Donald Simanek, "The Moon Illusion, An Unsolved Mystery" Jan 11, 2002.

[2] Lee Krystek, "Experiment in Perception: The Ponzo Illusion and the Moon" 1998. Retrieved December 4, 2007.

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[2]

@@ Line 1: / Line 1: @@
-{{Claimed}}{{Started}}{{Contracted}}
+{{Copyedited}}{{Images OK}}{{Submitted}}{{Approved}}{{Paid}}
 [[Category:Politics and social sciences]]
 [[Category:Psychology]]
+[[Category:Illusion]]
+[[Image:Fdecomite - Moon and Venus at sunset (by).jpg|right|250px|thumb|High up in the sky, the moon appears smaller.]]
-The '''Moon illusion''' is a visual illusion (or [[optical illusion]]) in which the [[Moon]] appears larger near the [[horizon]] than it does while higher up in the [[sky]]. This optical illusion also occurs with the [[Sun]] and [[constellation|star constellations]].
+The '''Moon illusion''' is a visual illusion (or [[optical illusion]]) in which the [[Moon]] appears larger near the [[horizon]] than it does while higher up in the [[sky]]. This optical illusion also occurs with the [[Sun]] and [[star]] [[constellation]]s. Many explanations for this illusion have been proposed, some purely physical in nature, others involving innate or learned [[cognition|cognitive]] processes. Although the final solution to the phenomenon has not been agreed upon, the moon illusion has contributed greatly to research on how images are perceived and processed. In addition, the moon illusion has inspired in many a sense of wonder and amazement at the natural world.
+{{toc}}
+[[Image:IMG 0573 - Graz - View from Schlossberg.JPG|right|250px|thumb|When the moon is close to the horizon, it appears to be much larger.]]
+==Discovery==
+The moon illusion is seemingly as old as history itself. References to the moon illusion can be found as early as the seventh century B.C.E., and is mentioned in early Greek and Chinese writings. [[Aristotle]] also mentioned it in 250 B.C.E.<ref>Donald Simanek, "The Moon Illusion, An Unsolved Mystery" Jan 11, 2002.</ref>
-==Common misconceptions==
+==Description==
-It is a commonly held belief that the Moon appears larger near the horizon as a result of some kind of [[magnification]] effect caused by the [[Earth's atmosphere]]. This is not true, although the atmosphere does change the [[color]] of the Moon.
-Actually, the measured optical angle of the Moon's vertical diameter is smaller for the horizon moon than for the zenith moon, so when the rising moon is on the horizon it appears (both visually and in photographs) to be a bit squashed down (ovoid).  This occurs because the atmosphere here acts like a weak prism (not a lens).
-This same real optical 'flattening' also occurs for the rising and setting sun.
-In addition, the angle the Moon subtends is about 1.5%  smaller  when it is near the horizon than when it is high in the sky, because it is farther away from the observer by almost one Earth radius.
-The angle the moon subtends at an observer's eye is its [[angular size]] or [[visual angle]], V degrees.
+When looking at the [[moon]] while it is high overhead, it appears to be significantly smaller than when it is close to the horizon. In many instances, it is difficult to convince people that the moon has remained the same size without proving it to them. The constant size of the moon regardless of its position in the sky can be proven using a [[theodolite]], or by the much simpler method of holding a small [[coin]] at arm's length. The coin will cover the same amount of the moon no matter where the moon is in the sky. The [[illusion]] also becomes much less convincing when the horizon moon is viewed upside down (for instance, by bending over and viewing the moon through one's legs.)
-It can be measured directly with a [[theodolite]] to show that it remains constant overnight.
-It also can be calculated from the rule, V = tan S/D, where S  is the linear size of the moon's diameter (2160 miles) and D is its distance from the eye (which averages about 238,000 miles).  Thus, tan V = 0.009. So V = 0.52 degree, or 0.009 [[radian]].
+==Explanation==
+[[Image:GoldenMedows.jpg|left|250px|thumb|Clouds near the horizon are typically farther away from the viewer, while those high in the sky are closer, giving the impression of a flat, or gently curved, sky surface.]]
+It is sometimes thought that the moon appears larger near the horizon as a result of some kind of [[magnification]] effect caused by the [[Earth]]'s [[atmosphere]]. This is not true, although the atmosphere does change the [[color]] of the moon. The moon does appear slightly more ovoid in shape near the horizon, due to the weak [[prism]] effect of the atmosphere, but does not change significantly in size. Any change in size is a ''perceived'' change, not an actual visual change, and therefore cannot be attributed to atmospheric conditions.
-== Proof of illusion ==
+One of the oldest explanations for the moon illusion is the apparent distance theory, which was first clearly described by [[Cleomedes]] around 200 C.E. This theory proposes that we tend to perceive the sky as more or less a surface, but unlike a hemispherical surface, it does not seem to be equally distant from us at all points. When we see clouds, birds, and airplanes in the sky, those near the horizon are typically farther away from us than those overhead. If we see an airplane overhead, its image gets smaller and smaller as it nears the horizon. This results in the perception of the sky as a fairly flat surface. In other words, we perceive the sky near the horizon to be farther away than the sky overhead. This theory is usually illustrated by the well-known drawing of the "flattened sky dome." This theory is related to the [[Ponzo Illusion]], where a shape placed higher up on converging lines appears to be larger than a shape placed lower down. Because of perspective and distance cues, we expect shapes that are further away to be smaller; because of this, shapes that are the same size appear to be larger than they actually are.
-Besides actually measuring the moon's subtended 1/2 degree angle with a theodolite to prove that it remains constant overnight, another proof is that photographs of the moon at the horizon and higher up are exactly the same size (if taken with the same camera settings).
-Another way of demonstrating that the effect is an illusion is to hold a small coin at arm's length with one eye closed, positioning it next to the seemingly large moon. When the Moon is higher in the sky positioning the same coin near the moon will reveal no change in size.
+One of the problems with the apparent size theory is that, in addition to the moon looking larger, it should theoretically also look farther away. In reality, most people perceive the moon as not only being larger when it is close to the horizon, but also nearer. To explain this, some use the "relative size hypothesis," which suggests that the perceived size of an object depends partly on the size of objects in its immediate visual environment. Thus, any trees or buildings visible in the vicinity of the moon make the moon appear larger.
-Yet another way to see that the effect is an illusion is to view the Moon with one's head upside-down, say by bending over and looking through the legs, the moon on the horizon will appear smaller.
-==Possible explanations==
+Another attempt to explain the moon illusion was proposed by Restle, based on the assumption that size is always judged relative to other extents in the visual field. He suggested that differences in the grain of the visual scene, such that a distant horizon has finer grain than a closer one, result in terrain of different extents. These cause the observer to judge the moon close to the horizon as relatively larger.
-[[Image:GoldenMedows.jpg|right|250px|thumb|Clouds near the horizon are typically farther away from the viewer, while those high in the sky are closer, giving the impression of a flat, or gently curved, sky surface]]
-After reviewing the many different explanations in their 2002 book ''The Mystery of the Moon Illusion'', Ross and Plug conclude "No single theory has emerged victorious" (p 180). What follows is a brief summary of some of the better-known contenders. It is, of course, possible that the correct explanation is some combination of these and others described in the external links.
-===Apparent distance hypothesis===
+Trehub has postulated that we have an innate subconscious cognitive processing mechanism that causes the angular size of objects at eye level to be perceived as greater than the angular size of the same objects seen when looking upward. Such a process is advantageous as it leads us to devote more visual processing resources to nearby space, and space at eye level, but fewer resources to things seen at higher elevation. Although he suggested this is a preprogrammed biologically-based mechanism, it is also possible that such perceptions can be modified through experience and by particular sets of visual cues.
-One explanation is the ancient apparent distance theory, which was first clearly described by Cleomedes around AD 200. The theory proposes that we tend to perceive the sky as more or less a surface, but unlike a hemispherical surface, it does not seem to be equally distant from us at all points. When we see clouds, birds and airplanes in the sky, those near the horizon are typically farther away from us than those overhead. If we see an airplane overhead, its image gets smaller and smaller as it nears the horizon. This results in the perception of the sky as a fairly flat surface.  In other words, we perceive the sky near the horizon to be farther away than the sky overhead.  This assumption is usually illustrated by the well-known drawing of the "flattened sky dome."
-If we perceive the Moon to be like other objects we see in the sky, we would expect it to behave in the same way and get farther away as it approaches the horizon, which should result in a smaller [[visual angle]] and [[Retinal image]].  But since its retinal image is the same size whether it is near the horizon or not, the low moon appears a larger physical size (linear size).  This effect is known as Emmert's Law.  That is, if two objects have the same angular size (visual angle) at the eye but appear to lie at different distances from the viewer, the object which seems further away will appear a larger linear size.
+A more recent explanation regarding the moon illusion was proposed by Don McCready of the University of Wisconsin. McCready suggests that the illusion is due to [[oculomotor macropsia]] and [[oculomotor micropsia]]. Oculomotor macropsia causes objects to appear larger when they appear far away based on distance cues such as buildings or trees. Oculomotor micropsia makes objects appear smaller when we perceive them to be closer. Oculomotor micropsia exists in order to make it easier and faster to turn our heads the right amount to perceive a nearby object that might pose a threat. Oculomotor macropsia exists because it is a necessary byproduct of using oculomotor micropsia, and there is no harm done if we misperceive an object that is far away.<ref>Lee Krystek, [http://unmuseum.mus.pa.us/exmoon.htm "Experiment in Perception: The Ponzo Illusion and the Moon"] 1998. Retrieved December 4, 2007.</ref>
-However, very few people (perhaps about 5%) see the horizon moon as being both larger and farther away. Indeed vision researchers have found that most people (perhaps 90%) say the horizon moon looks both larger and closer than the zenith moon. And most of the rest say  it looks larger and about the same distance away as the zenith moon.  (Also a few people do not have a "moon illusion.")
-The apparent distance theory requires that, if the horizon moon "looks larger" it must also look farther away.  This common disagreement between that theory and the research data for the moon illusion (and other similar illusions) is known as the "size-distance paradox."  Accordingly, vision scientists have published other theories to describe and explain the type of illusion that most people experience.
-===Relative size hypothesis===
+Thus, while numerous explanations of the moon illusion have been proposed, there is no consensus. However, it is clear that the moon illusion is a robust and universal phenomenon, and has led to much research that has proven valuable in developing our understanding of our perceptual systems.
-The "relative size hypothesis" has been the most popular substitute for the unsatisfactory apparent distance theory.
-In loose terminology  it 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.
-But the term 'perceived size' can have two quite different meanings. It can mean perceived angular size, V' degrees, and it can mean  perceived linear size, S' meters.
-So, a 'relative size' hypothesis can refer either to relative angular size (in degrees) or to relative linear size (in meters).
-It now seems clear that, for most people the Moon Illusion begins as an angular size illusion.  So, what is most relevant is the relative angular size theory.
-===Oculomotor Micropsia hypothesis===
-Entry to be posted here soon.
-===Cognitive processes hypothesis===
-This general hypothesis applies to (is part of) all the above 'moon illusion' hypotheses.
-It applies, as well, to normal everyday seeing.
-It claims that visual illusions are due to the fact that the neural circuitry in our visual system evolves, by neural learning, to a system that makes very efficient interpretations of usual 3D scenes based in the emergence of simplified models in our brain that speed up the interpretation process but give rise to many optical illusions in unusual situations. In this sense, the cognitive processes hypothesis can be considered a framework for an understanding of optical illusions as the signature of the empirical statistical way vision has evolved to solve the inverse problem [http://www.richardgregory.org/papers/knowl_illusion/knowledge-in-perception.pdf].
-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 essentially 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.
-The retinal image is the main source driving vision but what we see is a "virtual" 3D representation of the scene in front of us. We don't see a physical image of the world. We see objects; and the physical world is not itself separated into objects. We see it according to the way our brain organizes it. The names, colors, usual shapes and other information about the things we see pop up instantaneously from our neural circuitry and influence the representation of the scene. We "see" the most relevant information about the elements of the best 3D image that our neural networks can produce. The perceived properties of the objects, such as brightness, angular size, and color, are unconsciously "determined" and are not real physical properties. The illusions arise when the "judgments" implied in the unconscious analysis of the scene are in conflict with reasoned considerations about it.
+==Applications==
+The moon illusion, like many visual and perceptual [[illusion]]s, helps scientists formulate theories on how images are perceived and processed. In addition to scientific study, the moon illusion has probably done more than any other illusion to inspire a sense of wonder and amazement at the natural world, particularly in poets and artists.
+==Notes==
+<references/>
 ==References==
-* Ross HE and Plug C (2002) ''The mystery of the moon illusion: Exploring size perception''. Oxford University Press. ISBN 0-19-850862-X.
+*Gregory, Richard L. 1997. ''Eye and Brain''. Princeton University Press. ISBN 0691048371
+*Hershenson, Maurice. 1989. ''The Moon Illusion''. Lawrence Erlbaum. ISBN 0805801219
+*Long, Kim. 1998. ''The Moon Book: Fascinating Facts About the Magnificent, Mysterious Moon''. Johnson Books. ISBN 1555662307
+*Restle, F. 1967. "Moon Illusion Explained on the Basis of Relative Size" in ''Science,'' Vol. 167, 1092-1096.
+*Robinson, J. O. 1998. ''The Psychology of Visual Illusion''. Dover Publications. ISBN 978-0486404493
+*Ross, HE and Plug C. 2002. ''The mystery of the moon illusion: Exploring size perception''. Oxford University Press. ISBN 019850862X
+*Trehub, Arnold. 1994. ''The Cognitive Brain''. MIT Press. ISBN 978-0262700498
 ==External links==
-* [http://facstaff.uww.edu/mccreadd/index.html  A vision scientist reviews and critiques moon illusion theories.]
+All links retrieved November 9, 2022.
-* [http://www.lhup.edu/~dsimanek/3d/moonillu.htm  A physicist reviews past and current theories]
-* [http://www.nasa.gov/vision/universe/watchtheskies/20jun_moonillusion.html Summer Moon Illusion - NASA]
-* [http://news.bbc.co.uk/2/hi/uk_news/magazine/4619063.stm  Why does the moon look so big now?]
-* [http://www.archimedes-lab.org/moon_illusion/moon.html  Inflatable Moon (explanation of an early illusion)]
-* [http://www.enane.de/empirhyp.htm Explanation visual gestalt effects, which may also apply to the moon illusion]
-* [http://www.straightdope.com/classics/a2_110.html Why does the Moon appear bigger near the horizon?] (from [[The Straight Dope]])
-* [http://www.netaxs.com/~mhmyers/moon.tn.html#moonillusion Moon illusion illustrated]
-* [http://www.badastronomy.com/bad/misc/moonbig.html Moon illusion discussed at Bad Astronomy website]
-*[http://niquette.com/books/sophmag/moonill.htm Four Answers to the Why Question about the Moon Illusion]
 *[http://antwrp.gsfc.nasa.gov/apod/ap020130.html APOD Moon Illusion page]
+*[http://niquette.com/books/sophmag/moonill.htm The Moon Illusion] by Paul Niquette
+*[http://www.archimedes-lab.org/moon_illusion/moon.html Inflatable Moon (explanation of an early illusion)]
+*[http://www.nasa.gov/vision/universe/watchtheskies/20jun_moonillusion.html Summer Moon Illusion - NASA]
+*[http://news.bbc.co.uk/2/hi/uk_news/magazine/4619063.stm Why does the moon look so big now?]
 {{Credits|Moon_illusion|156737864|}}