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

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[[Category:Politics and social sciences]]
 
[[Category:Politics and social sciences]]
 
[[Category:Psychology]]
 
[[Category:Psychology]]
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[[Category:Illusion]]
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[[Image:Fdecomite - Moon and Venus at sunset (by).jpg|right|250px|thumb|High up in the sky, the moon appears smaller.]]
  
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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.
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[[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.]]
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==Discovery==
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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>
  
The '''Moon illusion''' is an [[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]].
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==Description==
  
==Common misconceptions==
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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 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, it does not change its apparent size. Indeed, the true angular diameter of the Moon 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.
 
  
== Proof of illusion ==
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==Explanation==
A proof that this effect is an illusion is to establish the Moon's size relative to another object both near the horizon and when it is in a high position in the sky. As an example, when the Moon is near the horizon, it is only necessary to hold a small coin at arm's length with one [[eye]] closed and positioning it next to the Moon. Then, when the Moon is higher in the sky, positioning the coin next to it again will confirm that the Moon has the same size in both positions.  
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[[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.]]
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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.
  
==Possible explanations==
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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.  
[[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===
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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.
One explanation is the apparent distance theory which may now be rejected by modern vision researchers who have investigated the Moon illusion. [[Human]]s may 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 comparatively flat surface. In other words, we perceive the sky near the horizon to be farther away than the sky overhead.
 
  
If we perceive the Moon to be in the general vicinity of those other things we see in the sky, we would expect it to get equally farther away as it approaches the horizon as well, which should result in a smaller [[Retina|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 (visual)|perspective]], assume a low moon must be physically larger. This effect is known as the [[Ponzo illusion]].[http://www.space.com/scienceastronomy/top5_myths_020903-2.html]
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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.
That is, regardless of eye elevation, the Moon near the horizon will be perceived as being much larger than the Moon near zenith because the terrain is viewed as a plane extended outward from the observer. Thus, if two objects have the same projected size but appear to lie at different distances from the viewer, the object which seems farther away will appear larger.
 
  
Nevertheless, very few people (only about 5%) see the horizon moon as being both larger and farther away, and vision researchers have found that most people see the horizon moon as both larger and closer than the zenith moon. This common disagreement between the apparent distance theory and the available data is referred to as the "size distance paradox", and new theories are being developed to replace the apparent distance theory.
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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.  
  
===Relative size hypothesis===
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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>
This hypothesis states that the perceived size of an object depends not only on its retinal size, but also on the size of its immediate visual environment. However, relative size can be either relative angular size or relative linear size, or both, and the Moon illusion begins as an angular size illusion (see [http://www.lhup.edu/~dsimanek/3d/moonillu.htm]).
 
  
===Cognitive processes hypothesis===
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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.
This hypothesis claims that the illusion is 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.  
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==Applications==
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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.
  
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.
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==Notes==
 
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<references/>
In a nutshell the Moon illusion is due to '''failure of distance constancy'''.
 
  
 
==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.
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*Gregory, Richard L. 1997. ''Eye and Brain''. Princeton University Press. ISBN 0691048371
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*Hershenson, Maurice. 1989. ''The Moon Illusion''. Lawrence Erlbaum. ISBN 0805801219
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*Long, Kim. 1998. ''The Moon Book: Fascinating Facts About the Magnificent, Mysterious Moon''. Johnson Books. ISBN 1555662307
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*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==
 
==External links==
* [http://www.lhup.edu/~dsimanek/3d/moonillu.htm Review article of past and current theories]
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All links retrieved November 9, 2022.
* [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.lhup.edu/~dsimanek/3d/moonillu.htm Descriptions of several possible explanations]
 
* [http://facstaff.uww.edu/mccreadd/intro9.htm The moon illusion explained via oculomotor micropsia] (Presents an alternative argument to the usual)
 
* [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]
 
 
 
  
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*[http://antwrp.gsfc.nasa.gov/apod/ap020130.html APOD Moon Illusion page]
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*[http://niquette.com/books/sophmag/moonill.htm The Moon Illusion] by Paul Niquette
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*[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]
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*[http://news.bbc.co.uk/2/hi/uk_news/magazine/4619063.stm Why does the moon look so big now?]
  
  
{{Credits|Moon_illusion|113216088}}
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{{Credits|Moon_illusion|156737864|}}

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

  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.

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.


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