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

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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 [[constellation|star constellations]].
  
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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==Common misconceptions==
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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.
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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).  
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This same real optical 'flattening' also occurs for the rising and setting sun.
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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.
  
==Common misconceptions==
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The angle the moon subtends at an observer's eye is its [[angular size]] or [[visual angle]], V degrees.
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.
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It can be measured directly with a [[theodolite]] to show that it remains constant overnight.
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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]].
  
 
== Proof of illusion ==
 
== Proof of illusion ==
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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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).
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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.
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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==
 
==Possible explanations==
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===Apparent distance hypothesis===
 
===Apparent distance hypothesis===
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.
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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."
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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.
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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.")
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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.
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===Relative size hypothesis===
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The "relative size hypothesis" has been the most popular substitute for the unsatisfactory apparent distance theory.
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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.
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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.  
  
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 largerThis effect is known as the [[Ponzo illusion]].[http://www.space.com/scienceastronomy/top5_myths_020903-2.html]
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So, a 'relative size' hypothesis can refer either to relative angular size (in degrees) or to relative linear size (in meters).
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.
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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.
  
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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===Oculomotor Micropsia hypothesis===
  
===Relative size hypothesis===
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Entry to be posted here soon.
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===
 
===Cognitive processes hypothesis===
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].
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This general hypothesis applies to (is part of) all the above 'moon illusion' hypotheses.
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It applies, as well, to normal everyday seeing.
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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.  
+
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.
 
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.
  
In a nutshell the Moon illusion is due to '''failure of distance constancy'''.
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==References==
 
==References==
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==External links==
 
==External links==
* [http://www.lhup.edu/~dsimanek/3d/moonillu.htm Review article of past and current theories]
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* [http://facstaff.uww.edu/mccreadd/index.html  A vision scientist reviews and critiques moon illusion theories.]
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* [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://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://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.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.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.straightdope.com/classics/a2_110.html Why does the Moon appear bigger near the horizon?] (from [[The Straight Dope]])
Line 51: Line 76:
 
* [http://www.badastronomy.com/bad/misc/moonbig.html Moon illusion discussed at Bad Astronomy website]
 
* [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://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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{{Credits|Moon_illusion|113216088}}
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{{Credits|Moon_illusion|156737864|}}

Revision as of 18:09, 11 September 2007


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.

Common misconceptions

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. 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.

Proof of illusion

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. 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

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

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.

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

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 [1].

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.


References
ISBN links support NWE through referral fees

  • Ross HE and Plug C (2002) The mystery of the moon illusion: Exploring size perception. Oxford University Press. ISBN 0-19-850862-X.

External links


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