Moon illusion

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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 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, 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

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.

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 apparent distance theory which may now be rejected by modern vision researchers who have investigated the Moon illusion. Humans 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 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 a low moon must be physically larger. This effect is known as the Ponzo illusion.[1] 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.

Relative size hypothesis

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 [2]).

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 [3].

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.

In a nutshell the Moon illusion is due to failure of distance constancy.

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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