Possible Explanations for the Moon Illusion

Moon illusion

Possible Explanations for the Moon Illusion
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.


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]


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.)


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 un 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, 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 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.


The moon illusion, 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.


  • 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

All links retrieved October 20, 2018.

Источник: https://www.newworldencyclopedia.org/entry/Moon_illusion

Why the Moon Looks Bigger Near the Horizon

Possible Explanations for the Moon Illusion

For millennia, a perplexing sight has materialized in the evening sky: Sometimes, a gigantic moon appears to hover near the horizon, but as that overgrown orb climbs overhead, it shrinks to a fraction of its moonrise size.

This striking phenomenon is known as the moon illusion, and the fact that it happens has been well documented since at least the fourth century B.C. Obviously the moon isn’t actually changing size—but the precise reason it seems to be expanding and contracting continues to puzzle scientists today. (See 11 striking pictures of November’s record-setting supermoon.)

Here are the various explanations for the moon illusion and its enduring mystery.

Moon 101

Since long before the moment Neil Armstrong took his «one small step» in 1969, humans have been mesmerized by the moon. Get a crash course on lunar science.

Lunar Trickery

Back in the fourth century B.C., Greek philosopher Aristotle suggested that Earth’s atmosphere might be enlarging the image of the horizon moon, just as water can make immersed objects seem magnified to our eyes.

The Greco-Egyptian astronomer Ptolemy suggested something similar in his famous treatise Almagest, published during the second century A.D., as did Greek astronomer Cleomedes around the same time, though they both also ascribed the phenomenon to a change in the moon’s apparent distance.

However, we now know that Earth’s atmosphere does nothing of the sort. It may change the color of the moon, depending on how particles bend and filter moonlight, but that’s about all it does.

More recently, psychologists began to appreciate that the huge horizon moon is a true trick of the imagination—and it’s relatively easy to see for yourself that things are not as they seem.

Set a camera on a tripod and snap multiple images of the huge moon rising. When the moon is high in the sky, go back and compare the size of the moon’s disk in your photos. You’ll see that there is exactly zero difference.

Or, when the moon rises, roll a piece of paper so that when you look through the tube, the paper just hugs the humongous moon. Tape the rolled-up paper at that size and use it to view the rising moon. You’ll find that the moon never shrinks or expands inside that circle.

The moon always occupies roughly 0.52 angular degrees on the sky, or about the size of a thumb tip held at arm’s length. That changes by a minuscule amount between lunar cycles, with the moon’s apparent size getting up to 14 percent larger than normal during its closest approach to Earth. Still, the actual angular size of the lunar disk during a single moonrise will always be the same.

All in Your Head

In the 11th century, Arab mathematician Ibn Al-Haytham developed the first plausible theory for how the moon illusion works, suggesting that the size difference has to do with how our brains perceive distance, and then how we automatically adjust an object’s apparent size to match.

Al-Haytham suggested that when the moon is overhead, we perceive it to be closer and therefore smaller. But when the moon is rising over a distant horizon, we perceive it to be farther away and therefore larger.

One reason why the horizon might appear more distant than the sky overhead is that our brains perceive the shape of “space” as a gently flattened dome rather than a perfectly round sphere. That means we judge celestial objects that are overhead to be closer than celestial objects on the horizon.

In general, humans are terrible at estimating vertical distance; next time you’re gazing at a mountainous ridge, try and guess how high it rises above you.

“The idea of the perceived flattened dome of the sky is more promising, the apparent shape of the sky rather than the apparent distance to the moon,” says University of Oxford psychologist Brian Rogers, who experiments with the moon illusion in a planetarium.

Another possibility has to do with the way surrounding visual cues can trick our brains. This idea is best illustrated by the Ponzo illusion, in which two identical objects appear to be vastly different sizes the visual cues provided by their surroundings.

Trouble is, according to the apparent distance hypothesis, which psychologist Don McCready calls “popular but inadequate,” the horizon moon should appear to be both larger and farther away. But when people are asked how far away they think the moon is, they say the horizon moon appears larger and closer.

So, that doesn’t exactly work.

Universe of Theories

Another idea, called the relative size hypothesis, suggests that the illusion’s roots are in the sizes of objects in the foreground terrain, and that without houses, mountains, or trees, the lunar disk would not appear as large. This particular idea is similar to the well-known Ebbinghaus illusion, which demonstrates that two identical objects can appear radically different in size depending on what surrounds them.

A more recent idea even suggests that binocular vision could be the culprit, with our brains trying to compensate for the moon’s perceived position in front of a flat, distant sky by distorting the size of the moon.

The truth is, no one yet agrees about what’s going on.

“Although there have been written books and hundreds of articles on the topic, the issue is not fully resolved. Some factors are clear, other not,” says psychologist Claus-Christian Carbon of Germany’s University of Bamberg, who also studies the moon illusion using planetariums.

“What is really clear is that any kind of supermoon, such as the recent one where you could have been experienced a 14-percent larger moon due to the extreme closeness of the moon to the Earth (the closest since 1948), is ridiculously small when compared with a psychological supermoon.”

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Источник: https://www.nationalgeographic.com/science/article/moon-illusion-explained-horizon-size-supermoon-space-science

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