# Optical Forums > Ophthalmic Optics >  When the moon hits your eye Like a big pizza pie It's ...

## rinselberg

OK - it's not "Amore". Not in this post anyway. It could well be history's oldest puzzle in optics - even in all of science. And it could also be history's oldest such unresolved puzzle.

_Consider this text from 2005, which I lifted from a page on the UNESCO Natural Sciences Portal under the title "The Miracle of Light"._

Have you ever wondered why the moon looks bigger when it's low in the sky and close to the horizon?

This familiar observation, known as the '"moon illusion" or alternatively, the "horizon illusion", is arguably the oldest unsolved scientific puzzle today. It extends to the rising and setting sun, and also to star constellations. The ancients mistakenly attributed it to what they thought were certain magnifying properties of the atmosphere. _And just a day or two ago, that's what I would have thought._



*The moon illusion. This image has been edited to exaggerate the effect*.
Credit: http://slapnose.com/archives/2005/06...oon_illusion/.


But is this really a _physical_ effect? Is it optics?

Surprisingly, the answer is no. The moon illusion was correctly diagnosed by  the illustrious  "father of optics" Alhazen (965-1040) as an aspect of the _psychology_ of human visual perception. It's not a property of the light that enters the human eye. 

The illusion has been tested by comparing photos of the moon when it's on the horizon to photos when it's directly overhead.



_Photography reveals that to the camera, the angular diameter of the moon is always about 0.5 degrees, regardless of whether it's on the horizon or directly overhead. In this time-lapse multiple exposure, the moon was tracked across the sky above Seattle at 150 second intervals. Credit: Shay Stephens._


Most people are surprised to find that the size of the moon as recorded on film is almost exactly the same from one photo to the next.


The moon illusion: animated "gif".
Credit: http://www.muskingum.edu/~psych/psycfair/moon.htm.


A mind-bending explanation for the moon illusion was given by Alhazen in his Book of Optics.

First, he proposed what is now called the SDIH (Size-Distance Invariance Hypothesis), explaining why an object would appear to be larger if it is perceived to be further away: An effect of visual processing in the brain.

Most present-day explanations of the moon illusion are based on some version of SDIH.

Second, Alhazen explained why the dome of the sky appears flattened; i.e. why stars near the horizon seem to be further away than stars that are directly overhead. Paradoxically however, most people say that the "big" moon on the horizon actually seems _closer_ than the smaller looking moon overhead.

It's precisely this paradox which certain present day researchers are still trying to resolve.


_Moving on to other sources ..._

Archaeologists found clay tablets from the royal library at Nineveh, dating to the 7th century BC, with references to the moon illusion inscribed in cuneiform.

It was remarked by the ancients in what is now China and by Aristotle ca. 350 BC. 

Ptolemy considered the problem ca. 150 AD.

Roger Bacon, Leonardo da Vinci, Johann Kepler, Rene Descartes, Marin Mersenne, Christiaan Huygens, Leonard Euler, Alexander von Humboldt, Hermann von Helmholtz and Thomas Huxley II wrote discourses on the moon illusion.

Maurice Hershenson authored a 400 page book _The Moon Illusion_, published in 1989.

Internet searches keying on "moon illusion" have retrieved more than 2000 online reports.

One of the better (and longer) essays on this subject was posted online by Donald E. Simanek as The Moon Illusion: An Unsolved Mystery. It was revised as recently as 2002.


In 2000, PNAS (Proceedings of the National Academy of Sciences) featured Explaining the moon illusion from a father and son team:


> The Kaufmans ... designed two experiments to measure directly the perceived distance to the moon. Both tests used an apparatus built at IBM Research to project stereoscopic images of artificial moons from an IBM ThinkPad computer display to optical infinity so viewers could see them against an actual sky. Professor Kaufman then took people to a Long Island hilltop, where he made hundreds of measurements of their perceptions of the distance to the moon.


Father-Son Scientists Confirm Why Horizon Moon Appears Larger is a summary of the PNAS report that includes an animated binocular fusion experiment that you can try. It's based on the IBM moon simulations.



_Binocular fusion experiment. See Father-Son Scientists Confirm Why Horizon Moon Appears Larger for viewing instructions. Select the stereogram (above) for a high-resolution JPEG image._


Helen Ross and Cornelis Plug coauthored a 250 page book _The Mystery of the Moon Illusion_, published in 2002. It's said they researched the illusion for their entire working lives and published many smaller papers on the subject.


In 2005, BBC News took up the issue, opening with:


> It can put a man in space and land a probe on Mars, but NASA can't explain why the moon appears bigger when it's on the horizon than when it's high overhead.


They went on to report:


> Two main theories dominate. The first, known as the Ponzo Illusion - named after Mario Ponzo who demonstrated it in 1913 - suggests that the mind judges the size of an object based on its background.
> 
> Ponzo drew two identical bars across a picture of railway tracks which converge as they recede into the distance. The upper bar appears wider because it appears to span the rails, as opposed to the lower bar, which sits between the rails. In the same way, with a low-lying moon, trees and buildings in the distance, which are familiar foreground reference points, set up a contrast that makes the moon, as we perceive it in our brain, appear larger than the image of the moon that is projected by our eyes onto our retinas.
> 
> Skeptics of this theory point to sea and air-based observers (ship and aircraft crews) who also see the illusion, even in the absence of ground reference points (like trees and buildings) that are visible to land-based viewers.
> 
> Alternatively, there's a theory that the human brain perceives the sky as a flattened dome rather than the exact hemisphere that actually describes it. We perceive things that we see immediately overhead - flying birds for example - as closer to us than birds on the horizon. Birds flying overhead _are_ closer to us than birds on the horizon. When the moon is near the horizon, the human brain, conditioned by its memories of birds, miscalculates the moon's true distance and size; i.e. the brain compensates for the low-lying moon's seemingly greater distance by perceiving it as larger than the image that is received by the eyes.


This is illustrated with four diagrams that you can view in sequence.


In 2006, another moon illusion "crusader" criticized the Kaufman father-and-son report in PNAS (cited above) as too simplistic. Don McCready, Psychology Professor Emeritus at the University of Wisconsin (Whitewater), brought _oculomotor micropsia_ and _macropsia_ front and center in his explanation. His closing paragraph is remarkable:


> Roscoe and his colleagues have conducted many experiments which clearly show that the moon illusion illustrates oculomotor micropsia.
> 
> Roscoe's many publications have emphasized the largely overlooked role that oculomotor micropsia can have in aviation accidents when, to land the aircraft in low visibility conditions (weather), the pilot depends upon a "heads up" viewing device that shows an image of the landing strip ... This screen, viewed by the pilot at close range, induces oculomotor micropsia, so the pilot consequently "sees" the runway as further ahead than it truly is, and so may land (i.e. crash) beyond the runway (and some have).
> 
> A similar condition with the eyes unwittingly adjusting to a near distance can occur for an automobile driver with a wet windshield, especially at night, or when driving in fog. The resulting oculomotor micropsia can make objects in the road ahead look deceptively far away, so the driver tends to underestimate the safe braking distance, and may discover, too late, that an object was actually much closer than it appeared.
> 
> In other words, trying to understand the causes of the moon illusion is _more_ than just an idle academic pursuit.



In a sidebar on this webpage, McCready suggests that oculomotor micropsia and macropsia were selected in one of our evolutionary ancestors _ala Darwin_ because they offer survivability advantages in the natural environments that predated civilization. But the Internet blogger "slapnose" (Anthony Hecht) was more succinct in his assessment:


> Other, more "sciencey" explanations are around, but they're too long and contain words like _oculomotor micropsia_ ...




Additional sources:

Archimedes Lab: The Moon Illusion
Did Ptolemy understand the moon illusion?
Explanations of the Moon Illusion
Moon illusion (Wikipedia)
NASA: Summer Moon Illusion
New Thoughts on Understanding the Moon Illusion from Carl J. Wenning
Perception: Alhazen's contribution to the Moon Illusion
The Moon Illusion: A Literature Thesis by Bart Borghuis.
The Moon Illusion: The Internet Version by Paul Niquette.
Why does the moon appear bigger near the horizon?

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## chip anderson

Long as we be done to pondering such stuff.  Why are tides higher on the full moon?  The moon's gravitational effect shouldn't be any greater whether light or dark.
What is the difference between "dangerous lightning" and "deadly lightning"?   Is there a kind you can play with?

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## Barry Santini

SDIH is the most probable explanation. You can simulate the same circumstances as this moon illusion with a simple, 2x-3x magnifying glass. Simply hold the magnifying glass near your eye, and pull the object being viewed to (just slightly inside) the limit of the magnifier's focus. Then, while keeping the distance of the object and magnifier the same (fixed), pull the two items *in-tandem* away from your eye. The object will appear to grow in size, but in reality, your brain does another iteration of SDIH. The changing ratio of object-size-to-magnifier *frame* (surround) also compounds this illusion.

FWIW

Barry

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## Darryl Meister

> Size-Distance Invariance Hypothesis


I've always seen this referred to simply as _size constancy_ in vision science.

It is interesting to note that they teach you in art theory that objects positioned at higher heights in a scene tend to be perceived as lying at farther distances from the observer. This is one of the various techniques used to create a sense of depth by painters.

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## Robert Martellaro

> Have you ever wondered why the moon looks bigger when it's low in the sky and close to the horizon?


When you tilt your head back to observe the moon at the zenith, the lens of the eye is flattened by gravity and its magnifying power is decreased, resulting in decreased image size, making the moon look much smaller. When you look back down towards the horizon the lens returns to normal and you see the moon at its correct size.

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## icare

> Long as we be done to pondering such stuff.  Why are tides higher on the full moon?  The moon's gravitational effect shouldn't be any greater whether light or dark.


I used to ponder the same question - here you go:
http://en.wikipedia.org/wiki/Tidal_range

When the moon is either full or new its gravitational force is added to the gravitational force of the sun.

Interestingly, that's also the same reason that you can adjust a progressive twelve different ways and the patient keeps coming back.
:hammer:

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## rob.optician

yarrrp.

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## dbracer

> When you tilt your head back to observe the moon at the zenith, the lens of the eye is flattened by gravity and its magnifying power is decreased, resulting in decreased image size, making the moon look much smaller. When you look back down towards the horizon the lens returns to normal and you see the moon at its correct size.


The lens, whether focused, unfocused, flattened or unflattened or whatever does not create magnification, either angular mag or distance mag (well  possibly  the latter for near objects). 

If indeed the crystalline lens does change it would make the viewed object out of focus, since the moon is definitely a distance object which requires no focus. 

The moon thing is an optical allusion, since if you measure the moon on the horizon with a caliper, (ie view the moon and adjust the caliper in alignment with the image edges) then measure it again when it is overhead, you will find that for the latter you do not have to readjust the caliper. It is actually measures the same in both instances. 

Respectfully,
dbracer

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## chip anderson

While I agree with dBracer, this would only be true if the caliper were held at the same fixed distance from the eye in both senerios. 

Chip

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## dbracer

> While I agree with dBracer, this would only be true if the caliper were held at the same fixed distance from the eye in both senerios. 
> 
> Chip


You'll get no argument from me there, Chip. Yes the "viewing window" would have to be constant.

Respectfully 
debracer

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## rinselberg

There's no doubt that it's an optical illusion.

The challenge has been to explain how and why it comes about, in a way that everyone can agree with.

I'm not really "into" optical illusions, but I think that most of the illusions that are presented to us have been explained in a way that most learned parties find acceptable - how it comes about that this is the way we perceive what is put before our eyes - how our brain is being "tricked" or "deceived" by the light that enters our eyes.

I'm not sure that this moon illusion has yet been explained in a way that satisfies a concensus of the experts.

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## dbracer

> There's no doubt that it's an optical illusion.
> 
> The challenge has been to explain how and why it comes about, in a way that everyone can agree with.
> 
> I'm not really "into" optical illusions, but I think that most of the illusions that are presented to us have been explained in a way that most learned parties find acceptable - how it comes about that this is the way we perceive what is put before our eyes - how our brain is being "tricked" or "deceived" by the light that enters our eyes.
> 
> I'm not sure that this moon illusion has yet been explained in a way that satisfies a concensus of the experts.


Rinselberg,

Of course you're correct.  The mystery in explaining optical illusions is, "How do they happen?"  

But, Now you're asking questions rivaling:

1. Why is light bent toward the norm as it goes from a rare to dense media?

2. Why does light travel at 186,000 mi/sec regardless of the how fast the emitting device is traveling toward or from the measuring device?

3. Why does acceleration put the "special" in the Special Theory of Relativity. 

4. Why is sound a longitudinal wave and electromagnetics are transverse?

If you were walking in the forest and found a fully functional computer lying in the crook of a tree, you'd know that there is a computer builder somewhere. The world is far more complex and mysterious than a laptop. It further works with far more precision, accuracy, and profound consequence. And, it repairs itself constantly without the aid of a technician.

I like knowledge as much as the next guy. I have 9 years of higher education, but whether the secular progressive world likes it or not, there is a time to just say, "God made it that way," because further explanation is useless esoteric academics.

Respectfully,
dbracer

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## chip anderson

Anyone figured out why the meteorologists say tides are higher with the full moon?  Can't see why the moon's gravity should pull any more with or without more reflection from the sun.

Chip

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## talapanka

Maybe this BBC site could have the answer http://news.bbc.co.uk/1/hi/magazine/4619063.stm

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