
The Moon Illusion, an Unsolved Mystery - bmease
https://www.lhup.edu/~dsimanek/3d/moonillu.htm
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akeck
"The literature on the moon illusion is vast, indicating an obsession with the
problem that borders on lunacy"

Nice touch.

~~~
DougN7
I never understood what the word lunatic meant until I learned the German word
for it, which mean literally "moon addict".

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strathmeyer
While getting a Psychology degree I took a class learned "Perception" where we
learned there are seven cues the brain uses to tell how far away something is,
one of which is how elevated in the sky something is; higher things are assume
to be further away, which is why we get dizzy when there is no ground below
us. So a large object will look larger the lower it gets.

~~~
mrexroad
[Edit: depth perception article has more complete than monocular article]

Monocular Cues

[https://en.wikipedia.org/wiki/Depth_perception#Monocular_cue...](https://en.wikipedia.org/wiki/Depth_perception#Monocular_cues)

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elihu
My theory is that this is related to an illusion I experience looking through
a window at hills several miles away. When directly in front of the window,
the hills look a long ways off. When I'm 50 feet or so from the window, the
hills look like they're right there, immediately outside the window. By
narrowing the field of view, the hills appear much closer.

Maybe when the moon is close to the horizon, the sky looks less big because
local terrain encroaches on its boundaries, and so the moon in turn looks
bigger in a seemingly-smaller sky.

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cyberferret
Interesting article. I've always subscribed to the theory that nearer the
horizon, people tend to use other things on the terrain as reference points
for size (as the article explains).

However, I'd like to think that atmospherics has more to do with things than
the article goes into - For instance, (ironically), astronauts who have stood
on the moon itself have said that the lack of atmosphere and associated
dust/movement around them made it impossible to judge distances. Boulders,
rocks and mountains that they thought were only a few hundred feet away from
them were in fact miles away, and vice versa.

~~~
spdustin
If you've ever been to the Las Vegas strip and thought, 'it won't take long to
walk from Mandalay Bay to NYNY," you can make the same mistake on Earth, too.

:)

It's an interesting read, though, and I've also read the point you've made. I
wonder if there's any mathematical approach that can solve this, or if it's
one of those "only affects humans" sort of things.

~~~
cyberferret
I've read other theories on the astronauts dilemma. One train of thought
focuses on the curvature of the horizon. It was theorised that the moon (being
smaller than earth) had a more pronounced horizon curve, which lead to the
miscalculation of distant objects for the astronauts.

I am wondering if similarly, the very imperceptible curvature of the Earth's
horizon can also affect the perception of size depending on how close an
object is to said horizon. If so, it means that peripheral vision may provide
the brain with more cues than we realise.

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russellbeattie
This is a great explanation... could also be titled, "Why taking a picture of
the moon with your smartphone never looks as amazing as in real life."

~~~
thaumasiotes
You can't blame the moon illusion for that. Here's wikipedia's page image for
"Moon illusion":
[https://en.wikipedia.org/wiki/File:Harvest_moon.jpg](https://en.wikipedia.org/wiki/File:Harvest_moon.jpg)

~~~
pavel_lishin
Was that taken with a smartphone, or a camera with a nice zoom lens?

~~~
thaumasiotes
What would that have to do with the moon illusion? I doubt the moon has been
zoomed any further than the rest of the picture, if that's what you're
thinking.

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rangibaby
The further a lens is away from the film plane or sensor (this is what 50mm or
135mm refers to), the larger the background appears for the equivalent
foreground. It's called lens compression.
[http://www.bokehworkshop.com/public/charts/lens-
compression-...](http://www.bokehworkshop.com/public/charts/lens-compression-
chart-12mm-600mm-4k.jpg)

~~~
mortenjorck
Yep. It may seem counterintuitive that the moon could somehow be "zoomed
further," yet because of its vastly greater distance, that is effectively what
happens.

In the example above, the woman is standing the same distance from the
Starbucks, while the photographer moves further and further back with a longer
lens. As the woman recedes into the distance, the longer lenses are picking a
smaller and smaller angle out of the field of view, which makes the Starbucks
appear ever bigger behind her. The same thing happens, at a much larger scale,
with the moon.

~~~
drcongo
In movies this technique is known as a contrazoom or dolly zoom, most famously
used in Jaws [1], but most effectively used in Goodfellas [2] where it's done
incredibly slowly to give a terrible feeling of claustrophobia. One of my
favourite shots of all time.

[1]
[https://www.youtube.com/watch?v=sDWcQnaMMlM](https://www.youtube.com/watch?v=sDWcQnaMMlM)
[2]
[https://www.youtube.com/watch?v=udgF6hNoCRE](https://www.youtube.com/watch?v=udgF6hNoCRE)

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bitL
Our brain does non-trivial but deterministic visual processing. When you ask a
surgeon after performing an eye surgery, they often mention that their
patients see the world like through camera/with computer game perspective
projection (pinhole model etc., the usual "wrong" projection used nowadays).
After their eyes heal, vision returns back to normal (i.e. you don't see
curved walls like in a camera but they are nicely straightened for you by your
brain). Moon effect might be based on some of this processing; it seems
horizon might play a larger role in our vision.

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kartan
When you see how our visual system works with so much heuristics and hacks the
surprising part is that we are able to get things right the major part of the
time.

[http://www.michaelbach.de/ot/](http://www.michaelbach.de/ot/)

But that is not so scary as when you realize that the rest of things, like how
we reason, have the same kind of weird hacks and hacky solutions fine tuned to
live in Africa 250,000 years ago. So humans can design a super-complex
computer but fail to stop eating from an open bag of chips after deciding that
it was the last one. :)

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pgrote
In Depth explanation of the theories and reasons. Great read.

You can always ensure it's the same size by holding your thumb up to the moon
in the sky. Low on the horizon or high in the sky, your thumb will cover the
moon in the same space. It really does appear to be an illusion.

~~~
pklausler
You assume that one's thumb doesn't expand at lower angles.

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Raphmedia
Everyone knows that the moon is an illusion.

[http://www.revisionism.nl/Moon/The-Mad-
Revisionist.htm](http://www.revisionism.nl/Moon/The-Mad-Revisionist.htm)

~~~
77pt77
I have been looking for this for 15 years.

Thanks.

~~~
Raphmedia
You can keep looking, it doesn't exists!

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allthatglitters
When you next have an opportunity for a clear view of a full moon on the
horizon, turn your back to the moon, bend over, and view it from between your
legs... "upside down" so to speak. Not a joke - if you've never done this - it
will go a long way in explaining this "illusion"!

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dahdum
Wow, I've wondered many times why the moon looked massive sometimes. Never
thought it was an illusion.

~~~
CognitiveLens
If it wasn't an illusion, then the moon would literally be changing size
depending on when you looked at it, by definition.

~~~
thaumasiotes
That depends what you mean by illusion. A heat mirage, which is a shiny patch
floating just above the ground, is not an illusion -- in the sense that your
eyes are really receiving light from a weird floaty shiny area just above the
ground, and we can describe the physics that causes them to occur.

On the other hand, a heat mirage _is_ an illusion in the sense that, if you go
over and look for a reflective object, say a pool of water, at the location of
the mirage, you'll find there's nothing there.

The moon illusion, unlike a mirage, is an illusion in that first, stronger,
sense, but it didn't have to be. The image of the moon can change size in
reality (for example, when viewed through a lens) without affecting the size
of the actual moon.

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chm
The illusion could probably be easily tested with a VR headset.

The section entitled "Anisotropy of visual space" is very interesting.

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SilasX
This doesn't appear to discuss (what I understood to be) the accepted
resolution [1]: when it's near the horizon, you see objects near it, which
gives you a reference frame for sizing it, while higher up, it's just one
circle in isolation.

In fairness, that doesn't explain why the "compared" perception would be
_bigger_ than the "alone" one.

[1] relative size hypothesis:
[https://en.wikipedia.org/wiki/Moon_illusion#Relative_size_hy...](https://en.wikipedia.org/wiki/Moon_illusion#Relative_size_hypothesis)

~~~
alkonaut
The reference hypothesis has some merit, and is covered in the article.

~~~
SilasX
Good point, it's under "Contextual effects; reference cues in the field of
vision" and doesn't have diagrams for it -- usually they illustrate this
effect by showing a disk next to a bug, then a disk next to a skyscraper to
convey the intuition.

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Gravityloss
The web used to be like this. We expected a lot more from the audience.

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watermoose
This explanation dismisses refraction because it assumes the atmosphere is of
a single consistency. The atmosphere is composed of different types of
elements at different densities and temperatures depending on altitude, etc.

Also, our atmosphere contains an ionosphere where electrons, which are
involved in light propogation, could bend the light more depending on the
amount of ionosphere the light must travel through on the way to our eyes.

It's strange to me that these things were not discussed.

~~~
alkonaut
Any atmospheric effect is dismissed early on because (for example) the
illusion isn't visible in photographs. See picture in the atticle. It also
disappears if you hang upside down when you view the sky. Also in the article.

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chmike
The sun is subject of the same effect. When the sun is low on the horizont it
may some times look big, and even huge. Making a picture with a camera of it
shows only a small dot which is disapointing.

Another data is that the percieved diameter vary from day to day when looking
at the moon or sun from the smae spot. This excludes a psychological or
perspective effect.

The last relevant experimental data is that when the moon or sun appear big
and when you look at them through a small hole, they appear small again. It
works also when looking through a small hole made whith your fist.

My idea is that this effect is due to the combination of two factors. The
first one is strong humidity gradients of the air which can vary from day to
day. Living at the sea border, these gradients may have bigger amplitudes. The
same magnifying effect can be experienced when looking at the same mountains
on a long period.

I once saw a big low depth aquarium (20cmx1.5m). I could see the back of it
through the water or by looking just above it. There was a huge magnifying
effect when looking through the water giving the impression that the things in
the back were much closer and bigger then they really were. The effect was
still visible with only one eye open but with a slightly smaller magnitude.
Looking through a small hole I made with my fist, the magnification nearly
disapeared.

The second factor is due to the eye apperture and how our brain process Input
information. The exact mechanism is still unclear. This would explain that
cameras don't see the effect or that it vanishes when looking through a small
hole.

I suspect that our eye and/or brain is able to restore focus of images by
deconvolution to compensate out of focus images due to big apperture which we
need in low light. Our eye doesn't work exactly like a camera. It has a
powergul image processing unit behind the captor. The wide opening of our
pupil would allow to capture light of the moon from a much larger surface than
a conventional camera. And this would make our eye more subject to magnifying
effect resulting from humid air refraction.

looking though a small hole reduces the surface (apperture angle) of our pupil
and reduces the effect.

This idea should be tested. The first test should be to verify the correlation
of humidity gradient of the air with the magnifying effect. This could be done
in a lab. the second test should verify the correlation of pupil apperture
with the magnifying effect. Changing the ambient light in combination of a
fixed but significant humidity gradient should do the trick.

The fact that not everybody see the magnification effect could be due to
varing pupil apperture.

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uslic001
Just look at the moon upside down through your legs and the illusion goes
away. It is all in your head.

~~~
emmelaich
Mentioned in the article.

