The Moon Illusion, an Unsolved Mystery | Hacker News Reader

The Moon Illusion, an Unsolved Mystery

lhup.edu

118 pointsbmease42 comments

"The literature on the moon illusion is vast, indicating an obsession with the problem that borders on lunacy"

Nice touch.

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

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.

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

Monocular Cues

https://en.wikipedia.org/wiki/Depth_perception#Monocular_cue...

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.

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.

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.

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.

> However, I'd like to think that atmospherics has more to do with things than the article goes into

But, from the article:

> We know that this phenomenon is not a physical or atmospheric effect, as may be easily demonstrated by photographing the moon in the sky at various elevations and comparing its size on the negatives.

I agree with that observation. I used to think that the different thickness in atmosphere had some some sort of lens or refracting effect, but that is clearly not the case.

I was thinking more along the lines of the smoke/dust/haze looking across our atmosphere may actually bias our visual cues and sense of perception, much like the absence of atmosphere did to the astronauts.

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."

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

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

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.

If you're standing 100m from that fence and zoom in on it ("telephoto" in proper terms) you'll end up with a very different picture than if you're 2m away and using a normal or wide-angle lens. Kind of like holding your fingers in front of your eye and "crushing" people's heads.

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-...

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.

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 [2] https://www.youtube.com/watch?v=udgF6hNoCRE

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.

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/

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. :)

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.

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

Everyone knows that the moon is an illusion.

http://www.revisionism.nl/Moon/The-Mad-Revisionist.htm

I have been looking for this for 15 years.

Thanks.

You can keep looking, it doesn't exists!

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"!

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

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

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.

I just kept thinking my memory was poor since I knew the distance didn't change that much.

Or just getting further away, or closer.

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

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

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...

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

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.

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

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.

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.

It dismisses refraction because the size difference is not captured by a camera or other instruments.

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.

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

Mentioned in the article.

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