Moon Illusion | Hacker News Reader

Moon Illusion

en.wikipedia.org

129 pointsdcminter71 comments

In which I discovered an explanation for an illusion that always surprised me [0]: the fact that a partially illuminated moon does not seem to reliably indicate the direction of the sun.

[0] https://en.wikipedia.org/wiki/Terminator_(solar)#Lunar_termi...

I find the description of the phenomenon and its explanation incomprehensible. Can anyone explain more clearly what it means?

EDIT: here is a better explanation: https://chrisjones.id.au/MoonIllusion/

OK, the point is that when the moon and the sun appear to be the same height in the sky the sun is actually "above" the moon, because it's further away. Therefore the moon is lit from "above".

It doesn't have anything to do with distance to the objects. Straight lines in the sky are great circles that may appear curved to us because of our biased view standing on the surface of earth. Light rays traveling from the sun to the moon take a great circle path through the sky, and the path might look to us (if we could see it) like it goes "higher" in the sky than either object, but it's a straight line across the dome of the sky.

> It doesn't have anything to do with distance to the objects

Sure it does. If the sun were closer to us than the moon it wouldn't be possible for the sun to appear lower in the sky yet appear to light the moon from above. Indeed the opposite would be true: it would appear to light the moon from below.

So the relative distances to the objects is exactly relevant to determining from which direction the moon appears to be lit.

Their relative distances affect which portion of the moon is illuminated, but not what the line between the two looks like projected against the sky. That line being a great circle is what causes the terminator illusion.

I'm not sure I follow. You said

> It doesn't have anything to do with distance to the objects

yet if the moon and the sun were the same distance from the Earth the phenomenon would not occur, so I'm having trouble understanding what your claim means. It seems the phenomenon can only arise because of the difference in distance to the objects.

Oh no, I take that back. Even if they were at the same distance you could observe this phenomenon. So my explanation in terms of distance only works in a particular case, but the great circle explanation works for all cases. Interesting, thanks!

You had me doubting myself. I'm pretty sure this would happen with a gibbous Mars or Venus that is farther away than the Sun. But we can't see those phases with our naked eye, and the effect would be less because they would be pretty close to the Sun in the sky.

I reckon if we had a gibbous Venus that appeared to be on the same level as the sun, then it would "appear" to be lit from below(ish) (if we could even see it). Does that match what you expect?

EDIT: No, I think that's not even right. This is very mind bending!

It depends on where they are along the ecliptic in the observer's sky, I think. Objects appear to rotate with respect to the ground/horizon as the cross the sky.

This stackexchange question[1] has some interesting answers. The pictures where the lady is holding a globe and the one that shows the sky as a circle (round horizon) vs a flat horizon are my favorite.

[1] https://earthscience.stackexchange.com/questions/14809/what-...

Edit: but ignore the flat-earther in the comments.

This video is also quite interesting:

https://www.youtube.com/watch?v=Y2gTSjoEExc

When you come across this illusion, you can "break" it by getting a piece of string and holding one end over the moon and one over the sun, and you'll more easily see how the moon does after all point at the sun. Lying on the ground can also help.

The real paradox is that we think that we see the world as how it is, like a camera does, when in fact what we "see" is reconstruction based on surprisingly imperfect visual perception and equally imperfect recollection of prior experiences.

> reconstruction based on surprisingly imperfect visual perception

Unlike a camera? ;)

> equally imperfect recollection of prior experiences

I guess we (humans) put our understanding of the world into the mechanics of the camera, so somehow I see the mechanics of a camera like a summary of human "recollection of prior experiences" regarding visual imperfection.

> Unlike a camera? ;)

Cameras have higher overall resolution than the eye, as well as it being consistent across the sensor surface (unlike the eye). Eyes have pretty good dynamic range, but DSLRs are definitely catching up. Same with sensitivity. So I'm not sure whether I'd describe a camera as being all that imperfect.

But the point I was making is that we mostly navigate the world unconsciously believing that we perceive it directly via the light that hits our eyes. But we actually "see" an unfolding reconstruction that our brain pieces together from evidence contributed by our eyes and memories. If your vision is good, and especially when you're young, it's easy to be completely unconscious of the actuality of vision.

> I guess we (humans) put our understanding of the world into the mechanics of the camera, so somehow I see the mechanics of a camera like a summary of human "recollection of prior experiences" regarding visual imperfection.

Interesting question. I wonder if photography changed the way that people think of their vision - compared to previous methods of visual capture like painting or drawing? I suspect not but I don't know.

Camera captures light values of a scene at a specific moment in time.

While it’s better at that than your eye, that is vastly different from perceiving reality over time. Which is, incidentally, the only way humans perceive reality.

The way you perceive a scene to large extent depends on experiences you have had immediately before that (as well as in all of your prior life), your mood, what the air smells like, whom you are with, and so on. If you sit in a grey room for a long period of time and step into a garden, the colors will jump at you. If you grew up in a small town and stepped onto a rooftop in the middle of a modern megalopolis for the first time, you might initially be stunned by the amount of detail hitting you all at once; if you lived and seen that image every other day for years, you’d be inclined to filter out a lot and focus on what changed. You would not be able to be tricked by your mind into thinking the moon is surprisingly large when it’s near the horizon have you not had the prior experience of seeing the same moon overhead. So on, so forth.

Furthermore, the “objective” light values captured by the camera are fairly useless by themselves. We have to trust bundled software, film manufacturers, or indeed our own skills to interpret those raw values in terms of display space, and hopefully produce something resembling what we have experienced in the moment.

As an aside, it’s often frustrating how some believe that the output of camera’s JPEG engine at default settings is how reality is. It irks me when someone characterizes a dull[0] photo as “true to life”, and a photo interpreted to make the most use of P3 color space as “edited”, implying that it’s no longer representing reality objectively. There’s no objective representation of a single moment in time and there’s no useful reference point here; both outcomes are “edited”, but perhaps in the latter case the photographer tried to convey how they felt in the moment rather than defer to the view of whoever engineered their film or camera’s JPEG engine.

[0] A film or camera engine, while designed by undoubtedly skilled people, is optimized to provide acceptable outcomes for most diverse scenes and doesn’t really know much about final output medium, and as a result the outcomes tend to make a very conservative use of available color space.

Addendum: and yes, I do suspect that this aspect of photography has influenced (and not in a way I would characterize as “good”) our perception of reality and how we think about our vision.

Not only that, but human vision has a variable zoom - objects we focus upon receive a zoom, an enlargement distorting physical size. The human vision variable zoom is similar to the "3D hat" of a sine curve. The Fovo Renderer people discuss this here: https://www.fovotec.com/the-fovorender-story-part-1

I sometimes play with a simple experiment that I think illustrates this quite nicely:

Cover one eye and look at something, then uncover the eye, and notice the added "depth" information. Personally, I don't observe the depth information from stereo vision in a way that I can easily describe in terms of a (single) photographic image.

Apparently the images from the two separate eyes are combined after processing some of the depth information. My consciousness does not seem to have direct access to the entire pipeline.

(I made some goggles to experiment for longer durations. Great fun.)

Similar things apply to face recognition, instantly comprehending words from letters, mathematical formulas (possibly only for trained mathematicians), and perhaps even fear of spidery images.

You might be interested by the book "Consciousness and the Brain" by Stanislas Dehaene. In it, Dehaene tries to define Consciousness, based on a series of reproducible experiments designed to discriminate between conscious and unconscious processes.

For example, he is able to show that our unconscious processes that are responsable for parsing text and numbers are able to assign conceptual meaning to words/numbers (ie "five" and "5" lead to the same meaning), before any conscious awareness.

Although my summary hardly does it justice (as I'm still half-way through reading it), I highly recommend Scott Alexender's book review on Astral Codex Ten[0], it gives an exhaustive overview of the book and the subjects it covers.

[0]: https://astralcodexten.substack.com/p/your-book-review-consc...

Your covering-one-eye thing works particularly nicely if you use it in a place where you can see the tops of lots of trees at various distances overlapping each other. Cover one eye and suddenly all those canopies become one depthless blob, like it's one gigantic treetop.

It sounds like you might be interested in experiments with deliberately redirecting your eyes. There's lots of interesting phenomena you can achieve by pointing one eye at one thing and another eye at another thing. Here's a paper of mine which can serve as a sort of stepping stone into this wide unexplored field: https://philpapers.org/archive/ALEDSK.pdf

Side fact… ketamin has an interesting side effect. On ketamin your brain cant synthesis the image of both eyes and you are seeing both images at the same time. I tried it once … didnt like it. But this was one of the effects… and beeing confused as it gets

Which is probably why art exists. Sometimes a painting seems to capture reality better than a photograph.

Still frames of UE5 are certainly getting there. (Put it in motion, and that's a different story for sure...)

> when in fact what we "see" is reconstruction based on surprisingly imperfect visual perception

… like a camera does :)

No. Nothing like a camera does.

https://www.cambridgeincolour.com/tutorials/cameras-vs-human...

Yes, like a camera. You are confusing what we "see", with interpretations that don't occur in the eyes. That's even why you put the word "see" in quotes. You used the wrong word. The word "see" explicity refers to what is observed by the eyes, not our later interpretations.

No, it doesn't. "See" refers to our visual perception, whether it occurs in the eyes, the brain, or anywhere in between.

From the standpoint of dictionaries (and history, usage, and etymology), "see" refers to the eye. Below are the first entries in three different dictionaries and an etymological entry. All refer to the eye and not the brain or other intermediate locations.

Dictionaries:

  merriam webster:  to perceive by the eye [1]

  dictionary.com: to perceive with the eyes; look at [2]

  cambridge: to be conscious of what is around you by
    using your eyes: [3]

Etymology:

  eymonline: Middle English sēn, from Old English seon (Anglian sean)
    "be or become aware of by means of the eye; look, behold;"
    and
    It is attested by late Old English as "be able to see with the
    eyes, have the faculty of sight, not be blind." [4]

[1] https://www.merriam-webster.com/dictionary/see

[2] https://www.dictionary.com/browse/see

[3] https://dictionary.cambridge.org/us/dictionary/english/see

[4] https://www.etymonline.com/word/see

> to perceive

Where does this "perceive" happen?

That's not relevant. Perception doesn't occur "in the eyes, the brain, or anywhere in between". When something is perceived, that means the person has attained awareness of that thing. This awareness of an object's existence is entirely different from what was asserted, some sort of superimposition of prior understandings or other morphological change onto the eye's perception of that object's existence.

> From the standpoint of dictionaries (and history, usage, and etymology), "see" refers to the

> merriam webster: to perceive by the eye [1]

> perceived ... attained awareness

Perception is part of the definition you provided. How do you come to call it not relevant now?

Does the eye of a dead person see anything? Is it like a camera that clicks in a forrest with no-one to hear?

The notion of eye mentioned in the definitions you quoted is to distinguish "see" from other senses such as "hear" or "taste." Even at a physical level, the eye alone is little like a camera. It has an uneven distribution of light sensors with spaces without coverage (optical nerve).

"The retina is actually a piece of the brain that has grown into the eye and processes neural signals when it detects light. Ganglion cells carry information from the retina to the higher brain centers; other nerve cells within the retina perform the first stages of analysis of the visual world. The axons of the retinal ganglion cells, with the support of other types of cells, form the optic nerve and carry these signals to the brain."

"Investigators have known for decades that there are 10 to 15 ganglion cell types in the retina that are adapted for picking up different movements and then work together to send a full picture to the brain. The study estimated the amount of information that is carried to the brain by seven of these ganglion cell types."

https://www.eurekalert.org/news-releases/468943

> Perception is part of the definition you provided.

I was providing an answer to your question. The location of perception isn't relevant to the definition of the word "see". This conversation was about the word "see" applying to the eyes.

Wouldn't that make the word "see" essentially useless since we cannot know what we've seen?

"I saw a cow outside" - "No, you perceived a cow. You have no idea what you saw because you cannot access that information."

Does this seem like a good definition of "see" according to you?

> Wouldn't that make the word "see" essentially useless since we cannot know what we've seen?

Of course that's not true.

> "I saw a cow outside" - "No, you perceived a cow. You have no idea what you saw because you cannot access that information."

No. You both saw and perceived a cow. You also thought the cow outside was too skinny for being a Chianina. We perceive things through the use of the senses, and then we interpret, discern, and think.

You perceived a cow through the senses of sight, smell, and maybe sound and touch but probably not through taste. Your sense of sight was used to see, making you aware of the cow in that way.

Why so much discussion about the basic meanings of words that can be viewed in a dictionary? I find it odd that on HN people want to debate the meanings of words, when those words are already well defined.

Earlier you said: “You are confusing what we "see", with interpretations that don't occur in the eyes. That's even why you put the word "see" in quotes. You used the wrong word. The word "see" explicity refers to what is observed by the eyes, not our later interpretations.”

Are you saying that the brain interpreting signals from the eyes (real-time) cannot be affected by past experiences (i.e. interpreted)?

If that’s not what you’re saying then I’m not sure what your opinion is, feel free to re-state it if you want to have more pointless discussions ^^

Wow, it's been a while since so many ads annoyed me.

But the nature of the imperfect reconstruction is very very different.

A camera doesn't "see" the world "how it is".

Depending the sensor or array of sensors, cameras arguably see more of the world 'how it is' than humans are capable of.

Night Vision, Heat Maps, LIDAR, and Telescopic lenses can capture more of the world 'as it is' than humans could ever hope to achieve. It's why we developed them! :)

Hard disagree.

Cameras and their sensors are just different from the human eye. This doesn't make them capture reality "how it is", they just have different capabilities and they make different trade-offs. They are certainly different, but not necessarily "more real".

Cameras don't capture "reality".

An insightful link on this topic, shared previously here on HN, which is very illuminating: "Photography is not objective" [1]

[1] https://aaronhertzmann.com/2022/03/17/photography-is-not-obj...

A beautiful, big full moon was visible over Christchurch, New Zealand tonight, and I was just talking about this with the kids. It fascinates me that scientists have sent us up to the moon, but can't figure out exactly why it appears bigger when it's rising.

Maybe objects at low angles are perceived with another computational unit, since they are part of the scenary and we need to extrapolate, and the sky is a big whatever.

But really maybe there's no simple explanation other than "that's how the brain works", and we assume there has to be one.

Just came to say I saw the same full moon of startling clarity in Christchurch.

I am also in Christchurch and was wishing my iPhone was able to capture the moon in any amount of detail tonight

Key word is "appears." Brain function is another frontier of knowledge just like all those encountered in a space program.

The Ponzo illusion is one possible explanation of the Moon illusion, with objects appearing "far away" (because they are "on" the horizon) appearing bigger than objects "overhead". However, some have argued that explaining one perception ("appears far away") in terms of another ("appears bigger") is problematic scientifically, and that there are probably complex internal processes behind these illusions.

https://en.wikipedia.org/wiki/Ponzo_illusion

if you are still young and flexible enough .... compare the apparent size of the full moon near the horizon when standing normally and then upside down, by looking between your legs, or standing on your head.

It is how proved to myself my eyes can not be trusted really

>Conversely, if the more distant object did subtend the same angle as the nearer object then it is normally perceived to be twice as big.

herein lies I think the real cause of this illusion - It's the way our brain perceives size in things that are considered to be at a disconnected distance from each other, and things that are seen to be 'connected' being perceived as being the same distance. As soon as the moon comes close enough to the horizon to be perceived as 'connected' in our minds, the perception of it's size is shifted dramatically.

This same effect occurs with tall islands - If you're standing on the shore and look towards the island, it appears one size. If you move inshore and lose sight of the water, viewing the island over rooftops and such, it appears much more massive.

I live two blocks away from Lake Michigan and I’ve noticed the same effect with sailboats on the horizon. When I’m walking towards the beach and can only see a small sliver of water, sailboats on the water look absolutely massive, but as I get closer and see them in full context, they seem to shrink.

Yep, I witnessed the effect when living out near Kapiti island, which is pretty big, quite tall, and very close to the mainland. The effect some evenings was drastic, especially when you had the moon and island on the horizon. It reminded me of the cover for the film Synchedoche, New York. That sense of the scales all being very wrong.

I love that the proof of illusion section doesn't even try to prove that there's an illusion.

You could probably test out some of the perception in VR by doing things like putting the moon straight overhead but also put the horizon or some buildings in the sky overhead to see if it has the same enlarging effect as at the real horizon

People who think AR/VR is a 'fad', won't last; etc - really miss out on the potential for simulation and work in legitimate scientific and research fields.

Screw games, I'm more interested in stuff like VR/AR software that's helping surgeons and mechanics. That's the real future to me.

Such VR applications were foreseen even three decades ago

http://www.youtube.com/watch?v=h58lRIVHhGc&t=2m47s

Does this happen to everyone? Because i don't see this effect for the skyline, cloud and monochrome diagram picture in the article (despite the captions), only a tiny bit in the Ebbinghaus diagram.

It depends on how far away the camera is. The further away the camera is, the more visible the effect. The moon illusion effect is quite visible in this photo of the Manhattan skyline, taken from 15 miles away in West Orange, New Jersey.

https://cdn.theatlantic.com/thumbor/r3PYEwue-SKDCmd_v7HgzEF2...

That's not really what the illusion is about, it's just perspective compression.

AFAIU perspective compression is a key element of the illusion - when the moon gets closer to the horizon/treeline/skyline, our mind creates a sort of vignette with limited FOV for perceiving it, in which the relative size of the moon appears larger (because our focus has become inadvertently and subconsciously narrower).

I don't think it can "happen" to someone and not the person next to them, i.e. I don't think each individual observer sees it differently. I have seen what the article calls a harverst moon a few times. Every time everyone I asked from the same city has also seen it. I've seen it on the news, captured by photo and video cameras, so I'm pretty confident it's as optical as using lenses to zoom into things.

Oh, it's definitely just an illusion. You can take a suggestive picture of it, if that's your intention, but in fact if you took a couple pictures with a digital camera, few hours apart, you'd note that the Moon has the exact same pixel width in the pictures, regardless of it position in relation to the horizon. Moreover, the near-horizon pictures would actually show a lower pixel height, due to the atmospheric distortion.

It isn't happening for me at all in the examples.

In the same page they clearly state that the full moon's size varies due to its eccentric orbit. We all know the moon is sometimes clearly larger and that's the reason, this illusion stuff strikes me as mostly subtle b.s.

It should vary over the course of the month, but not a few hours. Many people experience the feeling that the moon is bigger when it’s near the horizon. It’s a feeling. You might not personally experience it. That’s OK. But other people report it, so unless they’re lying, the illusion exists empirically.

> It isn't happening for me at all in the examples.

You mean looking at the images on the wiki page ? Yeah it doesn't work for me either, seeing it in real life works every single time on me though

Just my thoughts on the illusion in case it's helpful.

I saw this illusion tonight when I suddenly saw the moon behind the top of a distant tree. I think the suddenness adds to the illusion, and the explanation of the brain's image processing reasoning about the size because it is compared to something distant feels right. It does not feel just a small amount bigger - at least double the usual size. Even as I looked at it, the illusion weakened. It was obscured by trees and building as I walked towards it, and again I saw it rise again over a building a little way off. The illusion repeated, though not as strong. Much later I saw the moon in the sky, and it looked normal size.

I do not have the illusion anywhere near as strongly when I look at that skyline photo, although I can sort of see what they are trying to describe.

I think many photos of the moon zoom in, greatly increasing the apparent size, and I don't think this is the same as the illusion, although in a way, comparing the moon to a building which is known to be large may contribute to the illusion we perceive when seeing it in real life (not in a photo).

There is a view of the Taj Mahal from Agra fort where it looks larger when viewed from a particular window. Looks similar to this illusion.

It really is puzzling- when I’ve seen this, I’d swear I could make out more detail on the surface of the moon than when it’s higher. I’ve measured with a ruler and know that it’s the same size, but there is definitely something affecting perception. Maybe higher contrast when it’s lower, in addition to having the horizon to measure against?

In the uk there will be a perfect example of this at around 10:30 this evening

What's special about this evening?

Peak full moon coinciding with moon at closest point in its orbit to earth coinciding with perfectly clear skies

Does the "moon at closest point in its orbit to earth" mean that the moon will actually take up a little more sky than usual tonight?

It's not a huge effect, but it's about 14% larger when it's closest compared to when it's furthest away.

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