In which I disagree with cartoon Neil Tyson

By Phil Plait | November 14, 2011 11:59 am

Last week, I was checking my feed reader, catching up on all my favorite web comics. One of them is sci-ence, a comic you really should be reading. It’s drawn (in part) by artist and science afficianado Maki Naro, and (like xkcd and Saturday Morning Breakfast Cereal) it’s both funny and scilicious.

I got a snicker out of the comic he had just posted, dealing with my pal Neil Tyson and the Moon. Go read it!

Back yet? OK.

Now, I know that just last night I was praising Neil, and today I have no cause to bury him. But I will nitpick a wee bit…

First, of course, who hasn’t wanted to chase Neil Tyson down the street while yelling incoherently at him? But that aside, I must point out that this explanation of the Moon Illusion, while very common, is not actually correct.

The Moon Illusion is when the rising (or setting) Moon looks huge and fat, squatting on the horizon, but appears far smaller when up high in the sky. But it’s not because you’re comparing it with foreground objects! I’ve seen this illusion when out in the open plain, with nothing between me and the horizon but Kansas farmland, which is like a geometric plane, except flatter.

The real cause is actually a bit more subtle, and the combination of two different illusions. I wrote a pretty thorough explanation a while back… and I figured, why not let Maki know?

So I sent him a note. He promptly replied, saying he’d update the comic. And when he did, I got quite the surprise… click the panel to see the whole thing.

I’m honored to be included in the comic, which is almost entirely accurate. The science explanation is correct, and once I was lost in a park; ironically Central Park, not far from the American Museum of Natural History where Neil works… shortly before I was scheduled to give a talk there! I’ll note that although it’s not exactly my style to jump out of a bush at people — scaring folks and potentially getting shot are things I generally try to avoid — I am enough of a dork to interrupt a conversation about astronomy if it’s going astray.

Come to think of it, it’s kind of amazing I never have been shot at.


Related posts:

Why does the Moon look so huge on the horizon?
Very Large Moonset… but not why you think
Kryptonite for the supermoon
Great Tyson’s ghost!

CATEGORIZED UNDER: Debunking, Humor, illusion, Science, Skepticism

Comments (53)

Links to this Post

  1. Male Fantasies | Sci-ence! A Skeptical Comic and Blog. | November 14, 2011
  1. Physicalist

    Man, calling Tyson a “cartoon” . . . that’s pretty low.

  2. Mike

    I knew it. The moon illusion is just another example of an illegal Ponzo Scheme.

    lol

  3. VinceRN

    The artist thinks you are nicer about explaining thing than Tyson? He got chased away, you got thanked.

  4. Isn’t there also a factor where humans actually perceive the sky as a shallow bowl over our heads rather than a true dome? http://astronomy.stackexchange.com/questions/183/why-does-the-moon-sometimes-appear-giant-and-a-orange-red-color-near-the-horizon/1197#1197

    And another comic added to my reading list! So, how did you end up getting so lost?

  5. Now I have this mental image of vigilante astronomer superheroes stalking the city at night, swooping down to correct astronomical misconceptions.

    By the way, Maki is also one of the Mad Art Lab crew. We’re everywhere.

  6. DrFlimmer

    I really ROFLed when I saw Phil appearing behind the bush. That’s absolutely brilliant! :-D

  7. Thanks Phil!

    @VinceRN It’s all part of my plan to start a “Good Guy Phil” image meme.
    In serio, we love Neil, but are mercilessly capitalizing on the bad rap he got for the Pluto thing ;)

  8. Science smack-downs in the parallel comic universe. It is all so meta.

  9. Pete Jackson

    If you ever get a chance to see a total solar eclipse with the Sun (and Moon) low in the sky, grab it. The entire eclipse apparition is enlarged also, including the corona, making for an unique view of an eclipse.

    We saw this kind of total eclipse on the early morning of July 11, 1991, from a ship off the big island of Hawaii.

  10. QuietDesperation

    First, of course, who hasn’t wanted to chase Neil Tyson down the street while yelling incoherently at him?

    That would be me, I guess.

  11. Joseph G

    Phil wrote: First, of course, who hasn’t wanted to chase Neil Tyson down the street while yelling incoherently at him?

    Depends on how emotionally attached to Pluto you are :D

    Personally, I never understood all that hullabaloo. Though I do think that “clearing the neighborhood” is way too vague to be a part of any good scientific definitions. Just my $.02.

  12. @#5 Steve: Now I have this mental image of vigilante astronomer superheroes stalking the city at night, swooping down to correct astronomical misconceptions.

    Now you have that mental image? You mean I was the only one picturing Phil sprinting across moonlit rooftops in tights, before today?

    I… need to reflect on some stuff.

  13. @#11 Joseph: Well, I did already have a supersuit designed for him: http://madartlab.com/2011/09/06/fashion-plait/ (It’s a picture of a meteor, honest!)

  14. Bah! My last comment got moderated. I bet Phil did that just to get back at me for the paper doll incident. :-D

  15. MadScientist

    “… which is like a geometric plane, except flatter.”

    Flatter than Illinois? Now did I read an explanation for this in Bad Astronomy or was it Heavenly Errors …

    The moon illusion is an interesting psychological phenomenon and maybe it has to do with people’s perception of the zenith. Ask someone to point 45 degrees above the horizon and they’ll point about 20 degrees as if the zenith were the top of their field of view rather than directly above them and out of view. Perhaps the horizon plays a part as well – the horizon may be processed by the brain as the nadir (though no human would point at the horizon if you asked them to point down). That would result in a fairly narrow angle being processed by the brain as an entire hemisphere (or should I say a ‘view’), whereas if we look upward and have less of the horizon, our false hemisphere grows. Of course that theory can be tested …

  16. Renee Marie Jones

    The Ponzo Illusion I can see (though I never knew what it was called!). But the Moon Illusion I cannot. I do not know, for some reason the Moon looks pretty much that same to me no matter where it is in the sky.

    Sigh

  17. Jason

    @Steve (comment #5): When you wrote “vigilante astronomer superheroes,” my mind naturally went to this xkcd comic: http://xkcd.com/663/

  18. Chief

    Meanwhile Dr. Phil is allowed one phone call as the authorities don’t believe this is a simple matter of correcting the science behind long ingrained beliefs. (but is has surely changed my mind on this, I must have missed the original article).

    I wonder if there is a way to force the eye/brain to correct for the perspective errors.

  19. Mike

    I recently had this conversation with a coworker who is fairly science-minded. He was absolutely convinced that it had to do with comparing to other objects around it, and at the time wouldn’t accept my suggestion that it was an illusion. He came back to me a week later after researching it for himself, and acknowledging that it was just that. I learned about it through one of Phil’s books.. we’re educating the world, one person at a time.

  20. noen

    This explanation: “The Moon appears larger because you’re comparing it with foreground objects!”

    And this explanation: “The Moon appears larger because of the Ponzo illusion” (which consists in comparing background figures like the Moon to foreground objects like trees.)

    Are the same. One just goes into more detail than the other. Therefore Neil Tyson is in fact correct. There is no actual disagreement, just pedantry.

  21. David

    Being lost in Central Park isn’t irony it’s just a coincidence.

  22. KaoS

    I thought this was caused by a lens-like effect of the atmosphere?

    No, guess not since you said so in the other article. I’m still confused. Gonna have to read over everything again.

  23. Rob P.

    Kansas gets a bad rap, it’s only 0.9997 flat, though it is flatter than a pancake. http://improbable.com/airchives/paperair/volume9/v9i3/kansas.html

  24. gdave

    @Noen:

    The Ponzo illusion does NOT consist of comparing background figures to foreground objects. The point of the illustration is that we are used to (and evolved for) seeing objects in 3 dimensional space, where parallel lines appear to converge in the distance. The Ponzo illusion is tricking our brains into perceiving the top horizontal bar as being farther away than the bottom horizontal bar, and then aut0-correcting by perceiving the top bar as larger. In fact, of course, they are both the same distance away and the same length. It has nothing to do with comparing foreground and background objects – note that in the illustration, both bars have the exact same objects around them (a pair of slanted lines).

    The moon illusion is similar in that our brains are tricked into perceiving it as being at a different distance. We are used to (and perhaps evolved for) the fact that an object (cloud, bird, plane, etc.) that is directly overhead is closer and appears larger than the same object on the horizon. The moon, however, is too far away for “overhead” and “on the horizon” to be significantly different distances, so it has the exact same size in our visual field, and our brain auto-corrects by perceiving it as being bigger when it is on the horizon. A cloud that is on the horizon and takes up the same space in our visual field as one overhead would in fact be bigger than the one overhead, and our brains auto-correct our perceptions for this – mistakenly, in the case of the moon.

    As BA points out in the post, the illusion is just as striking on a flat plain with no visible foreground objects – Kansas in his personal experience, Kuwait and Iraq in mine.

  25. gdave

    @KaoS:

    The atmospheric “lens” is responsible for a different horizon illusion – the apparently squashed shape of the rising and setting sun (and IIRC the dramatic colors). BA has written about this in several other posts.

  26. noen

    “The Ponzo illusion does NOT consist of comparing background figures to foreground objects.”

    Yes it does. There are no parallel lines in nature to trick us into seeing the red lines like those in the subway photo as being of different sizes. So how could the Ponzo illusion possibly work in a natural environment? Because we all have a background knowledge of the relative sizes of things. We see the moon low in the sky and then compare it to our mental image that we remember from when we saw it higher in the sky and…… wait for it……. *compare* them.

    Phil Plait (and the rest of you) owe Neil Tyson an apology. ;)

    We are not blank slates and our eyes are not video cameras. We bring a vast amount of previous knowledge to bear in order to help us know where things are, how fast they are moving and how big they are compared to other things. We can tell where we are in relation to other objects are on a flat featureless plain *only* because our background informs us. We can only know an object is in the distance if we can compare it to our mental image of how big it should be.

    Our eyes are constantly moving and comparing and we use those movements to inform us where things are. That’s why a candle in a dark room will seem to move around. Your mind mistakes you eye movements for the candle moving.

    The Ponzo illusion CONSISTS in comparing foreground to background and drawing a line between them. Then taking that line as a perspective line to relate it in three space. There are no perfectly flat, perfectly featureless plains in nature. Not even in Kansas.

  27. gdave has it right!
    But the squashed shape should (you’d think) make the Moon look smaller! So the Ponzo effect actually has an uphill battle in making the Moon look larger!
    Here is a Noguchi Soichi photo of the effect from the ISS:
    http://astrobob.areavoices.com/astrobob/images/Soichi_moonrise_1.jpg

    As for the Ponzo effect working where there are no parallel lines to lead us astray, imagine the Moon setting or rising behind a distant city skyline. We’d think to ourselves “My that’s a long walk to there!” Now imagine the Moon at the zenith, seemingly just out of reach, a tiny thing…

  28. @Kaos

    To add to the rest, being perfectly magnified requires light to refract horizontally as well as vertically. Since our atmosphere changes density along the vertical axis, light can only bend vertically, affecting only the vertical apparent size, so as Richard Drumm The Astronomy Bum Says: “the squashed shape should (you’d think) make the Moon look smaller!” So the atmosphere isn’t doing anything to magnify the moon.

  29. Wzrd1

    I only consider the last frame of the cartoon.
    The final word from the raggedy doctor. ;)

  30. heh… 29 Noel beat me to it. Yes, Snell’s law, a little geometry, and you’ll find the squat/flattened moon a little smaller on the horizon than it would be above, higher in the sky.

  31. Actually, Phil, you have been shot at, repeatedly, with rubber bands.

  32. Mephane

    I cannot remember ever perceiving the sky as a hemisphere. Even from my earliest childhood memories, it looked more like a rather shallow bowl upside down. I guess the clouds where giving it away – I could see them being smaller at the horizon, but always expected them to be of the same (however unfathomable at that time) height above the ground. That obviously wouldn’t work if the sky were a hemisphere.

  33. @14 Steve: Well, I did already have a supersuit designed for him: [URL removed] (It’s a picture of a meteor, honest!)

    Hahaha, that’s awesome!!
    And yes, I was hoping that was a meteor ;)

  34. Arun

    Hi, first time commenter here.

    Going from the explanation from the comic, should the moon then not ‘look smaller’ when it’s on the horizon – just as the clouds or people ‘look smaller’ further away. However, from the penultimate image, from what I read, it sounds like two opposite things are being said . Can someone kindly advise, as I think I am missing something. Thanks

    Arun

  35. First, of course, who hasn’t wanted to chase Neil Tyson down the street while yelling incoherently at him?

    Yeah, when it comes to some of his meaner comments on Pluto I’m definitely in that category! ;-)

    (No, case anyone’s wondering, I wouldn’t really hurt him but NdG Tyson certainly isn’t my favourite astronomer. Not at all.)

  36. Jenny

    Neil’s not bad, he’s just drawn that way ;)

  37. DennyMo

    “Kansas farmland, which is like a geometric plane, except flatter”

    Don’t you mean “like a pancake, except flatter”?

    http://improbable.com/airchives/paperair/volume9/v9i3/kansas.html

  38. Bob

    Oh – you have been shot at Phil – they just keep missing….

    :)

  39. D Hunt

    I always thought it was the thicker atmosphere magnifying the moon making it appear bigger near the horizon. Instead it’s just my brain making another poor assumption.

  40. Nigel Depledge

    Noen (21) said:

    This explanation: “The Moon appears larger because you’re comparing it with foreground objects!”

    And this explanation: “The Moon appears larger because of the Ponzo illusion” (which consists in comparing background figures like the Moon to foreground objects like trees.)

    Are the same. One just goes into more detail than the other.

    This is quite correct.

    Therefore Neil Tyson is in fact correct. There is no actual disagreement, just pedantry.

    But this is wrong. Both explanations are wrong.

    The Moon Illusion is nothing to do with the act of perception in relation to lines or buildings or trees or power cables or anything like that (the moon never looks bigger when you observe it adjacent to a nearby overhead power line, does it?).

    Hell, you can hold up your hand in fornt of the moon when it is high in the sky and observe it between your fingers and it won’t look as big as it does when it is on the horizon.

    Mephane (33) is pretty close to the best understanding of the Moon Illusion.

    The human brain models the sky as a shallow upturned bowl. Thus, objects in the sky near the horizon are perceived as being far away, while objects in the sky that are high overhead are perceived as being nearer. If one considers the way the sky looks on a cloudy day, this makes a kind of sense – those clouds overhead are very much closer than the clouds one sees near the horizon.

    The moon is, for all practical purposes, the same distance from the observer whether it is near the horizon or high overhead (in fact, it is very slightly closer when it is high overhead). But it is persistently perceived as being larger when on the horizon than when it is high overhead. This is because the brain recognises its actual angular size and compensates the image in our world-view according to whether we think it should be farther away (i.e. when it’s near the horizon) or closer to us (when it is overhead). This compensation is manifested as differences in the perceived size.

    It is an illusion not a real effect, because you can measure the angular width of the moon near the horizon and high in the sky, and find that it is pretty much the same width.

    The Ponzo Illusion relies on the same effect, but requires the presence of some reference system (lines or whatever) to happen. The Ponzo Illusion does not explain the Moon Illusion. Rather, the explanation of the Ponzo Illusion is the same as part of the explanation of the Moon Illusion. Perhaps I’m being a bit picky there, but I really feel that the simple diagram is not an explanation of the Moon Illusion.

  41. Robby

    Hey #41 @Nigel: If you clicked through and read Phil’s full explanation, you’d have noticed that you’re repeating exactly what Phil has already said, not correcting anything.

    Here, lemme re-link it for you.

    http://blogs.discovermagazine.com/badastronomy/2010/05/13/why-does-the-moon-look-so-huge-on-the-horizon/

  42. gdave

    @Arun (#35):

    The illusion is created precisely because your brain “expects” the moon to “look smaller” when it is on the horizon, just as clouds “look smaller” when they are on the horizon, and thus further away, than when they are directly overhead. However, the moon is in fact so far away that the additional distance to the horizon makes no difference in how big it looks. Our brains then subconsciously apply the same rules to the moon as they do to clouds, and over-correct. Our brains “expect” the moon to take up a smaller portion of our visual field when it is further away. Since it doesn’t take up smaller portion of our visual field, but our brains are perceiving it as being significantly more distant, our brains interpret it as being a larger object. For clouds, this correction works. A cloud on the horizon that takes up the same area of our visual field as a cloud directly overhead would be bigger the cloud overhead. For the moon, it’s an over-correction which creates an illusion of greater size.

  43. noen

    Shorter gdave: “The Moon appears larger because you’re comparing it with (the expected size of) foreground objects.”

    We compare our memory of the moon when overhead (foreground) with our current experience of it on the horizon (background) and add in the expectation that it should be smaller. It isn’t, therefore our brain concludes it must be larger than before.

    Which was what Neil Tyson said in the first place only without the tiresome pedantry.

  44. Pedantry….mayyyybe…. fun to talk about? Yes. We have a running conversation at work about it.

    I added to the comic for clarity and to touch upon the “shallow bowl” effect. The illusion has nothing to do with objects, it’s about how our brain maps the space around us and interprets objects based on perceived distance, just as it does with the ponzo images.

  45. Nigel Depledge

    @ Robby (42) –
    Hah!

    I guess that serves me right for not re-reading all of Phil’s old post.

    I just looked at the Ponzo Illusion bit and thought, no, that’s not the explanation.

  46. Nigel Depledge

    Noen (44) said:

    Shorter gdave: “The Moon appears larger because you’re comparing it with (the expected size of) foreground objects.”

    We compare our memory of the moon when overhead (foreground) with our current experience of it on the horizon (background) and add in the expectation that it should be smaller. It isn’t, therefore our brain concludes it must be larger than before.

    Which was what Neil Tyson said in the first place only without the tiresome pedantry.

    Not really.

    If this were correct, then merely having a nearby foreground object (say, a house or a tree or a power cable) close to the moon in your field of view when the moon is high in the sky should make the moon look bigger, and it does not.

    The Moon Illusion has nothing to do with foreground objects.

    And, hey, since when was pedantry tiresome? Oh, that’s right – since it became cool to be wrong. :-(

  47. gdave

    @Noen:

    “We compare our memory of the moon when overhead (foreground) with our current experience of it on the horizon (background) and add in the expectation that it should be smaller. It isn’t, therefore our brain concludes it must be larger than before.”

    No. “Foreground” and “background” in the “Tyson effect” refer to actual objects we are actually seeing at the same time in the foreground and background of our visual fields. It posits that when we see the moon overhead, there are no foreground objects with which to compare it, so our brains judge its size and distance purely by the space it occupies in our visual field – so it appears small. Near the horizon, the moon is in proximity to foreground objects and sometimes behind them (in the background) which we know to be large but far away, so our brains perceive the moon as being larger but further away – so it appears bigger than when directly overhead.

    But the illusion persists without foreground objects. The flat, featureless Kansas plains may not literally be flat, featureless geometric planes, but they’re close enough for human perception (as are, in my personal experience, areas of Kuwait and southern Iraq), and even here the illusion persists. And as Nigel Depledge (#47) points out, “merely having a nearby foreground object (say, a house or a tree or a power cable) close to the moon in your field of view when the moon is high in the sky should make the moon look bigger, and it does not.”

    You seem to be using “foreground” and “background” to refer to comparing memories of seeing an object to the current experience of seeing an object – which is NOT how they are used in Neil Degrasse Tyson’s conjecture. And that’s not really what’s going on according to the “Plait effect” either. See my previous posts, or Nigel Depledge at #41, or BA’s original post linked in the current post.

  48. noen

    There simply isn’t one explanation on which everyone agrees.

    Moon illusion
    “A central question pertaining to the Moon illusion, therefore, is whether the horizon moon appears larger because its perceived angular size seems greater, or because its perceived physical size seems greater, or some combination of both. There is currently no firm consensus on this point.”

    The reason that I objected in the first place was because it seemed to me that Phil and other commentors were unfairly ganging up on Mr. Tyson. Why I don’t know but I would guess it’s because they are still upset over Pluto. Or perhaps it’s because he is insufficiently “pure” for the internet Village Atheist’s He-Man club.

    The relative size hypothesis is an acceptable explanation for the Moon illusion. You (3rd person) all owe Neil Degrasse Tyson an apology because you’re being d!cks about the whole thing.

    —–
    “But the illusion persists without foreground objects.”

    “The Moon Illusion has nothing to do with foreground objects.”

    On Earth we are never without foreground objects. With the exception of looking straight up on a cloudless night everywhere we look there *must* be a foreground in our field of vision. The ground at our feet is visually different than the horizon. We take these as distance cues. The reason the Moon does not appear larger when it is overhead even when behind powerlines or house eves is because there are no distance cues. So our eyes adjust to a resting focus of about a meter away. When we look on the horizon there are tons of cues for distance so our eyes focus to infinity, which makes an object’s visual angle appear larger. We then *compare* that with our memory of it overhead and judge the Moon to be both larger and closer.

    The other effects, the Ponzo illusion and the Ebbinghaus illusion, then compete with each other for dominance based on the viewing conditions.

    Thus endeth the lesson for today.

  49. Matt B.

    Could someone please tell this cartoonist and Zack Weiner, who does Saturday Morning Breakfast Cereal, how to spell “whoa”?

    For the record, I get the same illusion when I see Orion on the horizon.

  50. gdave

    @noen:

    “There simply isn’t one explanation on which everyone agrees.”

    Fair enough. I think Phil Plait’s explanation is better than Neil Degrasse Tyson’s, but I won’t rehash that. My main point was that they are substantially not just semantically different, and although I’m not sure if you now agree about that, I won’t rehash that either.

    “Phil and other commentors were unfairly ganging up on Mr. Tyson. Why I don’t know but I would guess it’s because they are still upset over Pluto. Or perhaps it’s because he is insufficiently “pure” for the internet Village Atheist’s He-Man club….You (3rd person) all owe Neil Degrasse Tyson an apology because you’re being d!cks about the whole thing.”

    Frankly, this seems to come out of left field. I literally have no idea what you’re talking about. All that Phil Plait did was to say that Neil Degrasse Tyson’s explanation of the Moon illusion was off-base. He even refers to “my pal Neil Tyson” in the post, and links to another post where he praises Mr. Tyson and again refers to him as a friend. No one in the comments attacked him, as far as I can see. Where in the world are you seeing anyone “unfairly ganging up on Mr. Tyson”?

    I have no idea what the “internet Village Atheist’s He-Man club” is. I am, I suppose, a member of the “internet Village”, but I certainly don’t consider myself an Atheist or a He-Man, much less a member of any sort of anti-Neil Degrasse Tyson club. Furthermore, I personally don’t think Pluto should be classified as a planet. I just think that Mr. Plait’s explanation of the Moon illusion is better than Mr. Tyson’s – and I freely admit that I could be wrong about that.

    What comments, specifically, do you think constitute being “a d!ck”? What comments, specifically, do you think warrant an apology to Mr. Tyson?

  51. Nigel Depledge

    Noen (49) said:

    There simply isn’t one explanation on which everyone agrees.

    Well, feel free to disagree. That is your prerogative.

    However, I think you will find that reality agrees on only one explanation of the illusion.

    “A central question pertaining to the Moon illusion, therefore, is whether the horizon moon appears larger because its perceived angular size seems greater, or because its perceived physical size seems greater, or some combination of both. There is currently no firm consensus on this point.”

    This is out of date. Or just simply wrong.

    Its perceived angular size is the same whether it is near the horizon or high in the sky (you can measure it’s angular size and find it to be so close to the same that the human eye cannot perceive the difference). You can even take the simple step of viewing the moon through a cardboard tube and observing the distance between your eye and the tube to get the moon’s diameter in your field of view to equal the diameter of the tube in your field of view – you’ll get the same result whether the moon is low or high in the sky.

    The only thing that changes is the way the human brain interprets the data it receives from the eye. And, in the ambiguous terminology of your quote, it is the moon’s “physical size” that is being modelled by the brain.

    The reason that I objected in the first place was because it seemed to me that Phil and other commentors were unfairly ganging up on Mr. Tyson. Why I don’t know but I would guess it’s because they are still upset over Pluto. Or perhaps it’s because he is insufficiently “pure” for the internet Village Atheist’s He-Man club.

    No. It is simple. His explanation does not match reality.

    The relative size hypothesis is an acceptable explanation for the Moon illusion.

    Rubbish!

    It’s plain wrong. It does not explain the illusion. The “relative size” hypothesis predicts that, if the moon is high in the sky and you view it in close proximity to a nearby foreground object, it will appear larger. The moon does not do this – you can try it for yourself if you don’t want to take my word for it. Conversely, the “relative size” hypothesis also predicts that, when the horizon is flat and featureless (such as is experienced on a calm sea), the moon will appear smaller than if there are other objects (such as buildings, trees, or hills) on or near the horizon. The moon does not do this either.

    You (3rd person) all owe Neil Degrasse Tyson an apology because you’re being d!cks about the whole thing.

    And you and he are still wrong.

    I have not seen anyone here being a dick about it. We are simply pointing out Tyson’s – and your – error.

  52. Nigel Depledge

    Noen (49) said:

    “But the illusion persists without foreground objects.”

    “The Moon Illusion has nothing to do with foreground objects.”

    On Earth we are never without foreground objects.

    Nonsense.

    Go visit the middle of a flat a featureless plain. Or the middle of a calm sea, out of sight of land. Or a mountaintop above a smooth layer of cloud.

    Unless you count the horizon itself as a “foreground object”, there are several ways to view the horizon without foreground objects.

    With the exception of looking straight up on a cloudless night everywhere we look there *must* be a foreground in our field of vision. The ground at our feet is visually different than the horizon.

    So what?

    One cannot observe the ground at one’s feet while gazing at the horizon. It is irrelevant.

    We take these as distance cues.

    Not really. What we take as distance cues is the way distant objects look fainter and more haze-obscured than nearer ones. In very clear air on high ground, something that is 20 miles away looks much the same as something that is 30 miles away.

    The Apollo astronauts found that, on the moon where there is no atmospheric haze to give them distance cues, judging distance was incredibly difficult. The appearance of the ground itself is only a useful distance gauge for a few hundred metres. It is useless at larger distances.

    The reason the Moon does not appear larger when it is overhead even when behind powerlines or house eves is because there are no distance cues.

    What?

    Of course there is a distance cue – a nearby object (the power line or the house). It doesn’t matter whether your distance cue is 2 metres away or 20 kilometres away – it is still a tiny, tiny fraction of the distance to the moon.

    So our eyes adjust to a resting focus of about a meter away. When we look on the horizon there are tons of cues for distance so our eyes focus to infinity, which makes an object’s visual angle appear larger.

    No.

    I’ve seen the moon set behind a tall mountain (well, tall for England, it was about 600 m) while I was in a low valley, so I was looking upward at a steep angle to view the moonset. According to your conjecture, I had the entirety of the slope of the mountain to use as “distance cues”, but the moon still appeared small. You can try this for yourself if you choose not to take my word for it.

    Your hypothesis fails.

    We then *compare* that with our memory of it overhead and judge the Moon to be both larger and closer.

    Not consciously, we don’t. How many people look at the moon when it is high overhead? Probably not much more than 10%. But how many people will comment on how large a full moon appears to be at moonrise or moonset? Nearly anyone who observes it.

    The other effects, the Ponzo illusion and the Ebbinghaus illusion, then compete with each other for dominance based on the viewing conditions.

    Thus endeth the lesson for today.

    Not only does your hypothesis fail on comparison with reality, but you do not address the real explanation of the moon illusion, that it arises mainly through a combination of the brain’s model of the sky with the Ponzo Illusion.

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