Why does the Moon look so huge on the horizon?

By Phil Plait | May 13, 2010 7:00 am

moonillusionI love illusions, and I love astronomy. So what could be better than combining the two?

If you’ve ever seen the Moon rising over the horizon, looking so fat and looming that you felt like you could fall right into it, then you’ve been a victim of the famous Moon Illusion. And it is an illusion, a pervasive and persuasive one.

So, how does this thing work? Ah, step right up.

One of my favorite brain-benders is the Ponzo Illusion. You’ve seen it: the simplest case is with two short horizontal lines, one above the other, between two slanting but near-vertical lines. The upper line looks longer than the lower line, even though they’re the same length.

ponzo_schematicThe illusion works because our brains are a bit wonky. The slanted lines make us think that anything near the top is farther away; the lines force our brain to think those lines are parallel but receding in the distance (like railroad tracks). The two horizontal lines are physically the same length, but our brain thinks the upper one is farther away. If it’s farther away, then duh, our brain says to itself, it must be bigger than the lower one. So we perceive it that way.

While procrastinating on reddit, (you do look at reddit, don’t you, especially the science section?) I found this beautiful example of Ponzo:

ponzo_illusion

Heehee! You’d swear up and down* that the red vertical line on the right is much longer than the one on the left, wouldn’t you? It looks almost twice as long to me. It’s a very powerful perception.

ponzo_illusion_detailBut they’re not! I cut out the two red lines and put them side by side. They’re pretty much exactly the same length (well, they’re off by a bit due to resolution issues in the image, but not by nearly as much as your brain likes to think).

This example is a great one because it uses a real-life image. You can see the wall tiles getting smaller with distance, and the horizontal layout of them, complete with the lines between them, forces your brain to see the line on the right as farther away. Bang! Ponzo.

This illusion plays out all the time… including when the Moon is rising (you were wondering when I’d get back to that, weren’t you?). The Moon Illusion is in part due to this same effect, but weirdly, you also need to understand how we perceive the sky.

If I were to ask you what shape the sky is above your head, you’d probably answer "a hemisphere". But in fact, almost everyone perceives it as an inverted bowl, flattened at the top. Put it this way: if the sky were a hemisphere above you, you’d say the horizon was as far away as the zenith. But in fact most people perceive the horizon being farther away than the point straight over their heads; test after test has shown this. This isn’t too surprising; think of a cloudy day. The clouds over your head are maybe two or three kilometers above, but near the horizon they may be 100 kilometers away!

See where I’m going with this? When the Moon is on the horizon, your brain thinks it’s far away, much farther than when it’s overhead. So the Ponzo Illusion kicks in: your brain sees the Moon as being huge, and it looks like you could fall into it. The Illusion works for the Sun, too. In fact, years ago I saw Orion rising over a parking lot, and it looked like it was spread across half the sky. It’s an incredibly powerful illusion.

Oddly enough, when it’s on the horizon, the Moon actually is farther away than when it’s overhead. Not by much, really, just a few thousand kilometers (compared to the Moon’s overall distance of about 400,000 kilometers). Behold my Photoshop skillz:

moon_overhead_horizon

The guy at the top of the Earth in the diagram sees the Moon on his horizon, and the guy on the side of the Earth sees it overhead. But you can tell the distances aren’t the same: the Moon is closer to the guy who sees it as overhead (by an amount roughly equal to the Earth’s radius). That’s no illusion! That’s science, baby.

So the Moon Illusion is just that. It’s not the air acting like a lens, or foreground objects making it look big by comparison. It’s just the way we see the shape of the sky together with the well-known Ponzo Illusion.

Hmmm, is there a metaphor I’m sniffing here? Science taking something we perceive as real, breaking it down, and showing it to be an interesting but decidedly unreal illusion? Well, that’s what science does! It helps us not only understand the world better, but it also makes the world cooler, too.



*Haha! "Up and down!" Haha! Man, I kill me.

Moonrise image from Jorge-11’s Flickr photostream.

Comments (135)

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  1. ozprof

    Hi BA,

    Nice to see you mentioning that constellations also look bigger at the horizon. I am surprised at how little that is mentioned when the Moon illusion is discussed.

  2. L.Long

    I love illusions. This one has puzzled me and I thought the ‘lens thing’ was a fair reason.
    But thanks for the long explanation.
    I’ve seen many illusions and the best ones are the ones you know how they work but no matter how hard you try you can’t make your brain ‘see them’, its always fooled.
    I think skeptx work the same way. We know our brain can be fooled (the delusion of religion-psychics-etc) so we have to rely on the techniques of science to ‘see’ the illusion properly.
    Keep up the excellent work!!

  3. Albert J. Hoch

    Very effective Phil. The example also shows how a similar mind bender can be easily constructed with a ruler. Make two three inch marks. Then a line from each end of the center inch of one mark to each end of the other mark! Put that in your photo-shop and stroke it.

  4. mike burkhart

    The moon has been the subject of many illusions on the surface amatures have clamed to see walls a bridge on the mare cirsum all caused by the light and shadow .Also the biggest illlusion the man in the moon .The moon under certain atmosspheric conditions can apper strange,and has often been the cause of many UFO reports.

  5. Oscar Ferro

    The Ponzo explanation is awesome, but I think that there must be some other illusion taking place. I’ve heard that the moon illusion disappears when you’re upside-down, and the Ponzo illusion doesn’t.

  6. Phew… at last.
    I’ve been pondering over this for as long as I can remember, the question always resurfacing when Google is not close at hand :)

  7. I had always heard and believed the “comparing to objects on the ground” theory. Very fun to discover that it’s wrong! I did have an idea to test the theory, but never got around to it. Basically, get a box and open two ends so you can look through it. On the inside, build a model with houses, trees, etc. When the moon is high, look at it through the box and see if comparing it to the objects in the box makes the moon look bigger. Maybe even make different models with objects of different sizes to see if that makes a difference.

    Kinda silly, I know, but the only way I could think to test it.

  8. What evolutionary reason could there be for programming our brain to assume the “horizon” above us is farther away than the one on the ground? I can’t think of what advantage that could provide. I wonder if all animals have this mis-perception?

  9. Pete

    So the Moon illusion is just a Ponzo scheme?

  10. Christine P.

    I missed the pun until I saw your footnote. Then I groaned! Good one, Phil! :-)

  11. Peptron

    To Non-Believer:
    I think you meant the opposite (the “horizon” above us being assumed as closer than the ground’s horizon). My idea is that the important thing to “know” about the sky from an evolutionary perspective is “where the clouds are”. And clouds over your head really are closer to you than clouds away in the horizon, it’s not an illusion. Then this view of the sky as being “where the clouds are” just persists for things that aren’t clouds. It’s really only an illusion for things outside of the atmosphere, since anything inside of the atmosphere really is closer when it’s overhead versus close to the horizon.

    So, we’d have evolved to consider things inside our atmosphere preferencially over things outside of our atmosphere, most likely because they are a more immediate concern to our survival.

  12. RE: “Comparing to objects on the ground theory”

    This is not necessarily wrong. There is no reason that two different mechanisms could not be acting simultaneously. The comparison idea may be a stronger influence on perception. When the moon is on the horizon, the brain is comparing it with objects of known size in the foreground, like trees or buildings. Would this not make it look larger then when it is all alone in a vast sky?

  13. Raymond

    You can produce the same effect when you have an after image. If you look at a light (please don’t damage your eyes!) and then look at the dark after image on your hand it will look small but look at it on a distant wall it will look large.

    I think the illusion is that our brains do not perceive the sky as a spherical dome probably because the clouds move across the sky in a plane. So because clouds directly overhead are closer than clouds on the horizon the moon and starts are perceived to be closer overhead than on the horizon.

    I think this is related to why viewing the stars in a planetarium looks wrong because although the starts are pretty much in the correct location their perceived distance is wrong as they are projected onto a dome.

  14. The illusion is strong, but not as strong for me as it is for you. I’d say the long line looks 5/4 as long as the short line, definitely nothing like twice as long.

  15. Scott B

    This always fascinated me as a kid when I was in Texas and had a good view of the moon near the horizon. I just had to use my hand or a piece of paper to block the ground and any buildings or trees sticking up near where the moon was and the moon looked like normal. Remove my hand and the moon would almost appear to grow right there. Kind of disorienting to realize how much your mind can trick you.

    Love the picture of the wall there Phil. Probably the most dramatic example of this illusion I’ve seen. I had to use paint to cut out the close line and move it next to the other to prove to myself they were even close to the same size.

  16. Timmy

    I was on a camping trip about 20 years ago and couldn’t sleep so I went for a walk. It was about 4 AM and as I turned onto a trail, the rising moon was framed perfectly at the end of the path. It looked like it was only 20 feet away and about 8 feet tall. That image will stay with me for a long time…

    And you just ruined it for me with your vaunted science mumbo jumbo. Thanks Phil… :)

  17. hhEb09'1

    Weirdly, when I compared the two bars by cutting and dragging, they seemed ’bout exactly the same, no resolution problem. Maybe the illusion is really strong!

  18. 24601

    What I always found interesting was trying to take a photograph of the “large” moon on the horizon. No matter how big it seemed to be to me, when the photograph was later developed, the moon in the picture was just this tiny dot that one had a hard time locating. Granted, I’ve never had the type of camera that you can focus and adjust things like aperture and shutter speed, just one of those cheap little point-and-shoot cameras, but it certainly reinforced the idea that *something* weird was going on, either inside the camera or inside my head.

  19. So, wait. The real-life Ponzo example, everything in that is in-camera and not added, correct? If so, while the image itself is a Ponzo illusion, I’m pretty sure the far orange line is, in reality, quite larger than the foreground line, which makes the actual illusion one of forced perspective, making the longer line appear to be the same size as the shorter one. Two illusions for the price of one! Paradox! It’s messing with my head. (That’d also explain the size discrepancy better than resolution issues)

  20. Michel

    I´ve pointed this out many times.
    And it is always fun to show the difference between a rising moon and a setting moon.
    A rising moon always seems bigger (and redder). Always fun to show people the little wonders of the universe.
    And I think it´s common knowledge. But no. They are really awed by such a little thing like this.
    And some get back, wanting to know more… which is even more fun.

  21. @Daniel Comparison with foreground objects may add to the illusion, but if you have ever seen the moon rise or set on the ocean you will perceive the same enlargement. In fact, since the horizon appears so much further away (due to a lack of foreground objects) the illusion intensifies.

  22. Nemo

    But, the Moon is far away, and it is huge. So, isn’t the “illusion” when you don’t perceive it that way?

  23. Selagon

    Your photoshop skillz are just fine BA! Great article! I specialized in sensation and perception in college and as many times as I have tried to explain this illusion to friends, I don’t think they ever really “got it”. Your visuals are great for really illustrating the concept. Forwarding this to them ASAP so they will finally believe me!

  24. JWB

    Okay, then . . . why does it look squashed/flattened when nearer the horizon? Is THAT the atmosphere bending the light?

  25. Dave

    @JWB, the atmosphere does have that effect, but probably not perceptibly. Then again, I’ve never perceived it. Maybe you do perceive the refraction effect. In any event, that means that the actual image of the Moon is slightly smaller when it’s on the horizon both for Phil’s reason (distance) and because of the refraction, which slightly shortens it in the direction perpendicular to the ground but does not affect the horizontal size.

  26. kirk

    When I took the Cub Scouts out to look at the moonrise I always had some toilet paper rolls so we could dispel the illusion. Cub Masters rule.

  27. TSFrost

    Some nights the moon is near the horizon and looks huge, and some nights the moon is near the horizon and doesn’t look particularly big. This is what’s always bothered me about the ponzo illusion.

  28. Dr. Dan

    Ditto on the Orion. I saw it rising above the horizon last summer. It was very late and I was driving across Northern Arizona (*excellent* sky viewing conditions!) I immediately thought of this illusion (although I didn’t remember the “Ponzo”). Yes, it does looks extremely vast. Very cool, too!

  29. Bigfoot

    I once saw the full moon behind the very distant downtown Los Angeles skyline, and it has never again looked so utterly huge to me! I think knowing how tall the skyscrapers were mentally embiggened the image even more for me.

    And I have seen the full moon on the horizon not looking particularly big, but the horizon was plain and not well-defined. So I think that, while foreground objects may not enhance the illusion, a good far-distant horizon image (the appearance of a far-away skyscraper skyline adds a lot of mental distance in my case) makes the illusion much more effective.

  30. BA….. the BEST article on the Moon illusion I have ever read! Infact the ONLY proper explanation I have ever read. Most other ‘explanations’ make a big play on the fact that the Moon looks larger when down near he horizon because it is easy to contrast it solely with nearby terrestrial objects — houses and trees etc. This has never quite cut the ice with me!

    A superb posting… I had heard that the Moon was a few thousand kilometres further away when on the horizon compared to around zenith.

  31. Jixin

    I don‘t understand. The red vertical line on the right is fixed on the wall. Compared to the the wall tiles, the right one is five times bigger than the left one. How can you move the two red lines without magnifying the right one?
    If the two lines are not fixed on the wall, they are just put in the photo after the picture is taken, then it is really a demonstration.Thx.

  32. Pete (#9):

    I think a Ponzo scheme makes your chances of getting rich look bigger than they are.

  33. JOE

    I understand the Ponzo Illusion. But in it you have to points of view that your brain compares. With the moon there is no other reference point to compare.

  34. Michel

    @24
    Your asian genes.

  35. Measure

    If that line was actually painted in those two places on the wall, the line that looks longer actually would be longer upon closer inspection.

  36. BGC

    @35
    I agree. The righthand line in the photo covers five sets of the wall tiles. The line on the left is only one tile height, so the righthand line is no representative of the same height at the other end of the corridor. If we’re to use the horizontal black lines formed in between the tiles as our visual guide, then including vertical space at the other end that is outside the horizontal lines is not the same as shown in the Ponzo diagram.

  37. Paul from VA

    Errr…. I thought the cause of the moon illusion was still an area of current research? When I took honors psych in undergrad, my term paper was on the moon illusion and it seemed that there wasn’t a single explanation for what caused it yet. Wikipedia lists at least two plausible theories other than the apparent distance:

    http://en.wikipedia.org/wiki/Moon_illusion

  38. Anonymous Backstabber

    I think you are incorrect on this. The Ponzo illusion describes an *illusion*: in that illusion, the length of the lines as they fall on the retina are *identical*, and it is the brain that misinterprets their lengths because of the lines’ context.

    Your moon theory, however, clearly relies on the fact that the diameter of the moon’s image on the retina is *actually* larger for the (closer) horizon observer than it is for the (more distant) overhead observer. It is *in fact* 1.5% larger on the horizon observer’s retina due to his position, but that is *completely* unrelated to the Ponzo illusion, which, again, describes *an illusion*.

    The vantage point theory you describe can account for, at most, a 1.5% difference in perception of the moon’s size at the horizon versus overhead: The angular size of the moon in the sky (and its footprint on the observer’s retina) *is* in fact larger when viewed from at the horizon, but not by much.

    Most people that have experienced the illusion would attest that the horizon moon appears to be *much* more than only 1.5% larger than the overhead moon. So, unfortunately, your theory is both confused *and* lacking. (IMHO)

  39. Anonymous Backstabber

    Damn, I just reread my comment and was trying to edit it. Disregard it please. Instead:

    The Ponzo illusion describes an *illusion*: in that illusion, the length of the lines as they fall on the retina are *identical*, and it is the brain that misinterprets their lengths because of the lines’ context. It’s an illusion of context.

    Your moon theory, however, seems to rely the fact that the diameter of the moon’s image on the retina is *different* for the two viewers. (It is actually about 1.5% larger on the overhead observer’s retina due to his closer position, which works *against* the moon illusion) I think you’re mixing two conflicting, unrelated possible explanations.

  40. Thanks for the shout-out, Phil!

    Excellent article :)

  41. Next time you see a giant moon on the horizon, close one eye and hold your thumb up next to the moon and watch it shrink!

  42. jcm

    “The illusion works because our brains are a bit wonky.”

    That’s a fine example of Intellegent Design.

  43. Mary Peed

    My 3rd grader has argued with me that, not only is the moon bigger and closer when it’s on the horizon, but that it’s bigger than the sun because the sun is only as big as a house when it comes up but the moon is as big as a house and 1/2.

    Let’s not discuss where he thinks they are when they’re ‘sleeping’.

  44. Darren

    Regarding the Moon illusion, all the reasons discussed are generally accurate/relevant, but there is more disagreement than acknowledged regarding what exactly causes the effect.
    See third segment: http://www.ottawaskeptics.org/the-reality-check/62-episodes/162-the-reality-check-17-zombies-crime-stats-and-moon-illusion

  45. Pete

    I would add that I do think that the comparison explanation is partially true, because I once was driving up a hill towards the full moon and it looked larger as I started up the hill (and it was framed by trees) than when I reached the top and it was away from any terrestrial objects.

  46. Torbjörn Larsson, OM

    It’s interesting that the Ponzo illusion is the dominant one for most (or else we could place the Moon farther away by the horizon illusion alone). I see that the Wikipedia page Paul from VA gave sort of discusses that.

    That makes sense, as the Ponzo is the more useful one, the need to model perspectives is nearly ubiquitous.

    @ AB:

    Your moon theory, however, seems to rely the fact that the diameter of the moon’s image on the retina is *different* for the two viewers. (It is actually about 1.5% larger on the overhead observer’s retina due to his closer position, which works *against* the moon illusion)

    I don’t see how you extract that model from the text. In fact Plait discusses such modifications later, _after_ discussing how we perceive horizons. (The Moon Illusion page that Paul from VA points to use it as the null hypothesis showing that there must be an illusion going on.)

    And by the cloud comparison, it seems that our visual processing can blow up the horizon with a substantial factor. But that all happens after retina detection, ergo it’s an illusion (that drives the Ponzo illusion).

  47. I don’t think this explaination fits. Both the red lines are the same measurable length on the photo, thus proving it is an illusion. The moon however, when photographed on the horizion and in the sky (on the same zoom level) will measure much bigger in the horizion photograph ! Thus disproving that it is an trick on our brains alone. Something else is going on.

  48. Andy

    In discussion about this some years ago, an astronomer friend disputed the illusion explanation by saying that he needs to advance his zoom more, to fill the field of his scope, when the moon is overhead, than when it’s on the horizon. He claimed that it was due to a lensing effect by the atmosphere.

    Anyone out there care to test this ?

  49. olderwithmoreinsurance

    There seems to be no universally accepted (and testable) explanation for the moon illusion. For a really nice discussion (that argues pursuavively that the BA’s argument is at best, incomplete) check out
    http://www.lhup.edu/~dsimanek/3d/moonillu.htm

  50. Dr. Rocketscience

    See, this is why “3d” movies are pointless. Our brains already perceive a movie image in 3d. Otherwise, this illusion wouldn’t work.

    Also, Avatar was a terrible movie.

  51. Benjie

    My wife doesn’t see these “illusions”. When in college, my Psychology class has a website dedicated to all kinds of illusions and I think she saw one but the rest she could glance at and accurately and correctly say whatever is the correct whatever.

  52. Buzz Parsec

    Antony@49, No it doesn’t! Try taking a picture of the Moon on the horizon and when it’s high in the sky, and you’ll see it’s exactly the same size on the film/print/screen. (Use the same camera with the same lens and zoom factor.) If you measure carefully, you’ll see it’s actually slightly bigger (1.5%) on the overhead shot, even though it looks much smaller overhead when you just use your eyes.

    Andy@50, are you sure? Doesn’t match my experience.

    older@52, That’s what I’ve always heard before, but I never heard Phil’s explanation before. Is it a new theory, or did I just not understand it previously?

  53. A couple years ago I was at a talk by a rocket scientist that did some work on the Mars Lander.After, at a question/answer I patiently waited and asked him about the illusion.He dismissed it as a illusion relative to trees/buildings etc.I said I thought there was more going on than that and his interest evaporated.I don’t think it’s dialed in yet although the illusion has persisted ,probly,for the entire human history.I heard about turning around and bending over,looking at it from upside-down between your legs but that seems silly.

  54. Alexander Finch

    ‘If I were to ask you what shape the sky is above your head, you’d probably answer “a hemisphere”. But in fact, almost everyone perceives it as an inverted bowl, flattened at the top. Put it this way: if the sky were a hemisphere above you, you’d say the horizon was as far away as the zenith.’

    That seems to me a misunderstanding of where people perceive themselves to be in the hemisphere. If I follow correctly then you’re putting the person in the center of the sphere that the hemisphere defines. I’d say that the sky above me is a hemisphere but that I’m very close to one of the edges of that hemisphere. In this case that edge would be the zenith. The edges of my bowl are the horizon and because I’m not at the center they seem much further from me than the zenith does, and they are.

    Anyway that doesn’t change the point of your article. I used to wonder why the moon changed size when I was young. I found it hard to believe that it was in my head when I first found out.

    ‘Hmmm, is there a metaphor I’m sniffing here? Science taking something we perceive as real, breaking it down, and showing it to be an interesting but decidedly unreal illusion? Well, that’s what science does!’

    Would this be a hint of scoffing at religion? I used to enjoy your blog regularly before being eventually driven away by posts that were as much (or more) against creationism than they were for astronomy.

  55. Jeffersonian
  56. It is a trick of the brain, but I don’t think it’s the Ponzo illusion, Phil. If you see the Moon or the Sun near the horizon when you are out to sea, or looking out over the ocean from a beach, the illusion goes away. The Moon looks the same size as it always looks overhead. But when you see it from land, where buildings or trees are in your field of view, the moon looks larger than it does overhead. It’s context that lends it larger than normal size.

  57. Ruggy

    The sun becomes large at sunset because of all the daylight it has to suck up.

  58. Washingtonian

    Nice post, Phil, but do you have any references? I mean, the Moon Illusion has been around for over 50 years, and many scientists have offered many ideas to explain it. If you’re saying this explanation is the correct one, I think you’ll need references to studies which show that. Otherwise, you’re just scientist number n+1 offering yet another explanation.

  59. XavierAM

    AM I WRONG?: i believe youre partly right. However, i learned some time in middle school that it was actually the earth’s atmosphere that was acting as a big magnifying glass.

    COMBINE the second picture (the one with the earth and two little guys and moon), with this idea. Also, picture another circle(atmosphere) outside of the earth circle in the diagram. though it may only be a slight distance straight up from the stick figure on top, the distance between the stick figure on top and the second circle closer to the other stick figure is much farther.

    KIND OF LIKE when you hold a pair of reading glasses out, just past your nose and look at an object. This also accounts for the oblong shape of the moon in your picture.

    thoughts?

  60. molybdenumfist

    Antony… have you actually done the experiment?
    Have a look at this:
    http://apod.nasa.gov/apod/ap020130.html

  61. Jeff

    Nice theory about illusions. Did you miss out the density of air? The light from the moon/sun at the horizon passes through the air densier than that of the moon/sun directly on top.

    Thickened atmosphic air ‘magnifies’ the moon/sun and also reduces its clarity.

  62. “The Ponzo explanation is awesome, but I think that there must be some other illusion taking place. I’ve heard that the moon illusion disappears when you’re upside-down, and the Ponzo illusion doesn’t.”

    I once tested this. A friend came home and mentioned a huge full moon on the horizon. Sensing an opportunity, I suggested we all go to a nearby hill for a good look. I then suggested that everyone face away from the moon and bend down and look at it through their legs. All reported that it then looked smaller.

    While Phil’s explanation is probably correct for most people, some people don’t have the flattened-bowl perception. I think the comparison with nearby objects at least adds to the illusion, and the disorientation takes away this effect. Of course, it can also look big at sea, so this is not the whole effect. It is difficult to compare the perception when it is on the horizon and when it is at zenith, since a few hours are in-between. This might be the biggest effect, but looking through one’s legs shows that it looks smaller that way (though perhaps not as small as at zenith).

  63. This may be a REALLY stupid question but if the moon looking bigger is an illusion caused by our brains how come it still looks bigger in photographs? Shouldn’t whatever bias our brain has toward horizons and what-not be negated by isolating the image in a photograph? Yet the moon looks enlarged when compared to houses and other features.

  64. Jodo Kast

    Good try, but you’re still wrong. The photos from the space shuttle clearly show how much the moon is distorted when viewed through the atmosphere. From the space shuttle or space station, if you look through the atmosphere and see the moon on the other side, it looks oblong and oval shaped. It looks ‘painted’ on to the inside of the atmosphere in the space shuttle photos.

    And none of this has anything to do with the Ponzo Illusion. Look for the photos of the moon through the atmosphere from the space station or shuttle for proof that the atmosphere is definitely acting like a lens. It’s image #7 on this page: http://www.boston.com/bigpicture/2009/06/recent_scenes_from_the_iss.html and it’s proof that the atmosphere is acting like a lens.

    Also, even in the first image on this page, the moon is clearly squashed vertically by the lens effect. Next you’ll be telling us that you’re anti-vax! Please.

  65. Jodo Kast

    That is not an illusion, the lens effect is bending light. Now that’s science!

  66. Matt

    So why not include 2 photos of the moon. One when it appears large on horizon and one when it appears overhead, and compare the sizes of each side by side to dispel the myth?

  67. Rock

    Philip,

    Growing up I was in a rather wacky astronomy club and some of our members had a running contest/joke concerning how many unsuspecting people they could photograph testing the moon illusion in this manner at the same time. We eventually got photos with around 40 people testing the moon illusion. I guess you take your humor where you can get it.

  68. Kujo

    Antony: That would be good counter evidence if true, but every example I can find in google, like http://upload.wikimedia.org/wikipedia/commons/0/0b/Lunar-eclipse-2004.jpg shows the moon the same size across the sky. http://antwrp.gsfc.nasa.gov/apod/ap090616.html claims that the moon is always measured to be about 0.5 degrees

  69. eKim

    @64

    I had a similar experience. I was once watching a rising full moon at a point between a hill and the horizon. I then turned around, preparing to look between my legs, when I suddenly noticed multiple full moons rising at the top of the opposite hill! Amazing! How do you explain that?!?

  70. XavierAM

    I really expected a lot more from DISCOVER. Sad to think atmospheric conditions, the sun, etc. were all left out and play a huge role.
    Somebody have a talk with Phil Plait. Or tell him to have a talk with a highschool science teacher before he embarasses himself again. ZING!

  71. boppa

    interestingly- altho the horizontal ponzo works (left line looks shorter than right line)
    the vertical one doesent- to me both lines look the same
    I tried several different ones I found in google images, and in every case it only works for me if the `sloping lines are horizontal’ and the `measure me’ lines are vertical

    even if I tilt my head over, once I get to about 1/4 of the way the lines are shrinking/growing until at 90 degrees they look the same length

    anyone else get this??

    edit to add- this was with the subway tiles picture at first, but all images google returned had the same effect…

  72. Tim G

    I wonder if most people would be surprised to learn that a dime at arm’s length easily covers up the moon.

  73. cjl

    XavierAM (@73):

    First, this is Phil Plait’s blog, not Discover Magazine. It’s hosted on Discover’s site, true, but it’s more of a personal blog than a magazine/corporate one.

    Second, the atmospheric conditions and the sun really have nothing to do with it. It’s all just an optical illusion. If you need proof of this, try using a camera (with constant zoom of course) and taking pictures of the moon at various times. It will always be exactly the same size in the photos, which would not be the case if there was a significant contribution from actual physical (rather than illusory) effects.

  74. Mark

    Are you sure that that’s your Photoshop skillz and not MS Paint?

  75. Jodo Kast

    Here’s a better shot of the lens effect of the atmosphere when viewing the moon from space… http://twitpic.com/1bz3nm

    This is the same reason it looks big and squashed in the morning… the angle light travels through the thick atmosphere.

    Your brain is not tricking you. It’s not an illusion. That is how light behaves through a lens, in this case, the atmosphere.

  76. Alys

    We could easily work out the true effect (or lack thereof) of atmospheric lensing and buildings on the horizon if we could examine this illusion from the Moon looking at the Earth. Did the astronauts on the Moon take any comparable photographs of the Earth rising and the Earth overhead? Or did they report anything about an illusion of the Earth looking huge on the Moon’s horizon?

  77. …That’s what she said…

  78. Torbjörn Larsson, OM

    For a really nice discussion (that argues pursuavively that the BA’s argument is at best, incomplete) check out http://www.lhup.edu/~dsimanek/3d/moonillu.htm

    That discussion confuses perceived size with perceived distance (at least for “the mental sky-dome model”), and makes the same mistake that it identifies in others (“playing verbal games”).

    The illustration on that model is fine though, and shows exactly why we would perceive the moon larger when combined with the Ponzo illusion.

  79. JupiterIsBig

    #78 That photo is through a LOT of atmosphere at a position you won’t ever get your eyes – unless you’re very very lucky.
    There can be some significant lensing due to temperature layers and the associated refraction effects., but the moon illusion is always there.

    I have shown my kids lots of times the illusion by measuring it with my thumb or even marks on a piece of paper a couple of hours apart.

    Isn’t the illusion related to the converse ? … when looking from a height, 400m looks much scarier than kilometres across the planet. We evolved on the ground – threats from above can only come from relatively close, but things a long way away on the ground are important, water, trees, food, lions, etc.

  80. Warm moon colour on horizon due to setting sun and cold moon colour up above. Wavelength theories – refraction from the moon and the results of wavelength from sun, add to affect by subtle change in colour of the moon from cold blue hues, that are seen when moon is seen from above, to warmer tones whilst moon is on horizon. Comparison theories of size, length and in this case also colour. Colour perception – the warmer hues another clue as to why the moon appears subconsiously nearer to us. Warmer colours, like red and orange advance whereas colder colours, blue green recede. Both theories could be connected. Might not be obvious at first glance given the moon is in one place at any one time.

  81. By the way this was a guess. I haven’t gone to wikipedia.

  82. peggy

    Thank You! I knew I would find the answer someday. In october in the late 80’s, on a full moon night, was driving home. Wow, there it was, so big and so close I could almost reach out and touch it. I read every newspaper I could find. I asked people I thought might know; did you see how close the moon was last night? Have never forgotten it. Have never found the answer. Til now. Thanks again.

  83. Yvonne McChesney

    Why has no one mentioned that if this effect was the Ponzo Illusion it would be the same 100% of the time. However, the moon looking larger on the horizon doesn’t happen every time. It seems to me to depend on the condition of the air. I am no astronomer, just a casual observer.

  84. Ricardo

    I can appreciate Gestalt psychology, but you guys are taking the concept of an illusion too far. Sometimes, the moon does appear larger for reasons other than what you are suggesting.

  85. “Why does the Moon look so huge on the horizon?”

    The same reason a woman might ask a man, “Does my bum look big in this?”

  86. joseph hovey
  87. Chris Lamb

    I just learned that the horizon (at sea level) is ALWAYS only just over 3 miles away.

  88. Ahmed

    Why does it fool the camera though. The camera capture it the way we see it. If you compare two picture of the moon, one from the top and one from the horizon, I think the horizon one would be larger like in the picture. So it must be some physical effect, not a mind game. I would be glad if you can explain this.

  89. Your explanation of the so called moon illusion totally ignores the fact that light rays from the Moon are refracted by the convexity of the Earth’s enveloping atmosphere. (when observed from the tangent )

    The object viewed is larger. This applies to the Sun too, and explains why it has an ellipsoid shape when setting. Psychology cannot explain that.

    When observed from Earth, the true location of the setting sun on the horizon is actually lower due to the refraction of light.

    Science trumps psychology anyday.

    I’m surprised this psychobabble of illusion still persists.
    .

  90. Arthur

    Hello, gents! I have a question and can’t find the answer. I even tried to call our local Lick Observatory, but no one ever returned my call. May be my Q is too simple for them. Anyway, three years ago on August 24th, I was in Half Moon Bay, CA. Around 4pm I stepped on the balcony (ocean view) of the hotel I was staying in and couldn’t believe it. The moon and sun were on the same level (horizontally) above the horizon. Full size both. Beatiful view and very unusual, since neither did I or a few hundred people on the beach saw this event before. Do you have any explanations for this event, how often this happens and why?

  91. Mr H

    I can’t see how this holds up because I’ve used a ruler at arm’s length to measure the diameter of the Moon both low and high in the night sky.
    By my measurements the Moon’s diameter has been at least a 3rd bigger when on the horizon than when high in the sky. And I speak of only the recent measurements I’ve taken, I’ve seen it even larger on occasions.

  92. Kevin

    Ok…I am in total agreement with the size being the same in the sky…but this doesn’t explain why you can see the moons features more clearly and distinctly when it is “larger” than when it is at high “noon”

  93. Gammie Sharon

    Only twice have we observed this gargantuan moon illusion. The first time it was startling as I looked out of our 12 foot bank of windows to see a moon filling nearly half of the space. Not long after it seemed smaller and a bit later it was ‘back to normal’ size. The second time we were driving cross country one evening and , once again, saw a huge moon. Our son has sent us several links to smarten us up, but it is still astounding to witness. (I am easily entertained!) This article, Phil, is very helpful.

  94. Jegbert

    But wait a second! I hold a coin out at the full extent of my arm’s length, and it neatly eclipses the moon with a little overlap when I’m lying on my back and the moon’s high up. Yet, when I do the same thing standing up and the moon’s on the horizon, I can see the moon around the edges.

  95. BrustyBnail

    Compare the size of the moon in this multiple exposure photo. The one at the top has a smaller radius in pixels than the ones at the bottom do.

    http://mm04.nasaimages.org/MediaManager/srvr?mediafile=/Size4/NVA2-4-NA/6017/eclipse_5_15_03lg_damast.jpg&userid=1&username=admin&resolution=4&servertype=JVA&cid=4&iid=NVA2&vcid=NA&usergroup=NASA_Astronomy_Picture_of_the_Day_Collecti-4-Admin&profileid=16

    Clearly there’s more than mere psychology at work.

  96. gal with a camera

    24601- if you’re using a compact, then your camera’s lens is too wide angled to get a decent shot of the moon. Given that it is so far away, it’s going to look tiny in a photograph shot with the sort of wide-angle lens on your average compact (wide angle lenses distort the image to make the horizon appear further away than it naturally does). You’d need something like a 500ml lens, at least, to get a decent shot. I find about 900ml gets the moon to fill about a third of the frame in total and shows up nice detail. Of course, you’re looking then at a lens about the size of your head and about as long as your arm (if it’s a decent lens). Compare that to what you’re using and I hope you’ll be comforted by the knowledge that it’s not your skills at fault! Your options? Spend a few thou on some swanky kit, or strap your compact onto a cheap(ish) telescope with a dinky little adaptor (Google telescopic adaptors for more info). Hey presto – you should be getting much better shots of the moon, whereever it appears in the sky.

  97. What a great article. Thanks!

    Dr. S.

  98. bettyg

    I SAW THE SUN LAST THURSDAY AND IT WAS VERY HUGE AND RED….. I KNOW THAT IT WASN’T THE MOON BECAUSE I SAW THE MOON IN THE EAST AT THE SAME TIME….COULD THE EARTHQUAKE IN JAPAN SHIFT THE EARTH AND CAUSE CHANGE TO THE EARTH…NOW TODAY 3/19/11 THE MOON IS SUPPOSED TO BE VERY HUGE SOME SPEAKS OF IT BEING AN ILLUSION…I FEEL A MAJOR CHANGE TO THE EARTH AS WE KNOW IT ABOUT TO TAKE PLACE..COULD THIS BE THE BEGINNING OF THE END?

  99. TheLoneIguana

    bettyg: No. And quit shouting.

  100. Sandeep

    please tell me more about theory of everything and quantum foam.

  101. DRG

    I find it quite funny that some people have completely missed the point, although the description could be little clearer.

    Firstly, we need to realize that our brains are used to working in the realm of a few centimeters up to a few miles.

    In this world in which we live, picture a basketball directly above your head so that it appears to be about the same size as the moon.

    Now imagine the ball floating forward towards the horizon in front of you.

    The ball will appear smaller the further away it gets.

    THIS IS WHERE IT GETS INTERESTING!!!!

    Now imagine the moon above your head.

    Picture it moving towards the horizon in front of you.

    IT WILL STAY THE SAME SIZE!!!! (not withstanding the negligible increase due to the relative negligible increase in distance)

    Now think back to the basket ball, which gets smaller.

    Now think of the moon that doesn’t.

    Now picture them both moving next to each towards the horizon.

    The moon therefore appears bigger because it doesn’t get smaller! Our brains our used to objects getting smaller as they get further away (because we’re used to living in the realm of a few centimeters up to a few miles). The moon doesn’t appear to do this because it’s relative distance doesn’t change (very much) because it’s already far away.

    The basket ball appears to be getting smaller – The moon appears to stay the same but we perceive it to be at a greater distance and therefore we perceive it to be bigger.

    Surely this is obvious. Now re-read the above article and realize that what it says is correct :-D

    Sorry for the rant. I just feel I had to drive the point home! Just think about what we expect to see as things get further away. The moon doesn’t follow what we expect.

    The picture on the left of the wikipedia article article for moon illusion illustrates this quite well but with clouds instead of basketballs.

  102. Robby

    Couldn’t it be that because of the bending of light rays as was decribed by Albert Einstein? The bending being caused by the presence of the earth. If the earth (mass) tells space how to curve.
    Just a thought.

  103. So…I am pretty late to comment on this blog post, but I recently read it after reading the blog post titled “Watch the lunar eclipse Saturday”, which mentioned the Moon Illusion. I decided to read this blog post in order to better understand the Moon Illusion.

    @Robby: I thought your question concerning Albert Einstein’s theory of light rays bend was a legitimate question to consider. However, atmospheric distortions would actually make the Moon appear to be smaller along the horizon. Here is a little explanation I found outside Bad Astronomy: http://www.lhup.edu/~dsimanek/3d/moonillu.htm (the explanation is in bold near the beginning of the page and includes a diagram).

    It is quite interesting and amazing how much an effect perception has on the way we see things. It is also interesting how the Moon Illusion involves this perception and relates to the Ponzo Illusion. However, I still cannot seem to fully wrap my head around how the object that appears to be farther away looks much bigger. I realize it has to do with the human brain’s perception, but again I don’t fully understand how what is perceived as farther away (the Moon on the horizon) can be seen as bigger than something that is closer (the Moon directly overhead). I guess it is because I don’t fully understand the human brain itself.

  104. me

    In the genesis… Let the moon and the sun be for signs.. Not for illusions. This Ponzo mumbojumbo is an illusion. It makes absolutely no logical sense. That red bar picture thing??? It also makes no sense.. if the light would take up 1 tile (as it does when it is close to you)… it would be a lot smaller at the further end. That light at the further end takes up 5 tiles, of course it may seem the same size because it is bigger and at a further distance… meaning you will perceive it as smaller (than it actually is)… but in reality, its 5 tiles comapred to 1… IT just means things that are closer are bigger. Sheesh

  105. So how do you explain when the moon hits your eye like a big pizza pie?

  106. NG

    How about when there is no horizon? NASA link: http://tinyurl.com/5r94osh

  107. I haven’t noticed this as much as most people.
    I’m not wired like this and do perceive a hemisphere.
    I don’t see the moon much bigger on the horizon at all, in fact I don’t understand people who do.

    The red lines look the same length(especially when I look in between them).
    Hm.

  108. kmax

    @ Michel

    Whoa! Hold it right there! If the moon appears bigger when rising than when setting, then the ponzo illusion explanation incomplete at best, and probably wrong. But I suspect you are wrong. Most people aren’t awake at dawn to see a setting full moon. Just my guess.

  109. Dan

    I thought I was a genius when I thought the moon illusion was magnification through our atmosphere….I believe my eyes!! You say we are closer to the moon when it’s directly overhead….Then the moon should look closer, or larger…On the curvature of the earth, or moonrise and moonset, we are farther away, so it should be smaller, or further away…..I have been reading all day on this…I’m keeping my theory of magnificaton…..The people of the 17th century were right!! I know that when we observe something closer, then it looks LARGER…..when it is farther away, it looks smaller…..As a professional truck driver of over 40 yrs, I have seen the sun and the moon in the morning and evening, more than the average person….Furthermore, when looking straight up we are looking through approx. 20 miles of atmosphere…(less magnification)….When looking through the atmosphere to the horizon, we are looking through approx. 50 miles, of atmosphere. Our atmosphere contains water particles…The thicker the maginfying glass, the larger the object appears…I chose the 50 miles distance as an example….I don’t know the actual miles, but you get the idea!

  110. Dan

    I have been reading the replies….Some of you wonder why the moon and sun changes color…..In the dawn and in the evening, we are viewing the moon and the sun through our atmosphere at length(see my last reply)….We are looking not only through more water particles, which magnify, we are also looking through more particulate matter…(pollution)…Which changes the color of the light our celestrial neighbors that are illuminating at us….Just like putting on a pair of dirty sunglasses!! I’m sure light angle has something to do with this as well….Why is our sky BLUE?? Light is seen by us in a very narrow wave length….(white light)…Our atmosphere breaks down the wavelengths…..Mainly red, and blue….We see the blue, because blue wavelengths are more visible to our human eye. If green wavelengths were more prominent, then the sky would be GREEN!!
    If sunlight reached the earth without being filtered by the atomosphere, it would appear yellow…..(or white). All of the plants on earth look green to us, right? If our blue atmosphere was white, or (yellow), then our plants would be what color?? Combine the color yellow with the color blue…what do you get?? (green) Take the blue away, and you get yellow…..Our plants would be yellow!! Everything is an optical illusion!!

  111. Ray

    Evolution. A storm on the horizon is much less a threat to you and your family, (calm & small), because it is much farther away. As it gets closer it begins to move across the sky until overhead, and, the danger increases to, in the case of a tornado, (panic & big). Same with dangerous animals. The perfect evolutionary response.

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