Changeable moon

[Rolfe]

I can't find the link to the page I'm thinking about, but I want to ask forum members about the question of whether the size of the full moon appears to change depending on how high it is in the sky.

I know there's a page somewhere claiming that this does not happen, and the idea that it does is an optical illusion caused by the moon appearing large when close to objects on the horizon such as trees, but not when it's far from those objects. The page suggests cutting out a circle in a piece of paper corresponding to the apparent size of the full moon, and looking through it to show that the apparent size doesn't change. There was a link to the page given in a previous thread.

I don't think I agree with this. I admit I haven't tried the paper cut-out bit, but that seems chancy to me in any case, with a lot of variation inherent in exactly how far you hold it from your eye. But my understanding is that the apparent change in size is real.

I don't mean that the moon itself actually changes size of course! However, it was always my understanding that the apparent size of the disc when seen from Earth varies considerably depending on the thickness of the atmosphere layer through which the light from it has to pass before it reaches our eyes. More atmosphere when the angle is low (moon close to the horizon) causing a lot of refraction, less when the angle is high (moon high in the sky) causing less refraction. So the disc genuinely appears bigger to our eyes when the moon first rises, shrinking as it travels higher across the sky. As far as I remember, we were taught this in Physics class at school.

The same thing happens with the sun too, but because the solar disc isn't something you want to look at in too much detail, it's not nearly so noticeable. And non-full moons don't rise so high in the sky, so again the effect is less noticeable because of that.

I was thinking about this the other night when I was driving for several hours from soon after moonrise to about ten o'clock. Not only was the apparent change in size much too great for me to accept easily that it was purely an illusion brought on by the background objects, I noticed that the size appeared constant at any given moment irrespective of whether or not the lie of the land actually did place trees and a "horizon" close to the disc or not. If the relatively high moon happens to be close to a high horizon of mountains, or you deliberately create this by crouching down to place a line of trees close to its position, it doesn't suddenly seem to grow to the size of the newly-risen moon.

Anybody able to say with confidence whether I'm right, or whether the "it's all an illusion" page is right?

Rolfe.

[Rolfe]

OK, I've had a bit of a Google, and found several pages propounding the "it's an illusion, stupid" theory with apparent authority, and one saying that the refraction theory doesn't really work.

I'm still not convinced. Persuade me. (Preferably by demonstrating that the refraction theory is untenable.)

Rolfe.

[Cecil]

I've heard about half a dozen difference explanations. It is definitely an illusion, not an actual change in the angular diameter of the moon. Cameras do not detect a difference.

I don't like the idea that it's caused by comparison by buildings/trees near the horizon, for the reason that you mentioned - it appears when the moon is low in the sky, regardless of the existence of nearby objects.

The explanation that seems to be the most well supported, and that I like the best, is the following one: In our everyday experience, objects that are near the zenith are rarely more than several kilometers away, and certainly no more than 10. The distance to objects near the horizon, however, is only limited by the curvature of the Earth, and can be around a hundred kilometres, or more if the object in question is a cloud. Consider the angle a bird subtends on your retina when it's directly above you, then imagine how huge that bird would have to be if it were to subtend the same angle, but near the horizon.

Thus, we perceive the sky as a flattened dome. Our mind determines that the moon is actually closer when it's overhead, and since it always subtends the same angle, we perceive it as being smaller than when it's low in the sky.

It's interesting to note that the illusion disappears when you turn around and look at the moon between your legs.

And non-full moons don't rise so high in the sky. A nitpick. This is certainly not the case. The phase of the moon is irrelevant to its possible positions in the sky. It's just that new moons (and crescent moons) are close to the sun, so when they're high in the sky so is the sun, which makes observation difficult. ;)

[Cecil]

The best website I've ever found on this subject. It delves into extreme detail.

http://www.lhup.edu/~dsimanek/3d/moonillu.htm

[Capsid]

I thought this would have an easy answer but I am surprised at how much discussion has taken place. I've started reading this long and fascinating article

http://facstaff.uww.edu/mccreadd/

[Earthborn]

Anybody able to say with confidence whether I'm right, or whether the "it's all an illusion" page is right?I can say with confidence that you are wrong, and that it's all an illusion. If it was a real optical effect, then it would show up on photos too, and that's demonstratably untrue.I admit I haven't tried the paper cut-out bit, but that seems chancy to me in any case, with a lot of variation inherent in exactly how far you hold it from your eye.That's why you should do such experiments with a fully stretched arm. You probably don't need a paper cut-out. Just compare the apperent size of the moon with a small coin at arms length, or pretend to squeeze the moon between your fingers at arms length.

If you are still not convinced, take photos or study the moon through the viewfinder of a good telescope that shows a scale to the wire cross.OK, I've had a bit of a Google, and found several pages propounding the "it's an illusion, stupid" theory with apparent authorityHave you found this (http://www.lhup.edu/~dsimanek/3d/moonillu.htm) one?

[Rolfe]

The phase of the moon is irrelevant to its possible positions in the sky. It's just that new moons (and crescent moons) are close to the sun, so when they're high in the sky so is the sun, which makes observation difficult. ;)OK, OK, fair point, I was talking about observations made after dark. But my observations suggest that the effect is present and proportional to the height of the astronomical body whether it's the sun or the moon, and irrespective of the phase of the moon.

I'd really like to know whether the refraction theory really doesn't work, and if not why not. I have non-detailed memories of its being expounded in class, complete with diagrams and other real cool stuff.

Maybe I should try taking some photographs next time there is a clear night with a full moon.

I'm glad you agree with me about the "comparison with objects on the horizon" theory not looking so good. I really can't buy that one at all, though I'd have to think about your alternative suggestion a bit more carefully.

Rolfe.

[Rolfe]

Wow, this is a lot more complicated than I thought. Massively interesting, but not right now as it's past midnight.

Pray continue, I'll read more when I've time. Thanks everyone.

Rolfe.

[Cecil]

I'm still not convinced. Persuade me. (Preferably by demonstrating that the refraction theory is untenable.)

From http://www.licha.de/astro_article_atmosphere.php :

Distance to Zenith in Degrees Refraction
80 5' 31"
85 10' 15"
88 19' 17"
89 25' 36"
90 36' 38"

The refraction is vertical only, and it's displacement, not magnification. So the moon could appear "squashed" vertically by about 1/6 of its size (it's half a degree in diameter, so the difference in displacement between its bottom at 90 and its top at 89.5 would be about 5'), but only while it's setting/rising. Once it's 5 degrees above the horizon, the refraction is negligible, and certainly unable to account for the 50% increase in size that most people estimate.

[Patricio Elicer]

I don't mean that the moon itself actually changes size of course! However, it was always my understanding that the apparent size of the disc when seen from Earth varies considerably depending on the thickness of the atmosphere layer through which the light from it has to pass before it reaches our eyes. More atmosphere when the angle is low (moon close to the horizon) causing a lot of refraction, less when the angle is high (moon high in the sky) causing less refractionThat was my understanding too. I thought it was just matter of light refraction, but it is not. The illusion is astonishing, the moon near the horizon looks so "clearly" bigger than when it's up in the sky, that it's hard to believe it's an optical ilussion. But well, ....

To make things worse, I've read it's still an unsolved mystery how the ilussion works.

[Rolfe]

The refraction is vertical only, and it's displacement, not magnification. So the moon could appear "squashed" vertically by about 1/6 of its size (it's half a degree in diameter, so the difference in displacement between its bottom at 90 and its top at 89.5 would be about 5'), but only while it's setting/rising. Once it's 5 degrees above the horizon, the refraction is negligible, and certainly unable to account for the 50% increase in size that most people estimateOK, I'm getting that. This link http://facstaff.uww.edu/mccreadd/intro9.htm seems worth getting to grips with, possibly offering a credible explanation (unlike that "objects on the horizon" stuff, which seems to be agreed to be inadequate).

Rolfe.

[Soapy Sam]

Why would you expect atmospheric refraction to result in magnification?
Dimming, yes; distortion, maybe, but why enlargement?
2000 word explanation please, plus diagrams. Tomorrow will do.

Just went up on the helideck for a look. Moon (just past full) is next to Mars about 70 deg up. Other than that, only Sirius and Orion are clear through the rig lights.

The moon DOES look pretty small that high up. I can't see how refraction could make such a big difference compared to my shaky memories of full moonrise.
I agree with Earthborn that the full moon looks much the same size in my photos, irrespective where it is in the sky.

Nope. I'm sticking with the optical delusion theory. So are reported UFOs near the horizon described as larger than those reported overead I wonder?

[Rolfe]

Why would you expect atmospheric refraction to result in magnification?
Dimming, yes; distortion, maybe, but why enlargement?
2000 word explanation please, plus diagrams. Tomorrow will do.Look, how should I know! I said I only had a dim memory of a physics class where I thought there had been an explanation along these lines. And I know what you think of the quality of my memories of my schooldays already, thankyouverymuch.

I only really started to think about it when I read the "comparison with objects on the horizon" explanation and decided I didn't buy that. It's not an adequate explanation. But I can see it's more complicated than that, and there may be no one agreed accepted explanation.

Rolfe.

[Patricio Elicer]

Why would you expect atmospheric refraction to result in magnification?Dimming, yes; distortion, maybe, but why enlargement?Just didn't put enough thought on it, and accepted the idea as "plausible", maybe because of certain analogies. Have you noticed that objects submerged in a pool sometimes look larger than they really are?. Sounds "reasonable" that if water acts as an enlargement lens, then air should as well. All in all both are fluids.

Well, but if the illusion holds for the moon, it should also hold for the sun disc, shouldn't it?. However, I've never heard of the "sun illusion".

[Rolfe]

Well, but if the illusion holds for the moon, it should also hold for the sun disc, shouldn't it?. However, I've never heard of the "sun illusion".It does. I've noticed it, and one of the more detailed web sites confirms this. It's just that it's not too clever to look intently at the disc of the sun for any length of time.

Rolfe.

[Charlie in Dayton]

An excellent place to start in researching this phenomenon is the legendary website that's connected to the discussion board (http://www.badastronomy.com/bad/misc/moonbig.html), namely Badastronomy.Com, with commentary by our own beloved Bad Astronomer, aka HRMWOTAS.

[Rolfe]

I wish I'd never started this! The Moon Illusion doth murder sleep!

It occurs to me that my OP was a nice little example of the false dichotomy fallacy. That is, I've read this implausible explanation for a phenomenon. It's not credible. Therefore the "belief" I'd held unthinkingly for years about the subject must be true.

Er, no. "Neither of the above" is also on the checklist. And looking stronger by the minute.

I realise this is such a complex subject that I'm unlikely to come to a settled understanding of it in five minutes. Especially as senior academics in the field appear still to be arguing about it. But a few things have occurred to me.

Nobody seems to mention the other part of the illusion, which is that the moon also appears to change colour as it rises, from yellow or gold to pure white. Am I right in assuming that that is just a feature of the greater amount of polluted air between the observer and the disc when the moon is on the horizon?

I don't just perceive the moon as getting smaller as it rises, I also perceive the moon on the horizon as very close (sort of looming over the trees, if trees there are), compared to the zenith moon as being distant. This also argues against the "comparison with objects on the horizon" theory, which would imply that the horizon moon is being assumed to be further away. I agree with one of the references, where it is suggested that in fact the brain really is "seeing" the horizon moon as larger, and hence interpreting this as closer.

It was interesting to read that not everyone actually sees the illusion. Is there any reference to whether people blind in one eye see it? I think I still see it if I close one eye. I must ask my friend's husband, who lacks stereoscopic depth perception because of an uncorrected "lazy eye" in childhood, whether he sees it. I'm obviously going to have fun some time in the future trying those tricks about looking at it upside down and so on, to see how that alters the perception.

I'm also keen to try some photography. Now, photographs lack stereoscopic depth perception, and they are also close to the observer rather than light seconds (or minutes, for the sun) away, they have a frame round them, and overall they generally fill much less of the field of vision than a real event. These all contribute to the fact that a photo of a spectacular view loses a lot of the "wow factor" of the real thing. (If you don't believe me, look at a photo of the Monch-Jungfrau-Eiger range, then book a trip to Murren.) I wonder what contributes most to the losing of the illusion in the photograph? (It's not inevitable - after all, many of the illusions used to illustrate the points are clearly present in pictorial or diagrammatic form.)

Mmm, time to try that sleep thing again I think.

Rolfe.

[Soapy Sam]

That sleep caper can be habit forming. Just a word of caution.

The colour change is what I meant by "dimming". That IS just the thick layer of (probably polluted) air you see it through- though I can believe refraction would be involved there. Moonlight is reflected, (so part polarised) sunlight, so I expect it will be filtered rather differently by the atmosphere than sunlight. And it's so much fainter. I imagine there's a lot of imaginary "filling in" going on in the bonce when looking at the moon.

Moonset / rise over the sea looks just as impressive to me as on land- and there are no distance cues to speak of in a seascape at night.

The upside down between the legs thing is intriguing. I think if we understand that , we'll have an answer. Must admit I haven't tried it yet.
I envisage a spate of arrests all over the world , on grounds of insanity.

[Jeff Corey]

This guy in the office across the hall from me has been doing research on this for a few decades. Well, he's been there only a few years.
http://domino.research.ibm/com/comm/pr.nsf/pages/news.20000103_moon_illusion.htm
Well, that doesn't work for some season. Goolgle Lloyd Kaufmann moon illusion.

[ceptimus]

It's clearly impossible for everything low over the horizon to be magnified. Imagine a ring of low clouds just above the horizon occupying a full 360 degrees of panorama. If they were magnified, they would appear to occupy more than 360 degrees, and you'd need some weird geometry to allow that to happen.

[Rolfe]

It's clearly impossible for everything low over the horizon to be magnified. Imagine a ring of low clouds just above the horizon occupying a full 360 degrees of panorama. If they were magnified, they would appear to occupy more than 360 degrees, and you'd need some weird geometry to allow that to happen.I think that's a red herring, because aren't clouds relatively low, so that we're not looking at them through the full thickness of the atmosphere? And anyway, since the "magnification" would itself be a type of illusion, the geometry wouldn't need to be that weird.

However, it seems that the "magnification" idea has been comprehensively shot down by people who understand the matter very well, and what is actually at issue is the nature of the illusion which causes the appearance of a large moon on the horizon.

By the way, for a dissenting opinion, how about this (barely literate) page? http://www.zetatalk.com/index/earthm42.htmLast night I went out at round 6 PM and notice how small the Moon looked, and dim it looked. Then at 2 AM I went out again and this time the Moon was at least 2 times larger and much brighter. .... I am talking about the Moon looking really small at Moon rise, and very large hours later.Riiiight....

Rolfe.

[Deetee]

Remember, with each elliptical earth orbit, there are times when the moon is closer to earth, so will appear larger (and the converse applies).
(This should be the case wherever the moon is in the sky however, and obviously not be confined to the newly risen moon on the horizon..)

This shows the "real" difference in appearance between a perigee moon and an apogee moon.
http://antwrp.gsfc.nasa.gov/apod/ap991222.html

[The Bad Astronomer]

Here is an article I wrote for Night Sky magazine about the Moon Illusion (http://nightskymag.com/ns_pdf/200405056058n.pdf). Bottom line: it's a combination of the well-known Ponzo illusion with the perception that the sky is not hemispherical, but bowl-shaped (the zenith is closer than the horizon).

[Jeff Corey]

I found LLoyd Kaufman's last paper on this. http://www.pnas.org/cgi/content/full/97/1/500
There's a cool stereogram in there that illustrates the effect.

[Pepper's Ghost]

If it truly is an illusion, why are the lunar features more prominent when the moon is at the horizon? You can clearly see the light and dark spots and yet when the moon is high overhead, it is merely a bright, white disc.

[Soapy Sam]

If it truly is an illusion, why are the lunar features more prominent when the moon is at the horizon? You can clearly see the light and dark spots and yet when the moon is high overhead, it is merely a bright, white disc.

When the moon is high in daytime do you still find the features hard to see?
I think this is just overexposure- when you look at the high moon in a night sky, you are staring at the biggest and brightest light in the sky. Because your eyes are dark-adapted, you get dazzled.

The plot thickens, that diagram in Phil's article looks awfully like the one the chap in Rolfe's link claims to disprove.

Has anyone considered the possibility that something about a horizon moon makes the Earth look smaller?

[CurtC]

The phase of the moon is irrelevant to its possible positions in the sky. It's just that new moons (and crescent moons) are close to the sun, so when they're high in the sky so is the sun, which makes observation difficult. ;)It might have to do with the time of year that Rolfe could be out looking at a full Moon high in the sky. Since the full Moon is always opposite the Sun, a full Moon in the summer will be fairly low in the sky, while a full Moon in winter will be high in the sky. Maybe it's too cold wherever Rolfe is for her to be out in the winter in the dead of night.
Nobody seems to mention the other part of the illusion, which is that the moon also appears to change colour as it rises, from yellow or gold to pure white. Am I right in assuming that that is just a feature of the greater amount of polluted air between the observer and the disc when the moon is on the horizon?The full Moon appears orange near the horizon for the same reason the Sun appears orange near the horizon, which is the same reason the sky looks blue. Air molecules scatter light, and light on the blue end of the spectrum is scattered much more than the red end. When they're near the horizon, they light is travelling through more atmosphere, so more blue light has been scattered away, and just the more-red light is left travelling the direct path to your eye.

[YoPopa]

snip.. the moon also appears to change colour as it rises, from yellow or gold to pure white. Am I right in assuming that that is just a feature of the greater amount of polluted air between the observer and the disc when the moon is on the horizon?
Thanks for a very interesting thread Rolfe. You've caught my imagination. Just one little fly in the ointment. Everyone is so brainwashed (I've done it myself) by the constant drumbeat of "pollution has ruined everything" that we often think pollution when we see something perfectly natural that we don't fully understand.

CurtC nailed the color question:
The full Moon appears orange near the horizon for the same reason the Sun appears orange near the horizon, which is the same reason the sky looks blue. Air molecules scatter light, and light on the blue end of the spectrum is scattered much more than the red end. When they're near the horizon, they light is travelling through more atmosphere, so more blue light has been scattered away, and just the more-red light is left travelling the direct path to your eye.

I've photographed moon rise at tourist spots a good many times. It's been my experience that the BIG full moon rising is observed and commented on almost invariably just before sunset on the day before the moon reaches the actual moment of "fullness". Under these conditions there is still plenty of ambient light so that the landscape is illuminated at about the same level as the moon is. This produces the most spectacular photos and gets the greatest emotional reaction from folks watching the show live as well. I've watched lots of people come back the next night and be severely disappointed that the moon is now rising about one hour later and is coming up in the dark. Not nearly as exciting. I like the theory that Cecil favors with the added element of romantic/emotional excitement thrown in.

[ceptimus]

I think that's a red herring, because aren't clouds relatively low, so that we're not looking at them through the full thickness of the atmosphere? And anyway, since the "magnification" would itself be a type of illusion, the geometry wouldn't need to be that weird.Okay then, imagine a 360 degree ring of moons (if your imagination is good enough).

If you can imagine a full 360 degrees of moons all magnified but still fitting into 360 degrees, then you have a better imagination than most.

[Notrump]

Okay then, imagine a 360 degree ring of moons (if your imagination is good enough).

If you can imagine a full 360 degrees of moons all magnified but still fitting into 360 degrees, then you have a better imagination than most.

Good one. You beat me to it. I tried to post it yesterday but my registration here was not yet verified. It's the example that I normally use which quiets the believers in magnification. I thought it was original. Great minds must think alike. Yes, simply imagine 720 Moons in a ring around the Earth and spread out along your entire horizon, each Moon 0.5° wide. Magnification in the horizontal direction would not be possible.

Phil Plait (The Bad Astronomer) mentioned the Ponzi illusion. Perhaps in this case it can be better visualized by thinking of an airplane at constant altitude and coming toward you in a straight line. At first it appears low on the horizon and small. It then seems to grow to maximum size when over your head. Actually, the Moon does the same thing, but its increase in angular diameter is not really perceptible since the percentage of change is so much less. Your mind expects it to grow in the manner of an airplane image. When it does not, you perceive the illusion of shrinkage.

Indeed the eccentric (off-centered) orbit of the Moon causes it to appear about 10% larger at perigee than at apogee (about two weeks apart.) The use of the term elliptical (non-circular) orbit is somewhat misleading since a person located well above the Moon's orbital plane would not notice any deviation from a perfect circle. Nevertheless, it would be quite obvious that the Earth is not located precisely at the center of the Moon's orbit. ;)

[Skeptical Greg]

OK, I've had a bit of a Google, and found several pages propounding the "it's an illusion, stupid" theory with apparent authority, and one saying that the refraction theory doesn't really work.

I'm still not convinced. Persuade me. (Preferably by demonstrating that the refraction theory is untenable.)

Rolfe.Have you tried holding a measuring stick at arms length and checking the apparent diameter ?

It will prove it's an illusion..


What are the principles behind the illusion? I don't know..


-------------------------------------------

Yes, Patricio, the illusion occurs with the sun also.. Just harder to view..

[Rolfe]

Have you tried holding a measuring stick at arms length and checking the apparent diameter ?

It will prove it's an illusion..


What are the principles behind the illusion? I don't know..


-------------------------------------------

Yes, Patricio, the illusion occurs with the sun also.. Just harder to view..Oh, I'm quite happy to accept that it's an illusion, given the amount of evidence presented. I just didn't buy the only explanation I'd seen presented, which I now see nobody who has really examined the phenomenon buys either.

It's curious that there isn't a definite, accepted explanation. It would be nice and satisfying if there was, but I don't buy the simplistic ones and I don't understand the complex ones, so I think I'll just wonder for a bit.

Rolfe.

[Beady]

The distance to objects near the horizon, however, is only limited by the curvature of the Earth, and can be around a hundred kilometres, or more if the object in question is a cloud.

Er, not quite. The distance, in statute miles, to a water horizon is 1.221 multiplied by the square root of the altitude, in feet, of the observer's eyes. Standing on the beach with his toes in the water, for someone 6' 4" tall the horizon is ~3.5 miles away. However, he could see the head of another 6' person who was ~ 7 miles away. How far away a cloud could be seen would be a function of its altitude. If it's at, say, 1,000 feet, then our observer would be able to see it from a distance of ~40 miles (the distance from the cloud to the cloud's horizon, to the observer).

Meanwhile, a star which is also just above the horizon is considerably farther from the observer.

ETA: The reason cameras do not detect the optical illusion is that their lenses are not attached to a brain, therefore cameras cannot interpret.

[Rolfe]

The reason cameras do not detect the optical illusion is that their lenses are not attached to a brain, therefore cameras cannot interpret.But photographic images are observed and interpreted by brains.

Many famous optical illusions are paper presentations. Usually drawings, though I don't see why they couldn't come in the form of photographs. The interesting question is, of all the things which distinguish the photograph from the real sight, what is it that prevents the brain interpreting the illusion into the photograph when it's viewed?

Rolfe.

[Beady]

But photographic images are observed and interpreted by brains.


:bricks:

[Jeff Corey]

But photographic images are observed and interpreted by brains.

Many famous optical illusions are paper presentations. Usually drawings, though I don't see why they couldn't come in the form of photographs. The interesting question is, of all the things which distinguish the photograph from the real sight, what is it that prevents the brain interpreting the illusion into the photograph when it's viewed?

Rolfe.

You are generally correct. Many of the famous illusions are famous because they have been pictured in textbooks. I even saw a paper placemat in a diner with the Muller-Lyer, Ponzo, Poggendorff and Horizontal - Vertical Illusions.

But the Moon illusion is difficult to show in photographs, with the possible exception of the stereogram in the Kaufmans' paper that I linked before.

There are so many distance cues that fool us into judging the moon to be farthest away at the horizon. The "sky as a flattened bowl" effect. Height in plane, where object closer to the horizon are usually further away. And factors that produce the Ponzo illusion come into play.

If an object produces the exact same retinal image as another, but is seen to be farther away, it will appear larger.

[YoPopa]

But photographic images are observed and interpreted by brains.

Brains and hearts. I just took another look at one of my favorite moonrise photos and realized that what I remember about the size of the moon in the photo and what is really there are two different things. I've worked with this image a dozen times, printed it, entered it in competition and have a copy hanging in my hall. But when I looked again in the context of this discussion it shrank!

The experts have done a good job of explaining the optics but there is a whole Gestalt thing going on.

Try this experiment. Next full moon book a room where the coconuts grow. Maybe the British Virgin Islands, Cane Garden Bay, Tortola is perfect for this. Sit at the Barefoot Man Bar until the moon is well up into the sky. Use this time to convert whatever currency you are carrying into Rum Punch with fresh nutmeg on top. Share Rum punch equally with your SO or find someone at the bar if you have to. Now go out and lay down on the beach lounge chairs with SO and check out that moon. If you get to work on this now you should be able to report back this time next month.

[Paul C. Anagnostopoulos]

Damn you, Yo! You spied on me while as was at Cane Garden Bay.

~~ Paul

[Rolfe]

There are so many distance cues that fool us into judging the moon to be farthest away at the horizon.But that's most certainly not how I perceive it. I see the moon on the horizon as very close, while I see the zenith moon as far far away. According to this page here, http://facstaff.uww.edu/mccreadd/sectionI.html, this is how the majority perceive the illusion.Over those 28 years, more than 800 participants took part.
In a large audience, at least 75% (and often 90%) chose both "larger" and "closer."
About 5% to 15% chose both "larger" and "same distance."
Only about 5% chose both "larger" and "farther" (McCready, 1983, 1986).
It must be noted that some people report no relative "size" illusion for the moon.The author speculates that this is because the illusion "really" produces a perception of "larger", and then the brain assumes that larger must mean closer.

Rolfe.

[Jeff Corey]

Yes. This is because the many monocular cues for depth make it look larger.
But since we know it can't be, we feel it's closer.l

This has been kicking around for a long time and early attempts to explain this and similar anomalies were known as "The size-distance invariance hypothesis"

[Rolfe]

Yes. This is because the many monocular cues for depth make it look larger.
But since we know it can't be, we feel it's closer.But I don't know it can't be. Until getting into this thread, I thought that an optical effect of the atmosphere really did make it "larger" by magnification.

Rolfe.

[Jeff Corey]

If you really want to know more about the details of this, see
http://facstaff.uww.edu/mccreadd
{Warning, may produce brain fog}

[CurtC]

Here's one of my favorite factoids about bserving the Moon. Ask yourself (or someone else) this question - how far away from your eye, would you have to hold a US dime coin, in order to just block the full Moon? Rolfe, I assume you probably know about how big a dime is - it's the smallest US coin.

Go ahead, come up with an answer, without actually trying it. Just imagine it.

Most people will say something like two feet, or less than a meter - something they can easily reach with their arm. The answer is six or seven feet - two meters.

[Jeff Corey]

The same distance as to block out the sun.

[Notrump]

Here's one of my favorite factoids...
Which is the factoid? The misperception or the truth? ;)

[Rolfe]

If you really want to know more about the details of this, see
http://facstaff.uww.edu/mccreadd
{Warning, may produce brain fog}Yes, well. if you actually look at my posts, you'll see that this is the site I've been trying to work my way through. And it obviously isn't a simple, easy-to understand explanation. I just got to the bit about all objects being affected, and the illusion also involving the apparent size of objects on the horizon.

Rolfe.

[Capsid]

Yes, well. if you actually look at my posts, you'll see that this is the site I've been trying to work my way through. And it obviously isn't a simple, easy-to understand explanation. I just got to the bit about all objects being affected, and the illusion also involving the apparent size of objects on the horizon.

Rolfe.
I quoted it too. But I struggled to follow it. Any chance of a layman's summary?