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Changeable moon

Rolfe

Adult human female
Rolfe
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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.
 
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.
 
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.
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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. ;)
 
Rolfe said:
Anybody able to say with confidence whether I'm right, or whether the "it's all an illusion" page is right?
Click to expand...
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.
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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 authority
Click to expand...
Have you found this one?
 
Cecil said:
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. ;)
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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.
 
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.
 
Rolfe said:
I'm still not convinced. Persuade me. (Preferably by demonstrating that the refraction theory is untenable.)
Click to expand...

From http://www.licha.de/astro_article_atmosphere.php :
Code: 
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.
 
Rolfe said:
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
Click to expand...
That 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.
 
Cecil said:
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
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OK, 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.
 
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?
 
Soapy Sam said:
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.
Click to expand...
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.
 
Soapy Sam said:
Why would you expect atmospheric refraction to result in magnification?Dimming, yes; distortion, maybe, but why enlargement?
Click to expand...
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".
 
Patricio Elicer said:
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".
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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.
 
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.
 
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.
 
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.
 
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