Changeable moon

[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.htm

Last 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. 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..


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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.

 

[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"