Big moon, little faith...

January 31, 2006 2:20 AM

A few years ago I read an online article that explained that the common belief that the moon appears larger at the horizon because the Earth's atmosphere acts as a magnifying lens is wrong, and that this effect is due to an optical illusion caused by moon's visual proximity to much closer objects on the horizon. However...

This has left me wondering if photographs that show a large moon hanging over a city or landscape have been altered in some way? In other words, is it reasonable to assume that the large moon has been added after the fact?

posted by planetthoughtful to Science & Nature (33 answers total)

Er, zoom lens?
posted by salmacis at 2:32 AM on January 31, 2006

Another explanation is the photographer is using double exposures. Basically you take a shot of the moon with a long lens (at least a 400mm), then trick the camera into thinking that the film has advanced so you can release the shutter a second time. Slap on a different lens (say a 50mm) frame your shot and release the shutter. This site will give you the nitty gritty details on how to go about doing it this way.
posted by squeak at 2:48 AM on January 31, 2006

What salmacis said. If you move a long way away from the city and use a zoom lens, the city will be much smaller (because you are now many times further away), but the moon will be about the same size (because the distance to it will be about the same as before).
posted by cillit bang at 2:49 AM on January 31, 2006

cillit bang's got it. See the suspension cables and tower of the Golden Gate Bridge in your example picture? In order for those to be in the same picture as downtown San Francisco, you have to be on the other side of the bridge. That implies a rather significant distance.

That picture was taken from several miles away. Guessing from the angle, probably somewhere in the Marin headlands, where without some beefy magnification the downtown area would only be about the size of your hand at arm's length.
posted by majick at 2:59 AM on January 31, 2006

Telephoto lenses (or aggressive cropping) can produce this kind of illusion without the need for photoshoppery. Here's another example of the same phenomenon (the planes were actually well apart). In short you assume, presented with a photograph, that it is roughly what you would see with the naked eye rather than through a powerful telescope.
posted by grahamwell at 4:05 AM on January 31, 2006

Do you have the web page that plane picture came from?
posted by cillit bang at 6:37 AM on January 31, 2006

The news item (for the planes photograph) is here, though the photo itself is no longer linked. Google's cache provided the picture, here. Does that help?
posted by grahamwell at 7:00 AM on January 31, 2006

At least some of the time, the moon does show much larger near the horizon, and yes, it's because of atmospheric distortion, not just nearby ground objects. Have none of you witnessed this? I have seen the rising moon with an aspect easily double the size it shows after it's overhead. It's not the effect planetthoughtful's online "experts" describe.
posted by Kirth Gerson at 7:01 AM on January 31, 2006

While the telephoto lens technique certainly is often used to get a big moon, I know a professional photographer who likes wide angle lenses. When he wants a prominent moon in a photo he has a library of moons he chooses from to photoshop in the image he wants. Many photographers (even film photographers) have sky libraries they use to add clouds to an otherwise dull sky.
posted by TedW at 7:01 AM on January 31, 2006

Snopes looked into the plane photo and has it there.
posted by TedW at 7:04 AM on January 31, 2006

I have seen the rising moon with an aspect easily double the size it shows after it's overhead.

You measured the angular size with instruments both times? The whole point of it being a perceptual illusion is that you can't rely on your eyes and judgment.

I thought that the standard answer didn't have anything to do with comparison to nearby objects. Rather, the story I consistently see is that an object (say a bird) on the horizon or near the horizon is far away, but an object or bird overhead is relatively close -- the bird on the horizon is miles and miles away, but the one overhead is maybe half a mile away. It's the same actual size both times, but when the moon is low your brain thinks that it's farther away, and if it's the same apparent size but farther away then it must be bigger, so your visual system puts lots of [BIG] tags on it and it seems bigger even though the angular size is just the same.
posted by ROU_Xenophobe at 7:27 AM on January 31, 2006

It's not the effect planetthoughtful's online "experts" describe.

"Experts" like, oh, NASA? You're right, dude. What do they know?
posted by LionIndex at 8:01 AM on January 31, 2006

Umm, Yeah, What planetthoughtful and LionIndex said. Go get a nice telescope with a built in device for measuring celestial distances. You'll find that the moon's diameter doesn't change a whit. Whether it's on the horizon, or at it's zenith in the sky...
posted by asavage at 8:06 AM on January 31, 2006

Kirth: I was taught in Astro 101 that the "magnifying atmosphere" theory was considered false and that the "comparison to objects on the horizon" theory was rapidly gaining acceptance. That was 25 years ago. Since then, The Straight Dope has tackled the issue here and come to the same conclusion.
posted by The Bellman at 8:06 AM on January 31, 2006

Kirth Gerson: Atmospheric distortion actually makes the moon/sun smaller, not larger. That's what makes them appear flattened vertically (not stretched horizontally) when very close to the horizon. Also, if you consider the geometry, the moon is actually farther away from the observer when at the horizon than when it's higher in the sky.

The moon illusion is still not fully understood. It is an illusion, however, as is easily proven when some fixed point of reference (say, your thumb at arms length) is used to compare its size at various elevations.

The illusion is also said to disappear when the orientation of the observer is changed. (Lie on your back, tilt your head back, and look at the moon upside-down. It will not appear enlarged.)

As far as photographs go, as others have said, long telephoto lenses will enlarge and flatten distance. It takes at least 400mm to get the moon/sun a reasonable size on 35mm negative. 1000mm or so will give you a nice image, and 2000 mm (focal length of an 8-inch Schmidt-Cassegrain telescope) will fill the frame vertically.
posted by AstroGuy at 8:07 AM on January 31, 2006

What you can do to prove this to yourself is this:

The next time you see a big moon near the horizon, stick your arm all the way out and measure the moon with your thumb. Then lean back so your arm's pointing up at the sky, and imagine a moon the same size on your thumb-ruler up there. Hey, it's much smaller now!.
posted by Capn at 8:07 AM on January 31, 2006

In th book Understanding Exposure which is usually in the top 100 on amazon.com the author has a few example where he shoots a city scene and then later shoots the moon at night in the same shot, since the moon is the only white thing in the image the rest of the image will be unaffected.

Its a pretty common trick.

The article you read about the size of the moon was probably this one from the straight dope.
posted by skrike at 8:11 AM on January 31, 2006

You often see the shortening effect of a telephoto lense in sports photography.
posted by teg at 8:15 AM on January 31, 2006

It is an illusion, however, as is easily proven when some fixed point of reference (say, your thumb at arms length) is used to compare its size at various elevations.

Astroguy, that's exactly what I did, and the moon was much bigger at the horizon than it was later, high in the sky. I am not talking about every time the moon rises, I'm saying that sometimes (see where I said "some of the time" up there?) it covers more sky near the horizon. I guess you'd have to actually go outside and stuff to see it.
posted by Kirth Gerson at 9:42 AM on January 31, 2006

At least some of the time, the moon does show much larger near the horizon, and yes, it's because of atmospheric distortion, not just nearby ground objects.

I'm saying that sometimes (see where I said "some of the time" up there?) it covers more sky near the horizon.

Holy jessum its incredible how this stuff gets around and how convinced people can become that its true.

The Moon is, in point of fact, slightly smaller in the sky at the horizon because it is actually farther away from the viewer. (draw a picture if you can't figure out why.)

The common explanation for why we percieve the moon as larger at the horizon is that while we frontal-lobe "think" of the dome of the sky as just that - a hemispheric dome - perceptually it appears to us as flattened dome, elipsoid. This is because there are visual markers in between us and the "end" of the sky at the horizon, so we percieve that it is further away. There is no visual marers between us and the sky directly overhead, so we can't really make as accurate a judgement of how far it is, so we percieve it as being closer than the sky at the horizon.

So, when we have two objects that take up the same amount of space on our retina, and we assume that one is closer, we therefore assume that it is smaller. Think about it: a dime-sized shape in our visual field we percieve being a foot away we would think is about the size of a dime. A dime-sized shape in our visual field we perceive being a mile away we would think must be several hundred feet in diameter. Therefore, because we percieve things at the limit of our perception on the horizon as farther away than the things at the limit of our perception directly above us, we also percieve them as bigger. Hence the moon appears larger at the horizon.

As to you somehow 'measuring' the moon as 'actually' bigger on the horizon, next time I suggest you do it with a more reliable object than your thumb - as I've mentioned, a dime or other small coin works great.
posted by ChasFile at 10:50 AM on January 31, 2006

There's a book devoted to the issue of the Moon Illusion.
posted by euphorb at 11:06 AM on January 31, 2006

I guess you'd have to actually go outside and stuff to see it.

Ummm, I've spent quite a bit of time in the last 40 odd years doing exactly that. (See: username, profile.) It's an illusion. Sorry.

b1tr0t: Great example. Photography easily proves this.
posted by AstroGuy at 11:11 AM on January 31, 2006

This looks relevant:

THE MOON ILLUSION EXPLAINED
posted by jockc at 11:26 AM on January 31, 2006

I am very familiar with the way the moon normally looks somewhat larger low in the sky. I am not talking about that. I'm talking about its appearing much, much larger. Apparently, it is a form of superior mirage, called a telescopic or looming mirage.

Superior (Upper) Mirage. The light rays are bent downward from a layer of warm air which is resting on a cold lower layer. Since the bending occurs through a large air mass, the images tend to be more stable and clear. Superior mirages are normally seen resting above the horizon. On occasion, they can convey images past the horizon and many hundreds of miles away; such mirages are known as looming mirages. Rarer forms of superior mirages have the ability to magnify across great distances through the atmosphere. They are called telescopic mirages.

Looming is another way of defining a superior mirage. An object is said to be looming if the mirage makes an object appear larger or more elevated than they really are. The way this would occur would be in a situation where there's a shallow layer of air at the surface that is much colder than the air just above it.

superior mirage—A mirage in which the image or images are displaced upward from the position of the object. . . Although textbooks sometimes suggest that what is seen is an object and an even number of images, all are images, and have positions and magnifications that differ from that of the object.

There's lots more. Please note that it's an atmospheric effect. And, yes, it's an illusion, but not the one you're all talking about. In this case, the moon's image is measurably larger near the horizon.
posted by Kirth Gerson at 12:07 PM on January 31, 2006

Kirth, I've observed the sun setting through telescopes (watching transits, etc.) If what you contend is true, the image would get bigger in a scope. It doesn't. It shrinks in the direction perpendicular to the horizon. Atmospheric effects, are, by definition, not illusions but are real phenomena that can be observed, photographed, and measured.
posted by AstroGuy at 2:07 PM on January 31, 2006

I wonder if Kirth Gerson will actually come back here and apologize, and stuff, when he realizes he's wrong.

Dude, it's an illusion. Thinking it's real doesn't make you stupid, it makes you human.

To modify the thumb measurement, the next time you see the big moon effect, find a coin that is the same apparent size when in your outstretched hand. Then 5-6 hours later compare it to the moon again.

disclosure: astro nerd, I built an 8-inch Dobsonian from scratch
posted by intermod at 5:26 PM on January 31, 2006

You guys are ignoring the fact that I am not talking about the same thing you are. Maybe it's because you didn't read the three times I said that sometimes the aspect of the moon is magnified. I am not talking about the usual appearance of the moon near the horizon, which is what you insist on arguing about. I am talking about an unusual appearance resulting from atmospheric anomalies, one I have witnessed several times. Not every time.

I don't dispute that it's an illusion. Here, I will repeat what I said in my last comment: "And, yes, it's an illusion, but not the one you're all talking about."

intermod, here's an apology: I'm sorry our educational system failed to train you in basic reading comprehension. Dude.
posted by Kirth Gerson at 6:51 PM on January 31, 2006

Looking for something else, I came across this page. I guess the moon question has been studied more than I thought. As for the influence of mirages, as far as I can fathom, mirages manipulate earth-bound objects rather than celestial objects. Sorry, to beat a dead horse.
posted by namret at 12:11 PM on February 1, 2006

Kirth, at first I agreed with those who were dissing you, but the more you describe it, I wonder if you are talking about something like this? If not, perhaps you could find/post some links/pictures to show us what you mean. By the way, there is lots of neat stuff at the site I linked, which I first saw at MeFi, many moons ago.
posted by TedW at 9:41 AM on February 2, 2006

TedW - yes, that's exactly what I was talking about. I'm guessing that it's harder to take photos like that of the Moon, beacause it's so much dimmer.

Thanks for having an open mind.
posted by Kirth Gerson at 12:28 PM on May 11, 2006

I was wondering if you ever saw my post. I'm glad you feel vindicated. Of course, now I am going to wonder if you ever see this post; the thread is getting a little long in the tooth.
posted by TedW at 1:22 PM on May 14, 2006

Oh, and to add another comment that will probably never be seen, it should be easier to take pictures like that of the moon, because the moon is simple a gray subject in full sun, so the "sunny sixteen" exposure should apply (f /16 aperture, shutter speed approx 1/ISO sec.) I would assume the temperature inversion necessary for the phenomenon is less common, though, since the moon rise is usually when it is cooler outside. Taking pictures at sunset is notoriously difficult because the sun is so bright compared to the rest of the earth and sky. Our eyes can handle the dynamic range but film and digital sensors cannot.
posted by TedW at 1:27 PM on May 14, 2006

I s'pose you mean that taking pictures of the Sun at sunset is difficult, because the Sun is so bright, etc. I'm not that much of a photographer, but my attempts to take photos of the Moon have been uniformly disappointing. (No doubt you could tell me just where my technique was deficient, but I think it's better for everyone if I just concede defeat.)
posted by Kirth Gerson at 7:56 PM on May 15, 2006