When does the moon actually appear largest?

[darron_young]

You know when you see a full moon, and it is usually fairly low in the horizon, and it is HUGE! Not just a full moon, but a gigantic full moon. Dows this happen once a month, or a few times a year?

 

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Thanks

Darron

[jerrold_hartman]

My understanding is that this happens whenever the moon is low in the sky and near objects to objects that our brain knows the size of. It is completely fooling your mind that you are seeing it large. It hasn't been distorted by the atmosphere or anything and if you somehow measured it when it moved up higher in the sky and looks smaller, it's actually the same.

[russ_arcuri]

This is an optical illusion which happens when the moon is low on the horizon. It happens twice a day, when it rises, and when it sets. The times of moonrise and moonset vary each day, with both events happening approximately 50 minutes later than they did the day before. (This is because the moon actually revolves around the Earth in a west to east direction. If the earth stood still, the moon would rise in the west and set in the east. However, the earth is spinning faster than the moon is revolving, so the moon rises in the east and sets in the west.) The moon is only full or close to full about three days a month, though.<p>

 

Note that because the "giant moon" is actually an optical illusion, you won't get any better photographs of it at the horizon than you do when it's high in the sky. The image size will be exactly the same on film and in your pictures. Try it and see.

[nes_suno]

<P>Go check out any book on astronomy. You'll see that the moon's orbit is pretty close to circular. There is no way an average camera can capture the differences in image size due to the slightly elliptical orbit that the moon takes. Scientists measured such distance changes using lasers pointed at mirrors that Apollo astronauts set up over twenty years ago on the lunar surface.

[_____41]

It appears largest when you're standing on it with a Hassy!

[bobatkins]

As others say below, it's just an optical illusion. On film it will

look the same size as usual! You get about 1mm of moon diameter for

every 100mm of focal length, so to get even a 5mm moon you need a

500mm lens.

[mike_sisk]

As others have pointed out, the huge moon on the horizon phenomena is an illusion. But the size of the moon does vary somewhat. A not-terribly relavent but interesting nonetheless explanation can be found at <a href="http://www.fourmilab.ch/earthview/moon_ap_per.html">http://www.fourmilab.ch/earthview/moon_ap_per.html</a>.

[darron_young]

Thanks, for your comments but...

 

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Are you guys trying to tell me, that if I take a picture with the moon near the horizon, when it appears largest due to its releative objects, trees, or buildings, that it will appear small, porpotional to mm length, not actually show up on film relative to the other objects therefore rendering a small moon?

 

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Darron

[russ_arcuri]

To answer your most recent question: The moon does not fill any larger a portion of your field of vision when it's next to the horizon than it does when it's high in the sky. Your brain is, however, interpreting it differently. This is a very powerful optical illusion, but it's an illusion nonetheless.<p>

 

Also, this illusion is not dependent on there being buildings or trees with which to compare it. It can be seen even on a relatively flat horizon, like over water or desert areas. If you take a picture of it near the horizon and again several hours later when it's risen up higher in the sky, both images will be the same size on the film frame.<p>

 

The sun appears to do this as well, but most people haven't noticed it because the sun is only dimmed enough to look at directly when it's very low to the horizon.

[mark_ci]

Darron,<P>It shows up the same size on the film regardless. It actually gets squished a little bit flat when very close to the horizon, but that makes it smaller (in the vertical direction), not larger.<P>The moon illusion is a purely psychological phenomenon, not an optical one in any way. The previous poster is right -- it doesn't seem to depend on there being objects on the horizon to compare the moon with, since it occurs over the ocean. What seems to be happening is that we perceive the sky as a sort of flattened dome. The dome is closest to us overhead. Go out some night and ask yourself what seems to be closer: a star near the horizon or one overhead? You'll perceive the overhead one as somehow "closer." Our brains are trained to regard more distant objects as being larger than they appear, so we naturally think the moon is larger when it is "further away" near the horizon.

[tom_benedict]

Here's a neat mental-floss exercise: Next time you're outside and see the moon, hold your thumb at arm's length and hold it next to the moon. Get a feel for how big it is.

 

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Next time you see the moon low on the horizon and say, "Hot diggity, that moon is HUGE!", stick out your thumb at arm's length and hold it next to the moon for comparison...

 

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It's not so much a case of, "Your camera won't record what your eyes see," so much as it is a case of, "But my photo doesn't convey what I felt when I saw it!"

 

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Here's another example: A knee-jerk reaction a lot of people have when presented with a wonderful sweeping vista is to stick on the shortest, widest lens they have and try to capture it all. The resulting photograph is usually pretty flat, and in no way conveys the grandeur the photographer might've felt at the time.

 

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The moon on the horizon is the same kind of thing. The angular size of the moon isn't going to change that much, but the way in which you percieve it will. Rather than saying, "Hot diggity, that moon is HUGE!" say, "Hot diggity, that moon looks HUGE compared to the tree beside it." Now you're getting there. If you can use a long enough lens to make that tree big in the frame, the moon will be big as well.

 

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Tom

[dan_smith]

While the moon does not change size the appearance on film can approximate that "huge moon" feeling if you work with your exposure, film and use longer lenses. The feeling that it is bigger near the horizon or in relation to foreground features can be taken advantage of in your composition. The pastels in the sky, reflections off water and other objects can all help in the impression that the moon is larger than it really is. Then, using a longer lens will help a lot. If your film is capable of rendering the pastels, alpenglow and early or late light with saturation then the moon becomes part of the 'feeling' of the light and in projection or print will seem larger than the same moon 30 minutes later against the black night sky. (or earlier and the morning sky) By composing with a reference the illusion of a big moon can be maintained. A long lens, 400 and up, is a big help here. The viewer doesn't have to know it was a tele lens, they only see a large moon & that bristlecone pine, lake shore or whatever you use for the anchor point. If you shoot with the color and saturation in mind & use films that can render the pastels very cleanly, the moon will appear as part of a soft yet intense light and will seem larger. there are usually 6-7 days per month where this is viable or somewhat viable, so get out & try it.

[bobatkins]

Watch your depth of field when trying Dan's suggestion. The moon

is at infinity (approx!), the trees aren't. The HFD for a 600mm

lens at f8 is 1800m (over a mile!).

[don_schmidt]

One way to get around the DOF problem Bob mentioned is to shoot the moon with your long lens without any foreground objects in it, then put on a shorter lens, recompose and make a double exposure without the moon. (Is this cheating? Whatever it is, it can get you a neat picture if you get the moon in the right place.)

[martin_davidson]

Not only is it an optical illusion, but it's one of the oldest naturally occurring optical illusions known to man -- the Ancient Greeks wrote copiously about it.

 

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It's also the source of the big goof in APOLLO 13. Tom Hanks in his backyard, spell-bound by the moon, holds out his hand, but produces an impossible piece of perspective (but Ron Howard had dramatic license...)

[glen_johnson]

At the photo club competition last night there were several slides that included a disproportionately large moon. They were either done by the double exposure technique someone mentioned above, or by montage with two separate frames.

[bobatkins]

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<img src="http://bobatkins.photo.net/gallery/img0015.jpg">

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Real or fake? Can you tell?

[russ_arcuri]

Fake.

[edward_tufte]

You can supress the moon illusion by looking at the moon backwards with your head between your legs when the moon is close to the horizon. This is not a joke. For the literature, begin with an article in Scientific American perhaps 12-15 years ago on the moon illusion.

[bobatkins]

Russ: What about the image makes you think it's fake (double

exposure)?

[russ_arcuri]

The sun is off to stage right, as the rocks are casting shadows to the left. The moon can't be full if it's being lit from the right... it'd be about a quarter moon or a bit past quarter moon. For the moon to be full, it has to be in the exact opposite side of the sky as the sun. Since that would place the sun directly behind the camera used to take the picture, the rocks would cast no shadows to the left or right (relative to the camera). This is either a double exposure or a composite.<p>

[bobatkins]

Well spotted Russ. It's *almost* a real exposure, in that the moon

and rocks were shot from the same place only a few seconds apart.

Just long enough for me to switch between a 300mm and a 75mm lens

and make a double exposure.

 

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It's an almost accurate representation of what was realy there, but

as you say, the moon was probably off to the left somewhere. To get

the "real" shot, I'd have had to walk off the edge of a cliff and

back up a mile or so!

 

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Location: Badlands of South Dakota