Why the moon looks bigger at the horizon

[daniel_i_l]

For a while I've been wondering why the moon looks so big on the horizon. I looked around the web and found 2 answers:
1) At the horizon the brain thinks that it's very far way compared to when it's above your head in the sky, but since they're really the same size in both places the brain corrects for this and assumes that the moon near the horizon is bigger.
2) When the moon is low down you have a lot of things on the ground to compare it with and it looks big relative to them. But when it's high in the sky you have nothing to compare it with.
Is there any way to check which explanation is correct?
And is it really just an illusion with no optical explanation?
Thanks.

 

[D H]

It's an optical illusion. A very, very good one, too. The two answers say more-or-less the same thing. What's the conflict? The second is just a bit more descriptive.

 

[DaveC426913]

1) is the answer I have heard too.

Our perception of the sky - in the absence of visual clues - is that of a low dome, its roof closer to us than its rim. Thus, when we see the Moon above us, we perceive it to be closer than when it is at the horizon. Because we think it's closer yet its disk is the same size, we interpret that as if it is smaller over our heads.

Fig 2 on this page explains it.

[ EDIT ] Huh, it goes on to explain an effect I had not thought of. Clouds over your head are in fact much closer than clouds over the horizon. This lends plausibility to the idea that our brains might perceive the sky to be closer above us than near the horizon - since that's actually true. It is only necessary to suppose that the effect remains part of our expectation even when there are no clouds in the sky.

 

[DaveC426913]

It's an optical illusion. A very, very good one, too. The two answers say more-or-less the same thing. What's the conflict? The second is just a bit more descriptive.

Well, the second one doesn't actually explain the effect; it simply hand-waves it. So, it's not that 1) and 2) are the same, its that 2) says nothing of value.

 

[DaveC426913]

Also interesting:

"My personal contribution to the issue (and my only published contribution to the science of astronomy, as printed in Sky & Telescope, May, 1985, letters) is the following observation: use a tube over one eye to eliminate the foreground (close the other eye); note how small the moon suddenly appears. Now open the other eye. The eye with the unobstructed view sees a larger moon, and you have the slightly unnerving experience of seeing two different-sized moons at once. This observation proves that the illusion occurs independently in each eye, and places limits on any mechanism proposed to eplain it."

 

[D H]

Well, the second one doesn't actually explain the effect; it simply hand-waves it. So, it's not that 1) and 2) are the same, its that 2) says nothing of value.

I saw (1) as being pure hand-waving and saying nothing of value while (2) gives some clues regarding the brain does what it does..

 

[rcgldr]

I tried hand-waving while looking at the moon and it made no difference. Will have to go with one of the previously posted reasons, or with the reasoning that at the horizon, there are other objects to compare the moon with (for apparent distance from the viewer), while once it's up high enough a direct comparason can't be made (can't tell if the moon is in front of or behind other objects unless there are clouds).

 

[daniel_i_l]

Thanks for the replys! I can't wait until the next time I see it so that I can try what Dave posted with the tube.

 

[JeffKoch]

As I recall, this has been tested in planetariums, and sure enough, if you cast a photo of the moon horizontally, people will almost always think it's bigger than if you cast it straight up.

 

[Tempus Fugit]

A related phenomena I find when I look at the moon is that when the dark side of the moon is more visible (i.e. when you get a more three dimensional sense of the moon) the moon looks bigger (in addition, you really get the sense of «hey, there's a goddamn gigantic sphere up there!»).

 

[LURCH]

I can't agree with answer #2, since the Moon looks bigger when it's at the horizon over perfectly smooth water, when there are no objects with which to compare it. I also can't agree that the two statements are saying the same thing.

Therefore, I tend to accept the first explanation, that our mind uses the horizon as a marker for being really far away, and anything beyond it is perceived as being very distant.

 

[t-money]

I also read that in the brains eye, the Earth is dome shaped so anything appearing closer to the horizon seems larger when compared to being perpendicular to the observer. I guess I would have to agree with explanation number one.

 

[raylphscs]

I think nobody has said the right thing so far. Even you lie down on the ground to look into the moon that is in the middle of the sky dome, it is also small.
the real science behind this phenomina is that the Earth has a transparent atomosphere acting like an optical lens causing telescoping effect. only when the moon or sun on the horizon is a convex lens standing between you and the moon or sun, because the atomosphere is a transparant round ball suround the earth. The light beam has different behaviour in the air than in the vacuum outside the atomasphere. Asume the atomsphere ball are water or glass, you will sure understand what I am saying. When the moon is right on the sky dome, light beam goes to you without bending toward the center of the sphere because it is purpendicular to the surface of the spheric convex lens. but when it is on the horizon, your are at one tip of the lens because the center of the lens is the center of the earth, you are too far away from the center because you are on the Earth surface. We sure know the moon did not change size, but the light beam from it changed direction to reach our eye causing a bigger visual angle so it looks larger. Our brain will not feel anything bigger if the visual angle doesn't increase. It is not an illusion. We have a brain but a real camera hasn't, the real camera get a big sun at suset, do you think the camera had an illusion?

 

[Jack21222]

1 is correct, 2 is completely wrong.

http://www.badastronomy.com/bad/misc/moonbig.html

*edit* Just saw I responded to a necropost. Curse you, raylphscs! Check the dates of a thread before responding.

 

[nonequilibrium]

[ EDIT ] Huh, it goes on to explain an effect I had not thought of. Clouds over your head are in fact much closer than clouds over the horizon. This lends plausibility to the idea that our brains might perceive the sky to be closer above us than near the horizon - since that's actually true. It is only necessary to suppose that the effect remains part of our expectation even when there are no clouds in the sky.

Well that doesn't really make sense, because imagine you were a person in the time before they knew clouds were closer than the moon: you'd still perceive this optical illusion; so it doesn't seem to be contributing at all.

EDIT: lol just saw the previous post and its edit (wasn't there when I posted)

 

[DaveC426913]

I think nobody has said the right thing so far. Even you lie down on the ground to look into the moon that is in the middle of the sky dome, it is also small.
the real science behind this phenomina is that the Earth has a transparent atomosphere acting like an optical lens causing telescoping effect. only when the moon or sun on the horizon is a convex lens standing between you and the moon or sun, because the atomosphere is a transparant round ball suround the earth. The light beam has different behaviour in the air than in the vacuum outside the atomasphere. Asume the atomsphere ball are water or glass, you will sure understand what I am saying. When the moon is right on the sky dome, light beam goes to you without bending toward the center of the sphere because it is purpendicular to the surface of the spheric convex lens. but when it is on the horizon, your are at one tip of the lens because the center of the lens is the center of the earth, you are too far away from the center because you are on the Earth surface. We sure know the moon did not change size, but the light beam from it changed direction to reach our eye causing a bigger visual angle so it looks larger. Our brain will not feel anything bigger if the visual angle doesn't increase. It is not an illusion.

Your facts about the atmosphere, while true, are a red herring. They lead you to the wrong conclusion about the Moon illusion.

It is an illusion.

We have a brain but a real camera hasn't, the real camera get a big sun at suset, do you think the camera had an illusion?

A camera is how you prove it's an illusion. A camera is not fooled.

The moon does not change size as it nears the horizon.

Take as many pictures as like, they will show the same thing. (The critical thing to ensure though, is that the images of the Moon high and Moon low must be in the same picture - i.e. with the same camera settings - for comparison.)
[PLAIN]http://www.lifeinthefastlane.ca/wp-content/uploads/2007/06/summer_moon_illusion.jpg

 

[Rebound]

Thanks for the replys! I can't wait until the next time I see it so that I can try what Dave posted with the tube.

You should also try this: after observing it at the horizon normally, bend over and look at it from between your legs. It will look normal size again. It has everything to do with our perception that things that are nearer are larger. I was just reading about this effect the other day, in fact: http://www.michaelbach.de/ot/sze_moon/index.html"

 

[D H]

the real science behind this phenomina is that the Earth has a transparent atomosphere acting like an optical lens causing telescoping effect. ... We have a brain but a real camera hasn't, the real camera get a big sun at suset, do you think the camera had an illusion?

A real camera does not get a big celestial object (sun, moon, or even a constellation) when the object is at the horizon. There is an atmospheric effect, but it is rather small and it opposes the illusion. Objects near the horizon have a slightly smaller angular size due to atmospheric effects.

 

[PhanthomJay]

Has anyone ever seen the full 'harvest' moon rising over the Atlantic? It is orange, and it is HUGE...appearing much larger than an ordinary full moon rising over the Atlantic horizon, and appearing much much larger than a moon overhead. Now the moon doesn't change size, and it doesn't have anything to do with the apogeee or perogee. It's got to be more than an illusion, it must be due to the length of sight through the atmosphere when looking toward the horizon, and atmospheric conditions in particular at the autumn equinox when the Harvest moon appears.

Watch for it this month on the 22nd I think, and you'll see for yourself!

Note: I'll be out there with a toilet paper roll peering through from one eye, hoping that the neighbors aren't watching!:tongue:

 

[Janus]

I think nobody has said the right thing so far. Even you lie down on the ground to look into the moon that is in the middle of the sky dome, it is also small.
the real science behind this phenomina is that the Earth has a transparent atomosphere acting like an optical lens causing telescoping effect. only when the moon or sun on the horizon is a convex lens standing between you and the moon or sun, because the atomosphere is a transparant round ball suround the earth. The light beam has different behaviour in the air than in the vacuum outside the atomasphere. Asume the atomsphere ball are water or glass, you will sure understand what I am saying. When the moon is right on the sky dome, light beam goes to you without bending toward the center of the sphere because it is purpendicular to the surface of the spheric convex lens. but when it is on the horizon, your are at one tip of the lens because the center of the lens is the center of the earth, you are too far away from the center because you are on the Earth surface. We sure know the moon did not change size, but the light beam from it changed direction to reach our eye causing a bigger visual angle so it looks larger. Our brain will not feel anything bigger if the visual angle doesn't increase. It is not an illusion. We have a brain but a real camera hasn't, the real camera get a big sun at suset, do you think the camera had an illusion?

While the atmosphere does have some effect on the Moon near the horizon, it isn't what you think it is. For one, the index of refraction for air is pretty small (1.0003), For the Other the effect it does have is to bend light slightly around the horizon, so that we see objects that, in a straight line, are below the horizon and objects just at the horizon are seen higher in the sky. This bending effect gets stronger as your line of sight nears the horizon. When the Moon is sitting right at the Horizon, the effect is stronger at the bottom of the Moon than it is at the top, so the actual effect is to "flatten" the Moon slightly, not enlarge it.

 

[Oldfart]

Just to add to the confusion, if you bend down and look at the rising moon between your legs it no longer looks large.

 

[Jack21222]

It's got to be more than an illusion

No, it doesn't. Don't post such nonsense when that nonsense has already been debunked earlier in the thread.

 

[PhanthomJay]

No, it doesn't. Don't post such nonsense when that nonsense has already been debunked earlier in the thread.

I didn't mean to say it wasn't an illusion, I said it was MORE than an illusion. It is a combination of an optical illusion plus atmospheric conditions. See for yourself on the 22nd. Are you the ultimate authority on this?

 

[Janus]

I didn't mean to say it wasn't an illusion, I said it was MORE than an illusion. It is a combination of an optical illusion plus atmospheric conditions. See for yourself on the 22nd. Are you the ultimate authority on this?

No, it isn't, for the reasons I stated above.

The following is a rendering done with a ray-tracer. It shows the Moon near the horizon in both the top and bottom. The Top was done with no atmosphere surrounding the Earth.

The bottom was done by adding a transparent layer with an refractive index of 1.0003.

[PLAIN]http://home.earthlink.net/~parvey/sitebuildercontent/sitebuilderpictures/illusion1.gif

Other than raising the Moon slightly higher from the horizon, there is no noticeable change from the top image.

The next image was done by increasing the index of refraction to 1.0015.

[PLAIN]http://home.earthlink.net/~parvey/sitebuildercontent/sitebuilderpictures/ilusion2.gif

Notice how the Moon is perceptively higher above the horizon, and noticeably flattened.

 

[PhanthomJay]

Well, ok, but how do you explain the fact that the full 'harvest moon' (at the autumnal equinox) will appear larger on the horizon than a full moon appears say in July on the horizon. The harvest moon is noticeablyapparently larger than 'prior' full moons appearance on the horizon, as it rises in the east. Does it have to do with latitude of viewing? Or other? Surely you've seen days when the full moon appears larger on the horizon than the prior months full moon appeared on the horizon. I would think that if illusion was the sole cause, then the moon would always appear to be the same size on the horizon, regardless of the month, except perhaps for apogee/perogee.

 

[Redbelly98]

The following is a rendering done with a ray-tracer.

Nice! Did you use a commercial package? Which one?

Just to add to the confusion, if you bend down and look at the rising moon between your legs it no longer looks large.

I thought of a suggestive comeback to this, but we forum moderators need to set a good example.

 

[DaveC426913]

Well, ok, but how do you explain the fact that the full 'harvest moon' (at the autumnal equinox) will appear larger on the horizon than a full moon appears say in July on the horizon.

How exactly do you compare these two phenonena, which are a half year apart?

It is a rhetorical question. You can't.

 

[DaveC426913]

I thought of a suggestive comeback to this, but we forum moderators need to set a good example.

I was thinking of the member's username when he posted this...

 

[PhanthomJay]

How exactly do you compare these two phenonena, which are a half year apart?

It is a rhetorical question. You can't.

Well, I saw the full moon rise over the horizon last month, August, 2010. Due to the moon illusion, it appeared larger than when it later was overhead. Fine. I got a good sense of its apparent size. Big, but not too big.
Now this September 22-24, at moonrise just around sunset, I'll peer out over the horizon to view the full moon. If this moon is similar to last years harvest moon, it will be orange and HUGE...easily noticeably larger in comparison to August's full over the horizon. About nearly the size of the sun if one dare look at the sun. One thing's for certain...I'll be out there at moonrise on the seawall with my head between my legs holding a tube of toilet paper for viewing through, and if i don't get arrested, i'll inform you of the results.

 

[DaveC426913]

it will be orange and HUGE...easily noticeably larger in comparison to August's full over the horizon. About nearly the size of the sun if one dare look at the sun.

This is a damning comment.

First, you say "it will be HUGE" and "noticeably larger". These are your words, right?

Next you say "nearly the size of the sun". Your words.

Clearly you do not realize that, because of the quirk in their diameters and distances, the Moon's disc is always almost exactly the same size as the Sun's disc within just a few minutes of arc. (That's why we get eclipses.)

You have demonstrated that you are very bad at judging the comparable sizes of two objects seen less than 12 hours apart. How can you possibly continue to think your (or anyone else's) estimation of sizes of objects in the sky is anything but utterly untrustworthy?

 

[PhanthomJay]

Yes, when I said about the size of the sun, I was comparing it to what I thought was the size of the sun when I last viewed it during a sunset through a layer of clouds, some years ago. So you are correct, I can't compare the size of the moon with the size of something I saw years ago. I was really comparing it to the bright sun shining fully, which I can only look at with a glancing eye. So I'll correct my statement : The harvest moon last year appeared greater than the size of the sun. I can't believe that no one else on this forum has ever viewed the Harvest Moon at moonrise over the horizon. IT IS HUGE. Next week when I view it, I'll eat crow if I'm wrong. Of course, i won't be able to prove it appeared huge, so I guess you should look yourself and give me your take on it.

 

[john.phillip]

On the Southern Hemisphere, the Harvest Moon was on March. How could you see the moon larger on the Northern Hemisphere, on August, while it had a normal size for viewers on the South, and vice-versa on March ?

http://www.archimedes-lab.org/atelier.html?http://www.archimedes-lab.org/moon_illusion/moon.html

 

[Janus]

Nice! Did you use a commercial package? Which one?

I thought of a suggestive comeback to this, but we forum moderators need to set a good example.

I used POV-Ray.

 

[DaveC426913]

The harvest moon last year appeared greater than the size of the sun. I can't believe that no one else on this forum has ever viewed the Harvest Moon at moonrise over the horizon. IT IS HUGE.

No one is arguing that you looked at the Moon and saw it was huge. In fact, that is exactly what we're saying.

But without any objective measurement, who are you to claim that what you saw was not simply an illusion?

 

[DaveC426913]

On the Southern Hemisphere, the Harvest Moon was on March. How could you see the moon larger on the Northern Hemisphere, on August, while it had a normal size for viewers on the South, and vice-versa on March ?

I think you're missing his point.

He is claiming it's a phenomenon of atmosphere and viewing angle, which will be specific to one's location.

Not that it's right, but you're missing his argument.