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View Full Version : Moon illusion re-re-revisited



JohnOwens
2005-Jun-25, 01:24 AM
I noticed something new somewhat recently, that might connect to the illusion of the Moon seeming larger near the horizon. I actually observed this a month or two ago, so Gemini was still up a bit shortly after sunset.
I was looking for Castor and Pollux near Saturn, and although I was looking right at the one on the left, the brighter one, umm... Pollux, I couldn't seem to find Castor where I expected it (and the sky was dark enough that it wasn't just because I couldn't make out second magnitude stars yet). After a few moments looking around, I realized that that star wa-a-ay over to the right was Castor, and realized that the whole configuration looked rather bigger than when I'd been looking at it earlier in the year, when it was high up in the sky, even allowing for Saturn's apparent motion against the stars.
As I've looked for this kind of thing amongst the stars since then, I've noticed it with other constellations & asterisms, too. So, I'd like a few others to go out & look at the stars as they set, and tell me if I'm just crazy, or if the bigger-near-the-horizon effect does hold for shapes and patterns of point-sized bodies, and not just for a single big round body. It might help if you look on separate nights, rather than continuously watching a constellation as it sets, although that doesn't seem to be necessary with the classic Moon illusion.
I'll make it easy to keep track, and toss a poll in here. Please vote only after you've gone and looked for the effect, though. For the current sky, I might recommend as a test case, let's see... maybe Leo or Bootes, if you can make the rest of the constellation besides Arcturus out well enough.

Added: And just as soon as I started looking at other things off the BABB again, I found this Slashdot item (http://science.slashdot.org/article.pl?sid=05/06/24/2131256) linking to this BBC article (http://news.bbc.co.uk/2/hi/uk_news/magazine/4619063.stm), all about the "big" Moon of the past few days.

Maksutov
2005-Jun-25, 02:39 AM
The third choice.

I've always noticed that my favorite constellation, Scorpius (wonder when the astrologers are going to learn how to spell?), looks huge as it starts to appear over the horizon. Plus now looking at it from a location about 10 of latitude south from where I first observed it, it has a nice, sinister "rearing-back-to-strike" orientation when it's coming up over the SE horizon. :D

Same thing applies to Orion as it appears due east. The Moon's always looked big when skirting the horizon, whether full or crescent.

JohnOwens
2005-Jun-25, 02:50 AM
I've always noticed that my favorite constellation, Scorpius (wonder when the astrologers are going to learn how to spell?), looks huge as it starts to appear over the horizon. Plus now looking at it from a location about 10 of latitude south from where I first observed it, it has a nice, sinister "rearing-back-to-strike" orientation when it's coming up over the SE horizon. :D
I'd been looking at Scorpius lately too, but living where I do, it's always near the horizon, when it's visible at all. So that kind of invalidates any comparison for my own firsthand experience. But I suppose I shouldn't have taken such a Milwaukee-centric approach in my post. :wink:

gopher65
2005-Jun-30, 02:18 AM
Hmm this is interesting. Next time I am at my parent's house (so I can actually see a star or two:P) I'll check this out.

CJSF
2005-Jun-30, 03:25 PM
I think it works for constellations too. I've noticed it in the past. For me, though, since reading about the Moon illusion, I haven't really experienced it. It's like reading about the cause has made me unable to see it.

But I think I have noticed the constellations looking bigger since then. When we finally get out of this ridiculous rainy pattern (extreme even for FL), I'll check them out.

CJSF

bad novice
2005-Jun-30, 10:06 PM
I've seen that with orion near the horizon. Never bothered to link it to the moon illusion.

jkmccrann
2005-Oct-21, 11:02 AM
The moon looks a lot larger near the horizon at times, not to mention a lot yellower.

George
2005-Oct-21, 04:31 PM
I think the constellations look a little bigger, but I'll make a point to veify it.

When Mars appears as large as the Moon, we can add this to the poll, right? ;)

Gildamere
2006-Jan-31, 11:23 PM
I have two constellations that I look at regularly and that is Orion and Cassiopeia. And while I never caught Cassiopeia near the horizon I remember a few instances when I saw Orion close to the horizon and I didn't notice any difference in apparent size.

It is logical to assume the same mechanism that makes the moon seem bigger should apply to apparent distances between stars as well. I think that the phenomenon is not that widely noticed is that quite a few people (including me) have problems in spatial perceptions. It is easier for me to judge the size of solid objects (given some reference) than spatial relations, especially distances, between objects.

I think that can be attributed to the fact that with most solid objects, people have a basic preconception of their dimensions because of everyday experience. And the situations in which people judge the dimensions of a solid object occur far more frequently than situations in which one has to judge distances, so the average human being tends to err a lot more in distances than in sizes. Thus the "moon effect" is not that obvious to them in regards to constellations in the night sky.

Amateur and professional astronomers encounter apparent distances between objects more frequently than others so they are the most likely candidates to observe the "moon effect" in constellations.

HenrikOlsen
2006-Feb-01, 02:02 PM
That would be true if the effect is caused by an external physical effect.
If the effect is caused by how perception works, it doesn't have to work the same for apparent distance between things as it does for size.

Gildamere
2006-Feb-03, 04:34 PM
That would be true if the effect is caused by an external physical effect.
If the effect is caused by how perception works, it doesn't have to work the same for apparent distance between things as it does for size.


I do think the effect is caused by how perception works. I think that only a few people notice that effect when looking at constellations is that most people are infinitely more familiar with the moon than some "obscure" alignments of dots in the sky.

When I worked as an electrician I was able to accurately gauge the sizes of screws and nuts, whereas others that were less familiar with screw sizes had a hard time distinguishing m8 and m10 and had to find the right wrench by trial and error.

So most people won't notice the effect simply because they are not familiar with constellations and don't notice if a star was apparently further away on the horizon than it was high in the sky.

PhantomWolf
2006-Feb-06, 03:10 AM
Actually I can say for certain that Consellations do show the same effect. I went and checked with the Southern Cross. It looks WAY bigger when it is near the Horizon than when it is directly overhead.

The Saint
2006-Feb-12, 05:12 PM
Although these photos look too good to be true, could they theoretically and practically be real?
http://www.airliners.net/open.file/653742/M/
http://www.airliners.net/open.file?id=888011

beskeptical
2006-Feb-12, 10:13 PM
I found, by accident, that when the Moon is on the horizon and looking bigger, if you cover one eye, it looks smaller instantly.

I wonder if the illusion of the larger Moon isn't only that the brain compares it to the nearby structures and adds depth to the perception, but that the brain has to be perceiving depth at the time as well.

Chip
2006-Feb-16, 05:07 PM
Although these photos look too good to be true, could they theoretically and practically be real?
http://www.airliners.net/open.file/653742/M/
http://www.airliners.net/open.file?id=888011

Yes. Although the moon is very far away, it is much bigger than the aircraft. The aircraft are also far from the camera when photographed.

Images like these can also be made in Photoshop, however pictures like these have also been taken in the real world. You just have to be at the right place at the right time, and at the right distance, to line up the plane with the moon.

suntrack2
2006-Feb-16, 05:35 PM
very nice topic, john.

noha
2006-Feb-17, 06:53 AM
Does where your living have an effect on this?

beskeptical
2006-Feb-17, 06:44 PM
Does where your living have an effect on this?
This? As in Moon illusion?

No except having traveled to the other side of the equator from where I live, the Moon looks upside down down under. (Or up side down here if you're from there.) We have some built in ideas of what we expect to see that affects our perception. In photos, craters look like mounds if the shadow is on the top and mounds look like craters if the shadow is on the bottom because we expect to see certain shadowing rules.

(If I have that backwards, go ahead, point it out, I can take it. ;) )

Ken G
2006-Feb-19, 09:16 PM
I found, by accident, that when the Moon is on the horizon and looking bigger, if you cover one eye, it looks smaller instantly.

I wonder if the illusion of the larger Moon isn't only that the brain compares it to the nearby structures and adds depth to the perception, but that the brain has to be perceiving depth at the time as well.
This agrees with one standard explanation, that your brain goes through the logic, "it looks small, but I can tell it's very far away, so it's really big, so I perceive it as big". The key step in there is the perception of great distance, which is where the depth perception comes in. But then the question is, why do you need the reference object in the foreground, when your depth perception already tells you the Moon is very far away (no parallax). I'm not sure I believe any of the explanations I've heard, to be honest.

beskeptical
2006-Feb-19, 09:21 PM
This agrees with one standard explanation, that your brain goes through the logic, "it looks small, but I can tell it's very far away, so it's really big, so I perceive it as big". The key step in there is the perception of great distance, which is where the depth perception comes in. But then the question is, why do you need the reference object in the foreground, when your depth perception already tells you the Moon is very far away (no parallax). I'm not sure I believe any of the explanations I've heard, to be honest.Try covering one eye when the Moon is on the horizon. I think it may convince you. It is truly bizarre that you brain should adjust the size of what you perceive.

PhantomWolf
2006-Feb-20, 02:21 AM
Although these photos look too good to be true, could they theoretically and practically be real?
http://www.airliners.net/open.file/653742/M/
http://www.airliners.net/open.file?id=888011

The first is definately faked, and a poor one at that. There is an obvious border about the plane's silhouette and the moon itself looks like an Apollo shot. There is far too much detail and it is the wrong colour for it to be taken with a normal camera from Earth. Also the focus just doesn't seem right. I suspect that the plane was just pasted in and the contrail created with the spraycan tool.

The second is either an incredibly well done fake, or real. The moon is certainly the right colour and detail for a real image, and there is no obvious tell tail signs of the plane being pasted into the image, but rather its edge is mixed well with the colour of the moon, just as would be expected in a real image. I'm not going to say it is definately real, I have a few reservations, but if it's fake, then the artist did on excellent job, right down to the heat haze distortion of the lower part of the moon.

noha
2006-Feb-20, 06:46 PM
I meant like how big it appears

mahesh
2006-Feb-22, 12:50 PM
may i put in my tuppence...
no noha. the size differs visually...depending on whether the moon is at perigee, apogee and elsewhere along its elliptical orbit around the earth.

it differs also due to atmospheric conditions, when seen nearer the horizon, the light passes thru' denser layer of air, the refraction causes this illusion.
(the sun displays same illusion..but hey do not look at the sun directly, please)

and beskeptical is right about the orientation of the moon..... upside down etc

NEOWatcher
2006-Apr-10, 12:16 PM
snip
it differs also due to atmospheric conditions, when seen nearer the horizon, the light passes thru' denser layer of air, the refraction causes this illusion.

While looking at the sky this weekend, I had a similar thought. Part of the moon illusion is the fact that the sky "looks" closer at higher elevations. Along with all the other explainations, I was also thinking of the clarity of the stars. I noticed that the higher the stars, the clearer and brighter they are (not much of a surprise there). But I started thinking, that this adds to the illusion. The lower the stars, the further they are, since they are harder to see (or so it seems)

beskeptical
2006-Apr-10, 07:05 PM
While looking at the sky this weekend, I had a similar thought. Part of the moon illusion is the fact that the sky "looks" closer at higher elevations. Along with all the other explainations, I was also thinking of the clarity of the stars. I noticed that the higher the stars, the clearer and brighter they are (not much of a surprise there). But I started thinking, that this adds to the illusion. The lower the stars, the further they are, since they are harder to see (or so it seems)I don't think you have this correct.

I used to think the larger Moon on the horizon was due to the fishbowl effect of atmospheric distortion. The BA's information corrected my incorrect information about that.

There is more atmosphere to look through laterally than directly vertical. And stars twinkle more and my less than spectacular telescope notes the difference. But how would you see such a significant difference that your brain interpreted distance based on atmospheric distortion? Venus on the horizon is certainly brighter than Mars directly overhead.

It is the objects next to the sky object you are looking at that makes the difference. Try the one eye test I described earlier. When the Moon is on the horizon, switch between one eye and two eyes to look at it. When you eliminate the parallax your brain is interpreting size of a distant object by, you see an immediate decrease in the perceived size of the Moon.

It makes an interesting statement about how your brain interprets visual data. You know intellectually what is affecting what you see, but your brain interprets the size of the Moon irrespective of what you consciously know about the illusion. For many optical illusions, (http://www.mit.edu/people/tere/graphics/illusions/impossible_seat.jpg) you can refocus on what you see with or without seeing the illusion. But certain things, like the size variation certain pictures evoke cannot be so easily refocused. (For example in this picture (http://www.bobjude.co.uk/entertain/illusion/circles.gif) the two inner circles are the same size.) Nor can the perception of tone in this famous illusion (http://www.suplido.com/joel/images/696781_07b22774de.jpg) where the shaded light squares are actually the same shade of gray as the dark squares not shaded.

Gildamere
2006-May-06, 09:51 AM
Nor can the perception of tone in this famous illusion (http://www.suplido.com/joel/images/696781_07b22774de.jpg) where the shaded light squares are actually the same shade of gray as the dark squares not shaded.

I think that illusion is due to our brain's optical processing ability. We encounter shadow/light situations everyday and our brain is used to the fact that objects in shadow are usually brighter than they appear, so we autocorrect. This particular illusion cashes in on that and our experience with checkerboards. We know by experience that a standard checkerboard alternates between light and dark squares. Add to that the autocorrection and everyone assumes that square A must be darker than square B. It might be easier to see if there wasn't a letter printed on the squares.

Fascinating nonetheless.

I once saw a similar illusion on the internet in which two apprently different colors were actually the same (e.g. green/red were actually green/green). Does anyone know where to find that?

EDIT: And to get back on topic: I think the moon illusion is due to the fact that most of the people that live in the city are not used to seeing objects more than a few blocks away. I notice that when I'm travelling I seldom look beyond a few meters. Looking straight ahead to the horizon when walking was a whole new experience for me.
Our perception and our ways of thinking seem to be dominated by habit and experience. That's why it is so important to look at things from a different perspective once in a while.
That, by the way, is not only true for how we see our surroundings but also for other people's cultures, beliefs and points of view.

Gillianren
2006-May-06, 04:39 PM
EDIT: And to get back on topic: I think the moon illusion is due to the fact that most of the people that live in the city are not used to seeing objects more than a few blocks away. I notice that when I'm travelling I seldom look beyond a few meters. Looking straight ahead to the horizon when walking was a whole new experience for me.
Our perception and our ways of thinking seem to be dominated by habit and experience. That's why it is so important to look at things from a different perspective once in a while.
That, by the way, is not only true for how we see our surroundings but also for other people's cultures, beliefs and points of view.

Okay, but then why does it work on people who've lived in the country their whole lives? (For what it's worth, I grew up in the suburbs, yes, but on a slope leading up to the San Gabriel Mountains. From the right places in my home town, you can see all the way to the Pacific. Where I live now, you can see Mount Rainier, which is at least 20 miles away.)

The skeptic
2006-May-06, 04:55 PM
I've often noticed how the plough (big dipper) looks so much bigger when it's close to the horizon than it does when it's higher up...Hence, I went for the 'both look bigger' option.

beskeptical
2006-May-06, 09:23 PM
.... I notice that when I'm travelling I seldom look beyond a few meters. Looking straight ahead to the horizon when walking was a whole new experience for me. ....Man, I have a hard time driving in open country at night because I can't keep my eyes off the immense sky full of stars.

The Bad Astronomer
2006-May-06, 10:55 PM
The first is definately faked, and a poor one at that. There is an obvious border about the plane's silhouette and the moon itself looks like an Apollo shot. There is far too much detail and it is the wrong colour for it to be taken with a normal camera from Earth.

I wouldn't be so hasty. The color of the Moon changes in every shot I have ever taken, and yellow-ish isn't so uncommon. The detail in the Moon looks about right for a telescopic shot to me. And the fuzziness around the plane could be an atmospheric effect, especially if the shot was taken low to the horizon, which it may very well have been (that would account for the Moon's color as well).

I would say this may very well be real, but I can't tell either way. The photographer has many more shots like that one, with descriptions, BTW.

Skyywatcher
2006-May-06, 11:59 PM
Now that's one I'll check I have a very flat horizon.

zephyr46
2006-May-07, 04:19 AM
So, It must be in my head.

Mind you the film was a disposable camera, and it was light still.

I like the mystery in something that questions our ability to explain something we see, but does not exist, only in the mind.

And what is actually there that we can't see?

I hope it has everything to do with gravity myself.

beskeptical
2006-May-08, 09:27 AM
I wouldn't be so hasty. The color of the Moon changes in every shot I have ever taken, and yellow-ish isn't so uncommon. The detail in the Moon looks about right for a telescopic shot to me. And the fuzziness around the plane could be an atmospheric effect, especially if the shot was taken low to the horizon, which it may very well have been (that would account for the Moon's color as well).

I would say this may very well be real, but I can't tell either way. The photographer has many more shots like that one, with descriptions, BTW.The second one looks real to me.

But the first one, how do you get that much lunar detail without a very out of focus jet? I'm not a photographer but I know what I see when my telescope is focused on the Moon and a tree branch is in between. Perhaps two legit shots superimposed?

ToSeek
2006-May-10, 02:37 PM
The second one looks real to me.

But the first one, how do you get that much lunar detail without a very out of focus jet? I'm not a photographer but I know what I see when my telescope is focused on the Moon and a tree branch is in between. Perhaps two legit shots superimposed?

Once you get beyond a certain distance, the focus might as well be at infinity. The jet is thousands of feet away, while the branch might be dozens.

beskeptical
2006-May-11, 06:47 AM
Once you get beyond a certain distance, the focus might as well be at infinity. The jet is thousands of feet away, while the branch might be dozens.The trees are much closer than a jet, I suppose. That's interesting given the lunar clarity in the image.

Gildamere
2006-May-25, 10:51 AM
Okay, but then why does it work on people who've lived in the country their whole lives? (For what it's worth, I grew up in the suburbs, yes, but on a slope leading up to the San Gabriel Mountains. From the right places in my home town, you can see all the way to the Pacific. Where I live now, you can see Mount Rainier, which is at least 20 miles away.)

Well, I limited my ramblings to people living in urban environments, so you're certainly right.

And as you say it does work for people living in the country then there must be a physiological/psychological explanation to that. But I still think that the environment is a contributing factor.