Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Possible Duplicate:
Why does the moon sometimes appear giant and a orange red color near the horizon?

I made a little research about this and found this article http://en.wikipedia.org/wiki/Moon_illusion that states the explanation of the moon being bigger at the horizon is still debated.

I found this which looks pretty 'big' http://www.psychohistorian.org/img/astronomy/deep-sky/photos/ayiomamitis/20090804-moonrise.jpg but however this kind of repetitive shots http://apod.nasa.gov/apod/image/0706/UludagMoonrise_tezel.jpg show no decrease in the aparent moon radius.

What I am trying to understand is if it is a real illusion, if the atmosphere makes the image bigger or what other explanation could be possible and of course plausible.

share|improve this question
2  
I am editing this since you must not mean zenith . Zenith is right overhead: merriam-webster.com/art/dict/azimuth.htm –  anna v Feb 11 '12 at 16:55
    
I think your first link's photo must be photo shopped.It is known that in photos the moon subtends the requisite angle, as your second link shows and the wiki article explains. It is our perception that changes –  anna v Feb 11 '12 at 17:02
1  
Is there a particular reason you think wikipedia might not be on top of this? It's a popular question (and has been for more than a a century) and one I'd expect wikipedia to have good, up-to-date article on. Further, as anna's comment suggest this is, perhaps, not a physics question at all. Opinions from the commentariat are solicited. –  dmckee Feb 11 '12 at 17:29
    
@annav Thank you for the comments! I do not think that it is photoshopped, or at least not very much because it is a phenomena that I have observed myself. –  mugur Feb 11 '12 at 17:38
    
@dmckee I trust wikipedia, but however I was wondering if there is any certain explanation since Wikipedia sais it is still debated. –  mugur Feb 11 '12 at 17:39
show 3 more comments

marked as duplicate by David Z Sep 13 '12 at 1:47

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

3 Answers

up vote 4 down vote accepted

As it turns out, this is not so much a physics question as it is a psychological one. If you use a ruler or some such held at a fixed distance from your eyes, you'll find that, as demonstrated in the repetitive shot image you linked, the moon has approximately the same apparent size across its entire path across the sky.

The optical illusion arises from the lack of reference points in the sky. Basically, when near the horizon, there are terrestrial objects (trees, hills/mountains, houses, etc.) whose size we recognize as large. The moon, looking larger than these objects, appears even bigger by comparison. When high in the sky, however, the only thing to compare the moon's apparent size to is the sky itself, which is much larger than the moon. The result is that the moon appears smaller.

share|improve this answer
    
That is a good one. I will try that. thank you! –  mugur Feb 11 '12 at 17:40
1  
More accurately, we can only comoare the moon with the clouds. Clouds are pretty huge, though they don't look huge, so they dwarf the moon' –  Manishearth Feb 11 '12 at 17:40
    
I believe I've heard (although I can't remember where) that when the moon is high up in the sky, you also compare it with the distance between the moon and the horizon. –  David Z Feb 12 '12 at 2:44
    
Is this true? While it is correct that the cause is "psychological", the actual mechanism is not known. In particular, it is my understanding that the mechanism you cite has been ruled out. –  raxacoricofallapatorius Sep 7 '12 at 22:10
add comment

It is an illusion of apparent angular size (rather than a geometrical or atmospheric effect), the precise mechanisms and reasons for which are unclear. Though it has been suggested that it is a form of the Ponzo illusion, or a consequence of the presence of foreground queues that we know to be large, several lines of evidence argue against this.

While there is no single agreed to explanation, it appears that the illusion is a form of oculomotor micropsia, the same process that causes a camera flash after-image to appear large against a distant wall, yet small against your hand. In the case of the Moon illusion, cues that create perceived distance (regardless of whether they provide scale, as in common explanations) at the horizon, work like the distant wall for the flash after-image, while in the absence of such queues in the sky overhead, the eyes adjust to a perceived "resting focus" distance, like the hand for the flash after-image.

share|improve this answer
add comment

For me the visual size heavily depends on the brightness or redness. More bright Moon/Sun appear smaller than less brigh/more red.

I notices that reddish moon in zenith also looks big.

share|improve this answer
add comment

Not the answer you're looking for? Browse other questions tagged or ask your own question.