The common perception that a rising or setting Moon is magnified by the atmosphere is an optical illusion: Click link: http://en.wikipedia.org/wiki/Moon_illusion

The apparent angular diameter of the Moon actually increases slightly as it rises higher, because the rotating Earth is bringing you closer to it. When near the horizon the lower limb of the Moon is refracted higher than the upper limb, thus it appears squashed in the vertical dimension. There is no distortion in the horizontal dimension.

When you see a rising Moon, extend your arm toward it and compare the Moon to the size of a dime. If you do the same thing a few hours later, you will see that the size of the Moon has hardly changed at all.

Regarding an alleged horizontal magnification of the Moon when near the horizon, consider this thought experiment. Imagine a giant torus (hula-hoop) with its larger diameter the same as the Moon’s orbit. Picture the smaller diameter of the tube being the same as the diameter of the Moon. Then paint 720 alternately colored striped rings around the tube of the torus, each would be the same width as the diameter of the tube/Moon. To an observer at the center of the torus, the painted rings would appear to be squares.

Now imagine you are on a boat in the ocean and the torus is placed so that it is visible along your entire 360° horizon with you at its center. It would be positioned so that the bottom of the torus nearly meets the horizon with the rest of it extending upward. If the atmosphere actually magnified objects horizontally, how would that change the appearance of the rings that are supposed to look like squares from your perspective? Differing refraction levels for the top and bottom of the torus would make it appear squeezed vertically, but there is no room for the alternately colored rings (squares) to push into each others’ space. The result is that the squares would be seen as rectangles with the same width as the squares but with a lesser height.

The above thought experiment demonstrates how a rising or setting Moon may appear squashed vertically but with no distortion of its horizontal dimension.

It is true that one Full Moon can appear more than 10% larger than another one seen some months later. The difference is due to whether the Moon is near apogee or perigee. The recent Full Moon occurred near apogee making it appear smaller than usual. During the coming months the Full Moons will appear increasingly larger until the December Full Moon happening near perigee. The occasion of Full Moon at perigee (or apogee) recurs over a cycle of 14 lunar months (1.13 years). See my illustration by clicking: http://www.curtrenz.com/astronomical.html

Below is a photo I took from Arlington Heights, Illinois of the Full Moon rising on 2007 JUL 29 at 20:33 CDT. That was 18 minutes after sunset and 8 minutes after moonrise. The altitude of the center of the Moon would have been 0.9°. The Moon’s horizontal diameter was 0.5°. The apparent compression in the vertical dimension is obvious in the photo. Beneath it is a diagram I created with self-written software to show the scene as it would have theoretically appeared, taking into account the differing refraction factors for the upper and lower limbs but allowing no change in the horizontal dimension.

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Curt Renz - "Centaur"
For monthly astronomical calendar visit:
www.CurtRenz.com/astronomical.html

There is a website associated with the forum.

(Just giving you a hard time - the BA used to say that a lot when he was more active on here. Meanwhile, thanks for the cool photo!)

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Everything I need to know I learned through Googling.

Reading the links, it seems like its still open to debate.
I wonder if part of the illusion can be explained by the detail we see on the Moon.

I once got a "3x telescope" from a box of Cracker Jacks. Trying out my new toy, I first assumed I was looking through the wrong end because things appeared smaller, not bigger. But I was holding it right. Keeping my other eye open, I could confirm that objects through the telescope were indeed 3x bigger. But they weren't clearer: ie, if I was trying to read the words on a sign 20 meters away, I had a better chance of doing it naked-eye, rather than with the 3x telescope. Because of its cheap plastic lens, you actually lost detail despite gaining size. And this tricked my mind into thinking that the telescope actually made things look smaller.

I only notice the Moon illusion on full moons. When the full moon is high in the sky, at first glance, it is completely over exposed to my eyes. I can't see any detail. After staring at it, my pupils closed down, and some detail comes into view. But on the horizon, the atmosphere will blur the Moon a bit, but it also dims it to the point that surface detail is easier to see, especially when it is against a sky that is not yet completely dark. So at first glance, its much easier to differentiate the dark fields from the bright highlands when the Moon is on the horizon rather than when its high in the sky. And based on my "3x telescope" observation, more detail means things appear larger.

As an example, look at Centaur's photograph. Although the Moon is somewhat blurred by the atmosphere, there's lots of surface detail to be seen. Although the photograph seems to be exposed properly for the Moon, there's still lots of detail visible in the foreground trees. Because it's not yet dark, your pupils will not yet be dialated, allowing you to see Moon surface detail much easier than if you stared at a full Moon against a black sky.

Furthermore, I don't seem to notice the Moon illusion on waxing gibbous moon rises, when the Moon rises while the Sun is still up. At this point I think the atmosphere robs the Moon of contrast, making detail more difficult to see, negating the moon illusion.

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www.gravitysimulator.com

Actually; the wiki and the BA page seems to be quite short on the subject.

I have a book with an entire chapter on the subject... I think that's where BA got his ideas. [pauses to allow the groans and sighs to finish]

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Numbers are not case sensitive. (me)

Don't miss Wednesday's huge moon illusion
Now this is national news?
I don't know if I should be happy at the good science, or frustrated that it's such a big story.

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Numbers are not case sensitive. (me)