vision - Why does the moon appear bigger close to the horizon, rising or setting?

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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.

Answer

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.