Why does the moon appear to be bigger on the horizon than in the sky?

The so-called "moon illusion" has nothing to do with atmospheric distortion. The phenomenon can be observed with terrestrial objects, such as mountains or tall buildings (like the Empire State Building), which when viewed at long distances appear much larger than when viewed at closer distances. Here is a fact: the angle subtended by the moon's width when it is near the horizon is THE SAME as when it is directly overhead, even though it appears to be larger when low in the sky. One evening when the moon is low in the sky and appears large, hold your thumb up at arm's length and note the moon's size compared to your thumb. Later, when the moon is high in the sky, do the same thing. You will see that the moon's apparent size does not change. It's a somewhat disturbing experiment, since the data -- your thumb measurements -- defy what your eyes are telling you. Scientists are unsure what causes this phenomenon, but it is obviously related to how the brain interprets images of large, distant objects viewed low in the sky or that appear near the horizon. I observed this phenomenon myself when I visited Seattle for the first time. Mount Rainier, when viewed from Seattle, appears quite huge. It's a big mountain, no doubt, but it appears disproportionately large when viewed from Seattle. As you drive to the mountain, it looms quite large until you get into close proximity, when it doesn't seem quite as massive any longer. Anyone who has been to the New York City area has also observed this phenomenon when viewing the city's skyline from a distance, when the Empire State Building seems preposterously huge. But when the observer gets closer to the city -- say, right across the Hudson near the entrance to the Lincoln tunnel -- the ESB appears only somewhat taller than the other buildings and skyscrapers. answer It's a depth-perception glitch - your mind percieves it as being closer because you are viewing the moon and the relatively closer horizon in the same scope of vision. This is delightful. The moon looks larger at the horizon because it seems farther away. Yes, I know you think I stated that incorrectly. You will see how it works in a moment. Keep in mind the idea stated above of the angle subtended by moon being the same no matter where you see it. Now, when you see the full moon high in the sky, there is nothing readily visible to give you any cues about size or distance. It's just this beautiful white disk floating up there.

Now imagine how we interpret the size of images in our everyday experience. The angle sub-tended by the object is only a part of the picture. Try this "thought experiment". You are in the middle of Death Valley. Your best buddy is standing 100 yards away from you. Get an idea of the size of the image that your buddy makes on your retina. Now imagine a person standing 500 yards from you, but the person is producing the same size retinal image as your buddy at 100 yards. You know this imaginary person is farther away because you can see that he is nearer to the horizon. The image is a little higher up, so you may even see it against the backdrop of distant mountains. You can see that this person will appear to you to be gigantic. And if he could actually exist, he WOULD be gigantic. But the retinal images for him and your buddy are the same. When we see the moon through trees, or above a horizon of mountains, we have a 'cue' to the distance on this image. The cues tell us: "That thing is far away. And consequently, the brain understands it to be-- Really big...