From what I’ve read, my perception is consistent with a large percent of the population, but I’ve never seen a poll or study to determine exactly how much of the population sees what. I am reasonably certain that different people see different things but don’t have a reference for that.

Don McCready has a website (http://facstaff.uww.edu/mccreadd/) that explores the idea of macropsia being the root of the illusion. This idea seems like the best one I’ve heard so far, but simple experiments with fingers and pictures tell me it’s not the only cause, because the effect just isn’t big enough. By the way, I emailed him a while back, and I guess he gave all that to somebody else, so he doesn’t have any new info on it. Unfortunately, I can’t find the email he sent me, so I’m not sure who is doing the research.

When it started rising, he said “Oh it is much bigger!” and I told him it was just the moon illusion and it always does that near the horizon. I thought it was odd that he’d never noticed that before–I’ve noticed it since I was 4 or 5.

A bit off-topic, I’ve had some beefs with your explanation of the moon illusion for a while now, but now I have a good excuse to write about it:

Something I think is funny: the WWdN site you linked to quotes you as saying it’s the Ponzo effect combined with the sky-dome illusion causing the illusion, but the lhup.edu link on your moon illusion page dismisses both as not true (possible contributors, but not The Cause). I can’t get to the griffithobs.org page to see what they say; I get the error “You are not authorized to view this page”.

Different people see different thing, so the causes are likely different for different people, or at least in different proportions (a fact oddly not mentioned in most discussions of the moon illusion). But it’s definately not a Ponzo illusion for me: the Ponzo illusion explanation predicts that the moon would appear to be further away, physically larger, and about the same angular size; what I actually see is that the moon appears to be about the same physical size, closer and a larger angular size. Predictions fail horribly; hypothesis is wrong. Also, in a Ponzo illusion, if I look back and forth at the two objects, I can see through the illusion; it’s only if I look at the entire image that the illusion shows. With the Moon illusion, I can look at the sun on the horizon and the moon in the sky (or vice versa), and the object on the horizon consistently looks closer and angularly larger, even though they are nearly the same angular size.

I’m not sure exactly what does cause the illusion, but I know the biggest contributor is, by far, the presence of foreground objects. In a place with lots of foreground objects, the moon illusion can persist even at 70 or 80 degrees above the horizon. The moon seems the biggest when there are mountains or such surrounding it as it rises. On flat, barren terrain like west Texas, the moon illusion is very slight.

I also have a couple beefs with the average description of the sky-dome illusion that accompanies moon illusion “explanations”: first, the sky-dome is where clouds and planes fly–not where the stars and moon sit. So any illusions related to the sky-dome are not necessarily related to the moon illusion. Second, the average description of the sky-dome illusion states that clouds on the horizon are farther away than clouds straight up, and leaves the reader to guess what effect that has on anything. In reality, this is not an illusion; the clouds on the horizon really are much further away than the clouds directly overhead. The illusion is that the brain perceives the clouds on the horizon to be closer than they really are, and the clouds above you to be further than they really are, because the brain can’t truly comprehend the vast distances involved, nor the angles represented. The reality is that the visible portion of the skydome is very nearly flat (if my math is correct, the clouds 50 miles away are about 35 feet lower than the clouds directly overhead, due to the Earth’s curvature), while the illusion is that the visible portion of the sky-dome is very rounded. If anything, the sky-dome illusion reduces any effect a Ponzo illusion might have, by making the apparent difference between the sky above and the horizon much smaller than it actually is.

The space-dome (the one on which the stars, moon and sun appear to be fixed) appears to my eyes to be somewhat farther away straight up than at the horizon. This seems to correlate with the moon illusion, and the sky-dome illusion, but the effect is not directly proportionate in magnitude. So it’s possible that all three illusions have a common root cause, or at least a common contributor, but I can’t see how any of the 3 would cause each other. Furthermore, none of them seem to be related to the Ponzo illusion in any way.

http://anothermonkey.blogspot.com/2007/07/one-degree-of-separation.html

The night before Saturn was directly above Venus, but now it’s at the 2:00 position! Seeing that sort of apparent change in Saturn’s position gives you a sense of how quickly Earth and Venus are moving in their orbits!