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star_shine10
2006-Apr-13, 06:31 PM
At the time of the moonrise, why is it that the moon appears big above the horizon? Also why does the colour of the moon change from orange, when it's above the horizon, to white when it's finally well above the horizon?

korjik
2006-Apr-13, 06:34 PM
The size is an optical illusion and the reddening is Raleigh scattering in the atmosphere. same reason dusk is red

WorseAstronomer
2006-Apr-13, 06:42 PM
The illusion is because at moonrise, when the moon is right at the horizon, we see it in front of actual things: trees, houses, buildings, and in comparison, it looks really, really big.

When the moon is higher up in the sky, it's not surrounded by anything else except a big, empty sky. So it appears smaller, since we don't have a visual frame of reference in front of it.

NEOWatcher
2006-Apr-13, 06:59 PM
At the time of the moonrise, why is it that the moon appears big above the horizon? Also why does the colour of the moon change from orange, when it's above the horizon, to white when it's finally well above the horizon?
I see you came from UT...Have you checked out BA? There's lots of information there (yeah it's kind of old, but so is the universe)
The moon illusion is here. (http://www.badastronomy.com/bad/misc/moonbig.html) (some links to follow there)
I also suggest browsing the whole site. There's a lot of knowledge in non-technical terms, excellent for the average person.

Nowhere Man
2006-Apr-13, 10:00 PM
You can do an experiment: When held at arm's length, the nail of your little finger is just a bit bigger than the moon, roughly. Compare the moon to your little fingernail at moonrise, and when the moon is at its highest.

Fred

cjl
2006-Apr-13, 11:26 PM
The illusion is because at moonrise, when the moon is right at the horizon, we see it in front of actual things: trees, houses, buildings, and in comparison, it looks really, really big.

When the moon is higher up in the sky, it's not surrounded by anything else except a big, empty sky. So it appears smaller, since we don't have a visual frame of reference in front of it.

This is actually not the reason, if you think about it. For example, this does not explain why it still looks bigger even at sea or in a desert, with nothing to compare it to. The BA has the best hypothesis that I have heard - that when we think of the sky, we think of it as a dome. And this dome (in our minds) is a shallow dome. When we see the moon on the horizon, we think of it as really far away, and so we think it is bigger. When we see it overhead, we think (unconsiously) that it is closer, yet the angular size remains the same, hence the perception of a smaller moon.

rob
2006-Apr-15, 06:19 AM
cjl, I agree with you, and I agree with the Bad Astronomer's explanation ( http://www.badastronomy.com/bad/misc/moonbig.html )

I came here looking for an explanation, since I noticed the Wikipedia page on the moon illusion (http://en.wikipedia.org/wiki/Moon_illusion) has become quite a mess. Some time ago I wrote the two paragraphs following "possible explanations", then someone else came in and wrote that my explanation is "discredited by experts", and added all the stuff below it, which....well...just seems like a bunch of garbage, to be blunt. :) (note the request that you "do a web search"....wow that's professional!)

So I figured I'd pop in here (at my old gradeschool friend Phil's place, no less!) and get some opinions. Is my explanation pretty much the same as the BA's? Is the moon illusion really unexplained as others on Wikipedia seem to suggest?