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Low-Hanging Moon Explained
gollum123 wrote to mention a BBC article which explains the low-hanging moon of the past few nights. From the article:"For the past few nights the moon has appeared larger than many people have seen it for almost 20 years. It is the world's largest optical illusion, and one of its most enduring mysteries. The mystery of the Moon Illusion, witnessed by millions of people this week, has puzzled great thinkers for centuries. There is still no agreed on explaination for why the moon appears bigger when it's on the horizon than when it's high in the night sky."
There's a similar illusion with mountains. When I look at Mount Rainier between some large trees, it looks huge. When I look at it while driving down the highway, it doesn't look all that big. I actually find it disappointing to stand at the foot of the mountain. From that vantage point, it doesn't look all that impressive. Having climbed Mount St. Helens, looking down on a mountain from the top, it looks huge. It's really weird.
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Perhaps when it is closer to the horizon, your line-of-sight to the moon also follows closer to the surface of the Earth. Because the atmosphere is denser at the surface, the denser atmophere has a greater lens effect?
No? Well, it was just a shot-from-the-hip thought.
Assuming a perfect non-eliptical orbit, the moon on the horizon is farther away than the moon directly overhead by almost half the diameter of the Earth.
Additionally, I wrote a college term paper about this illusion and in my research I found the illusion to be less pronounced in denizens of mountainous areas who have less exposure to things like train tracks that extend straight into the horizon. Without that frame of reference, they are less likely to think of objects near the horizon as necessarily being very away.
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So what you are saying is that this "Moon Illusion" is simply an occipital lobe processing error? Makes sense to me, there are obviously intances where our brain is incapable of properly processing information. This was the first hit on google. I recommend trying the full tour, its neat stuff.
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A lot of things change when you turn them upside down; I don't think it tells you much about the mechanism of the illusion; it's a wide-ranging and general visual processing hack.
For example, frightening movies totally lose their atmosphere if you tilt your head 90 degrees so the TV is sideways. You can see everything going on, but the images aren't alarming. At least, that's what I've found.
Read Mind Hacks for some interesting stuff on visual processing. The rotating-during-scary-movie thing I first noticed as a little kid watching Jurassic Park, but in Mind Hacks I learned things about how we recognize rotated shapes -- we have to do a lot of processing to flip them over, and the time this takes is proportional to the angle. So I think we get the images with too much lag for the brain to do a lot of the post-post processing it usually does -- i.e. being frightened, comparing sizes properly, etc.
The visual parts of the brain are surprisingly dependent on orientation.
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I actually figured the whole thing out after visiting both a Planetarium and a Bucky-Dome.
The first clue came at the planetarium. At the top of the dome was a small circle. If you visually estimated the size of the circle, you would assume it is 1-2 feet across. However, according to the planetarium guy, it is actually 6 feet across.
The second clue came at the Cinerama Dome. The dome, like all geodesics, is made up of identical hexagonal pieces. However, inside the dome, all the pieces look distorted and irregularly shaped.
The key here is that while both domes are semi-spherical, when you are in them, they both look like they are much wider than they are tall (sort of a squashed sphere shape). Your brain, for some reason, assumes that things directly above you are closer, and that things near the horizon are further, so the dome looks misshapen. With an improper mental image of distance, the tiles look distorted due to perspective, and the circle looks smaller because it is further than it appears.
Basically, what this means is that the moon is the correct size on the horizon, and this "bug" causes it to look too small when it is high in the sky.
And, if you think about it, this bug makes perfect sense. Most things your brain would see (think primitive man on the savanah here) that are straight ahead are going to be far away, or at least 10 meters or so away, so your brain adjusts accordingly. Similarly, most things you see when looking down are close, on the scale of a couple of meters, so your brain also adjusts from that. Most things you see looking up are the sky, and with no frame of reference, your brain assumes that looking up is just like looking down (after all, looking forwards is the same as looking backwards). Therefore, your brain associated things on the horizon as far, and therefore bigger than they appear, and things up or down as close, and smaller than they appear.
They left out the key word:
Not since June 1987 has the full moon been this low in the sky
Actually, they are not saying "this low in the sky". They are saying "hangs lower in the sky".
The difference is simple:
When the Moon is full (or nearly full depending on how long you have to wait for the Earth to rotate it into view), it can appear right on the horizon for any viewer (excepting those whose horizons block the Moon entirely). This happens roughly monthly, not every 20 years.
"Hangs lower in the sky" is referring to the arc that the Moon appears to travel as the Earth rotates. Since the summer solstice was a few days ago, the tilt of the Earth makes the Sun appear in its most northerly position. Consequently, the Moon appears in its most southerly position, and it appears to 'hang' lower in the sky than during winter months for viewers in the Northern hemisphere (this effect is reversed for Southern hemisphere viewers).
When the Moon 'hangs' lower in the sky, the illusion lasts significantly longer because the Moon appears to be closer to the horizon for a much longer period. As a result, far more people notice the illusion, even those who don't normally watch the Moon on a regular basis.
This is the lowest hanging full Moon in 20 years mostly due to the timing of the full Moon relative to the solstice.
Note: there is some slight magnification of the Moon at the horizon due to observing it through much more atmosphere than when the Moon is overhead. However, this effect makes the Moon look very slightly taller. The illusion being discussed here typically makes the Moon appear to be wider on the horizon.
Note: IANAA (I am not an astronomer), but I'm fighting the urge to sleep in order to become one!
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Photographers actually use a trick to capture the big moon: they zoom in on the moon alone in the sky, then use a double exposure to capture the scenery
The moon is actually rarely in the scenery when the moon is in the picure.
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We are completely incapable of estimating them, at all.
I don't know if it has anything to do with looking down, but that's an interesting theory.
But I have to point out that everything we can see up is either very close, maybe three hundred feet max, with most of it within ten, or was, for the vast majority of human existence, infinitely far away, like clouds and stars. So it's not just because downward is so close. Up is basically the same way, being very close, with a few weird exceptions for mountains. (Of course, down has the same exceptions.)
Whereas we've always been able to see things miles away and verify they are, in fact, that far away.
People think Douglas Adams' idea of a race that can't conceive of 'up' is a bit silly, but we have a fairly serious blind spot there.
For example, we think mirrors flip you around left to right. Well...it's just as correct to think they've flipped you around up to down. If you flipped an image in the mirror up to down, the person would be correct, although standing on their head. (Or flipped them front to back, but that's understandable, as you can only see one side of that in a mirror, so how you'd 'flip' that is a bit abstract.)
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