Low-Hanging Moon Explained 381
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."
Re:Bruce Almighty flashback (Score:5, Interesting)
Umm... maybe because a thicker atmosphere? (Score:5, Interesting)
No? Well, it was just a shot-from-the-hip thought.
Actually... (Score:5, Interesting)
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.
Explained? (Score:3, Interesting)
I'd really like to see a bit more attention paid to making Slashdot headlines accurate, both by submitters and editors.
Isn't it just because of the frame of reference? (Score:3, Interesting)
Re:Bruce Almighty flashback (Score:5, Interesting)
-kaplanfx
Re:Bruce Almighty flashback (Score:3, Interesting)
Speaking of thumbnails, a really good trick for estimating distances if you can estimate object size:
Measure the width of your thumbnail. Measure the distance between your thumb and your eye (arm extended, hitchhiker). Divide the thumb-eye distance by the thumbnail width. You'll get a number around 30.
To estimate distances, stretch out your arm and estimate the height of your thumbnail against an object at that distance. If there's a person standing on a boat, and my thumb is slightly wider larger than their height, my thumb is about 2 meters high. Times 30 puts them at 60 meters away. Best distance estimation trick I know, and great for the compulsive quantiphiles among us.
Re:Bruce Almighty flashback (Score:3, Interesting)
And the perception of depth goes away if you close one eye. And the appearance of continuous motion vanishes if you blink your eyes rapidly. In other words, if you literally change the way you are looking at the world, you will change the way your nervous system processes the light that enters your eye. Your example is intriguing, but not all that revealing.
I won't hypothesize why what you said works (since I haven't tested it), but I will point out that there's more to perception than thinking "That is a tree. That is a bird. The bird is in the tree. Therefore the bird is smaller than the tree. There is the moon..." Vision processing occurs in the retina and the optic nerve and on who knkows how many levels in the brain before conscious thought ever gets involved. Inverting an image completely alters its appearance; is it to much to suppose that the processing of rarely inverted images would be different from the processing of normally upright images?
Completely off-topic I once read that intelligence was nothing more than an overgrown hack on the optic nerve. I partly believe it. The optic centers of the brain imporant bits of the visual signals - there's some lines, there's some blue, there's some motion. Intelligence is little more than astract abstractions - feeding the abstractions back into the engine that produced the abstractions, mixing the levels of abstraction, and seeing what useful behaviors the whole process produces.
Re:Easy Fix (Score:5, Interesting)
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 [amazon.com] 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.
Damn, I was wrong. (Score:2, Interesting)
In the May 1999 issue of Sky and Telescope, there appeared a follow-up article which proved that Sky and Telescope had in fact created the current meaning by mistake in an article published in March 1946. The author of the 1946 article had misinterpreted a page of the 1937 Maine Farmers' Almanac.
By studying copies of the Maine Farmers' Almanac dating as far back as 1819, the authors of the May 1999 article showed that the compilers of the Almanac used the term to label the third Full Moon in a season which has four.
We have calculated the dates of this type of Blue Moon for the 20th and 21st centuries and put them in a list for you to browse.
It's a delightful irony that Sky and Telescope, in publishing an article in March 1999 on the history and folklore of Blue Moons, should turn out to be celebrating a "tradition" which it inadvertently created in an article 53 years before!
So which definition is "correct"? The authors of the May 1999 article admit,
With two decades of popular usage behind it, the second-full-Moon-in-a-month (mis)interpretation is like a genie that can't be forced back into its bottle.
And Charles A. Federer, Jr., the founder of Sky and Telescope magazine, adds,
Even if the calendrical meaning is new, I don't see any harm in it. It's something fun to talk about, and it helps attract people to astronomy.
http://www.obliquity.com/astro/seasonal.html [obliquity.com]
Re:Bruce Almighty flashback (Score:4, Interesting)
I actually figured the whole thing out after visiting both a Planetarium and a Bucky-Dome [bfi.org].
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.
Re:Bruce Almighty flashback (Score:2, Interesting)
It is more likely an illusion related to stereoscopic vision. Monoscopic cameras - which is virtually all modern camera equipment - do not see this illusion. There were experimental cameras in Victorian times with a stereoscopic lens arrangement, but there is no record of whether those see the illusion or not.
Stereoscopic vision works by seeing the same object from different angles, but it only works well if the difference between images is very small. If you raise a finger and move it close, you will see two fingers. Move it far enough away, and the images partially overlap - enough to see a single object, but a much larger one than is real.
When the moon is at a very low angle, is it possible that the eyes are seeing an overlapping image and therefore an apparently larger one?
Perhaps. The illusion does not exist if you close one eye. As noted above, a standard camera won't show it either. So, we have an illusion that appears to require stereoscopic vision to exist, which means that it is likely that it involves discrepancies between the images.
This makes some sense, as the effect also occurs only when the moon is very bright, which means that the physical boundaries aren't going to be as visible.
The easiest way to determine if the effect is mechanical in the way I describe is to use a stereoscopic camera, or to use two digital cameras with suitably different positions and settings, where you can recombine the image electronically on the computer. If the illusion is reproducible from the electronic images, then it is a mechanical illusion of the sort I've described and NOT one of interpretation by the brain.
Why argue over things that can be tested by experiment and proven by inspection? We're geeks, not politicians!
Re:"Low-hanging" moon? (Score:5, Interesting)
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!
Head standing optional (Score:3, Interesting)
The moon subtends about 2 solid degrees. By fortunate coincidence, this is more or less the same angle subtended by by most adult's fingers when their hand is held at arms length -- very rought it's true, but close enough.
So, just hold your index finger at arms length. It will be wide enough, approximately, to just cover the moon. Remember how it looked. Then look for the moon later when it's higher in the sky and try again. The moon looks much smaller in all that empty sky, but it will be about the same size compared to your finger.
The finger trick is useful for rough angle estimations. A hand width with closed fingers is about ten degrees, and a spread fingered hand (unless you have Marfan's syndrome) is about twenty. If you are really concerned about accuracy, you could calibrate it I suppose and multiply by some factor other than 2 degrees per finger width. I wouldn't use it for civil engineering purposes, but it will do for navigation and star hopping.
psychology? illusion? (Score:2, Interesting)
It's refraction... The same reason it's very difficult to catch fish with a spear or bare-handed..
When you look at the water in a stream at an angle, you're not seeing 'in a straight line', the refraction due to the surface of the water causes you to perceive the fish in a different spot than it really is.
This can also be observed in a fish tank (get real, this is Slashdot, do any of us EVER find ourselves wading in fish-bearing streams?) If you look at the tank from the outside, straight-on, you see the fish where it really is. If you look from an angle, however, your perceprion of where the fish is will be distorted in proportion to the angle at which you deviate from the perpendicular (with respect to the side of the tank)
If the surface of the tank were curved (like the atmosphere) you'd perceive the fish to be larger than it is as well.
In the moon's case, if you are looking straight up at the moon when it is directly overhead, you're experiencing as little atmospheric/curvature distortion as possible. As the moon gets lower in the sky, the refraction becomes more pronounced resulting in the perception of a larger moon...
I dub this the Archibald Castell Jr. (Dad) theory of moon illusion.
I assume I'll be contacted by the Nobel people soon...
Re:Bruce Almighty flashback (Score:3, Interesting)
The moon is actually rarely in the scenery when the moon is in the picure.
Re:Bruce Almighty flashback (Score:5, Interesting)
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.)
Re:"Low-hanging" moon? (Score:2, Interesting)
KoA
Alaska men should hit the trail for breasts [blogspot.com]
The question is WHY (Score:3, Interesting)
I would like to propose a hypothesis why this is actually not a "bug" but has a purpose: gravity and hand-to-eye-coordination.
Most of us may have noticed that when you throw things, the things won't keep going straight to that direction, but fall to ground. We are pretty good at throwing at things far away rather accurately. You don't need to calculate the "launch parameters" mathematically, but you just look at the target and your brain "just does it".
Now, if something is 20 meters up above, you need to throw a lot harder than when it's 20 meters away horizontally. Therefore, your brain makes it look like it's farther away to compensate. This may be a bit indirect way of compensating, but that's often how the nature works.
Just a 2.4132 cents worth from your AI guy.
Re:Bruce Almighty flashback (Score:2, Interesting)
1) Gravity gives us a natural bearing telling us what is up and down. We have no such natural bearing for left and right.
2) We are left-right symmetrical. A completely random shaped or regular (polyhedral or spherical) creature wouldn't have the notion of left or right with respect to itself.
3) On all standard shaped humans, left and right are basically interchangeable. Top and bottom aren't.
4) Consequently it disturbs our senses far less to consider left and right as being switched rather than up and down.
5) As evidence for this, lie on your side and look in a mirror. Lo and behold, everything is switched - top , bottom, up, and down.