Do clouds appear to form a flattened bowl because the sky appears to form flattened bowl, or does the sky appear to form a flattened bowl because the clouds do? I wouldn’t know. When the sky is completely overcast here in CO, it is raining, snowing, or foggy, and there is no horizon.
It seems to me that anything of (to the eye) arbitrary size and distance that covers the sky looks bowl shaped for the same reason. Yeah, we could be habituated to trees, etc. on the horizon, or not. (Does anyone know if the Professor or Gilligan had kids with Mary Anne or Ginger?) jfk, I’d go with the habituation theory, only if Micronesians, Samoans, etc., have the same habituation.
I would presume that clouds appear to form a flattened bowl because they really are, in actual fact, a flattened bowl. To see this: Draw two concentric circles, with close to the same radius. The inner one is the surface of the Earth, and the outer one is the cloud ceiling. Now draw a line tangent to the inner circle: This is the horizon. Anything above that line, you can see, and anything below it, you can’t. Now look at the part of the cloud ceiling that’s above the horizon line: It’s part of a circle, but it’s less than half. So it’s a flattened bowl.
The answer to this question lies in the warm blanket of air around the Earth called the atmosphere. The atmosphere consists of gases including nitrogen, oxygen, argon, carbon dioxide, water vapor and ozone, as well as various dust particles. The concentration of these gases and particles is greatest near the Earth and drops off as altitude increases. Atmospheric particles can either absorb, refract or transmit light as it hits them. Refracted light can change the way objects look. Random refraction of light will scatter light - making the source of the light look bigger than it actually is.
When you are looking at the Sun/moon on the horizon, your viewing angle is such that you end up looking through more atmosphere than if you were looking at the orb directly overhead or at any angle in between. To prove this just draw concentric circles - one to represent the Earth and one its atmosphere. Place a dot on the inner circle to represent yourself. Then draw two lines - one from the dot directly to the outer circle (viewing the Sun/moon overhead) and one from the dot to the outer circle perpendicular to the first line (viewing the Sun/moon on the horizon). The first line will be shorter. In addition to viewing the Sun through a longer distance of atmosphere, your line of site includes a higher percentage of the denser portions of the atmosphere. Additionally, because the density of the atmosphere in greatest closest to the earth, light near the Earth scatters more than that away from the Earth, resulting in a Sun/moon that looks wider than it is high.
In short, when there is more crap to scatter light, light scatters more. Cecil's answer is not wrong - perspective can add to this illusion, but it is not the primary cuplprit here. In a related note many of my fellow Arizonians feel as though the Arizona sunset is beautiful because of the way our pollution and that of L.A. on the western horizon scatter light - a prism effect bringing out many colors. In reality, most of the chemicals responsible for air pollution block light - hence the dark cloud. The arid desert air is the true culprit as there is less water vapor to refract the lower wavelengths of visible light away and we can see the full spectrum of light in the night sky. All except for green, but that's another question for another day.
On one hand you argue that, in the desert, one is comparing the size of the moon to what, sand? Now, you argue that on the horizon, “space” appears smaller? What is “space”, anyway? Nothingness, right? I don’t get your argument.
This issue was being batted around when I first joined the SDope in 1999. I mentioned that, while people notice the large moon, the same effect happens to the constellations, as well. The stars of a constellation appear quite spread apart when the constellation is close to the horizon. The debate continues…
As Cecil points out, the difference in the size of the actual image is insignificant. The “moon illusion” is called that because that’s what it is – an illusion.
Thanks, I get it now. In fact, I would assume the atmosphere looks like a flattened bowl for the same reason. The “suface” is not as obvious, but I assume the eye sees the bowl as “an edge” of blue scattering or something.
Yes I bothered to read Cecil’s column. Did you bother to read my response?
And for the record, I do not believe that the moon actually changes size; I agree that this is an illusion.
I also don’t dispute that this phenomenon can be PARTIALLY explained by our ideas of how big and how far away the moon is when compared to other objects.
I dispute the degree to which this matters. Support for Cecil’s arguements also supports my arguement - when you use a carboard tube, camera, or extend your view to arms-length, you see only direct light and reduce the amount of refracted light seen - and thus reduce the effect that refracted light has on one’s perception of the moon.
One thing I have noticed is that you can see more lunar detail when the moon is near the horizon. I am pretty sure this is not an illusion. In fact, you can purchase telescope filters intended to produce this effect to an even greater degree. It’s sort of like reading the print on a light bulb… you can’t see it as well on brighter bulbs.
The greater degree of lunar detail visible at the horizon suggests to your brain that it is in fact closer or larger than other times.
I would think that comparison to anything, including the bowl of the sky, is not neccesary for the explanation.
When looking at the horizon, we are used to making an unconscious correction for the fact that far-away objects will appear smaller than they are. However, with the moon this correction produces the wrong result, because the moon is just as far away when it is high up in the sky as when it is near the horizon. That is something which it shares with no other object, including airplanes and the like. Except for stars and planets, which are just pinpricks of light and have no visible dimensions, and the sun, which you shouldn’t look at directly.
This also explains why looking at the moon near the horizon through your legs destroys the illusion. Because most of us are not used to this stance, we have no expectation of how small or large things will appear, and so we don’t make the unconscious correction for objects near the horizon.
Ah, yes, but why don’t we also make that unconscious correction when the Moon is high in the sky? That’s where the comparison to other objects comes in.
And I suppose that arrangements of stars also have the same property of “same distance no matter where they are in the sky”, but I’ve never heard of a “big Pleiades” illusion.
My point is that the other objects don’t need to be present at the moment of the illusion – it’s enough that you are used to the fact that objects appear smaller when near the horizon, so you correct for that when estimating their true size. Since you’ve been doing this all your life, you are now doing it automatically even when there are no objects of known size present to compare against.
When it comes to how an object’s apparent size changes as it moves upwards, however, we don’t have a lot of reference material, so we’re not very good at estimating its true size, so the automatic correction does not happen.
Maybe you would have heard of it, if a lot of people had a habit of seeking out the Pleiades each night? However, most people couldn’t point out the Pleiades to begin with, and a lot of nights it’s not visible anyway. The moon, on the other hand, is kind of hard to miss.
Or maybe the fact that the Pleiades consists of a lot of apparently dimensionless dots against an even more dimensionless background, throws your internal size-estimating machinery off-kilter enough to mess up the illusion?
Maybe, if you made a point of seeking out the Pleiades each night and noting its apparent size, you would also see the same illusion? Maybe people who are outside all the time and spend a lot of time looking up at the sky are less susceptible to the moon illusion? Those would be interesting experiments to validate or falsify my hypothesis. Unfortunately, as a city boy, I can’t do much work here myself.
I’m a stargazer, and I do seek out the Pleiades at night whenever I can. I do notice the “big moon” illusion, but not a “big Pleiades” illusion. Folks not noticing it probably does have something to do with this, but I don’t think it’s all. Maybe it is because the individual stars are dimensionless; I dunno. Or maybe I’m just a freak.
I’ve had this argument with a number of friends before, have read the writings of the Master on the subject, and probably read many posts about the phenomenon on the message board in the past.
But I’ve had a new and almost original idea:
Things which are near the horizon are probably on the ground. “Things on the ground” encompass most of what was relevant to humans when our brains evolved. Most predators and viable prey were not high in the air, certainly not when viewed at a distance. Hence, things on the ground are more important than things in the air. We perceive them as such.
Something like a less extreme version of the problem the people of Krikkit had with looking up in the H2G2.
I have plenty of times, in the UK. Most memorably when the moon was visible, through trees, just above the roofline of a row of houses a couple of hundred yards away. It looked simply enormous, presumably because its apparent diameter was so big compared to the houses. So yes, the illusion does happen here.
To see the “big moon” illusion, you need to be looking at it with both eyes. I tried covering one eye and it really did seem to make the illusion disappear.