# Sky **URL:** \ **Category:** Factual Questions\ **Created:** [April 8, 2000, 11:14pm UTC](https://boards.straightdope.com/t/sky/10554 "2000-04-08T23:14:00Z")\ **Posts on this page:** 17\ **Page:** 2
**Author:** ![Rysdad](https://avatars.discourse-cdn.com/v4/letter/r/7cd45c/32.png) [@Rysdad](https://boards.straightdope.com/u/Rysdad)\ **Post date:** [April 12, 2000, 2:36am UTC](https://boards.straightdope.com/t/sky/10554/21 "2000-04-12T02:36:00Z")
My 7th grade science teacher said that one of the reasons that the sky was blue was because our atmosphere is 80% nitrogen, and nitrogen relects blue while absorbing most of the other colors in the visible wavelengths. (Color photos from space would’ve disproved this hypothesis, but we hadn’t seen any of those yet in 1967.) He explained red sunsets/sunrises by saying that the angle of the sun’s rays were closer to the ground, and since there was more dust closer to the ground, the light was scattered even more. I wish I would’ve thought to ask him why Hawaiian sunsets were red even though the Pacific ocean isn’t very dusty. Where does one go to request a refund for the the taxes paid for education levies? ---
**Author:** ![douglips](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@douglips](https://boards.straightdope.com/u/douglips)\ **Post date:** [April 12, 2000, 2:37am UTC](https://boards.straightdope.com/t/sky/10554/22 "2000-04-12T02:37:00Z")
What the heck happened here? scr4 has already explained this most of the way, but let me address Jinx: > [@](#): > > All this talk about scattering, etc., still does not preclude the sky from being violet. So, why isn’t the sky violet? Isn’t it’s wavelength even less than blue’s? (True!) This is a great question, I don’t know for sure the answer my WAG is that the sun simply doesn’t have much violet light in its spectrum compared to blue due to the shape of the black body curve. Another WAG would be that the mix of light being scattered ‘averages out’ to blue - i.e. your eye is taking the average of scattered rays. > [@](#): > > Also, if scattering is the deal: Is it meant to say that the other colors are scattered outward from Earth? If so, why don’t these colors appear in photos of Earth from space? i.e: The earth should not be a blue marble with all this scattering. That’s exactly wrong - the sky should appear blue from space too. It is the blue that is scattering, not “the blue scatters towards us and the red scatters away from us”. So, if you look at a chunk of atmosphere from any direction it will appear blue. > [@](#): > > Everything I’ve read here is just the rhetoric from rote memory as to what we were forced to believe was the whole story…as a simple answer. No, scr4 is actually giving you layman’s terms for deep physical truths you can learn about in 3rd year college physics, which I assure you is not rote memorization. I remember sitting in that class and saying “Wow! Now I **really** understand why the sky is blue!” If I remembered any of the math I might be able to help answer the violet question as well. > [@](#): > > I saw a demo in college, also, where white light was shown through a tank comtaining some aqueous solution. From the side, the light appeared blue, but head-on, the light appeared yellow. This was attempting to explain why the sun doesn’t appear blue. But, I still don’t get what this is all about. Let’s try it again. In space, there is no scattering, so you only see light coming from the sun and not from the sky. Therefore, the sky is black. Light is passing through space all around you, but you only see the light coming from the sun. Let’s simplify the ideas greatly - the sun is shooting light ‘particles’, some red, some blue. The red particles shoot straight through the atmosphere, but the blue particles get bounced around some. It’s like separating wheat from chaff - the blue particles are ‘lighter’ and so get bounced around more, the red particles are heavier and go right through. So, if you look at the sun, red and blue (remember only some blue particles get bounced) particles are streaming straight into your eyeball. If you look at the sky, no red particles are coming towards your eye, but a few blue ones will be, so you see blue (and a lot less light than when you look at the sun.) When the sun sets, the light passes through much much more air than at noon, so almost all of the blue particles have bounced around and gone away, and only the red particles are able to get straight to your eyeball. > [@](#): > > Originally posted by OgreFade: > actually the most abundant wave lenght happens to be green. Thats where the peak of our vision is… > > Hmm, yet another mystery of light! In high school, I was taught that the eye is most sensitive (responsive) to the green frequency because it is in the middle of the visible spectrum. OK, but not really substantial proof. I’ve never been able to decipher Ogre’s posts, so I’ll work with yours. All I know about visual perception is that we have receptors for red, green, and blue light. There may be some uneven distribution, i.e. green receptors are more numerous. > [@](#): > > In college, the same Hopkins physics prof as I mentioned above said that the eye is most sensitive (responsive) to yellow light because the sun is a yellow star, and our eye as “evolved” to respond to this frequency. Sounds like a reasonable guess to me, but I don’t know that this is known for sure in these circles. > [@](#): > > Also, about the constant exposure to yellow from the sun, couldn’t I also argue that the eye should be least sensitive to yellow? The eye tends to lose sensitivity to a particular wavelength(s) when over-exposed…the eye fatigues, in a sense. Or so, this is how I was taught about why the eye “sees” an after-image after staring at an object. You are confusion evolution (development of eyesight in general over many many generations of organisms) to the behaviour of one individual organism. It makes sense that we would evolve to see yellow light and not say, gamma rays, but it is a completely different thing to say that our eyes get less sensitive over time to exposure from the sun. One process is driven by natural selection, the other is reversible (just close your eyes) and probably also useful - it helps you see the non-green animal in the forest of green for example. ---
**Author:** ![douglips](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@douglips](https://boards.straightdope.com/u/douglips)\ **Post date:** [April 12, 2000, 2:41am UTC](https://boards.straightdope.com/t/sky/10554/23 "2000-04-12T02:41:00Z")
Sorry - I didn’t mean to throw in “black body curve” without explaining. A hot object radiates light in a distribution - more in the middle of its frequency, less at either end. A hotter object has a shifted, brighter distribution. So, the sun emits mostly yellow light, and a whole lot less blue and red light than yellow light, for example. A hotter star might emit mostly blue. The curve is more gentle on the red side, so you get more red than violet out of the sun. ---
**Author:** ![douglips](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@douglips](https://boards.straightdope.com/u/douglips)\ **Post date:** [April 12, 2000, 5:27am UTC](https://boards.straightdope.com/t/sky/10554/24 "2000-04-12T05:27:00Z")
My first post was a simulpost of scr4, and he has as usual done an excellent job. ---
**Author:** ![Lexicon](https://avatars.discourse-cdn.com/v4/letter/l/ac8455/32.png) [@Lexicon](https://boards.straightdope.com/u/Lexicon)\ **Post date:** [April 12, 2000, 6:03am UTC](https://boards.straightdope.com/t/sky/10554/25 "2000-04-12T06:03:00Z")
Oh, Moses smelled the roses, people! You can read this: > [@](#): > > Dear Cecil: > > Why is the sky blue? --Bob Balfanz, Dallas > > Cecil replies: > > Ah, jeez, Bobberoonie, can’t you ask your old man anything? It’s because sunlight striking air molecules is scattered in all directions. For abstruse technical reasons > no Texan can hope to understand, the blue portion of sunlight is scattered with greater intensity than other wavelengths. Since scattered light is the only reason the sky > is illuminated at all (the airless lunar sky, as you no doubt recall, is black), what you see is blue. Similarly, when sunlight takes a very long path through the > atmosphere, as at sunset, so much blue is diverted out of the direct path that you get a shift to the other end of the spectrum and the normally yellow-white sun > appears red. > > –CECIL ADAMS here: [www.straightdope.com/classics/a810828b.html](http://www.straightdope.com/classics/a810828b.html) And _that’s_ the straightdope, from the Man himself. Search the archives, why dontchya? 😉 * * * Sticking feathers up your butt does not make you a chicken. ---
**Author:** ![moriah](https://avatars.discourse-cdn.com/v4/letter/m/b2d939/32.png) [@moriah](https://boards.straightdope.com/u/moriah)\ **Post date:** [April 12, 2000, 6:45am UTC](https://boards.straightdope.com/t/sky/10554/26 "2000-04-12T06:45:00Z")
Here’s a more scientific answer: [http://math.ucr.edu/home/baez/physics/blue\_sky.html](http://math.ucr.edu/home/baez/physics/blue_sky.html) Peace. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [April 13, 2000, 5:26am UTC](https://boards.straightdope.com/t/sky/10554/27 "2000-04-13T05:26:00Z")
> [@](#): > > Originally posted by scr4: > \*\*So what does this white light, the light from our sun, look like? It is a continuous spectrum which has a broad peak in the green. That is, it has a lot of green, less blue and red, even less ultraviolet and infrared, and not much of anything else. > > So yes, green light is the most abundant. And yes, our eyes did evolve to be sensitive in this region, because - well, why try to see colors which are poorly illuminated by the sun?\*\* Wait a second, SCR4! I’m 😕 confused here! Peak in the green? Then, why is the sun a yellow star? (I’ve heard of Red Giants, but are there Green Giants in space, little sprout? ho-ho-ho!) …Sorry, I couldn’t resist adding that quip! 😃 * * * “They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [April 13, 2000, 5:29am UTC](https://boards.straightdope.com/t/sky/10554/28 "2000-04-13T05:29:00Z")
> [@](#): > > Originally posted by scr4: > \*\* …yes, our eyes did evolve to be sensitive in this region, because - well, why try to see colors which are poorly illuminated by the sun? \*\* Oh, also, SCR4, I need you to explain: By that logic, then why don’t we see in UV and IR ranges? …Just being the devil’s advocate, here, to understand your point of view. * * * “They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [April 13, 2000, 5:48am UTC](https://boards.straightdope.com/t/sky/10554/29 "2000-04-13T05:48:00Z")
Sorry, but one more question to confuse everyone! I’m not quite sure how to pose this, but here goes: OK, if all this blue light is scattered, as explained, with the remaining colors coming straight through the atmosphere… Then, wouldn’t we expect the blue rays to be superimposed onto the rays of the remaining colors prior to, or perhaps while, becoming incident upon my eye? So, taking this into ocnsideration, I must ask why should one then see the sky as any one distinct color? My WAG would be to say that the eye is tracing the light rays back to their point of origin, but if the superimposition is occurring as I suggest…wouldn’t it then be impossible for the eye to trace back a distinct blue ray from the muddle of colors in a superimposed mess? (If I’ve lost you, I’ll see if I can state my question a different way.) * * * “They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV ---
**Author:** ![Flat\_U.Lance](https://avatars.discourse-cdn.com/v4/letter/f/90db22/32.png) [@Flat\_U.Lance](https://boards.straightdope.com/u/Flat_U.Lance)\ **Post date:** [April 13, 2000, 5:57am UTC](https://boards.straightdope.com/t/sky/10554/30 "2000-04-13T05:57:00Z")
The sky is blue because if it was green you wouldn’t know where to stop mowing your lawn. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 13, 2000, 1:01am UTC](https://boards.straightdope.com/t/sky/10554/31 "2000-04-13T01:01:00Z")
> [@](#): > > Originally posted by moriah: > [http://math.ucr.edu/home/baez/physics/blue\_sky.html](http://math.ucr.edu/home/baez/physics/blue_sky.html) > \*\* Thanks for the link, moriah. It appears that I was wrong on one important detail - the scattering is from molecules, not dust particles. > [@](#): > > By that logic, then why don’t we see in UV and IR ranges? …Just being the devil’s advocate, here, to understand your point of view. I don’t think there are clear-cut answers when it comes to the end results of evolution. As for UV though, most natural objects don’t reflect UV very well, and it isn’t terribly useful. A notable exception is flowers - many of them are very bright in UV, and many insects evolved UV vision to find them. Or rather, they evolved together to better help each other. As for IR, one problem may be that the eye itself is at body temperature and emit some IR. It will be difficult to see through the glowing eyeball. This is less of a problem for cold-blooded animals, and I heard some species of snakes can “see” IR. Another answer may be that ultra-violet and infra-red are by definition just outside the visible range, and that’s why we can’t see them… > [@](#): > > Peak in the green? Then, why is the sun a yellow star? Yellow or white light is a mixture of all colors of light. If you take a lot of green light and a bit less of red and blue light, it becomes a yellow light. It’s a very broad peak, you see. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 13, 2000, 1:17am UTC](https://boards.straightdope.com/t/sky/10554/32 "2000-04-13T01:17:00Z")
> [@](#): > > OK, if all this blue light is scattered, as explained, with the remaining colors coming straight through the atmosphere… Then, wouldn’t we expect the blue rays to be superimposed onto the rays of the remaining colors prior to, or perhaps while, becoming incident upon my eye? So, taking this into ocnsideration, I must ask why should one then see the sky as any one distinct color? It might help to look at the diagrams in the link moriah posted. I’ll give it a shot though. Let’s say you are admiring a sunset in Boston. The sun looks orange. Why? Because the blue light has scattered off the path before it gets to Boston. What happened to this scattered light? It’s raining down from the sky onto, say, Chicago. Which is why the sky in Chicago is blue. Of course the Boston sky is blue too. After dumping the blue portion onto Boston, the beam of sunlight continues to Nantuckett where it is admired as a red sunset. In other words, scattered light hits your eyes from every direction - that’s why it makes up “the sky.” You can only see unscattered light when you look directly at the sun. ---
**Author:** ![Hunsecker](https://avatars.discourse-cdn.com/v4/letter/h/d78d45/32.png) [@Hunsecker](https://boards.straightdope.com/u/Hunsecker)\ **Post date:** [April 13, 2000, 3:55am UTC](https://boards.straightdope.com/t/sky/10554/33 "2000-04-13T03:55:00Z")
> [@](#): > > By that logic, then why don’t we see in UV and IR ranges? …Just being the devil’s advocate, here, to understand your point of view. The explanation that I’ve heard for the reason that the visible spectrum is at about 400-700nm is that its the most useful range in which water is transparent. A real nice theory, considering that we evolved sight while in the ocean. (Unfortunately, none of my color theory books say diddly about why the spectrum is where it is, so I can’t back this up.) ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [April 17, 2000, 11:52am UTC](https://boards.straightdope.com/t/sky/10554/34 "2000-04-17T11:52:00Z")
Boy, the smorgasbord of evolution theroies has an answer for everything! I bet the creatures who evolved from the sea have the best vision in the hindsight range, too! 😉 * * * “They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [April 17, 2000, 10:03pm UTC](https://boards.straightdope.com/t/sky/10554/35 "2000-04-17T22:03:00Z")
scr4 said: > [@](#): > > This is less of a problem for cold-blooded animals, and I heard some species of snakes can “see” IR. FYI - the snakes in question are called pit vipers. They include rattlesnakes, water moccasins, and copperheads. They do not see IR with their regular eyes. They have small indentations in their face below their eyes. These indentations, or pits (and thus the name), detect IR. All of this is in a standard encyclopedia. Those IR pits are an early form of eye. They are patches of skin sensitive to a certain wavelength of EM radiation, with a concave shape that functions to help detect the direction of the source. So the next time you get in a debate about evolution and someone throws out the argument about irreducible complexity and the eyeball, talk about pit vipers and their IR eyeballess sockets. [/hijack] ---
**Author:** ![moriah](https://avatars.discourse-cdn.com/v4/letter/m/b2d939/32.png) [@moriah](https://boards.straightdope.com/u/moriah)\ **Post date:** [April 18, 2000, 6:26am UTC](https://boards.straightdope.com/t/sky/10554/36 "2000-04-18T06:26:00Z")
> [@](#): > > Originally posted by Jinx: > \*\*Boy, the smorgasbord of evolution theroies has an answer for everything! I bet the creatures who evolved from the sea have the best vision in the hindsight range, too! 😉 > \*\* Yes, evolution and genes are too readily called for to explain everything from black athletes to why the chicken crossed the road. However, in this case, there is a cogent argument. Water blocks out most of the em bandwidth except for what we know as visible light. Since the underwater is ultraviolet dark, there’d be no advantage for an ultraviolet sense. That’s why evolution can explain that sight covers the visible light range, because the visible light range is somewhat transparent in water, in which we evolved. ## Peace. _Daddy, why is the sky blue? I mean, is blue light scattered because its wavelength is the same size as the ubiquitous nitrogen atom or because blue is susceptible to the frequency of the dipolar magnetism of the air molecules?_ ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [April 18, 2000, 8:25am UTC](https://boards.straightdope.com/t/sky/10554/37 "2000-04-18T08:25:00Z")
Actually, you would see in the UV if it were not for the fact that the lens of the eye is opaque at those wavelengths. The blue receptor does respond to UV light, although its peak sensitivity is in the blue. UV light is one of the causes of cataracts. A cataract is usually fixed by replacing the lens with a plastic one. Usually this plastic one is transparent to UV, so those people can then see those wavelengths. [Previous page](https://boards.straightdope.com/t/sky/10554.md?page=1)