Why is the sky dark at night?

So, the sky looks blue because of the particular gases in our atmosphere reflect and scatter the blue wavelengths of light from the Sun. Fair enough. But that leads directly to a second question that, I'm ashamed to say, I never really thought to ask — why doesn't the light from all the stars in the Universe reflect and scatter off our atmosphere, producing a blue sky, all the time?

This Minute Physics video provides a great explanation, which is grounded in both the timey-wimeyness of astrophysics and the limitations of our own human biology.

Via BrainPickings


  1. I find the “scatter the blue wavelengths of light” line to be unsatisfying.  Is it not much more correct to say “scatter the blue and violet wavelengths of light, except the human eye can’t actually see the violet wavelengths very well” ?

  2. I am honor-bound to point out that the blueness of the Earth’s sky is not really related to the particular mix of gasses, it has to do with the fact that when electromagnetic waves interact with a continuous medium in which positive and negative charges (i.e. nuclei and electrons) react differently to electric fields, short wavelengths (blue) scatter more strongly than long wavelengths (red). So the blue light goes all over the place, and the red light mostly goes straight, and what you see is the blue light coming at you from all over the place. 

    Look up “Rayleigh scattering” on wikipedia — I tried to provide a link, but it got mangled.

  3. I’ve noticed that occasionally a big bright disc appears in the night sky. Some say brave men have visited there, I agree. 
    My question is: Why does this light source not scatter at least some blue light in the atmosphere as does the sun?

    1. A brief, possibly imperfect, search of the wiki reveals the sun is approximately 400,000 times brighter than the moon. Eyes accommodate all that and more. XKCD did a what if a couple of weeks ago about shining lazers at the moon wherein he showed that in order to change the apparent brightness of the dark moon would require approximately all the energy on Earth. I have a feeling that these things are beyond mere common sense.

          1. “So, you want to know about quantum entanglement? Just read The Book!” – “I couldn’t find anything about quarks in there, the first chapters are all about who begat whom…”

    2. My gut tells me that there is a big difference between radiated light, and radiated light reflected off of a spheroid covered with substances of varying degrees of reflectance.

      EDIT: I do recall moonlit skies as being lighter. I think there is a color called Midnight Blue

  4. I’m not sure that this explanation is 100% accurate. I’ve always heard it explained as more of a fractal problem. Imagine drawing dots in the 4 corners of a sheet of paper. Now, zoom out such that those dots clump together really tightly, such that they themselves look like one dot. You could draw 4 of these dot groups on a sheet of paper like before. It would have 4 times as many “stars” but is still largely empty. You can repeat this process as much as you want, but it will always appear largely empty since the dots are so distant.

    Space itself is like this. It is by and large, empty (hence we call it “space”). Distant stars shrink to points that are so far away, they take up significantly less visual space than if they all lied on a flat surface with each other. You can always find more stars, but they will always be further away then what you had seen before.

    1. True for actual points, but stars do have a finite, albeit small area.

      Imagine a volume of space that’s a thin spherical shell, centred on the earth.  It contains some particular number of stars.  A  similarly thin shell with, say, twice the radius should contain 4 times as many stars, each of which appears to us as half the size (and so has a quarter of the area).

      So if we imagine space as a collection of these concentric shell volumes, each should contribute the same constant amount of “star coverage” to the sky.  Add up an infinite number of these, and you get an infinite sum.

      1. But as they get further away, they contribute fractionally less light. Adding up an infinite number of decreasing fractions does not guarantee infity – try adding 1/2+1/4+1/8… The sum asymptotically approaches 1. Adding up a different geometric series will approach different limits.

        The dots (not points) that I am imagining do take up a finite area. That area simply shrinks as they move away from you.

        Evidently, this all falls under “Olber’s Paradox”. Wikipedia has mention of the fractal explanation: https://en.wikipedia.org/wiki/Olbers%27_paradox#Fractal_star_distribution

        1. That’s interesting – according to the wiki, every possible explanation has been dismissed.  However, I will point out that Genesis 1 only mentions two lights in the sky.  Therefore, stars do not exist.

    2. I agree that this explanation rings hollow. Isn’t it a much simpler explanation that the earth only absorbs a minuscule fraction of energy from distant stars as explained by the inverse-square law? 

      The video makes a fair point about the light from extremely distant stars (beyond the observable edge of the universe) not reaching us, but the whole red shift thing makes it sound like the amount of radiation striking the earth from distant stars is equal to the amount of radiation we get from the sun.

      1. Surface brightness doesn’t change with distance.  Surface area (or angular diameter) does.  So if every point of the sky contains a star, regardless of distance, the entire sky will have the surface brightness of a star.

  5. The idea that there should be a star visible in every direction, giving a uniformly bright sky is a couple of centuries old and is named Olbers’ Paradox. 

    The blue colour during the day has already been explained by Rayleigh scattering of sunlight.  There are subtleties to the colouring – note how the sky is darker and bluer higher up, but lighter and whiter near the horizon.  It’s because you’re looking through more atmosphere near the horizon and the light gets scattered and mixed up a lot more.

    With moonlight, the sky should be coloured similarly.  But even a full moon is a million times less bright than the sun, so it’s a very dark blue.  And remember that our low-light vision is monochrome, because it relies on rod cells, of which there’s only 1 type (unlike the 3 cone cell types used for colour vision).   

    Without a moon, and just starlight…that’s a tricky one because the light sources are in a variety of directions.  I’d speculate there’s still some kind of colour gradient to the sky, but will patiently await an actual physicist poster…

    1. I thought it was odd that they would put together a video like this without referring to it as Olbers’ Paradox.

  6. Hey, no mention of interstellar dust and all that? I mean, it’s the reason why we can’t see most of the bright stuff in the center of our own galaxy. What chance has some light from the Timbuktu galaxy got?

    1.  that’s been covered too. eventually all the interstellar dust would be excited by the infinite amount of light and start to glow as well. dust can’t explain the darkness of the night sky.

  7. Because you masturbate.  

    Also, there seems to be some confusion as to how one posts a link in this here thread. Allow me to demonstrate:

    1.  Thank you! Now I understand… nothing.
      But hey, while we’re at it; will someone explain how to upgrade my generic ‘sideways G’ icon?

          1. It looks like the profile pic is only a Disqus feature, not a WP feature. I guess that you can’t change it unless you create a Disqus account. Sorry. I had suggested at one point that we change the default avatar to Jill, but nobody ever got a round to it.

          2.  Thanks for the follow up… Looking about WP support forums for the answer, maybe a plugin adjustment by BB might allow us a profile pic w/o dealing with Disqus?

  8. Seems to me the video raises more questions than it answers.

    First, while I agree the visible spectrum is red-shifted away from visible, why does that matter? Stars emit radiation in the non-visible spectrum of higher frequencies than visible as well, and those would get red-shifted into the visible range.

    Second, the video seems to start with the assumption that we already understand the theory of infinite stars, jumping right into why infinite stars doesn’t work. But lots of people who haven’t thought hard about the “why is the night sky dark?” question haven’t bothered to think about the infinite stars issue in the first place, nor are even convinced there really is an infinite number of stars (I agree the number of stars is humongous…but the universe has a finite age, and so I’d think a finite number of objects, including stars, in it).

    I think the more intuitive question to ask is why even when there’s a moon, the sky isn’t blue with scattered moonlight (answered above), but the video jumps right to assuming we know “the sky ought to be blanketed with a complete coat of stars” and then proceeds to show why that isn’t the case. For someone who wasn’t thinking the sky ought to be blanketed with a complete coat of stars in the first place, it’s not as interesting to hear someone explain why it’s not. :)

    1.  It doesn’t say that there’s an infinite number of stars because there aren’t.  It just says that the universe doesn’t have an edge.  If you were a two-dimensional being walking on the surface of a very large sphere, you’d never find an end of it, but it would still be finite.  We’re three dimensional beings in a bounded universe, but if we can move through it as much as we want and will never find an edge.  It’s just assuming that there’s enough stars that it should fill the sky.  I’m not sure that’s justified given distance and spread.  It is true that there is energy coming from all directions because some of it came from the time of the big bang when everything was more densely packed, but I’m not sure that energy is very bright.  I did some web searches to try to answer that, but didn’t have any luck.

      About the red-shift, there is an upper limit to how energetic photons produced by stars are.  For distant enough galaxies, the red-shift is sufficient to shift all of the photons below the visible range.  For example, when we look for light emitted very near the time of the big bang (and thus currently coming from very far away), we use telescopes capable of seeing microwaves, which is a range whose frequency is below the range of visible light.

      1. Right. Like I said, raises more questions than it answers.

        I.e. it’s well and good for you or someone else to be able to explain these things, but the video does not.

  9. So I asked my wife the other morning, “What does kick-ass” mean. And I explained, I had challenge my high school engineering class to devise a kick-ass Halloween costume.  They looked at me blankly and asked, “what do you mean by kick-ass?”

    My wife, the English major, replied without a moment’s hesitation, “kick-ass means you’d be kicking yourself in the ass for not thinking of it yourself.”

    That was a kick-ass question. That the comments have more questions than answers is proof of it.

  10. I can see that kind of blackness in photographs but I can’t see it in the stars. Somehow I see too much red interference to see the milky blackness of night sky in the country. I remember seeing it as a kid though. No eye specialist I have talked to has ever been able to explain what’s going on.

  11. Fun fact: a couple of days ago I was about to drop my 3-year-old off at daycare.  I stopped the car, and he pointed to the windshield and said “daddy, why is the sky blue?”  I was really excited to tell him all about it, but then I thought a joke might be better, so I said “because if it was green, we wouldn’t know where to stop mowing.” [Heard that one many years ago on Night Court.]

    Turns out he was talking about the bluish sun-blocking window tint.

  12. As can be seen from everyone above, it’s scientifically a more complex question than might appear. My question is about the production value of the video: the speed, the breathless talking, the whirlwind scribbling and generally manic feel of the presentation — halfway through I was already going WTF, is this how you explain a complex scientific question?

    1. well, the series *is* called “minute physics” so i guess the fast talking kind of comes with the territory.

  13. The light from all the stars in the Universe does reflect and scatter off our atmosphere producing a blue sky, all the time.  If you set out a camera and set the exposure properly, then you will get a blue sky.  This is most easily seen when there’s significant moonlight, demonstrating that the Rayleigh scattering that results in the sky appearing blue during the daytime is every bit as active with the smaller amount of incoming light we receive during the night.  If your eye integrated like a camera does, you’d see blue skies. 

    Here’s a 13-minute exposure.  Looks blue to me.  Also note how the foreground looks like it’s illuminated by sunlight. Because it is, just much less of it than during the day.

  14. Why is it that, if you’re just on your way out of the Paths of the Dead or hanging out in the Orfalch Echor, you can see stars even in the middle of the day?

    1. Perhaps the narrow field of view can make it appear that the few bright stars visible from those locations are a sky full of stars, if by chance several stars of the first magnitude are in the field of view?  http://www.4information.com/trivia/stars-daytime-well/

      1. Also in the case of Orfalch Echor there’s the possibility that a flat Earth coincided with more visible stars of the first magnitude.

        1. Or Arien was screwing around that day and didn’t have her fruit turned all the way up.

  15. The sky is blue during the day because the sun’s light is brighter than the stars, and it’s light is scattered like a bright light placed in the sea at night. : )

  16. So the answer is the fact there is no visible spectrum light for us to see…umm isn’t that what “darkness” is to most people?  I mean from a science standpoint just because something is dark or black doesn’t mean there isn’t radiation/energy present, but to the average person sitting in a pitch black room there is just no light (ie they don’t make the distinction that there might be an infrared energy source, thus providing some type of energy input, just they know there isn’t anything they can see.)

    So when the kids ask why the sky is black at night, the realistically correct answer is there is no “light” (at least in the visible term).

  17. This video is factually incorrect about the cause of the red shift.  It is not caused by the doppler effect, it is caused by the space the photons are moving through expanding, thus lowering their wavelengths.  Distant galaxies are not moving away from us.  They are hardly moving.  We are also hardly moving.  But we are getting farther apart from them because the universe is expanding.  This is the cause of the redshift.  It is called Cosmological redshift.  The wikipedia description is reasonable. https://en.wikipedia.org/wiki/Redshift#Expansion_of_space

    1. Distant galaxies *are* moving away from us. Redshift is a measure of recessional velocity. You don’t get to pick and choose which kinds of motion are “real” and which aren’t. If I run away from you really fast and shine a light in your direction, you can measure the redshift of the light and be confident that I am moving away from you. It’s the same with distant galaxies – sure, the cause of the motion is cosmological in nature, but motion is motion. Those suckers are receding from us.
      I’m assuming what you’re getting at is that the motion of distant galaxies is due to the expansion of the universe, not dynamics. That the galaxies are not being repulsed by some force, but are being carried along by space itself.

  18. If light hasn’t reached us yet how could telescopes that are just magnifying from where we are see it earlier?

    1. They don’t. It’s not suggesting that the telescopes can see light that hasn’t reached us yet. They’re saying the oposite — that the light that has reached us from the edge of the universe has been travelling a really long time — 13 billion years — and is light from when the universe was young.

  19. doesn’t it have something to do with the fact that the star is a finite size and its light radiates?, so the farther the object is from the star the less light rays would hit it.

  20. Why is the sky dark at night?  

    Because we’re in our own shadow. :]

    (why it’s colder, too)

    Om buhr buvhatsva. Tat savitur vareniam. Bargo devasya demahi.  Diyo yoha ha pracho bayat!

    1. I think that refuting that common sense explanation was the main point of the video. 

      brhat-sama tatha samnam. gayatri chandasam aham. masanam marga-sirso ham. rtunam kusumakarah.

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