# Why is sky dark at night? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [October 2, 2012, 3:14pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476 "2012-10-02T15:14:36Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Indian](https://avatars.discourse-cdn.com/v4/letter/i/c0e974/32.png) [@Indian](https://boards.straightdope.com/u/Indian)\ **Post date:** [October 2, 2012, 3:14pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/1 "2012-10-02T15:14:36Z")
Cecil’s [column](http://www.straightdope.com/columns/read/1356/why-is-the-night-sky-dark). A new [video explanation](http://m.youtube.com/#/watch?feature=youtube_gdata&v=gxJ4M7tyLRE&desktop_uri=%2Fwatch%3Fv%3DgxJ4M7tyLRE%26feature%3Dyoutube_gdata) worthy of sharing. ---
**Author:** ![Powers](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Powers](https://boards.straightdope.com/u/Powers)\ **Post date:** [October 2, 2012, 3:23pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/2 "2012-10-02T15:23:48Z")
> [@Indian](#): > > Cecil’s [column](http://www.straightdope.com/columns/read/1356/why-is-the-night-sky-dark). > > A new [video explanation](http://m.youtube.com/#/watch?feature=youtube_gdata&v=gxJ4M7tyLRE&desktop_uri=%2Fwatch%3Fv%3DgxJ4M7tyLRE%26feature%3Dyoutube_gdata) worthy of sharing. [Non-mobile link](http://www.youtube.com/watch?gl=US&hl=en&client=mv-google&v=gxJ4M7tyLRE&feature=youtube_gdata&nomobile=1) Powers &8^] ---
**Author:** ![Indian](https://avatars.discourse-cdn.com/v4/letter/i/c0e974/32.png) [@Indian](https://boards.straightdope.com/u/Indian)\ **Post date:** [October 2, 2012, 3:27pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/3 "2012-10-02T15:27:31Z")
Thanks 🙂 ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [October 2, 2012, 4:06pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/4 "2012-10-02T16:06:22Z")
One error - he attributed red shift of the galaxies and cosmic background microwave radiation to the “Doppler Effect” - which isn’t really correct. Space itself expands, the galaxies are not moving within space, so the extension of space itself stretches the wavelengths. Otherwise quite nice (although I don’t like the voice all that much.) ---
**Author:** ![davekhps](https://avatars.discourse-cdn.com/v4/letter/d/a5b964/32.png) [@davekhps](https://boards.straightdope.com/u/davekhps)\ **Post date:** [October 2, 2012, 5:41pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/5 "2012-10-02T17:41:52Z")
> [@Francis\_Vaughan](#): > > Space itself expands, the galaxies are not moving within space Galaxies move in space. 😕 ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [October 3, 2012, 11:44am UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/6 "2012-10-03T11:44:24Z")
> [@davekhps](#): > > Galaxies move in space. 😕 Yes they do (although that wasn’t what Frankie was talking about). The Andromeda galaxy is heading towards us, for instance (or our Milky Way is headed towards them). ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [October 3, 2012, 12:43pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/7 "2012-10-03T12:43:47Z")
> [@Francis\_Vaughan](#): > > …the galaxies are not moving within space… > [@davekhps](#): > > Galaxies move in space. 😕 > [@njtt](#): > > Yes they do (although that wasn’t what Frankie was talking about). The Andromeda galaxy is heading towards us, for instance (or our Milky Way is headed towards them). Well, this is a confusing sequence. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [October 3, 2012, 3:59pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/8 "2012-10-03T15:59:38Z")
I think what **Francis Vaughan** is trying to say is that the reason the galaxies are getting farther away is that the space between us is growing. The galaxies are not themselves on a high speed trajectory away from us. Which is independent of the local motion of each galaxy. ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [October 3, 2012, 4:19pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/9 "2012-10-03T16:19:18Z")
> [@Irishman](#): > > I think what **Francis Vaughan** is trying to say is that the reason the galaxies are getting farther away is that the space between us is growing. The galaxies are not themselves on a high speed trajectory away from us. Which is independent of the local motion of each galaxy. An analogy would be that two cars can be rushing towards each other, while the planet earth is spinning. The galaxies are moving (and can collide), even though the universe is expanding. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [October 3, 2012, 4:57pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/10 "2012-10-03T16:57:00Z")
I have read and pretend to understand the balloon analogy, where all points of the balloon are moving away from each other, even through there’s no actual objective center point where those points all move from (like you would expect with an explosion). I admit, though, that this is still pretty confusing to me. Matter is mostly empty space. If space itself is expanding, what happens to the space inside matter? Is it that the expansion has no meaning on a small scale? On a larger scale, is the Earth expanding away from the Sun? Is the Milky Way expanding due to space expansion, or is that still too small of a scale? It seems like the distance between the Milky Way and Andromeda galaxies is still too small of a scale, since they are moving towards each other – would they be moving faster together if space were not expanding? Like dark matter and dark energy, these explanations seem kludgy to me, like we’re missing something more fundamental, although I have no idea what that could be. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [October 3, 2012, 5:27pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/11 "2012-10-03T17:27:42Z")
It does make one wonder. If space is expanding the same at all scales, then how would we know? If the space inside matter is expanding the same as the space between stars, then wouldn’t everything be expanding at the same scale, and thus the effect be invisible? Without a boundary to expand against or an internal reference, what would expanding even mean? I think the current model is that the expansion does not happen at the local scale, i.e. within matter - it is overwhelmed by the other binding forces. It only seems to play a role at the grand cosmic scales, where gravity alone is insufficient to contain it. Thus it is not relevant within Solar Systems or smaller, and galaxies seem to do okay holding together, but between galaxies gravity gets weak enough. Or something. I’m not certain. As for dark matter and dark energy, we have been able to identify regions of space where the pull of gravity appears to act as if there is a lot more matter there than we can see. The matter that isn’t visible is somewhat offset from the regular matter. This matter behaves somewhat differently than regular matter, thus “dark matter” is a reasonable description. “Dark energy” is pretty much just a placeholder for “we don’t really know what is going on here”. ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [October 3, 2012, 6:13pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/12 "2012-10-03T18:13:16Z")
> [@RitterSport](#): > > I admit, though, that this is still pretty confusing to me. Matter is mostly empty space. If space itself is expanding, what happens to the space inside matter? Is it that the expansion has no meaning on a small scale? At the subatomic level, quantum effects take over. An electron can be _here_ or _there_, but it can’t be in the space between. > [@](#): > > On a larger scale, is the Earth expanding away from the Sun? Again, no, because the Earth’s orbit is determined by its existing movement and by gravity > [@](#): > > Is the Milky Way expanding due to space expansion, or is that still too small of a scale? Gravity again. > [@](#): > > It seems like the distance between the Milky Way and Andromeda galaxies is still too small of a scale, since they are moving towards each other – would they be moving faster together if space were not expanding? I think so. It’s really only when you get to intergalactic distances that the effect becomes noticable. > [@](#): > > Like dark matter and dark energy, these explanations seem kludgy to me, like we’re missing something more fundamental, although I have no idea what that could be. Dark matter and dark energy are indeed somewhat kludgy, but when it comes to the expansion of the Universe, the facts are there. Either the Universe is expanding, or there’s something about the Milky Way Galaxy that all the other galaxies are afraid of. ---
**Author:** ![Freakenstein](https://avatars.discourse-cdn.com/v4/letter/f/b782af/32.png) [@Freakenstein](https://boards.straightdope.com/u/Freakenstein)\ **Post date:** [October 3, 2012, 10:28pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/13 "2012-10-03T22:28:44Z")
Or that everything but the space is getting smaller… ---
**Author:** ![Gaqua77](https://avatars.discourse-cdn.com/v4/letter/g/43a26b/32.png) [@Gaqua77](https://boards.straightdope.com/u/Gaqua77)\ **Post date:** [October 4, 2012, 7:25am UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/14 "2012-10-04T07:25:15Z")
Because sun is not providing its hydrogen light to us, at that portion of time. As all knows sun revolves on its own axis. Hope you understood now??? ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [October 4, 2012, 6:55pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/15 "2012-10-04T18:55:09Z")
> [@Gaqua77](#): > > Because sun is not providing its hydrogen light to us, at that portion of time. As all knows sun revolves on its own axis. Hope you understood now??? “Is this a statement that I see before me?”—̵Lewis Carroll ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [October 4, 2012, 10:18pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/16 "2012-10-04T22:18:15Z")
> [@Gaqua77](#): > > Because sun is not providing its hydrogen light to us, at that portion of time. As all knows sun revolves on its own axis. Hope you understood now??? If I understand your post, you are missing the point. Or else wrong. The Sun does not have a light side and a dark side. The Sun emits light in all directions. The Sun does have rotation of sorts, but not really the same as Earth rotating. If you read the column and watched the link, then you would realize that the question is not really about why we don’t see the Sun when the Sun is on the other side of Earth. The question is about why “space” looks black. The answer seems to be that the farther away we see, the more redshifted the light is, thus it shifts to infrared, so we cannot see it with the naked eye. The lack of light is black. Ergo, the sky looks black if there is not a visible light source in the way. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [October 5, 2012, 12:04pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/17 "2012-10-05T12:04:57Z")
> [@Irishman](#): > > If I understand your post, you are missing the point. You think? ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [October 5, 2012, 3:17pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/18 "2012-10-05T15:17:51Z")
> [@RitterSport](#): > > I have read and pretend to understand the balloon analogy, where all points of the balloon are moving away from each other, even through there’s no actual objective center point where those points all move from (like you would expect with an explosion). > > I admit, though, that this is still pretty confusing to me. Matter is mostly empty space. If space itself is expanding, what happens to the space inside matter? Is it that the expansion has no meaning on a small scale? On a larger scale, is the Earth expanding away from the Sun? Is the Milky Way expanding due to space expansion, or is that still too small of a scale? It seems like the distance between the Milky Way and Andromeda galaxies is still too small of a scale, since they are moving towards each other – would they be moving faster together if space were not expanding? > > Like dark matter and dark energy, these explanations seem kludgy to me, like we’re missing something more fundamental, although I have no idea what that could be. The problem with much of math and physics is that we grow up with and are taught arithmetic and Newtonian physics, the stuff that works in everyday life on earth, and then expect to be able to extrapolate that to everything in the universe, including the universe itself. Not surprisingly, people get frustrated, suspicious, even angry when they are told that they’re getting it completely wrong. The balloon analogy really is a good one. But people don’t think of it as a two-dimensional surface that is all there is with no inside or outside; they think of a balloon embedded in three-dimensional space. And they can’t picture the universe as a three-dimensional surface; our brains don’t work that way. Similarly, they can’t grasp the difference between matter expanding and the fabric of space itself expanding. There isn’t a real analogy for that and no everyday experience corresponds. Relativity and QM are even worse. The extremes of velocity, size, temperature, and other basics work in weird, counter-intuitive, seemingly impossible ways; explanations emerge from the math and the words applied to the math often fail. Scientists don’t help their cause at time. They use “dark” to mean “unexplained.” But dark is also used for a multitude of other astronomical events and its use here is hopelessly confusing. Even finding a deeper theory today won’t help. That theory won’t be as simple as E = MC[sup]2[/sup]. (General relativity itself is way beyond that.) When they looked for something whose math yielded relativity and QM automatically they came up with string theory. And string theory is not simple. The math is far worse that what came before. People have had a hundred years to learn how to put relativity’s and QM’s concepts into words, with the benefit of being taught them throughout college on up. Nobody even knows if string theory is correct, or which version of it, or if supersymmetry and other advanced concepts have to be part of it. If it isn’t true then something possible worse, like loop quantum gravity, is. Don’t wish for something new: you might get it. And dark energy and dark matter probably don’t need anything new to get explained. Just better observations on larger and deeper scales. They’re kludgy probably because we’re trying to explain the photoelectric effect using microscopes. The tools aren’t there yet. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [October 10, 2012, 12:40pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/19 "2012-10-10T12:40:32Z")
The dark night sky is proof that the universe isn’t infitely large and infinitely old (or, more precisely, doesn’t have an infinite number of stars, and hasn’t had that number for infinity). Cecil’s explanation is more precise, of course. Earlier cosmologists tended towards the “always there, infinitely large” hypothesis, possibly because anything that was always there doesn’t require a creation myth. Those with a more religious outlook tended to prefer some kind of creation myth, and were relieved when the Big Bang theory came about. IIRC, Hubble noticed that all galaxies are moving away from us, equally in every direction, and moving faster apart the farther away. (And by extension, everything was moving away from everything else.) Run that film backwards, and it implies that everything came from one point. It took a good bit more digging to realize it wasn’t just all the galaxies streaming away from some point, but that space itself was expanding – which is a rather mind-expanding concept to grapple with. Amusingly, my father, who likes to try to poke holes in my agnosticism, doesn’t like the Big Bang theory; he seems to think that it contradicts a theistic explanation. He’s a terrifically intelligent guy, but his grasp of theology is almost as bad as his handle on science. Ah well! ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [October 10, 2012, 8:35pm UTC](https://boards.straightdope.com/t/why-is-sky-dark-at-night/636476/20 "2012-10-10T20:35:07Z")
According to Phil Plait, Edgar Allen Poe was the first person to see the solution to Olber’s Paradox.