[HN Gopher] How does the sky turn dark at night?
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How does the sky turn dark at night?
Author : politelemon
Score : 53 points
Date : 2024-02-29 07:49 UTC (1 days ago)
(HTM) web link (apod.nasa.gov)
(TXT) w3m dump (apod.nasa.gov)
| HPsquared wrote:
| More interesting to ask "why is the sky light during the day?"
| shreddit wrote:
| Because the sun is "turned on" during the day, duuh
| tricky wrote:
| Olbers's paradox, also known as the dark night paradox, is an
| argument in astrophysics and physical cosmology that says that
| the darkness of the night sky conflicts with the assumption of an
| infinite and eternal static universe. In the hypothetical case
| that the universe is static, homogeneous at a large scale, and
| populated by an infinite number of stars, any line of sight from
| Earth must end at the surface of a star and hence the night sky
| should be completely illuminated and very bright. This
| contradicts the observed darkness and non-uniformity of the night
| sky.
|
| https://en.wikipedia.org/wiki/Olbers%27s_paradox
| ninkendo wrote:
| I don't really see how Olbers's paradox proves that the
| Universe is finite, merely that the _observable universe_ is
| finite. The observable universe is finite because (a) it's
| expanding and (b) the speed of light is finite. Both of these
| can be true in an infinite universe (an infinite universe can
| still expand everywhere, which can be counterintuitive but it's
| true.)
| russdill wrote:
| "and eternal static universe."
| Filligree wrote:
| Doesn't that assume that an infinite amount of time has
| already passed? Which seems like a contradiction in terms.
| HPsquared wrote:
| The observable universe is finite because the light started
| at some point in time (and the corresponding distance in
| light years).
|
| It says less about the size of the universe, than its age.
| kmm wrote:
| The line of sight argument is nice and succinct, and of course
| in some sense correct, but I've come to view the paradox in a
| somewhat different way: if all you have in a static and eternal
| universe are everlasting sources of energy, then of course
| you're going to run into trouble when considering equilibrium
| concerning energy, because it won't exist.
|
| So while it is true that the true resolution to the paradox is
| that our universe is finite in age and expanding, that doesn't
| mean a static and eternal universe is in principle untenable.
| One could for example imagine as of yet unknown sinks of
| energy, or perhaps starlight gets recycled back into new stars
| as the old stars disappear. Without speculating on the
| mechanism, basically any universe where conservation of energy
| holds will have (on a large enough scale) a constant energy
| density, and hence, a dark sky at night.
| sandworm101 wrote:
| The paradox also assumes several things about light,
| specifically that light propagates forever. Only in recent
| years have we managed to prove that assumption true. But if
| light did degrade over astronomical distances, a static and
| infinite universe could still have a dark sky at night. If one
| postulates that light degrades into lower and lower frequencies
| over time/distance, maybe we are indeed living in a non-
| expanding universe? Given the thermodynamic issues of an
| accelerating expansion (dark energy) photons that degrade over
| distance seems at least a less-strange option.
| sethammons wrote:
| > light propagates forever .... Only in recent years have we
| managed to prove that assumption true
|
| I thought the dark of night was explained by redshift (given
| distance, it shifts out of the visual spectrum). I guess the
| infra-red still continues on so that is in line with light
| propagating forever
| charcircuit wrote:
| >Only in recent years have we managed to prove that
| assumption true
|
| How? What if after 100 years light has a small chance to
| disappear.
| ars wrote:
| It also assumes that the density of stars is constant in an
| infinite universe.
|
| You can have an infinite universe, but with all the stars
| located in just part of it, i.e. finite mass, infinite space.
|
| Most of the light heads off into empty space, never to be
| seen again.
| kesava wrote:
| I miss the early academic internet with at least a couple of
| links every sentence. :)
| jetrink wrote:
| Nick Carver, a photographer, had a really interesting video
| recently where he explained the progression of light at sunset.
| Prior to watching the video, I understood the primary phenomenon
| of Rayleigh scattering, so I didn't really expect to learn
| anything. However, there's a lot more going on as the sun sets
| than I realized. The properties of sunlight (color, hardness,
| direction) shift in interesting and surprising ways even after
| the sun falls below the horizon.
|
| The segment begins at 7:10, but if you already have a basic
| familiarity with the science, you can probably skip to 12:00.
|
| 1. https://youtu.be/_q9i_HEOkxo?si=7YYRq6bK-Bcjiw_i&t=430
| MyFirstSass wrote:
| What a great video, thanks! Especially the airlight phenomenon
| is fascinating with the softness related to its size.
|
| I've always loved that magic almost pink shadowless very
| dreamlike illumination you'll sometimes get right when the arch
| is large, and the subtle change in feeling when it gets sharper
| and dimmer.
| javier_e06 wrote:
| At the lodges of the Grand Canyon they have pamphlets with the
| sunrise times for the whole year. If one arrives at night and
| leave the same night. One was not there really. But that prompts
| the question too: How does the earth turn dark at night?
| dhosek wrote:
| I took some pictures when my (then) wife got her first DSLR using
| long exposure times at night of some buildings in Mexico, e.g.,
| https://share.icloud.com/photos/0ccsqWIBoU-CmT60CaZMopaLA
|
| What surprised me was that with enough exposure time, the sky was
| blue, not black (I'd also note that this being 2008, the color
| change cannot be attributed to algorithmic processing of the
| image in camera).
| jandrese wrote:
| This shouldn't be that big of a surprise. The atmosphere is
| still there, it does the same thing to incoming light that it
| always does, your eye normally can't see it because the total
| amount of light drops the color filtering below the detection
| threshold of a human eyeball.
| dylan604 wrote:
| If that was surprising to you, have you seen full moon long
| exposure images? It looks just like the day time. I have long
| exposures during a full moon, where at first glance it just
| looks like a random daytime shot, but when you look more
| closely, you can clearly make out the stars/milky way/etc. The
| water also has that the telltale smoothing. I actually enjoy
| this style of photography, but it is so constrained by the
| short availability window. Essentially, it's a golden hour type
| thing. Once the moon gets too high, it's just overpowering. So
| just before/after moon rise is best.
| notum wrote:
| I forgot all about APOD, glad to see them still going strong.
| 1970-01-01 wrote:
| Total solar eclipses will flip the table on this banding, and you
| get 'night' darkness -without- the phenomena.
|
| https://www.youtube.com/watch?v=3Pfsy5nvkVY
| divbzero wrote:
| [delayed]
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