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How does the sky turn dark at night?
- HPsquared 3y agoMore interesting to ask "why is the sky light during the day?"
- shreddit 3y agoBecause the sun is “turned on” during the day, duuh
- jareklupinski 3y ago[flagged]
- Am4TIfIsER0ppos 3y ago[flagged]
- jareklupinski 3y ago[flagged]
- swasheck 3y ago[flagged]
- marcosdumay 3y agoWell, you can become one of today's lucky 10,000 if you search for it instead of assuming the answer is simple.
- tricky 3y agoOlbers'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 https://en.wikipedia.org/wiki/Olbers%27s_paradox
- ninkendo 3y agoI 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 3y ago"and eternal static universe."
- Filligree 3y agoDoesn't that assume that an infinite amount of time has already passed? Which seems like a contradiction in terms.
- ben_w 3y agoI don't see how it would be a contradiction in terms; we can easily speak of the possibility of an infinite amount of time in the future, why not also in the past? (It may be factually wrong, but that's not a contradiction in terms).
- HPsquared 3y agoThe 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.
- marcosdumay 3y agoIt also talks about red-shift. Olbers couldn't know that, but even if our universe was here forever we still wouldn't have a lit night sky.
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- kesava 3y agoI miss the early academic internet with at least a couple of links every sentence. :)
- jetrink 3y agoNick 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 https://youtu.be/_q9i_HEOkxo?si=7YYRq6bK-Bcjiw_i&t=430
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- MyFirstSass 3y agoWhat 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.
- hunter-gatherer 3y agoYeah this videonwas great. Some years ago I visited Kitt Peak observatory and the astronomer pointed out something I had never appreciated before. If you have a big enough open sky to observe and turn against the sunset you can observe the Earth's shadow in atmospher. A cool site that few l ever turn to appreciate.
- its_ethan 3y agoI immediately thought of this video too! Kinda funny that it's supposed to be a video talking about how he composed the photo but it turns into a 10 minute science lesson target.
- kinow 3y agoHey, thanks a lot for sharing Nick's video! I hadn't seen it before, but the details about colours, light and atmosphere were super interesting, especially if you are into photography and painting.
- javier_e06 3y agoAt 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 3y agoI 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 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 3y agoThis 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.
- dhosek 3y agoIt’s the sort of surprise that seems obvious once you see it, but isn’t so easy to predict. Without this experience, most people would guess that in a long exposure picture of the night, you would see a black sky with perhaps brighter luminaries and more easily seen objects on the ground, not a blue sky.
- aaronbrethorst 3y agohttps://xkcd.com/1053/ https://xkcd.com/1053/
- dylan604 3y agoIf 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 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 3y agoI forgot all about APOD, glad to see them still going strong.
- deleted 3y ago[deleted]
- divbzero 3y agoIf you look in the direction of the Sun, there can also be a “green flash” just as the Sun sets. https://en.wikipedia.org/wiki/Green_flash https://en.wikipedia.org/wiki/Green_flash
- SeriousM 3y agoThat means that if you travel into the sunset at the same speed as the sun sets, you could see a permanent greeen spot when the conditions are right?
- divbzero 3y agoYes I suppose so :) That would be faster than the speed of sound at the equator: https://www.wolframalpha.com/input?i=(Earth's+circumference+in+km)+/+(24+hours) https://www.wolframalpha.com/input?i=(Earth's+circumference+...
- ReptileMan 3y agonote to self: run this experiment when I become billionaire.
- tecoholic 3y agoI like how every link is a scientific article or such and there is a Easter egg with “don’t despair” ;)
- mmaunder 3y agoNow let’s discuss the green flash.
- nomel 3y agohttps://en.wikipedia.org/wiki/Green_flash https://en.wikipedia.org/wiki/Green_flash
- SapporoChris 3y agohttps://xkcd.com/766/ https://xkcd.com/766/
- decae 3y agoI shot a timelapse of the Tokyo skyline at sunset that is similar to this but with much more detail [0], and then motion tracked it so that time is traveling across the frame from left to right[1]. In a similar process to the linked NASA image, the video is made of strips, but each strip is 4 pixels wide. It turned out very different compared to how I imagined it would, but that is due to the stages explained in the linked article. Super interesting! [0] https://youtu.be/wTma28gwSk0 https://youtu.be/wTma28gwSk0 [1] https://youtu.be/v5HLX5wFEGk https://youtu.be/v5HLX5wFEGk
- stianlaa 3y agoThese timelapses are beautiful, thank you for sharing. Any particular place in Tokyo you filmed it from? Also, I find it interesting in the second timelapse how distinct the separation between day and night is.
- decae 3y agoCheers! I became a bit obsessed with timelapses and this technique during the pandemic. It was shot in Ichikawa, Chiba, close to the station. And yes, definitely, it's quite stark. I guess a combination of being shot in summer and Tokyo's location relatively close to the equator. I'm originally from southern Australia and the dusk there is far longer.
- tdudhhu 3y agoAfter writing a path tracer following 'Raytracing in a weekend' I checked out the paper of T. Nishita about implementing a sky model. It also describes how light is scattered. For those interested, a copy can be found at multiple websites: Display method of the sky color taking into account multiple scattering https://www.researchgate.net/publication/242513955_Display_method_of_the_sky_color_taking_into_account_multiple_scattering https://www.researchgate.net/publication/242513955_Display_m...
- pangolinja 3y agothis is why I love hacker news. the actual article is shit. NASA source, but shit. terms you fucking nothing about the "how". then you come to the HN comments and it's a bunch of smart shit, each comment 100iq higher than the clickbait NASA bullshit they are nested beneath.