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I am trying to imagine how dark outer space is. Looking at images of cosmonauts in space proved not very useful, as the exposure makes space look completely dark.

Thinking about it, I would imagine an object would be quite well illuminated, being bombarded by provided it's not in the shadow of a planet or some other celestial body. Is that correct? How about light scattering? Is cosmic dust abundant enough for the purpose?

EDIT: @zephyr made me realize that the quantity of light is a function of position, so let's say the question is with regards to the Milky Way. Still, I would appreciate some comments on what's the situation outside the Milky Way too.

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  • $\begingroup$ I assume by space you mean in orbit around Earth or perhaps on the Moon, not in some random void of the universe, far from anything else. $\endgroup$ – zephyr Oct 17 '16 at 13:36
  • $\begingroup$ I am not sure if I mean that. :) It was my understanding that light is ever-reaching and that generally such voids would be a rare occurrence? If that's not the case, does ``anywhere inside a galaxy'' (like the Milky Way) sound more reasonable? $\endgroup$ – goblin Oct 17 '16 at 13:40
  • $\begingroup$ Light is certainly pervasive everywhere, but the amount of light you might receive at any one location is highly dependent on how close you are to a light source. Thus the answer would be different if you were near Mercury versus Pluto, or inside our galaxy vs outside it. Just wanted to determine which situation you were referring to in your question. Feel free to edit if you want to provide something more specific. $\endgroup$ – zephyr Oct 17 '16 at 13:43
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    $\begingroup$ Measurement of what is known as the extragalactic background light is an entire area of study. Here's a recent review paper on the topic by Asantha Cooray. $\endgroup$ – Sean Lake Oct 17 '16 at 14:10
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Just to put in a simple answer:

if you mean (A) for astronauts just in orbit (at the space station and so on) or on the surface of the moon. Yes, it is incredibly bright. Just like on Earth in the Sahara at noon!

A point of confusion is that THE >> SKY << IS BLACK, but it is incredibly bright. On Earth in daytime, the sky happens to be blue, and it is incredibly bright. On Moon in daytime, the sky happens to be >> black <<, and it is incredibly bright. (Same deal in orbit as on the Moon.)

(If you are, say, explaining to a young person "is there daytime/nighttime on the moon" that's the salient point to explain: on the moon, confusingly, the sky is black during both daytime and nighttime. But everything's just as bright on the moon in daytime as on Earth in daytime, there's the same amount of light, i.e. you can trivially read a book or see the ground, just the same as on Earth.)


If your question is about (B) in the solar system - say "on Uranus" or the like. There is remarkably less light there than on Earth, the sun is much smaller. Many young people don't realize this; everything's basically dark all the time from about Saturn outwards!

Outstanding page explaining this:

http://www.astronoo.com/en/children/sun-apparent-size.html


Finally if your question is about either (C) inside our galaxy but not near a star, or, (D) midway between the galaxies:

That's too difficult.

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    $\begingroup$ Actually even though the Sun appears smaller the further out you travel in the Solar System, there is still a significant amount of sunlight. NASA have even created an app were you can calculate "Pluto Time" which is the time just after sunset when the sky is as bright as it is on Pluto. $\endgroup$ – Dean Oct 18 '16 at 12:47
  • $\begingroup$ Hey @Dean great app suggestion! Yes for any young folks reading there's a picture right here: astronoo.com/en/children/sun-apparent-size.html showing how dark it is on Pluto, cheers $\endgroup$ – Fattie Oct 18 '16 at 12:51
  • $\begingroup$ Also in relation to your point (D) I did a rough estimate for the brightness of Andromeda half way from the Milky Way and I got an apparent magnitude of 1.4, which is about as bright as the star Regulus. So if you assume the milky way is just as bright then you would effectively have two bright diffuse objects around 1-2 magnitude in the sky and not much else visible to the human eye. $\endgroup$ – Dean Oct 18 '16 at 12:58
  • $\begingroup$ Some very good point raised here, especially that black can be bright too. $\endgroup$ – goblin Oct 18 '16 at 13:44
  • $\begingroup$ Let's be clear here - the sky is black, and the illumination level is bright. But the black sky itself is not bright. It's like being in a room with a very bright overhead light but black walls and a black ceiling. It's plenty bright in the room, but the walls and ceiling are still dark. $\endgroup$ – Mark Foskey Oct 5 '18 at 14:05
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I think you can actually experience the answer to your question by going out into the deep wilderness late on a night where there is no Moon, make sure no artificial lights are anywhere nearby, and simply observe. That's how dark it is in a reasonably random location in the Milky Way! It will be very dark indeed, but you might still be able to see something (other than the stars, of course). It's a good question-- can you see your hand in front of your face by starlight alone? I don't think you can, but I haven't done the experiment!

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  • $\begingroup$ It is an outstanding point that, indeed, that is a random point in the Milky Way !! You simply hide our Sun (by putting the Earth there) and then it's pretty much identical to being anywhere in the Milky Way, not close to a star. Great post. $\endgroup$ – Fattie Oct 18 '16 at 13:04
  • $\begingroup$ Good analogy. Actually, when writing the question, I was thinking precisely of the woods at night. It's so dark you most likely won't be able to see anything! But then again, I would imagine that the situation is a bit different in outer space, as there are no trees or Earth to obstruct light. $\endgroup$ – goblin Oct 18 '16 at 13:40
  • $\begingroup$ There could be a factor of 2 or so difference, it's true, but that kind of variation you'll also get just from choosing random places in the Milky Way. $\endgroup$ – Ken G Oct 18 '16 at 15:23
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The reason the night sky is dark is our distance from the nearest stars (over 4 light years) which would be as bright as our sun (93 million miles) if they were not over 2500 times further away and only the brightness of a star, not the sun.

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  • $\begingroup$ Whilset superficially appealing that isn't a valid reason for why the sky is dark - please see en.wikipedia.org/wiki/Olbers%27_paradox for reasoning $\endgroup$ – adrianmcmenamin Oct 4 '18 at 14:43
  • $\begingroup$ @adrianmcmenamin I read it before I posted and after. Some stars we cannot see. If we are in a city many more we cannot see. The nearest and brightest are pinpricks and all more than 24 TRILLION miles away. $\endgroup$ – Mark Oct 5 '18 at 17:12
  • $\begingroup$ It doesn't really matter how far away they are - if the universe was of infinite age and infinite extent and matter was uniformly distributed we'd still have a bright night sky: individual photons do not grow dim over distance, so an infinite number of sources, no matter how far away, would burn your retinas out! $\endgroup$ – adrianmcmenamin Oct 7 '18 at 11:51

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