Why is the sky not totally bright?

Burnsy2023 said:
But special relativity tell us that they couldn't move faster than the speed of light...
Click to expand...

Special relativity only covers constraints of an object achieving the speed of light in respect of another point in the Universe. However, if the Universe is expanding and the objects are unobservable from each other it is perfectly possible (theoretically) that they could be moving apart from each other at greater than the speed of light.

That's why the objects would be unobservable from Earth.
 
OP: If you manage to spend a night in the middle of nowhere (like I did camping in Australia), then you'll be shocked at how much you can see in the sky at night. Light pollution drowns out much of what should be visible.

Must say this post made me scratch my head a bit though. The light from a star isn't like a laser, it diffuses (or is possibly reflected/scattered by particles in space) and hence is nowhere near as bright by the time it reaches us. At least that's how I think of it. ;)
 
Sorry, I can't help myself

(bout 30 seconds in specifically)

Need to also think about the way that different light sources disperse their light. So for example, in computer graphics we have the concept of a point light source or a global light source.

The latter is actually an impossibility, i.e. how do you produce perfectly uniform light that extends forever without reducing in intensity. All light must have a point source, and as it so happens because the mechanics of the universe has a habit of producing spherical things, the point source are often spherical in nature, meaning that from any angle you perceive a bright disc that reduces in intensity the further from the source you go.

Put these sources far enough away from each and consider the three dimensional nature of what you're seeing (two objects next to each other in the sky may in fact be billions of light years from each other) and it starts to become obvious why the sky as we see it does not appear brighter than it is.

Not to mention the fact our eyes only perceive a small portion of the overall spectrum that is actually out there.

As has been said, go somewhere that takes you away from sea level and pollution sources and you'll be amazed at just how bright the sky is! You can even just find yourself a field somewhere out in the sticks up north somewhere and not even go that high and be amazed!
 
Last edited:
manic_man said:
Sorry, I can't help myself

(bout 30 seconds in specifically)

Need to also think about the way that different light sources disperse their light. So for example, in computer graphics we have the concept of a point light source or a global light source.

The latter is actually an impossibility, i.e. how do you produce perfectly uniform light that extends forever without reducing in intensity. All light must have a point source, and as it so happens because the mechanics of the universe has a habit of producing spherical things, the point source are often spherical in nature, meaning that from any angle you perceive a bright disc that reduces in intensity the further from the course you go.

Put these sources far enough away from each and consider the three dimensional nature of what you're seeing (two objects next to each other in the sky may in fact be billions of light years from each other) and it starts to become obvious why the sky as we see it does not appear brighter than it is.

Not to mention the fact our eyes only perceive a small portion of the overall spectrum that is actually out there.

As has been said, go somewhere that takes you away from sea level and pollution sources and you'll be amazed at just how bright the sky is! You can even just find yourself a field somewhere out in the sticks up north somewhere and not even go that high and be amazed!
Click to expand...

Damn I was about to post the youtube vid... :p
 
Don't forget, that they are also not evenly distributed in the sky! You can see the centre of the milkyway, and it is incredibly bright, so bright its difficult to make out individual stars. Anything other than the 200-400 billion stars in the milkyway are confined to near enough single points of light in patches around the sky in galaxies.
 
This place always amazes me... Threads that at first seem completely stupid sometimes end up in a good bit of reading/self-education. :)
 
it's hard to believe our neighbour galaxy is Andromeda, which is about 2 billion light years away, and our milky way is part of a local group, check out the virgo cluster, there are literally loads of galaxies which look like there miles apart but in reality billion of years away. when i looked through my 8' dob they look like faint smugdes it's hard to believe that we can see them even though there so far away.
 
The sky in the uk is not as bright as it actually is, due to the large amount of light that is in the uk and europe.

I know when i go out in the middle of no where in south africa, the sky is so bright with stars that a picture can not do it justice. If you take a picture of the sky it will not be as good as looking at it, because the camera can not capture all the light.

No hotlinking!

If you go to the black areas and look at a clear night you will be blown away.
 
a lot of what has been said so far on this thread has been incorrect.

What you have briefly described is known as "Olber's paradox". It was an old philosophical way of arguing that the universe was not infinite in response to postulations by other famous scientists who belived the universe was infinite, isotropic and homogeneous.

The argument states that if the universe was infinite, the sky would be infinately bright, as no matter where you looked there would be an infinite number of stars and therefore high intensity of light.

Inverse square attenuation is not relevant, as if the number of stars was infinite the sky would remain bright, regardless of the apparent brightness of each star.

The universe is not expanding at a speed greater than that of light...however, the speed of individual galaxies could exceed the speed of light depending on the chosen frame of reference. The law is nothing can travel faster than the speed of light in an intertial frame of reference(iirc). However, take two galaxies at opposite ends of the universe and measure thier relative speed, and yes it will exceed c. Also, the argument that the light from the most distant starts may not have reached us holds some ground, becuase as we all know c is a fixed constant.

There is no such thing as "black space", if the reference is to "dark matter" than that does exist. However if the universe was infinite it would not stop the sky being bright at night-time. This is becuase dark matter is, in theory, a perfect black body, meaning any radiation (light) absorbed would be re-emitted back out at the same intensity (albeit scattered), causing no loss in net flux.
 
Last edited:
PianoBasher said:
OP: If you manage to spend a night in the middle of nowhere (like I did camping in Australia), then you'll be shocked at how much you can see in the sky at night. Light pollution drowns out much of what should be visible.
)
Click to expand...


I read somewhere that If you view the sky in Australia you are looking in towards the milky way whereas on our side of the planet we look outwards and away and as we are near the edge of the galaxy cluster we see far less stars.
 
I fancy boys said:
The universe is not expanding at a speed greater than that of light...
Click to expand...

I don't think anyone said it was. However, two points within the Universe could conceivably be moving away from each other at a speed at or greater than the speed of light. They would, however, be unobservable from each other.
 
Gilly said:
Are you sure? Can't find it here.

Looking at going to Galloway for it, but Kielder is closer...
Click to expand...

Maybe it isn't an official one then, at least not by that group. A lot of star camps are held there, and I've been camping in Kielder and Galloway and they were both great. Your not going to be missing out on anything by just going to Kielder.

http://www.kielderobservatory.org/

edit: Just seen this as well
http://bellingham.journallive.co.uk/news/kielder-observatory-looks-for-dark-sky-status.html
 
Last edited:
Kielder is also an official 'dark spot'. Took this there in Summer:
Click to expand...

Cool thanks i did not know there was a place in the uk to see all the stars. Will definitely visit in summer some time.

a lot of what has been said so far on this thread has been incorrect.

What you have briefly described is known as "Olber's paradox". It was an old philosophical way of arguing that the universe was not infinite in response to postulations by other famous scientists who belived the universe was infinite, isotropic and homogeneous.

The argument states that if the universe was infinite, the ....snip
Click to expand...

The universe could still be infinite because the light will hit at different times, as the distance between a light source and earth, is rarely if ever the same, due to the scale.
 
You must log in or register to reply here.
Back
Top Bottom