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Q: Why is the sky black at night? ( Answered,   2 Comments )
Subject: Why is the sky black at night?
Category: Science > Astronomy
Asked by: nonet-ga
List Price: $4.00
Posted: 26 Sep 2004 17:51 PDT
Expires: 26 Oct 2004 17:51 PDT
Question ID: 406675
Why is the sky black at night?
Subject: Re: Why is the sky black at night?
Answered By: larre-ga on 26 Sep 2004 18:29 PDT
There are actually several questions in one. 

Q. Why do we (humans) perceive the sky as black at night?

A. "When asked the color of the night sky, it is tempting to say, 
   ?black, of course!? However, that is not really correct. It looks 
    black to us because there is not enough light to stimulate the 
    color-sensitive cones in our eyes, even though there is light in 
    the nighttime sky, and it has color. Away from city lights or 
    other interfering light sources, the night sky, illuminated by 
    moonlight is blue, similar to the sunlit daytime sky."

What Color is the Sky?

Q. But it sure looks black to me. Why? 

A. At night we see through the atmosphere and out into space.

Q. So. Why is space black?

A. NASA has a great answer: "There are two things to think about here.
Let's take the easy one first and ask "why is the daytime sky blue
here on Earth?" That is a question we can answer. The daytime sky is
blue because light from the nearby Sun hits molecules in the Earth's
atmosphere and scatters off in all directions. The blue color of the
sky is a result of this scattering process. At night, when that part
of Earth is facing away from the Sun, space looks black because there
is no nearby bright source of light, like the Sun, to be scattered. If
you were on the Moon, which has no atmosphere, the sky would be black
both night and day. You can see this in photographs taken during the
Apollo Moon landings.

So, now on to the harder part - if the Universe is full of stars, why
doesn't the light from all of them add up to make the whole sky bright
all the time? It turns out that if the Universe was infinitely large
and infinitely old, then we would expect the night sky to be bright
from the light of all those stars. Every direction you looked in space
you would be looking at a star. Yet we know from experience that space
is black! This paradox is known as Olbers' Paradox. It is a paradox
because of the apparent contradiction between our expectation that the
night sky be bright and our experience that it is black.

Many different explanations have been put forward to resolve Olbers'
Paradox. The best solution at present is that the Universe is not
infinitely old; it is somewhere around 15 billion years old. That
means we can only see objects as far away as the distance light can
travel in 15 billion years. The light from stars farther away than
that has not yet had time to reach us and so can't contribute to
making the sky bright.

Another reason that the sky may not be bright with the visible light
of all the stars is because when a source of light is moving away from
you, the wavelength of that light is made longer (which for light
means more red.) This means that the light from stars that are moving
away from us will become shifted towards red, and may shift so far
that it is no longer visible at all. (Note: You hear the same effect
when an ambulance passes you, and the pitch of the siren gets lower as
the ambulance travels away from you; this effect is called the Doppler

NASA Goddard Space Flight Center | Why is Space Black?

I hope this helps!


Google Search Terms

oblers' paradox
why sky black night

Clarification of Answer by larre-ga on 27 Sep 2004 12:09 PDT
Additional Sources:

Wikipedia | Olbers' Paradox'_paradox

Relativity FAQ | Olbers' Paradox

Astronomy FAQ | Olbers' Paradox

Cosmology FAQ | Olbers' Paradox

Also see:

Paul Wesson, "Olbers' paradox and the spectral intensity of the
extragalactic background light", The Astrophysical Journal 367, pp.
399-406 (1991).
Edward Harrison, Darkness at Night: A Riddle of the Universe, Harvard
University Press, 1987

Scott, Douglas, and Martin White, "The Cosmic Microwave Background

Subject: Re: Why is the sky black at night?
From: neilzero-ga on 27 Sep 2004 06:00 PDT
Olber seems to be assuming that some constants times infinity divided
by infinity squared = 7 or some other number that would produce a
perceivable light level to the human eye. We divide by infinity
squared as the brightness of a light source decreases as the square of
the distance. If the Universe is infinately old, infinately large and
not expanding, we should expect the perceived light level to be very
 Finite size, expanding and 13.7 light years since the big bang also
yields a very low light level IMHO = in my humble opinion. The light
energy of too long a wavelength (red shifted) to be perceived by
humans is also of very low energy density. I would not regard NASA as
a reliable authority.  Neil
Subject: Re: Why is the sky black at night?
From: iang-ga on 27 Sep 2004 10:59 PDT
>Olber seems to be assuming....

Sorry, I don't follow that, but there's another way of looking at the
stats.  Also, bear in mind that Olbers, and Halley before him, started
from the assumption that the universe is both infinite and eternal.

Picture the universe as an onion, with each ring of equal thickness
and the Earth at its centre.  It can be shown that, assuming a uniform
distribution of stars, the light from each "ring" will be the same. 
Since there must be an infinite number of rings, and we can see the
light from at least one of them, there should be an infinite ammount
of light.  OK, the stars aren't uniformaly distributed, but you can
substitute galaxies for stars and the logic still holds.

The red shift is now seen as a small contributing factor rather than
the main reason, but it always had the problem that the shorter
wavelengths would be shifted into the visible band, so you couldn't
get rid of the light.

Ian G.

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