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?
http://www.opticsforkids.org/resources/Color_12.pdf
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
Effect)."
NASA Goddard Space Flight Center | Why is Space Black?
http://www.gsfc.nasa.gov/scienceques2002/20030328.htm
I hope this helps!
---larre
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