MadSci Network: Astronomy

Re: Why is the sky dark at night?

Date: Wed Dec 30 12:30:39 1998
Posted By: Bruce Lobitz, M.D., Emergency Physician
Area of science: Astronomy
ID: 913398422.As

You are right that the light from the sun is not red-shifted, at least not 
from our point of view.  It is the movement of the source of the light, 
depending on your frame of reference, that gives light a red or blue 

The night sky is black because there is no light.  It would seem that the 
sky should be bright white because of the billions of stars and galaxies 
in the universe.  The problem is that all the billions of stars are 
billions of miles apart; space is mostly that, empty space, which does not 
produce light.  Plus, the intensity of light, that is the energy which is 
responsible for light, decreases inversely with the distance squared.  
This means that the brightness of light from any source drops off quickly 
over distance.

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