MadSci Network: Astronomy

Subject: Why is the sky dark at night?

Date: Fri Dec 11 11:47:02 1998
Posted by Barry Hunks
Grade level: undergrad
School: No school entered.
City: Terrace State/Province: BC
Country: Canada
Area of science: Astronomy
ID: 913398422.As

  I had a prof. who was trying to make the point that the most 
obvious answer is not always the correct answer.  The obvious 
answer is the earth is casting a shadow , so there is no sunlight  
in the atmosphere.  He says that because the sun is behind us, 
the sunlight is travelling away from us so the light is red 
shifted; so, in fact, there is plenty of light but it is not in 
our visual spectrum because it is infrared.
    I say that light cannot be red shifted unless the source of 
the light itself  ( the sun )  is travelling away from the 
observer at a very fast rate; and that there is just as much 
infrared duting the day as there is at night.  Space is black 
unless there is something to illuminate it such as atmosphere or 
surface.  So, the fact that there is no sunlight in the 
atmosphere makes the sky dark.  Am I right? or is he? neither?
   He also says you can see starlight without an atmosphere or a 
surface.  I say your eye is the surface.   
   Also even if the light was red shifted, you would be able to 
look 90 degrees  beyond the horizon into space where the light 
would not be travelling away from you, hence the horizons would 
never go dark.


Re: Why is the sky dark at night?

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