From: kfl@KeithLynch.net   
      
   Henry Spencer  wrote:   
   > Beto  wrote:   
   >> According to my calculations, the difference between the longest   
   >> distance from earth to the sun and the shorstest one are about 4.8   
   >> millions of kilometers.   
      
   > That is at least roughly correct.   
      
   >> Why the temperature doesn't drastically change because of this?   
      
   > Partly because Earth has a lot of thermal inertia:  changes in the   
   > amount of sunlight don't change its temperature quickly.   
      
   > Partly because in the Northern Hemisphere, where most of the people   
   > live, this trend is opposed to the seasons: Earth is closest to the   
   > Sun in January -- Northern Hemisphere winter -- so the changes due   
   > to distance are hidden by the normal seasonal changes.   
      
   Also because 4.8 million out of 150 million is really not much   
   difference.  It makes a difference of about 3% in light intensity,   
   hence of about 0.075% in equilibrium absolute temperature, i.e.   
   about 2 degrees C, or 3 degrees F.   
      
   >> If this is because of radiation doesn't being too important this   
   >> far from the sun, Where can I find a equation which relates   
   >> radiation with distance?   
      
   > Light intensity varies (to a good approximation) inversely with the   
   > square of distance.   
      
   And equilibrium temperature varies with the fourth root of light   
   intensity.   
      
   Another way to look at it is:  take the fourth root of the proportion   
   of the sky (including the half below the horizon) taken up by the sun,   
   and multiply that by the surface temperature of the sun, and you get   
   what the temperature of the earth ought to be.  It won't be quite   
   right since the earth isn't a perfect black body, but it will be   
   close.  This works for all planets, asteroids, comets, space probes,   
   etc., so long as nearly all their heat comes from the sun, and so   
   long as they spend no time in the shade (e.g. of a nearby planet).   
   --   
   Keith F. Lynch - kfl@keithlynch.net - http://keithlynch.net/   
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