There are various effects in the atmosphere which contribute
to scattering. Therefore, a single media statement may not be
enough to simulate all kinds of views.

Here is a sample with two media blocks (not a great atmosphere
but it shows how combine uniform and directional scattering so
you can get more control how it looks in various directions.

I also fixed the scale for spherical as Alain suggested, and
used a macro so you can see for which height above ground you
are actually specifying densities:


#declare ATM_THICKNESS = 50;
#declare ATM_RADIUS    = 5140 + ATM_THICKNESS;

#declare Atmosphere = difference
{
   sphere {0, ATM_RADIUS}
   sphere {0, 5140}
   hollow
   material
   {
     texture
     {
       pigment
       {
         color rgbt 1
       }
     }
     interior
     {
       media
       {
         method 3 samples 10
         scattering {1, <0.8, 0.8, 1>/200}
         density
         {
           spherical scale ATM_RADIUS
           color_map
           {
             [(ATM_THICKNESS-50)/ATM_RADIUS rgb <0.0,0.0,0.0>]
             [(ATM_THICKNESS-20)/ATM_RADIUS rgb <0.8,0.9,0.9>]
             [(ATM_THICKNESS-00)/ATM_RADIUS rgb <0.9,1.0,1.0>]
           }
         }
       }
       media
       {
         method 3 samples 10
         scattering {5, <0.8, 0.8, 1>/100 eccentricity 0.4}
         density
         {
           spherical scale ATM_RADIUS
           color_map
           {
             [(ATM_THICKNESS-50)/ATM_RADIUS rgb 0]
             [(ATM_THICKNESS-00)/ATM_RADIUS rgb <0.9,1.0,1.0>]
           }
         }
       }
     }
   }
}

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