> I'm now working hard on my assignment for my CG class at college, in which I'm
> trying to make a realistic earth animation.
> I used "difference" to create something like atmosphere right outside earth, and
> in which I also used emission for making the atmosphere glow. And this is where
> I'm facing a problem.
>
> When I set a light_source from a distance, there is no shadow cast on earth.
> Here I paste the source code:
>
> light_source {
>  <100, 0, 50>
>  color White
>  spotlight
>  fade_distance 100
>  fade_power 10
>  point_at <0, 0, 0>
> }

Why use a spotlight?
Why use fading?

Given the distance from the Sun to the Earth, you are better using a 
parallel light without any fading. Think about it, the Sun-Earth 
distance is over 10000000 times the Earth's radius.
The difference in the illumination over the moon's orbit 's diameter is 
effectively negligible.

fade_power ONLY realistic value is 2.
fade_sidtance should be about the appearent diameter of a light's diameter.
If the light becomes to dark, you can increase it's intensity 
arbitrarily by as much as you need.

Try this one:
light_source{<100, 0, 50>*10 rgb 2 parallel}

>
> // earth
> sphere {
>     <0, 0, 0>, 3.5
>     texture {
>         pigment {
>       image_map {
>                 jpeg "textures/earth.jpg"
>                 map_type 1
>             }
>             scale 3.5

This scalling is useless.

>         }
>
>         finish {
>             ambient 1
>             diffuse 3

Beter use ambient 0, diffuse 0.6 here.
ambient 1 mean that your earth is actualy glowing.
diffuse 3 mean that it give off 3 times as much light as it receive from 
the Sun.

>         }
>
>         rotate y*-135
>         rotate x*35
>     }
>     // rotation
>     rotate <0, 360*clock, 0>
> }
>
> // atmosphere emission
> difference {
>   sphere { <0, 0, 0>, 3.54 }
>   sphere { <0, 0, 0>, 3.0 }
>
>   material {
>    texture{
>    pigment { rgbt 1 }
>    }
>
>    interior {
>      media {
>        emission 0.5

A GLOWING atmosphere? Last time I checked, Earth's atmosphere scatters 
the light and don't emit any.

>        //scattering { 5 color rgb 0.01 eccentricity 0.56 }

The proper model is scattering{4 rgb 0.01}

>        density {
>          spherical density_map {
>        [ 0.0  rgb 0.0 ]
>           [ 0.5294*0.25e-6  rgb <0.02, 0.05, 0.2>*0.07 ]
>           [ 0.5294*0.4e-6   rgb <0.02, 0.07, 0.3>*0.32 ]
>           [ 0.5294*0.5e-6   rgb <0.08, 0.18, 0.4>*0.5 ]
>           [ 0.5412*0.6e-6   rgb <0.08, 0.18, 0.4>*0.9 ]
>           [ 0.5471*0.65e-6  rgb <0.08, 0.18, 0.4>*1.5 ]
>           [ 0.5471*0.675e-6 rgb <0.08, 0.18, 0.4>*4.5 ]
>           [ 0.5471*0.71e-6  rgb <0.08, 0.18, 0.4>*12 ]
>          }
>      scale 100
>        }
>      }
>    }
>   }
>
>   hollow on
> }
>
> And below I attach the rendered result for now.
>
> How can I solve this problem? I'd appreciate it if someone could help.
> I'm not a native English speaker, so I'm very sorry for my poor English... I
> really wish I didn't use impolite expressions!
>
> Thanks.
>

Alain

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