I've just tried using light groups and negative rgb values for the lights,
and it does work. The problem with what I was trying, was that I didn't
realise at the time was that the dark spots (from the texture used) would
obscure parts of the lower sphere.

A better soloution would be "transparency lighting". By using the
appropriate keyword in the pigment{} code, the texture becomes more
transparent with more light. Now this would be best for creating nightside
lights.

sphere{<0,0,0> 2
  pigment{
    image_map{
      png "nightlights.png"
      map_type 1 // I think that's spherical mapping
      }
    transparent_light transmit 0,1
    // or use this for filter transparency
    // transparent_light filter 0,1
    }
  }

"transparent_light" - keyword for using transparent light

transmit/filter - transparency method to use

0,1 - transparency for no light, full brighness (e.g. 0.5,0.9 gives 50%
transparency in full darkness, and 90% transparency in full light)

This could be handy for creating special effects. In the case of the
nightside lights, they would fade in over the terminator, and be full
brighness on the dark side of the Earth. I think this is what you would see
looking at the terminator of Earth at night, the lights getting brighter as
it gets darker, as more people start turning on their lights.

Rohan _e_ii

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