Ken wrote:
> 
> The first sphere with the daylight image. Intersect the sphere with a fixed plane
> resulting with half a sphere - call it Left_Hemisphere
> 
> The second sphere with the night time image. Intersect the sphere with a fixed plane
> resulting with half a sphere - call it Right_Hemisphere.
> 
> Now you have two half-spheres that look like one whole sphere.

Would do it, yeah. The thing that might be an issue is the sharpness of 
the boundary.

Having made this same thing before I had to overlay image maps and use 
an extreme ambient on the nightside lights map while covering it with 
Earth, clouds, and masking texture. The real trick being the mask, it 
was a clear and black gradient which blended a little at the center to 
act as a dusk/dawn area.

To describe in layers, it went like:

1) nightside lights, highly ambient (ambient 3 or more)
2) masking, clear to black (index 0.47 to 0.53 or thereabouts)
3) Earth, transmit 0.05 for the underlying lights to squeeze through
4) Clouds, clear to white basically

The mask layer counter-rotated so the black (opaque) side always faced 
the Sun while the Earth layer rotated, as well as the clouds a little 
faster over that. The little bit of transparency added overall to the 
Earth layer wasn't enough to make it show anything through except for 
the highly ambient nightside lights portions. I'm going by memory here 
since it was years ago now and the numbers could be different, but tells 
basically what I did to get it done.

If you'd like to see it the animation I did of it is at 
http://hometown.aol.com/persistenceofv/anims.htm

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