Having done this before I know what you're trying to do.  But as of the
moment I can only think of to say what I did as an example.
The nightside lights image, same as how your clouds image was done for
transparency, except as a layer under the ground image layer (clouds were a
separate sphere) had a layer between the Earth surface texture.  It was half
transparent and half black, for night (transparent) and day (opaque).  So
there were 3 layers on one sphere.  Very high ambient was added to bring out
the brightness of the lights.  This of course means the overlying ground
image had to be slightly semitransparent too, then the large ambience
allowed for the lights to be seen through that.
The half transparent, half black gradient x (or z) texture was then rotated
opposite Earth rotation in animation.

Make sense?  Hopefully I described it right.

I know there must be plenty of ways to go about it but that's what I did.
For example, you might make use clipping or differencing of another sphere
(not smooth transition terminator though).

Bob H.

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