Does this shader parameter exist?

[Corey_Faure]

You know how phong shows according to the light hitting the model? Well, I was wondering if there was a way to have a texture kind of reveal itself when it was facing you/light. I'm not quite sure the best way to put it in words, but say I wanted to make a 3D Earth that had two textures, night and day. The lower the light level, the more the night texture would fade in on the low-lit side.

Is this possible?