How to change texture based on light level

I am trying to construct a to scale model of the solar system and I wanted to change the texture of the light and dark side of earth so the user can see the lights of the cities. Is there any way to have the side facing away from the sun have one texture while the side facing toward it have another.

 thanks

Top hit for “Unity Planet Shader”: http://wiki.unity3d.com/index.php?title=Earth/Planet ?