Connecting a texture with a light
 

Hopefully sombody might know this!
I made a rotating planet. On one side of the planet is daylight on the otherside is nighttime (I used a spotlight as the sun source). I put an Incandescense map on the planet to show cities lit up at night. The Incandescense map is also showing up in the Daytime part. Is there a way to connect the light source to the Incandescense map and make the map dissapear when the light hits it? So in other words I want the cities lights to go out when its on the sun side and go back on when it hits the night side.

I need the light source to move and controll the texture because I might need the sun to move around the earth. Thank you.