Say, I have a sphere, and I want the bright side to be one texture/shader and the dark side to be another texture/shader. Is there any way to vary this based simply on how much light is hitting it?

I've done some work with dot products, but the best result I have involved a seem in the sphere where the test pattern changes abruptly; furthermore, there are a lot of assumptions about where everything is, among other problems.

Basically, I'm looking to change the material based on the shading itself (ie, if there were a single color, the changes to that color that cycles makes to describe the object is what I want to base the changes on.)

I'd post my work, but it's getting obscenely complicated and it only looks good from certain angles - (none of which are the camera angles, that was one of the cross products.)

Thanks in advance.