Casting shadow of one sphere onto another

I’m trying to create a demonstration of eclipses for a school by casting the shadow of one sphere (planet) onto another. I’m using the spotlight to simulate the sum, but it doesn’t seem to be casting the shadow. Am I using the right type of light source and settings?

Make sure that you have shadow and Buf. shadowor or Ray shadow activated. Also I your using AO the shadow isn’t really that obvious. Could you show a picture?