Orbiting Sphere

The trick for making the moon appear to go 'behind' the planet is to split the planet into 2, placing one half if front and one half behind the orbiting Moon.

Finally, go to the timeline and drag the layer containing the top half of the planet so that it is above the Moon layer and the Motion Guide layer (see the illustration).

Note: A different effect can be obtained by reversing the positions of the PlanetTop and PlanetBottom layers.