Multi Element Animation - Spinning Globe around a sun -- Part 2 (Shooting the Moon)
By: David Spake
It can be a bit tricky to animate the moon. You can either asign it a motion path from the instructions listed for setting the entire group in orbit. (The next information after the link you just followed.)
You should start your animating with group elements then move to groups if possible. So we will assign the orbit pattern of the moon first, then return you to your regularly scheduled programing to assign motion to the entire earth/moon group.
However you will need to to be sure to go to the right key frame first. 
ANIMATE -- EDIT -- SCENE TIME (set to 0.0)
With "key object" function set your scene time to .2 seconds. Move the moon to the right of the earth, and rotate it 90 degrees on the Y-axis.
ANIMATE -- CREATE -- KEY OBJECT (select moon only)
MODIFY -- OBJECTS -- MOVE (Ctrl F8) Select moon and place to the right of the earth.
MODIFY -- OBJECTS -- ROTATE (Ctrl F-10) On Y axis rotate 90 degrees.
Repeat these 3 commands: Key Object, Move and Rotate. Moving the moon next above the earth, then to the left, followed by placing it back on it's originating point. Don't forget to rotate the moon, this is what lends the roundness to the paths. If you don't rotate it, the moon will follow straight lines from point to point.