• Start a new movie.
• Insert a new layer. Name the top layer moon and the bottom layer planet.
• Click in frame 1 of the planet layer and draw a large circle. Holding the shift key as you drag out your circle will make it perfectly round. Fill with a radial color gradient to give the appearance of a smooth sphere rather than a flat circle.
• Click in frame 1 of the moon layer and draw a smaller circle. Also give it a radial color gradient fill.

• Select the small sphere and press F8 to convert it to a Graphic symbol. Do not convert the larger sphere to a symbol!
• Add a motion guide to the layer containing the Moon. To add the guide, right-click on the Moon layer, and choose 'Add Motion Guide'. Click in frane 1 if the motion guide layer and draw an ellipse in the Guide Layer (use the null fill), and rotate it a little if you like. This is going to be the path around which the Moon will move.
• Drag the Moon so that it is centred on the path. A small circle will appear at its centre to show that it has locked on.

• Click on frame 1 of the planet layer, then right click in frame 20 and choose Insert KeyFrame.
• Click on frame 1 of the moon layer, then right click in frame 5 and insert a keyframe. Continue to insert KeyFrames in frames 10, 15, and 20 of the moon layer.
• Repeat the step above adding Keyframes to the Motion Guide Layer.
• Click back in frame 1 of the moon layer and at the Frames Panel (Window>Panel>Frames) select Motion from the Tween drop-down box.
• Click in frame 5 and move the moon a quarter way to the front. Click in frame 10 and move it another quarter, and so on until frame 20 you should be back right where you started.
• Add motion tween to frames 5, 10, and 15.
• Now test the movie (Ctrl+Enter). The Moon should move in an elliptical path.

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.

• Select the layer containing the Planet. Draw a contrasting line across the middle, splitting it into two (see illustration). You should be able to select the top half of the Planet and cut and paste it to a new layer (I called this PlanetTop).

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).

• Delete the line you used to split up the Planet.
• The motion guide should now appear to go behind and in front of the Planet.

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