help with motion path animation

Hi everyone! 

I'm emulating some cool interactions for a portfolio demo and have run across a problem I can't wrap my head around. 

This interaction is set up so that when you click the left and right arrows you "move" the little moon shape left or right (what's actually happening is a layer is shown and the background group of shapes moves along a motion path)

What I want to happen is that when the moon shape reaches the end of the background, a layer appears or something happens. I just can't figure out what trigger to set to make this happen! 

Hope I'm making sense - I've attached the story file. Thanks in advance!

 

Sue

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