Any tutorials on moving background out there?

I'm basically doing a space invaders shooting type thing, but I want the stars to be moving from top to bottom in the background. 

I figure using 2 sprites or something would be the best way right?  Do you make the canvas bigger than the screen size so you have "play room" 

What is the best method for making a moving background.  Thank you. 

see one of these examples https://groups.google.com/d/msg/app-inventor-developers-library/TwW0TZLFzp4/Ec28ONkoFH8J