How to "zoom out" in Star Walk?

When you first install Star Walk (I use it on iphone 3GS) and you use it to look at the night sky, objects on the screen look pretty distant. You can double-tap the screen to zoom in a little at a time, if you want.

But how the EFF do you zoom out? I have it zoomed all the way in, so that a star like Sirius has a nice big graphic, and I cannot get it to zoom back out to how it was at first. I can't find anything in the tiny help section about this, nor on the company's website. Please help!
 
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