Hoi, I use Skysafari 7 on my Macbook i like to give you feedback that zooming with two finger works but is no ideal. Please consider supporting the Apple mouse, a function with a slider which can be used with a mouse or finger if you have a touch screen.
Most of these kinds of mapping apps have this
you can have near "time" a zoom function, the slider which appears like time but than a slider.
Any way take your advance with this feedback