Based on where you live it shows you the stars where you point your mobile, even if it's below the horizon. In the evening I was looking below my feet at the suns location. Its great for a novice star spotter who whats to know what that brighter light in the sky is beside the moon, it turned out to be Jupiter! Sometimes it does judder, but thats more to do with the phone compass than the app. I find it best to look around the horizon first then raise the phone. You do get an advert when you start up ....... not a problem on a free app.