1) map vzethdev the sky is not the problem
2) to determine what is currently available you will need:
coordinates of the user
- direction of view
3) in order to do all the really cool need to learn to look for the level of the horizon, then it will be possible for him to do already. There will, again, work with the accelerometer and compass.
And about the option of analysis of stars from the camera can be easily forgotten.