title: Measuring tilt angle with CMMotionManager
body: <p>Suppose you are holding an iphone/ipad vertically in front of you with the screen facing you, in portrait orientation. You tilt the device to one side, keeping the screen facing you. How do you measure that static tilt angle using CMMotionManager? It seems a simple question which should have a simple answer, yet I cannot find any method that does not disappear into quaternions and rotation matrices.</p>
  
  <p>Can anyone point me to a worked example?</p>