The greater the background shift, the further away it is from the front icons. How is this different from parallax in astronomy? In that case, you would measure the change in angular position and ...

Parallax is the observed displacement of an object caused by the change of the observer's point of view. In astronomy, it is an irreplaceable tool for calculating distances of far away stars.

New Windows Phone 8.1 video reveals Start screen backgrounds, parallax view. News. By Daniel Rubino. last updated 21 November 2018.