Internal reference system

The internal cartesian reference system is described as follows: $XZ$ is the equatorial plane. $Z$ points towards the vernal equinox point, Aries (♈) where $ra=0^\circ$ . $Y$ points towards the north celestial pole ( $dec=90^\circ$ ). $X$ is perpendicular to both $Z$ and $Y$ and points to $ra=90^\circ$ .

All the positions and orientations of the entities in the scene are at some point converted to this reference system for representation. The same happens with the orientation sensor data in mobile devices.