[h1]Appendix C:  Astronavigation of the Empire[/h1]

The galactic plane is divided into North, North East, East, South East, South, South West, West and North West sectors. Star systems are designated their sectors based on their relative position to the view from the throne room on Terra the day the first Emperor ascended the throne. When a new dynasty takes over the throne all star systems get a new designation.

It's the only reasonable way to handle astronavigation and those alien races are ignorant to not acknowledge it.