The Galaxy is unspeakably vast, and only a fraction of it has been properly mapped. Most of the settled portions of space known to the Empire run along the major Hyperspace Lanes where their proximity to interstellar traffic has increased their chance of discovery.
Though every celestial body in the galaxy is constantly in motion, the position of these worlds relative to one another remain mostly fixed. Part of the job of Astrogation computers is to safely calculate the drift caused by the never-ending spiraling rotation of the galaxy.
Click on the map for a larger image.