I apologize for posting this question on a physics forum because it has a science fiction origin. However, I thought that real astronomers may have a real science answer to the question, so here goes. Imagine two parties from distant parts of the galaxy in communication with each other. They want to exchange their locations in the galaxy. How do they agree on a mutual coordinate system? In spherical coordinates, the radius from the galactic center is easy. But lattitude and longitude both need a zero degree reference. How to establish that reference? My thought is that the obvious candidate is a line connecting the center of the Milky Way Galaxy with the center of the Andromeda Galaxy.