The galactic plane is the plane in which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. Most often, in actual usage, the terms "galactic plane" and "galactic poles" are used to refer specifically to the plane and poles of the Milky Way, which is the galaxy in which the Earth is located.
Some galaxies are irregular and do not have any well-defined disk. Even in the case of a barred spiral galaxy like the Milky Way, defining the galactic plane is slightly imprecise and arbitrary since the stars are not perfectly coplanar. In 1959 the IAU defined the position of the Milky Way's north galactic pole as exactly RA = 12h 49m, Dec = 27° 24′ in the then-used B1950 epoch; in the currently-used J2000 epoch, after precession is taken into account, its position is RA 12h 51m 26.282s, Dec 27° 07′ 42.01″. This position is in Coma Berenices, near the bright star Arcturus; likewise, the south galactic pole lies in the constellation Sculptor.
The "zero of longitude" of galactic coordinates was also defined in 1959 to be at position angle 123° from the north celestial pole. Thus the zero longitude point on the galactic equator was at 17h 42m 26.603s, −28° 55′ 00.445″ (B1950) or 17h 45m 37.224s, −28° 56′ 10.23″ (J2000), and its J2000 position angle is 122.932°. The galactic center is located at position angle 31.72° (B1950) or 31.40° (J2000) east of north.
- Reid, M. J.; Brunthaler, A. (December 2004), The Proper Motion of Sagittarius A*. II. The Mass of Sagittarius A*, The Astrophysical Journal 616 (2): 872–884, arXiv:astro-ph/0408107, Bibcode:2004ApJ...616..872R, doi:10.1086/424960. See appendix for the numbers listed above.