Thegalactic plane is theplane on which most of a disk-shapedgalaxy'smass lies. The directionsperpendicular to the galactic plane point to thegalactic poles. In actual usage, the termsgalactic plane andgalactic poles usually refer specifically to the plane and poles of theMilky Way, in which PlanetEarth is located.
Somegalaxies are irregular and do not have any well-defined disk. Even in the case of abarred spiral galaxy like the Milky Way, defining the galactic plane is slightly imprecise and arbitrary since the stars are not perfectlycoplanar. In 1959, theIAU defined the position of the Milky Way's north galactic pole as exactlyRA =12h 49m,Dec = 27° 24′ in the then-usedB1950epoch;[citation needed] while in the currently-usedJ2000 epoch, afterprecession is taken into account, its position is RA12h 51m 26.282s, Dec 27° 07′ 42.01″.[citation needed] This position is inComa Berenices, near the brightstarArcturus; likewise, the south galactic pole lies in theconstellationSculptor.
Thezero of longitude ofgalactic coordinates was also defined in 1959 to be at position angle 123° from the northcelestial pole. Thus the zero longitude point on the galactic equator was at17h 42m 26.603s, −28° 55′ 00.445″ (B1950) or17h 45m 37.224s, −28° 56′ 10.23″ (J2000), and its J2000 position angle is 122.932°. TheGalactic Center is located atposition angle 31.72° (B1950) or 31.40° (J2000) east of north.
