Galactic coordinates are coordinates based on the plane of the Milky Way Galaxy, which is inclined about 63° to the celestial equator, and centered on the Sun, with the zero point of longitude and latitude pointing directly at the galactic center. Before 1958, the zero point of galactic latitude and longitude was taken to lie at R.A. 17h 45.6m, Dec. -28° 56.2' (in Sagittarius). Galactic latitude (b) is measured from the galactic equator north (+) or south (-); galactic longitude (l) is measured eastward along the galactic plane from the galactic center.
In 1958, because of increased precision in determining the location of the galactic center, based on observations of the 21-centimeter line, a new system of galactic coordinates was adopted with the origin at the galactic center in Sagittarius at R.A. 17h 42.4m, Dec. -28° 55' (epoch 1950). The new system is designated by a superior Roman numeral II (i.e., bII, lII) and the old system by a superior Roman numeral I.