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galactic coordinate system
A coordinate system used to study the structure, surroundings, and contents of the Galaxy. The fundamental circle is the galactic equator and the zero point lies in the direction of the galactic center (in the constellation Sagittarius) as seen from Earth (see illustration). The coordinates are galactic latitude and longitude.The galactic latitude (b) of a celestial body is its angular distance (from 0° to 90°) north (counted positive) or south (counted negative) of the galactic equator; it is measured along the great circle passing through the body and the galactic poles. The galactic longitude (l) of a celestial body is its angular distance (from 0° to 360°) from the nominal galactic center measured eastward along the galactic equator to the intersection of the great circle passing through the body.
The position of zero galactic longitude, i.e. the nominal galactic center, was agreed (1959) by the International Astronomical Union (IAU); it lies at RA 17h 45.6m, dec –28°56′.3 (2000.0). More recent observations suggest the actual galactic center coincides with a radio and infrared souce, Sagittarius A West, a few arc minutes from this nominal position; the nominal center is still used, however, as the zero point for galactic coordinates. The true center lies at l = –3′.34, b = –2′.75.