The galactic coordinate system (GCS) is a celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy. The "equator" is aligned to the galactic plane. Similar to geographic coordinates, positions in the galactic coordinate system have latitudes and longitudes.
The symbols ℓ and b are used to represent the galactic longitude and latitude, respectively. The galactic longitude is measured in the plane of the galaxy using an axis pointing from the Sun to the galactic center. The galactic latitude is measured from the plane of the galaxy to the object using the Sun as vertex.
The International Astronomical Union (IAU) defined the galactic coordinate system in reference to the Equatorial coordinate system in 1958 The north galactic pole is defined to be at right ascension 12h 49m, declination +27.4° (B1950), and the zero of longitude is the great semicircle that originates from this point along the line in position angle 123° with respect to the equatorial pole. The galactic longitude increases in the same direction as right ascension. Galactic latitude is positive towards the north galactic pole, the poles themselves at ±90° and the galactic equator being zero.
The equivalent system referred to as J2000 has the north galactic pole at 12h 51m 26.282s +27° 07′ 42.01″ (J2000) (192.859508, 27.128336 in decimal degrees), the zero of longitude at the position angle of 122.932°. The point in the sky at which the galactic latitude and longitude are both zero is 17h 45m 37.224s −28° 56′ 10.23″ (J2000) (266.405100, -28.936175 in decimal degrees). This is offset slightly from the radio source Sagittarius A*, which is the best physical marker of the true galactic center. Sagittarius A* is located at 17h 45m 40.04s −29° 00′ 28.1″ (J2000), or galactic longitude 359° 56′ 39.5″, galactic latitude −0° 2′ 46.3″.
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