Definition
See also: Galactic plane and Galactic centerIn 1958, the International Astronomical Union (IAU) defined the galactic coordinate system in reference to the 21-cm radio emissions of galactic neutral hydrogen, replacing a system first defined in 1932. In the equatorial coordinate system, for equinox and equator of 1950.0, the north galactic pole is defined at right ascension 12h 49m, declination +27°.4, in the constellation Coma Berenices, with a probable error of ±0°.1. Longitude 0° 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 galactic equator being 0°, the poles ±90°. Based on this definition, the galactic poles and equator can be found from spherical trigonometry and can be precessed to other epochs; see the table.
| right ascension | declination | constellation | |
| north pole (+90° latitude) | 12h 49m
(12h 51m.4) |
+27°.4
(+27°.13) |
Coma Berenices |
| south pole (-90° latitude) | 0h 49m
(0h 51m.4) |
-27°.4
(-27°.13) |
Sculptor |
| galactic center (0° longitude) | 17h 42m.4
(17h 45m.6) |
-28°.92
(-28°.94) |
Sagittarius |
| anti-center (180° longitude) | 5h 42m.4
(5h 45m.6) |
+28°.92
(+28°.94) |
Auriga |
Radio source Sagittarius A*, which is the best physical marker of the true galactic center, is located at 17h 45m 40s.0409, -29° 00' 28".118 (J2000). Rounded to the same number of digits as the table, 17h 45m.7, -29°.01 (J2000), there is an offset of about 0°.07 from the defined coordinate center, well within the 1958 error estimate of ±0°.1.
Read more about this topic: Galactic Coordinate System
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