Galactic Coordinate System - Definition

Definition

In 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.

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.