Galactic equator

The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way galaxy, and the fundamental plane approximately in the galactic plane. It uses the right-handed convention, meaning that coordinates are positive toward the north and toward the east in the fundamental plane.[1]

Galactic longitude

Longitude (symbol l) measures the angular distance of an object eastward along the galactic equator from the galactic center. Analogous to terrestrial longitude, galactic longitude is usually measured in degrees ( ° ).

Galactic latitude

Latitude (symbol b) measures the angular distance of an object perpendicular to the galactic equator, positive to the north, negative to the south. For example, the north galactic pole has a latitude of +90°. Analogous to terrestrial latitude, galactic latitude is usually measured in degrees ( ° ).


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.[1] 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.[2] 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°.[3] 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.

Equatorial coordinates B1950.0 / (J2000.0) of galactic reference points[1]
  right ascension declination constellation
north pole (+90° latitude) 12h 49m

(12h 51m.4)



Coma Berenices
south pole (-90° latitude) 0h 49m

(0h 51m.4)



galactic center (0° longitude) 17h 42m.4

(17h 45m.6)



anti-center (180° longitude) 5h 42m.4

(5h 45m.6)




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).[2] 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.

Rectangular coordinates

There are two major rectangular variations of galactic coordinates, commonly used for computing space velocities of galactic objects. In these systems the xyz axes are designated UVW, but the definitions vary by author. In one system, the U axis is directed toward the galactic center (l = 0°), and it is a right-handed system (positive towards the east and towards the north galactic pole); in the other, the U axis is directed toward the galactic anti-center (l = 180°), and it is a left-handed system (positive towards the west and towards the north galactic pole).[4]

In the constellations

The galactic equator runs through the following constellations:[5]

See also


External links

  • Equatorial/Galactic conversion tool.
  • Galactic Coordinate System - Wolfram Demonstration
  • Internet Encyclopedia of Science
  • Universal coordinate converter.
  • Fiona Vincent, University of St Andrews
  • Star Map in Galactic Perspective
  • An Atlas of the Universe