Galactic coordinate system

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Artist's depiction of the Milky Way Galaxy, showing the galactic longitude. A vector matching the plane of the galaxy, at 0° longitude, notably has the Galactic Center and intersects arms directly beyond. Far less of the galaxy lies at all points with opposing 180° longitude

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 parallel to an approximation of the galactic plane but offset to its north. It uses the right-handed convention, meaning that coordinates are positive toward the north and toward the east in the fundamental plane.[1]

Spherical coordinates[edit]

Galactic longitude[edit]



Galactic latitude[edit]

Latitude (symbol b) measures the angle of an object northward of the galactic equator (or midplane) as viewed from Earth. Analogous to terrestrial latitude, galactic latitude is usually measured in degrees (°).


The first galactic coordinate system was used by William Herschel in 1785. A number of different coordinate systems, each differing by a few degrees, were used until 1932, when Lund Observatory assembled a set of conversion tables that defined a standard galactic coordinate system based on a galactic north pole at RA 12h 40m, dec +28° (in the B1900.0 epoch convention) and a 0° longitude at the point where the galactic plane and equatorial plane intersected.[1]


Equatorial coordinates J2000.0 of galactic reference points[1]
  Right ascension Declination Constellation
North Pole
+90° latitude
12h 51.4m +27.13° Coma Berenices
(near 31 Com)
South Pole
−90° latitude
0h 51.4m −27.13° Sculptor
(near NGC 288)
0° longitude
17h 45.6m −28.94° Sagittarius
(in Sagittarius A)
180° longitude
5h 45.6m +28.94° Auriga
(near HIP 27180)
Galactic north pole.png
Galactic north
Galactic south pole.png
Galactic south
Galactic zero longitude.png
Galactic center

The IAU recommended that during the transition period from the old, pre-1958 system to the new, the old longitude and latitude should be designated lI and bI while the new should be designated lII and bII.[3] This convention is occasionally seen.[4]

Radio source Sagittarius A*, which is the best physical marker of the true galactic center, is located at 17h 45m 40.0409s, −29° 00′ 28.118″ (J2000).[2] Rounded to the same number of digits as the table, 17h 45.7m, −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°. Due to the Sun's position, which currently lies 56.75±6.20 ly north of the midplane, and the heliocentric definition adopted by the IAU, the galactic coordinates of Sgr A* are latitude +0° 07′ 12″ south, longitude 0° 04′ 06″. Since as defined the galactic coordinate system does not rotate with time, Sgr A* is actually decreasing in longitude at the rate of galactic rotation at the sun, Ω, approximately 5.7 milliarcseconds per year (see Oort constants).

Conversion between equatorial and galactic coordinates[edit]

An object's location expressed in the equatorial coordinate system can be transformed into the galactic coordinate system. In these equations, α is right ascension, δ is declination. NGP refers to the coordinate values of the north galactic pole and NCP to those of the north celestial pole.[5]

The reverse (galactic to equatorial) can also be accomplished with the following conversion formulas.

Rectangular coordinates[edit]

In some applications use is made of rectangular coordinates based on galactic longitude and latitude and distance. In some work regarding the distant past or future the galactic coordinate system is taken as rotating so that the x-axis always goes to the centre of the galaxy.[6]

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 anticenter (l = 180°), and it is a left-handed system (positive towards the east and towards the north galactic pole).[7]


In the constellations[edit]

The galactic equator runs through the following constellations:[8]

See also[edit]


  1. ^ a b c d Blaauw, A.; Gum, C.S.; Pawsey, J.L.; Westerhout, G. (1960). "The new IAU system of galactic coordinates (1958 revision)". Monthly Notices of the Royal Astronomical Society. 121 (2): 123. Bibcode:1960MNRAS.121..123B. doi:10.1093/mnras/121.2.123.
  3. ^ a b James Binney, Michael Merrifield (1998). Galactic Astronomy. Princeton University Press. pp. 30–31. ISBN 0-691-02565-7.
  4. ^ For example in Kogut, A.; et al. (1993). "Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps". Astrophysical Journal. 419: 1. arXiv:astro-ph/9312056. Bibcode:1993ApJ...419....1K. doi:10.1086/173453.
  5. ^ Carroll, Bradley; Ostlie, Dale (2007). An Introduction to Modern Astrophysics (2nd ed.). Pearson Addison-Wesley. p. 900-901. ISBN 978-0805304022.
  7. ^ Johnson, Dean R.H.; Soderblom, David R. (1987). "Calculating galactic space velocities and their uncertainties, with an application to the Ursa Major group". Astronomical Journal. 93: 864. Bibcode:1987AJ.....93..864J. doi:10.1086/114370.
  8. ^ "SEDS Milky Way Constellations".

External links[edit]