Galactic coordinate system

The anisotropy of the star density in the night sky makes the galactic coordinate system very useful for coordinating surveys, both those which require high densities of stars at low galactic latitudes, and those which require a low density of stars at high galactic latitudes
This panoramic view encompasses the entire sky as seen by Two Micron All-Sky Survey. The measured brightnesses of half a billion stars (points) have been combined into colors representing three distinct wavelengths of infrared light: blue at 1.2 microns, green at 1.6 microns microns, and red at 2.2 microns. This image is centered on the core of our own Milky Way galaxy, toward the constellation of Sagittarius. The reddish stars seemingly hovering in the middle of the Milky Way's disc -- many of them never observed before -- trace the densest dust clouds in our galaxy. The two faint smudges seen in the lower right quadrant are our neighboring galaxies, the Small and Large Magellanic Clouds.

The galactic coordinate system is a spherical reference system on the sky where the origin is close to the apparent center of the Milky Way, and the "equator" is aligned to the galactic plane. Similar to geographic coordinates, positions in the galactic coordinate system have latitudes and longitudes.


The International Astronomical Union (IAU) defined the galactic coordinate system in reference to the Equatorial coordinate system in 1959.[1] The north galactic pole is defined to be at right ascension 12h49m, 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 J2000 has the north galactic pole at 12h51m26.282s +27°07′42.01″ (J2000), the zero of longitude at the position angle of 122.932°.[2] The point in the sky at which the galactic latitude and longitude are both zero is 17h45m37.224s −28°56′10.23″ (J2000). 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 17h45m40.04s −29°00′28.1″ (J2000), or galactic longitude 359°56′39.4″, galactic latitude −0°2′46.2″.


The symbols l and b are used to represent the galactic longitude and latitude, respectively.

See also

* Galaxy formation and evolution

* disc (galaxy)

* bulge (astronomy)

* galactic halo

* galactic corona


1. ^ User Manual: The Galactic Coordinate System. Where is M13?. Think Astronomy (2007). Retrieved on 2008-02-07.

2. ^ Reid, M.J.; Brunthaler, A. (2004 2004). "The Proper Motion of Sagittarius A*". The Astrophysical Journal 616 (2): 883. The American Astronomical Society. doi:10.1086/424960. Retrieved on 2008-02-07. 


* Equatorial/Galactic conversion tool.

* Universal coordinate converter.


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