Editing Galactic coordinate system (section)

== Definition ==
{{See also|Galactic plane|Galactic Center|Galactic anticenter}}

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 [[right ascension|RA]] {{nowrap|12<sup>h</sup>&thinsp;40<sup>m</sup>}}, [[declination|dec]] +28° (in the [[B1900.0]] epoch convention) and a 0° longitude at the point where the [[galactic plane]] and [[equatorial plane]] intersected.<ref name="Blaauwetal" />

In 1958, the [[International Astronomical Union]] (IAU) defined the galactic coordinate system in reference to radio observations of galactic neutral [[hydrogen]] through the [[hydrogen line]], changing the definition of the Galactic longitude by 32° and the latitude by 1.5°.<ref name="Blaauwetal" /> In the [[equatorial coordinate system]], for [[Epoch (astronomy)|equinox and equator of 1950.0]], the north galactic pole is defined at [[right ascension]] {{nowrap|12<sup>h</sup>&thinsp;49<sup>m</sup>}}, [[declination]] +27.4°, in the constellation [[Coma Berenices]], with a probable error of ±0.1°.<ref name="Reidetal" /> Longitude 0° is the great semicircle that originates from this point along the line in [[position angle]] 123° with respect to the [[Celestial pole|equatorial pole]]. The galactic longitude increases in the same direction as right ascension. Galactic latitude is positive towards the north galactic pole, with a plane passing through the Sun and parallel to the galactic equator being 0°, whilst the poles are ±90°.<ref name="Binney" /> Based on this definition, the galactic poles and equator can be found from [[spherical trigonometry]] and can be [[axial precession|precessed]] to other [[Epoch (astronomy)|epochs]]; see the table.

{| class="wikitable" style="float: right;"
|+'''[[epoch (astronomy)|J2000.0]] [[equatorial coordinate system|equatorial coordinates]] approximating the galactic reference points'''<ref name="Blaauwetal"/>
| style="background:#e9d66b;"| &nbsp;
|  style="text-align:center; background:#e9d66b;"| '''[[Right ascension]]'''
|  style="text-align:center; background:#e9d66b;"| '''[[Declination]]''' 
|  style="text-align:center; background:#e9d66b;"| '''[[Constellation]]''' 
|- style="text-align:center;"
| style="background:#e9d66b;"| '''North Pole'''<br/>+90° latitude
|12<sup>h</sup>&thinsp;51.4<sup>m</sup>
| +27.13°
| [[Coma Berenices]]<br/><small>(near [[31 Comae Berenices|31 Com]])</small>
|- style="text-align:center;"
| style="background:#e9d66b;"| '''South Pole'''<br/>−90° latitude
|0<sup>h</sup>&thinsp;51.4<sup>m</sup>
| −27.13°
| [[Sculptor (constellation)|Sculptor]]<br/><small>(near [[NGC 288]])</small>
|- style="text-align:center;"
| style="background:#e9d66b;"| '''[[Galactic Center|Center]]'''<br/>0° longitude
| 17<sup>h</sup>&thinsp;45.6<sup>m</sup>
| −28.94°
| [[Sagittarius (constellation)|Sagittarius]]<br/><small>(in [[Sagittarius A]])</small>
|- style="text-align:center;"
| style="background:#e9d66b;"| '''[[galactic anticenter|Anticenter]]'''<br/>180° longitude
| 5<sup>h</sup>&thinsp;45.6<sup>m</sup>
| +28.94°
| [[Auriga (constellation)|Auriga]]<br/><small>(near [[Hipparcos|HIP]] 27180)</small>
|-
|colspan=4|
{|
![[File:Galactic_north_pole.png|160px]]<br/>Galactic north
![[File:Galactic_south_pole.png|160px]]<br/>Galactic south
![[File:Galactic_zero_longitude.png|160px]]<br/>Galactic center
|}
|}

[[File:2MASS LSS chart-NEW Nasa-added quadrants.jpg|thumb|Approx galactic quadrants (NGQ/SGQ, 1–4) indicated, along with differentiating Galactic Plane (containing galactic centre) and the Galactic Coordinates Plane (containing our sun / solar system)]]

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 {{math|''l''{{isup|I}}}} and {{math|''b''{{isup|I}}}} while the new should be designated {{math|''l''{{isup|II}}}} and {{math|''b''{{isup|II}}}}.<ref name=Binney/> This convention is occasionally seen.<ref>For example in {{Cite journal |last=Kogut |first=A. |date=1993 |title=Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps |journal=[[Astrophysical Journal]] |volume=419 |page=1 |arxiv=astro-ph/9312056 |doi=10.1086/173453|bibcode = 1993ApJ...419....1K |display-authors=etal}}</ref>

[[radio astronomy|Radio source]] [[Sagittarius A*]], which is the best physical marker of the true [[Galactic Center]], is located at {{nowrap|17<sup>h</sup>&thinsp;45<sup>m</sup>&thinsp;40.0409<sup>s</sup>}}, {{nowrap|−29°&thinsp;00′&thinsp;28.118″}} (J2000).<ref name="Reidetal"/> Rounded to the same number of digits as the table, {{nowrap|17<sup>h</sup>&thinsp;45.7<sup>m</sup>}}, −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 {{val|56.75|6.20|ul=ly}} north of the midplane, and the heliocentric definition adopted by the IAU, the galactic coordinates of Sgr&nbsp;A* are latitude {{nowrap|+0°&thinsp;07′&thinsp;12″}} south, longitude {{nowrap|0°&thinsp;04′&thinsp;06″}}. Since as defined the galactic coordinate system does not rotate with time, Sgr&nbsp;A* is actually decreasing in longitude at the rate of galactic rotation at the sun, {{mvar|Ω}}, approximately 5.7 [[milliarcsecond]]s per year (see [[Oort constants]]).