Editing Supergalactic coordinate system

[[File:SGP(50mln-ly.png|thumb|Galaxies and galaxy clusters {{nobr|< 50 [[mega-|M]][[light-year|ly]]}} away from Earth plotted in the supergalactic plane]]

The '''supergalactic coordinate system''' is a [[reference frame]] for the [[supercluster]] of [[galaxy|galaxies]] that contains the [[Milky Way]] galaxy, referenced to a local relatively flat collection of [[Galaxy cluster|galaxy clusters]] used to define the '''supergalactic plane.'''

The supergalactic plane is more or less [[perpendicular]] to [[galactic plane|the plane]] of the Milky Way; the angle is 84.5°. As viewed from Earth, the plane traces a [[great circle]] across the sky through the following [[constellation]]s:
{{div col begin|colwidth=13em}}
* [[Cassiopeia (constellation)|Cassiopeia]] (in the Milky Way [[galactic plane]])
* [[Camelopardalis]]
* [[Ursa Major]]
* [[Coma Berenices]] (near the Milky Way [[galactic pole|galactic north pole]])
* [[Virgo (constellation)|Virgo]]
* [[Centaurus]]
* [[Circinus]] (in the galactic plane)
* [[Triangulum Australe]]
* [[Pavo (constellation)|Pavo]]
* [[Indus (constellation)|Indus]]
* [[Grus (constellation)|Grus]]
* [[Sculptor (constellation)|Sculptor]] (near the [[galactic pole|galactic south pole]])
* [[Cetus]]
* [[Pisces (constellation)|Pisces]]
* [[Andromeda (constellation)|Andromeda]]
* [[Perseus (constellation)|Perseus]]
{{div col end}}

==History==
In the 1950s the astronomer [[Gérard de Vaucouleurs]] recognized the existence of a flattened “local supercluster” from the [[Shapley-Ames Catalog]] in the environment of the Milky Way. He noticed that when one plots nearby galaxies in 3D, they lie more or less on a plane. A flattened distribution of [[nebula]]e had earlier been noted by [[William Herschel]]. [[Vera Rubin]] had also identified the supergalactic plane in the 1950s, but her data remained unpublished.<ref>{{cite news |last=Scoles |first=Sarah|date=4 October 2016|title=How Vera Rubin confirmed dark matter |magazine=[[Astronomy (magazine)|Astronomy]] |url=http://www.astronomy.com/news/2016/10/vera-rubin}}</ref> The plane delineated by various galaxies defined in 1976 the equator of the supergalactic coordinate system de Vaucouleurs developed. In years thereafter with more observation data available de Vaucouleurs' and Rubin's findings about the existence of the plane proved right.

Based on the supergalactic coordinate system of de&nbsp;Vaucouleurs, surveys<ref>{{Cite arXiv |last=Lahav |display-authors=etal |first=O. |date=1998 |title=The Supergalactic Plane revisited with the Optical Redshift Survey |eprint=astro-ph/9809343}}</ref> in recent years determined the positions of nearby [[galaxy cluster]]s relative to the supergalactic plane. Amongst others the [[Virgo cluster]], the Norma cluster (including the [[Great Attractor]]), the Coma cluster, the [[Pisces-Perseus supercluster]], the Hydra cluster, the Centaurus cluster, the Pisces-Cetus supercluster and the Shapley Concentration were found to be near the supergalactic plane.

== Definition ==
The supergalactic coordinate system is a [[spherical coordinate system]] in which the [[equator]] lies in the supergalactic plane.

By convention, supergalactic latitude is usually abbreviated SGB, and supergalactic longitude as SGL, by analogy to {{math|''b''}} and {{math|''l''}} conventionally used for [[galactic coordinates]]. [[File:GalEclipt.png|thumb|Supergalactic and Ecliptic plane]]
* The zero point (SGL = 0°, SGB = 0°)<ref>{{Cite web|last=Abarkahkašân|first=A.|date=n.d.|title=An Etymological Dictionary of Astronomy and Astrophysics|url=http://dictionary.obspm.fr/index.php?showAll=1&formSearchTextfield=supergalactic|archive-url=|archive-date=|access-date=|website=}}</ref> lies in the constellation [[Cassiopeia (constellation)|Cassiopeia]], coincides with (''l''<sub>x</sub> = 137.37°, ''b''<sub>x</sub> = 0°) and was chosen for convenience to lie along the line where the supergalactic plane intersects with the galactic plane. In J2000 [[Equatorial coordinate system|equatorial coordinates]], this is approximately  [[right ascension|RA]] = 2.82<sup>h</sup>, [[declination|Dec]] = +59.5°.
* The origin essentially coincides with the Earth, because the supergalactic plane is identified as a plane observed from Earth.
* The north supergalactic pole (SGB = 90°) lies in the constellation [[Hercules (constellation)|Hercules]] at galactic coordinates ({{math|''l''<sub>z</sub>}} = 47.37°, {{math|''b''<sub>z</sub>}} = +6.32°), or approximately RA = 18.9<sup>h</sup>, Dec = +15.7°.

The transformation from a triple of Cartesian supergalactic coordinates to a triple of galactic coordinates is
:<math>
  \begin{bmatrix} x \\ y \\ z\end{bmatrix}_\text{gal} =
  \begin{bmatrix}
    \cos l_x \cos b_x & \sin l_z \cos b_z \sin b_x - \sin b_z \sin l_x \cos b_x & \cos l_z \cos b_z \\
    \sin l_x \cos b_x & \sin b_z \cos l_x \cos b_x - \cos l_z \cos b_z \sin b_x & \sin l_z \cos b_z \\
             \sin b_x & \cos b_z \cos b_x \sin\left(l_x - l_z\right)                       & \sin b_z
 \end{bmatrix}
  \begin{bmatrix}x \\ y \\ z\end{bmatrix}_\text{superg}
</math>
[[File:Crossing-Galactic-Supergalactic-planes.png|thumb|Supergalactic and Galactic plane]]
The left column in this matrix is the image of the origin of the supergalactic system in the galactic system, the right column in this matrix is the image of the north pole of the supergalactic coordinates in the galactic system, and the middle column is the cross product (to complete the right-handed coordinate system).

The corresponding [[cartesian coordinate system]] allows points to be specified by coordinates (SGX, SGY, SGZ). In this system the supergalactic z-axis points towards the north supergalactic pole, the supergalactic x-axis points towards the zero point, and the supergalactic y-axis is perpendicular to both.

==See also==
* [[Coordinate system]]
* [[Celestial coordinate system]]

== References ==
{{reflist|25em}}

== External links ==
* {{cite journal |first1=O. |last1=Lahav |first2=B.X. |last2=Santiago |year=2000 |title=The supergalactic plane revisited with the Optical Redshift Survey |journal=[[MNRAS]] |volume=312 |number=1 |pages=166–176 |doi=10.1046/j.1365-8711.2000.03145.x |doi-access=free |arxiv=astro-ph/9809343 |s2cid=17951943 |bibcode=2000MNRAS.312..166L}}
* {{cite web |author=Theuns, Tom |title=Nearest group of galaxies |publisher=[[Durham University]] |department=Institute for Computational Astronomy |website=icc.dur.ac.uk |type=course lecture notes |url=http://www.icc.dur.ac.uk/~tt/Lectures/Galaxies/LocalGroup/Back/galgrps.html |quote=This is a map of galaxies that lie within 20&nbsp;million light years plotted onto the supergalactic plane.}}
* {{cite web |author=Fisher, K.A. |date=17 July 2006 |title=Understanding the supergalactic coordinates |type=pers. site |url=http://fisherka.csolutionshosting.net/astronote/Structure/Supergal/Supergalacticcoords.html}}
* {{cite web |title=The universe beyond 10&nbsp;megaparsecs in Supergalactic coordinates |url=http://134.4.36.101/level5/Dev2/Dev6.html}}
* {{cite book |author=Tully, R.B. |pages=408–419 |year=1989 |section=Support for three controversial claims made by Gérard de&nbsp;Vaucouleurs |editor1=Corwin, H.G. |editor2=Bottinelli, L. |title=The World of Galaxies |publisher=Springer |place=New York, NY |isbn=978-1-4613-9356-6 |doi=10.1007/978-1-4613-9356-6_62}}

{{Celestial coordinate systems}}
{{Galaxy}}

[[Category:Astronomical coordinate systems]]
[[Category:Extragalactic astronomy]]