Editing Galactic plane

{{Short description|Plane on which the majority of a disk-shaped galaxy's mass lies}}
[[File:Femri 5 yr with ecliptic equators.jpg|thumb|[[Milky Way]] with galactic plane, [[ecliptic]] and [[celestial equator]].]]

The '''galactic plane''' is the [[plane (geometry)|plane]] on which the majority of a disk-shaped [[galaxy]]'s [[mass]] lies. The directions [[perpendicular]] to the galactic plane point to the '''galactic poles'''. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usually refer specifically to the plane and poles of the [[Milky Way]], in which Planet [[Earth]] is located.

Some [[irregular galaxy|galaxies are irregular]] and do not have any well-defined disk. Even in the case of a [[barred spiral galaxy]] like the Milky Way, defining the galactic plane is slightly imprecise and arbitrary since the stars are not perfectly [[coplanar]]. In 1959, the [[International Astronomical Union|IAU]] defined the position of the Milky Way's north galactic pole as exactly [[right ascension|RA]] = {{RA|12|49}}, [[declination|Dec]] = {{DEC|27|24}} in the then-used [[B1950]] [[epoch (astronomy)|epoch]];{{citation needed|date=November 2022}} in the currently-used [[J2000]] epoch, after [[precession]] is taken into account, its position is RA {{RA|12|51|26.282}}, Dec {{DEC|27|07|42.01}}.{{Citation needed|date=November 2022}} This position is in [[Coma Berenices]], near the bright [[star]] [[Arcturus]]; likewise, the south galactic pole lies in the [[constellation]] [[Sculptor (constellation)|Sculptor]].

The ''zero of longitude'' of [[Galactic coordinate system|galactic coordinates]] was also defined in 1959 to be at position angle 123° from the north [[celestial pole]]. Thus the zero longitude point on the galactic equator was at {{RA|17|42|26.603}}, {{DEC|−28|55|00.445}} (B1950) or {{RA|17|45|37.224}}, {{DEC|−28|56|10.23}} (J2000), and its J2000 position angle is 122.932°. The [[Galactic Center]] is located at position angle 31.72° (B1950) or 31.40° (J2000) east of north.

{{Multiple image
<!-- Essential parameters -->
| align     = right
| direction = vertical
| width     = 220

<!-- Image 1 -->
| image1    = NGC 4452.jpg
| alt1      = 
| caption1  = This edge-on view of the galaxy [[NGC 4452]] from Earth shows its galactic plane, with the nucleus at the center.

<!-- Image 2 -->
| image2    = NGC891HunterWilson.jpg
| alt2      = 
| caption2  = This edge-on view of the [[spiral galaxy]] [[NGC 891]] shows the profile of a dusty galactic plane.
}}

== See also ==
* [[Galactic coordinate system]]
* [[LHS 1815b]] example of  outside galactic plane exoplanet
* [[Zone of Avoidance]]
* [[supergalactic plane]]

== External links ==
* {{citation | last1=Reid | first1=M. J. | last2=Brunthaler | first2=A. | title=The Proper Motion of Sagittarius A*. II. The Mass of Sagittarius A* | journal=The Astrophysical Journal | volume=616 | issue=2 | pages=872–884 |date=December 2004 | doi=10.1086/424960 | bibcode=2004ApJ...616..872R |arxiv = astro-ph/0408107 }}. See appendix for the numbers listed above.

{{Galaxy}}

[[Category:Galactic astronomy|Plane]]