Talk:Galactic coordinate system

message: "As objects participate in this rotation their galactic coordinates will remain constant. However as these objects move about within this rotation their galactic coordinates will change."  Can this be worded a bit better?  It seems to arbitrarily state a duality where none exists, and is just plain overwordy. 22:55, 25 August 2006 (UTC)
replies: text: I'm sure the wording could be improved; it does sound a bit awkward. However the distinction is an important one: that be tween proper motion and improper motion. It's an issue in any coordinate system, but I was tyring to convey how typically  objects within our galaxy will be fixed relative to the galactic coordinate system, but ojbect outside our galaxy, like the Andromeda galaxy, will move relatively quickly through the coordinate sysstem, and that this distinction is due to the definition of the cooredinate system. --Cplot 18:31, 26 August 2006 (UTC)
thread_title:  Rewording 

message: Fellow coordinate fans: I'd like to propose an extensive rewrite of the galactic coordinates article.  Galactic coordinates are a means of expressing directions in the sky as seen from the solar system and earth, nothing more and nothing less; the present article has much to recommend it but drifts off into inaccurate, irrelevant, and confusing tangents about whether the system is rotating, what objects rotate with the system, and so on.  I think the confusions evident in the previous item on this page was exacerbated by the lack of clarity of the article as it stands.  The angular speed of the galaxy is so very slow that the motion of the system is ignored in its definition; practicing astronomers never use Galactic coordinates when high precision is required anyway.  I'd like to restructure the article to make it a little more didactic (I realize this is an encyclopedia, not a textbook), more accurate, and less misleading.  I'm a relatively inexperienced Wikipedia editor, so I'm posting this before boldly proceeding to allow for commentary.  So, any comments?  I'm having trouble logging on properly from home, but I'll ID myself here as user Jthorstensen.
:I agreed so much I actually did it.  I think some example positions could be added, and maybe a better graphic showing the equatorial and galactic schemes.  Anyone who wants to add some of the removed material back, please think carefully:  most of it honestly should be in other articles.  Do not be afraid of wikilinks!  mdf 15:06, 23 March 2007 (UTC)
replies: text: There is also the older "Ohlsson system", mentioned in the source that defines the coordinate system.  I can't find any reference to its definition online though, but would make an excellent addition to this article.  mdf 15:21, 23 March 2007 (UTC)
replies: text: More time to search:  Ohlsson, J. 1932, Lund. Ann. No. 3.  Probably a publication of Lund Observatory (http://www.astro.lu.se/), but they don't have it online (emailed).  Gum, Kerr and Westerhout's "A 21-cm determination of the principal plane of the galaxy", Monthly Notices of the Royal Astronomical Society, Vol. 121, p.132, cites the Ohlsson pole as α = 12h40m, δ = +28 (B1900), but no details the central meridian.  mdf 21:48, 23 March 2007 (UTC)
thread_title:  Proposed Major Revision 

message: This article pretends that there is only one system of galactic coordinates centered on the sun.  This is flatly wrong.
cf http://www.daviddarling.info/encyclopedia/G/galactic_coordinates.html
Coordinates based on the plane of the Milky Way Galaxy, which is inclined about 63° to the celestial equator, and centered on the Sun, with the zero point of longitude and latitude pointing directly at the galactic center. Before 1958, the zero point of galactic latitude and longitude was taken to lie at R.A. 17h 45.6m, Dec. -28° 56.2' (in Sagittarius). Galactic latitude (b) is measured from the galactic equator north (+) or south (-); galactic longitude (l) is measured eastward along the galactic plane from the galactic center.
In 1958, because of increased precision in determining the location of the galactic center, based on observations of the 21-centimeter line, a new system of galactic coordinates was adopted with the origin at the galactic center in Sagittarius at R.A. 17h 42.4m, Dec. -28° 55' (epoch 1950). The new system is designated by a superior Roman numeral II (i.e., bII, lII) and the old system by a superior Roman numeral I. 
) 05:20, 9 November 2009 (UTC)
thread_title:  Major Revision Needed  

message: There's a link on this page to the "Galactic Equator". Unfortunately, that link points straight back to the same article via redirect. This link should be removed. --Headrock (talk) 22:41, 9 April 2008 (UTC)
thread_title:  Redundant Link 

message: The zero of longitude for Galactic coordinates as currently defined as a point near the Galactic Center.  
Imagine defining Ecliptic coordinates that way, with longitude zero at the Sun.  It would not make sense 
because the record of locations of objects in that coordinate system would be forever changing with the 
motion of the Earth around the Sun.  
Yet in just that way, the locations of exterior galaxies in galactic coordinates are now changing each 
year with the motion of the Solar System around the Milky Way galaxy.  All the Galactic coordinates of 
all the millions of galaxies changing every year in the present, obsolete system.
Instead, the zero of longitude for the ecliptic was set some time ago at what is called the First Point 
of Aries, which was, then, the location of the Sun during the spring or Vernal equinox.
The Solar system has a 220 million year orbit around the galaxy. That is so long it leads to the assumption
that placing the zero of Longitude at the Galactic Center provides coordinates that are sufficiently stable 
for all foreseeable purposes.  Yet already the increasing stability and accuracy of measurements routinely
determine determines positions of objects within fractions of an arcsecond.
Human beings should not plan on waiting millions of years to establish a galactic coordinate system in 
which those coordinates are stable.  It is in this recent epoch, since about ten thousand years ago, 
that Earth is first beginning to determine its activities with a confident understanding of astronomy.
"Only" 65 million years ago Andromeda was in conjunction with the Galactic Center, and a million years
hence the events of present constructions will still be in evidence.
Instead, the zero of longitude for the galactic coordinates should be set at or near the Andromeda Galaxy,
which is a wonderfully visible bright galaxy that will be obvious to human beings for the next hundred 
million years and more, probably many orbits of the solar system around the Milky Way.  Then the galactic 
coordinates will be stable, anchored to a well known exterior objects.  
Just like the zero of Ecliptic coordinates are anchored to the First Point of Aries.
NASA has a fine detail image of a small region of deep space very close to the big M31 galaxy.  
Accurately located, famously popular, it is usable as reference near the zero of longitude  
for a new galactic coordinate system, if the Andromeda Galaxy itself is not.  
Even more, modern geological history is depicted in much the kind of time scale as that in which
Galactic rotation and the motions of the solar system are described.  The proposed reference for
Galactic Longitude will bring to the surface any events that have been influenced by the exterior
galaxies. 
The system of Ecliptic Coordinates which have been preserved through centuries of practice because they 
are based on fundamentally sound understanding of the dynamical of relationships between the solar system,
ecliptic and the exterior stars.  The most important exterior references are those which do not move at 
all or move very little.    
SyntheticET (talk) 22:25, 15 April 2008 (UTC)
replies: text: This isn't really the appropriate place to discuss revision of this standard. But if you are going to baseline it on a distant object, why not M87 at the center of the Virgo cluster (which is near the middle of the Virgo Supercluster)? That has the benefit of remaining fairly stationary over long timelines.—RJH (talk) 17:51, 5 October 2009 (UTC)
thread_title: ANDROMEDA GALAXY AS THE ZERO OF LONGITUDE FOR GALACTIC COORDINATES

message: and dropped it in Talk:Milky_Way for now. It is interesting, but of little relevance to the galactic coordinate system. Tfr000 (talk) 20:02, 23 July 2012 (UTC)
replies: text: The direction of rotation within the coordinate system is a crucial information missing here. I.e. is the Sun moving towards 90 or the 270 degree longitude (btw, this information is missing in the moved section, too, so it is indeed useless for this article in its current form)?--SiriusB (talk) 13:13, 27 November 2014 (UTC)
replies: text: The portion of the milky way where the sun is located is rotating towards the direction of Cygnus (90° galactic longitude).  --Lasunncty (talk) 12:28, 28 November 2014 (UTC)
thread_title:  removed off-topic section Galactic rotation 

message: Using the 1950 directions for Galactic North and Galactic Center, 
:GalacticNorth1950 = {12h, 49m, 0s, 27.4deg}
:{3.3554 radians polar, 0.47822 radians azimuthal}
:Direction vector: {-0.867601, -0.449722, -0.212178}
:
:
:GalacticCenter1950 = {17h 42.4m 0s, -28.92deg]}
:{4.63559 radians polar, -0.504749 radians azimuthal}
:Direction Vector: {0.456268, -0.850796, -0.062388}
:
I found that there is only a 75 degree angle between the vector toward the galactic center and the vector toward the galactic north pole.  i.e. they are not perpendicular.  What is commonly used in galactic coordinates, then?  A great circle in a plane perpendicular to the galactic north pole, or a great circle slanted 15 degrees toward the galactic center?  — Preceding unsigned comment added by JDoolin (talk • contribs) 18:34, 10 August 2013 (UTC)
replies: text: SIMBAD NGP J2000 Coordinates: 12h 52m 26.275s +27 07 41.70 Gal Coordinates 328.040482 +89.999997 Morbas (talk) 00:14, 24 April 2023 (UTC)
thread_title:  North Pole and Galactic Center not perpendicular. (Yes they are, sorry.) 

message: I believe the section Rectangular coordinates completely misreads the given source 1987AJ.....93..864J, which uses UVW to denote the velocity vectors of a star in a moving group, more specifically the Ursa Major Group (UMaG). Also: whether a left or right handed coordinate system is used, is not associated "to UVW" and "to author" in the source, I instead suspect this pertains to necessities regarding the historical form of available data, and is equally valid for xyz as for UVW, this is not a matter of author personal choice. Rursus dixit. (mbork3!) 18:37, 11 August 2013 (UTC)
thread_title:  Rectangular coordinates error 

message: The caption to the illustration showing the galactic latitude angle states:
"while the galactic latitude (b) measures the angle of the object above the galactic plane"
This is fundamentally incorrect because the angle is measured as observed from Earth, but Sol is not on the Galactic plane, it is some 20 parsecs above the plane. 
The Galactic Latitude is the angle between the target object an imaginary plane based on the observed location of Sagittarius A* (or, if you prefer the IAU defined center of the galaxy from Earth, and perpendicular to the galactic pole, as defined by the IAU convention.
This is an arbitrary designation, and is only somewhat similar to the actual galactic plane because Sol is relativaly close to the plane.
If someone else can word this better please do so.  — Preceding unsigned comment added by 198.177.6.246 (talk) 21:18, 27 April 2015 (UTC)
thread_title:  Bad Caption, Bad! 

message: You use the term galactic north in the intro, but it's difficult to see what you mean. I know the right-hand-rule, so I figured it out.  But most newbies reading that intro paragraph would be lost. Adding a north arrow to one of those first graphics would clarify things greatly. Pb8bije6a7b6a3w (talk) 03:04, 25 October 2015 (UTC)
thread_title:  NEED A GRAPHIC SHOWING NORTH 

message: The the section "Rectangular Coordinates", the current text says "n 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 anti-center (l = 180°), and it is a left-handed system (positive towards the west and towards the north galactic pole)." That is not supported by the source cited, which says "We will use a right-handed coordinate system for U, F, and Wf so that they are positive in the directions of the Ga- lactic center, Galactic rotation, and the North Galactic Pole (NGP), respectively. Use of a right-handed system enables the same matrix to be used to transform both coordinates and velocities. Also, a right-handed system seems preferable on pedagogical grounds. Some authors prefer a left-handed system for £/, F, and W, in which U is positive toward the Galactic anticenter." That is, the source says that the difference between the systems is that the direction of the U co-ordinate (corewards) is reversed, not that the direction of the V co-ordinate (east) is reverse. Reversing both would reverse chirality twice, thus returning to a right-handed chirality, but the source is explicit that the second system is left-handed. The assertion in existing Wikipedia text that the left hand system has a co-ordinate that is positive to the west is therefore unsupported by the source and contrary to reason. I shall change it by replacing the word "west" with "east".
Agemegos (talk) 00:16, 13 April 2017 (UTC)
thread_title:  CHIRALITY OF TWO VERSIONS OF RECTANGULAR CO-ORDINATES 

message: Someone who really understands this needs to include an explanation of the definition of galactic "north" in this coordinate system, which is what I came here for. It's obviously confused a lot of people here on the talk page, too. From graphics in other articles, it appears that galactic north has been defined in such a way as to violate the right-hand rule. If it obeyed the right hand rule, a view from galactic north would have the galaxy spinning counterclockwise (just as, viewed from the north pole, Earth spins counterclockwise), but images in other articles are quite definite that they are viewed from the north and the rotation is clockwise. If this is really true, that galactic north has been defined backwards from the way we define north with respect to planets, there needs to be a paragraph explaining it with a citation as to why it was done this way, ideally with links to alternative coordinate systems. Example image, used in Milky Way: https://en.wikipedia.org/wiki/Milky_Way#/media/File:Milky_Way_Arms.svg - caption is pretty clear, but has no explanation for the anomaly.
Hammerquill (talk) 21:08, 7 September 2019 (UTC)
replies: text: The IAU defines a planet's north pole as the pole lying in the same ecliptic hemisphere as Earth's north pole. Presumably something similar applies to the galaxy as well. It has nothing to do with rotation. They use the right hand rule to define what is called the positive pole. You are correct that the Milky Way's south pole is the positive one, and the north pole is the negative one. --Lasunncty (talk) 06:29, 8 September 2019 (UTC)
thread_title:  Need a definition of Galactic North with an actual graphic and explanation 

message: I was always taught that "Galactic" should be capitalized when used to refer to our Milky Way galaxy. In fact, the article on Galaxy actually notes this. I'll note that other wikipedia pages seem horribly inconsistent on this topic; however, I do believe it is common best practice in professional astronomy journals. 
DrCrisp (talk) 06:32, 4 December 2019 (UTC)
thread_title:  Capitalization of "Galactic"