Talk:Galactic coordinate system

The image made by Robert Hurt can be confusing when used as an example of the Galactic Coordinate System. Hurt uses the longitude angles in association with delineations in light years to create a polar coordinate system which defines the grid for the image. —Preceding unsigned comment added by 216.106.16.42 (talk) 10:59, 23 November 2009 (UTC)Reply

I've heard that our solar system is not directly on the plane but is slowly vibrating up and down through the galactic plane. Could anyone who knows more on this subject add factual information on this. 82.132.139.215 (talk) 15:09, 8 October 2009 (UTC)Reply

True, but not relevant to this article. - Rod57 (talk) 22:18, 4 December 2015 (UTC)Reply

If you consider the sun's orbit in three dimensions to be slightly tilted to the galactic plane, there will be times when the solar system is "above" the reference plane (currently by ~56·75 ± 6·20 light years), times when it is "below" and times (most recently ~2,500,000 ± 300,000 years ago) when it crosses that plane. The best estimate of the vertical component of solar motion is +16,220 ± 805 mph.

Although, as Rod57 says, that is not directly relevant to this article, it must be taken into account when deciding whether to use a 2D or 3D representation of the galaxy as it affects the true angle between the NGP and Sgr A* (90° in the 2D model, ~90·12° ± 0·03° in the 3D one). Astronomy Explained (talk) 23:43, 14 February 2017 (UTC)Reply

"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)

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)Reply

Assume your location is anywhere that your galactic latitude is +90 (Northpole, at this point the longitude is irrelavent). You are observing a point at latitude 0 and longitude 0 (with a distance from origin greater than zero obviously). The point you are looking at moves in the direction of positive longitude. Do you see it moving clockwise or counter clockwise? If someone can just provide the simple answer, I will be happy to add it to the content of the page. Thanx.

This may not sound simple. The Sun and entire Solar System orbit the Galactic Center together once in 220 million years or so,

with the interval imperceptibly diminishing.

If the galaxy were a carousel on a moving train Andromeda's Galaxy would be dead ahead on the tracks and your horse would be moving 31.17 degrees to the right relative to the train. At the present time Andromeda and the Galactic Center are 121.17 degrees apart along the Galactic Equator. About 19,000,000 years ago Andromeda and the GC were 90 degrees from each other and Earth was moving fastest toward M31. Sixty five million years ago when the dinosaurs disappeared NGC 224 was in conjunction with the Galactic Center. (Maybe the 'asteroid' was UGC 454 suddenly devolving.) Visually, Andromeda has been where it is now for billions of years while the Galaxy rotates.

Just now, Andromeda is -21.57 degrees in galactic latitude away from the plane of the Galaxy. The Milky Way Galaxy itself is moving in its entirety toward Andromeda. The Sun and nearby stars are moving and at a point in Galactic orbit somewhat past 90 degrees from the GC. The two velocities -- Galactic and Solar -- add vectorially. At this time the Sun is 31.17 degrees past the point where the velocity dead ahead is greatest.

About as far toward Casseopeia from as Deneb as Deneb is from Sadr and on the Galactic Equator is a point just 90 degrees from the Galactic Center. Relative to the Galactic Center that's where the Sun is heading right now. The direction changes slowly as the Milky Way Galaxy rotates.

Here's how to get a sense of it from the Earth and sky:

First get your mind ready with strongly contrasting images of great periods of time in the past and time in the future. Perhaps consider the building of the Pyramids. Then visualize a future images such as manned exploration of the solar system. Consider too what a small angle like six arcseconds means in an expensive optical system, say ESO's interferometer. Galactic rotation is only about six arcseconds in a thousand years. Get a good idea of what these are into your mind and make it substantial.

You should have a clear dark sky when the the Andromeda Galaxy and the Galactic Center are rising. Fortunately they rise nearly together.

Orient yourself in the Earth and facing East. Andromeda is to your left, and the GC is to your right. Now if you will, point your right hand toward the Galactic Center, and your left toward the Andromeda Galaxy.

With luck the Milky Way galaxy will seem to be or seem to have been in motion relative to the Andromeda Galaxy and myriad other less visible background galaxies. Now hang on.

I can add this:

When the Galactic Center in Sagittarius is visible, the part of the Milky Way you see is moving South relative to the invisible exterior galaxies behind it; the Milky Way is the east of Polaris, and Andromeda is east of the Milky Way or about to rise in the Northeast.

When the Milky Way is visible but Sagittarius and the GC are below the horizon, the part of the Milky Way you see is moving North relative to the invisible galaxies behind it, the Milky Way is west of Polaris, and Andromeda is not visible or is soon to set in the NorthWest.

SyntheticET (talk) 20:30, 19 December 2007 (UTC)Reply

We all know that the Earth rotates from west to east, which is anticlockwise when view from the north. I was under the impression that was the true definition of "north", "west" and "east" for all rotating bodies and systems. How is it that the galaxy's rotation is clockwise when viewed from the north? Is it because the Earth's rotation is approximately opposite that of the galaxy? Did someone make a colossal error in judgment when they defined "galactic north"? Now, how are we supposed to decide which way is north for a body whose rotation is tilted relative to Earth's rotation? Now, I am not sure which way east; is it clockwise or anticlockwise when viewed from galactic north. Onerock (talk) 05:26, 6 April 2010 (UTC)Reply

Your definition of East-West-North-South makes sense, but it's not the only possible definition. Astrographers have tended to reverse the conventions of geographers when drawing starcharts, for obvious reasons. Astrographer maps have followed the conventions of geographer maps in placing north at the top and south at the bottom, but have usually placed east at the left and west at the right, again for obvious reasons.

Selenographers have tended to follow the example of astrographers in putting west at the right. But note that an astronaut standing at zero degrees longitude on the moon and looking straight up at the earth, would have to look to the "west" horizon to see a rising star (or a rising sun) and to the "east" horizon to see it set.

Your question about how to decide about a body whose axis is 90° from the earth's is a good one, and I have no definitive answer, other than to point out that it is unlikely to come up in practice. But if it were, let me point out that it is incapable of being 90° in celestial, ecliptic, and galactic coordinates simultaneously.

Rwflammang (talk) 03:40, 22 November 2019 (UTC)Reply

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)Reply

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)Reply

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)Reply

Jthorstensen: I agree. The Galactic coordinate systems are at least in nascent confusion. It would seem that the best system for galactic coordinates would be based on the same general FORMS as have been used in the Ecliptic coordinates, which evolved for thousands of years before settling down to their present form. Galactic coordinates could use the Galactic North and South Poles as the references for Galactic Latitude, and the Andromeda Galaxy as the zero of Galactic Longitude. These would be stable for at least 35 million years, probably throughout the entire 220 million year orbit of the solar system around the Milky Way. The only reason to hurry this would be to minimize the existing early confusion. SyntheticET (talk) 05:20, 9 November 2009 (UTC)Reply

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) —Preceding unsigned comment added by PaulBHartzog (talk • contribs)

The sentence "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" makes me think that 0,0 is the center of the galaxy - yet I am told by others that the center is the Sun/Earth. Could someone who knows what they are doing make this article a little more specific? Thanks. --MatthewKarlsen 16:28, 25 June 2007 (UTC)Reply

It's two dimensional. 0,0 is on the celestial sphere. If you put in a third value, distance, it can name any place in the universe you want but still with a helio/geocentric bias. 0,0,0ly is the Earth.

In the latitude/longitude system 0ºN 0ºE would be like the galactic center and the center of the Earth would be like our position. Sagittarian Milky Way 18:26, 25 June 2007 (UTC)Reply

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)Reply

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)Reply

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)Reply

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)Reply

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)Reply

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)Reply

I think this is an important point. It anchors the mathematical coordinate system to the physical milky way. I think mention should be made in the article that longitude 90 degrees is in the direction of galactic spin, and that 270 degrees is trailing, It would be nice to have pictures of those two cardinal directions along with North, South, Center, and Anticenter. Rwflammang (talk) 01:21, 16 August 2024 (UTC)Reply

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)Reply

My apologies. I erred in my calculation, reversing the polar and azimuthal directions when calculating the vectors. My apologies. It does appear that the pole and the galactic center vectors are perpendicular. Having an angle of 89.9936 degrees between them. JDoolin (talk) 13:51, 11 August 2013 (UTC)Reply

One further question, though. From galactic north, would the rotation of the galaxy appear clockwise, or counterclockwise? Secondly, is galactic north defined arbitrarily, or according to magnetic fields, or according to rotation? JDoolin (talk) 13:51, 11 August 2013 (UTC)Reply

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)Reply

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)Reply

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)Reply

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)Reply

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)Reply

I tried to read this entire article, and linked articles when I thought it would help, to learn how this coordinate system worked. In the end I am not sure I understand it. For the sake of the editors, here is my account of how it goes.

A spherical coordinate system, ok I know what that is supposed to look like, so I only need to know the center and the orientation of the latitude/longitude grid. The Sun as its center, ok that is half the battle. Primary direction toward the center of the galaxy and fundamental plane parallel to and offset from the galactic plane. Um... At this point I had to look up what a fundamental plane was, though I am now confused why I need both a direction and a plane, since either defines the other. My best guess so far is that since we have a direction toward the center of the galaxy, the equator would go around that. Which is not parallel to the galactic plane so that would not make sense.

The image just shows a galaxy, no help there.

Maybe it will be clarified later in the article.

This section explains the concept of longitude and latitude in terms of north south east west and equators. While that makes sense in isolation I am still not sure what these mean in reference to the coordinate system at hand. Still I am not sure where the equator of the galactic coordinate system is.

The heading is promising, definitions are good. Unfortunately the entire section is actually a history of galactic coordinate systems. I wonder whether these historical systems have any resemblance to the current one, but since I don't yet understand the current one, I have no way of finding out.

The diagram shows latitude and longitude angles being measured. Ok extrapolating this picture and hoping I have latitude and longitude straight, I think I can imagine what the grid would look like. I would know I'm right if the picture somehow displayed a spherical grid. I can imagine someone objecting to one floating in space, but the other coordinate system pages have something similar. I only really came to this conclusion after reading the whole article several times and then writing this review up to this point.

The listing for north pole, south pole, and galactic center display three star fields. I am not really familiar with the sky, other than the galactic center normally looks brighter, but the pictures don't even support that expectation. Thinking about it again, at least I can infer that north pole or south pole do not coincide with galactic center as I first hypothesized.

Ok formulas are good eventually but they do not help get the basic picture at all.

The galactic equator runs through all these constellations, ok that is basically a review of the information on the milky way. If you don't understand this coordinate system by this point, there is not much hope left.

The picture of the milky way in the sky with no coordinate grid overlaid, useless. This picture would actually be a good picture to display the grid on, and not at the end of the article.

In the end I think I deduced what the galactic coordinates look like, but really, I still feel like I am guessing because any picture I have in my imagination is nowhere on wikipedia.

So... a fundamental direction (equinox like?) goes from the sun to the center of the galaxy. The equator is parallel to the galactic plane (offset 55ly north). The north pole is perpendicular to this plane, pointing to the side the earth's north pole is on? The fundamental plane hyperlink didn't really help at all.

Good luck with this article.

2600:8807:5480:7EB:EC33:C488:CDBE:2110 (talk) 20:37, 5 July 2019 (UTC)Reply

To define the orientation of the lat/lon grid, you need both a plane (the equator = roughly the plane of the galaxy) and a zero point (direction of zero longitude = roughly the center of the galaxy). One does not define the other, as you say.

The Milky Way image at the top does have a grid on it, but you have to zoom in to see it. Perhaps it could be enlarged or made with thicker lines so they are visible at the reduced scale.

I can see how the images showing the poles and the zero point might not be helpful if you were unfamiliar with the constellations. The red line in the third image is the galactic equator, but that is not stated anywhere. The blue lines are the RA/dec grid, but again, not necessary/helpful for this article. Do you think photographs of these points would be more helpful than the cartoon-like images?

--Lasunncty (talk) 06:21, 6 July 2019 (UTC)Reply

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)Reply

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)Reply

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)Reply