Talk:Galactic coordinate system

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[edit] rewording

"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)

[edit] Right Handed, Left Handed, What?

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)

[edit] Proposed Major Revision

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)
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)
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)

[edit] 00 00 00 +00 00 00

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)

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)

[edit] Redundant Link

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)

[edit] ANDROMEDA GALAXY AS THE ZERO OF LONGITUDE FOR GALACTIC COORDINATES

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)