Talk:Astronomical coordinate systems

U V W Space velocities

Wikipedia appears to have no definition of the U V W system for describing space velocities. I'm not sure of the canonical name of the system, or an official definition, but it appears that in the right-handed system,

 *  U is the component toward the galactic center
 *  V is the component along the galactic rotation
 *  W is the component toward the galactic north pole

Does anyone have more confidence on the canonical name and a reference for a definition? --NealMcB 03:36, 9 April 2007 (UTC)

I am not familiar with any galactocentric coordinate system for stars. However, it seems like this systems would be very difficult to use. For velocities, the main problem would be measuring velocities that are perpendicular to the line of sight, which is difficult to do for most stars. This may be worth writing an article, but it should not be used in articles on individual stars.

However, I can say that a general cylindrical coordinate system like the one above is used in creating models of disk galaxies. I can give references if needed. Dr. Submillimeter 09:58, 9 April 2007 (UTC)

JThorstensen here -- can't seem to log on from home. I think you're nearly correct, except the U component appears to be directed away from the Galactic Center, making the system left-handed. It actually is possible to get perpendicular velocities for stars provided you can measure a proper motion and an accurate distance, which is generally do-able for relatively nearby stars, e.g. those within parallax range -- you need to nail the proper motion down really accurately when solving for the parallax, anyway. There's a newly (as of 2007) revised article on the galactic coordinate system (l and b) which gives the exact IAU definition. Incidentally, as a fellow professional astronomer and wikipedia newbie I've found the quality of astronomy articles to be highly variable -- some are really excellent, some are very inaccurate. Caveat emptor!

A reference for the UVW coordinate system is Evidence from the motions of old stars that the Galaxy collapsed by Olin J Eggen et al. http://dx.doi.org/10.1086/147433 — Preceding unsigned comment added by Rauyran (talk • contribs) 15:59, 10 January 2014 (UTC)

Zodiac Coordinate System

This is a nice comparison article. May I suggest adding the Zodiac Coordinate system — used by Ptolemy and other ancient astronomers — to the table. It uses the ecliptic as it fundamental plane, the ecliptic poles as its poles but in contrast with the ecliptic coordinate system it uses stars (Antares and Aldebaran) as its fixed frame of reference and therefore requires no epoch. Actually, I believe there are two separate zodiac coordinate systems. One sidereal system using the fixed stars and requiring no epoch and one tropical zodiac using an epoch fixed to the vernal equinox around the time and Hipparchus. It's a useful coordinate system for those with very few modern day instruments. 41.248.252.130 (talk) 21:20, 11 May 2008 (UTC)

Prime Meridian

The suggestion of a zodiac celestial coordinate system also raises the issue of the prime meridian for the coordinate system. It might be worth having another column in the table for that too. 41.248.252.130 (talk) 22:40, 11 May 2008 (UTC)

Plural title?

It seems to me that the title should be plural, "Celestial coordinate systems", since the article refers to several. --George100 (talk) 12:00, 7 June 2008 (UTC)

I think singular is in accord with the article naming policy, we don't have birds except as a redirect to "bird", we have bird, irrespective of how many there are. An article Celestial coordinate systems should just be a redirect, but for editors it isn't needed since the link Celestial coordinate system could just be followed by an "s" and the link will just look perfect! ... said: Rursus (^mbork³) 14:04, 18 November 2009 (UTC)

Change with time

How can the coordinates of an object remain constant? Due to the earth's rotation, don't they change with time? —Preceding unsigned comment added by 122.169.1.73 (talk) 17:16, 24 March 2009 (UTC)

Some explanation needed

The article needs some explanations to purpose of respective coordinate system. Altazimutal is for the momentaneous observer on earth at a certain time. If a sailor measured up a star or planet position at a certain time and compared to his planetary tables, longitude and latitude on earth could be computed. Equatorial is for the astronomical observer. He/she generally has instruments adjusted to equatorial positioning of astro-objects. Ecliptical positioning is for Astrology, a practically obsolete usage, or, less obsolete, for a lot of solar system metrics. Galactic positioning is for interstellar astronomy that regards object distribution in the galaxy — that we could guess from the name! Extragalactic positioning is for whatfor, whatever or something else, that surpasses my knowledge... ... said: Rursus (^mbork³) 14:12, 18 November 2009 (UTC)

Did so μself. ... said: Rursus (^mbork³) 14:52, 18 November 2009 (UTC)

Request source of the illustration "Orientation of Astronomical Coordinates"

I tried to duplicate the illustration "Orientation of Astronomical Coordinates" in 3D with AutoCAD and I get an error, particularly that the earth north pole is tilted toward the center of the ecliptic in December, away from the center of the ecliptic in June, which seems wrong to me. So I question whether the autumnal equinox and vernal equinox are labeled correctly, or whether something else I am interpreting from this illustration is incorrect. Did this illustration come from some source that I can verify? Where can I verify the 98 degrees and the 123 degrees? Betweendust (talk) 18:29, 25 November 2011 (UTC)

I think the image is mostly correct (although I confess it required a bit of thinking on my part). The vernal equinox in the image is not the location of the Earth on the vernal equinox, but rather the location of the sun on the celestial sphere at that time of year. The placement of Earth in the diagram is confusing (and potentially wrong). The 98 and 123 degrees I can't verify without more careful calculation but it seems about right magnitude (angular distance between NCP and NEP comes out to ~23.5 degrees). Is that helpful? The diagram is a wikipedian's own work (not my own) so I can't read their mind for how the digram was made. Sailsbystars (talk) 22:33, 25 November 2011 (UTC)

Thank you, I was interpreting the vernal equinox as the earth's position, that's how I got turned around. After I complete my AutoCAD model I will post again.Betweendust (talk) 00:28, 26 November 2011 (UTC) OK I have completed my AutoCAD model, it is posted at http://www.engineeringanddesign.com/1/054.htm It meets my needs for now. I may or may not ever improve it. Thank you for your help.Betweendust (talk) 06:51, 26 November 2011 (UTC)

Origin of Coordinate Systems

Isn't the Earth the Center of any "Celestial Coordinate System"?

As such shouldn't the Celestial Sphere be Concentric with the Earth's surface

The diagram show's the Sun's orbital center as the center of the "Celestial Sphere"

This diagram is confusing to me, and seems inconsistent with this and other pages on Astronomical Coordinate Systems Daccadprose (talk) 23:50, 13 January 2012 (UTC)

There are at least three different origins of coordinates, the Earth, the Sun, and the Galactic center, and on the Earth there are the local rotating system (usually the topocentric one), the barycenter, and the center of some ellipsoidal reference frame (WGS84 and siblings). R. J. Mathar (talk) 10:19, 4 October 2012 (UTC)

Converting coordinates

I am considering pulling the various conversions from all of the coordinate system pages (horizontal coordinate system, equatorial coordinate system, ecliptic coordinate system, galactic coordinate system) and putting them here under the section Converting coordinates. This would allow us to show the similarity between the various conversions, and maybe we could work in Euler angles. We would, of course, point to the conversions here from all of the pages.

Also, the graphic here needs work. Confusing. A general graphic showing coordinates of some body in all of the different systems is a good idea... it may need to be several graphics, each centered at a different place - the Sun, the Earth, the galactic center. Tfr000 (talk) 18:43, 20 June 2012 (UTC)

Centralizing the conversion seems like a good idea to me. If you haven't already done so, you might want to search to see if there is already an article about conversions. Jc3s5h (talk) 20:08, 20 June 2012 (UTC)

Sounds like a good idea to me. I've thought about doing it before, but didn't have the time. I will say that personally I prefer the spherical trig version of the conversions, as they make what's going on easier to intuit. Speaking of long-needed updates, thank you Tfr000, for finally updating that image in Horizontal coordinate system that I never got around to! Sailsbystars (talk) 22:38, 20 June 2012 (UTC)

Actually, I found that image on Wiki commons, so I only get 1/2 credit. I also prefer the trig formulas... I know rotation matrices are very "in" now, but I don't really like them. They seem "black box" to me - you get something out without knowing what happened in there. We should probably include both, however. Tfr000 (talk) 12:30, 21 June 2012 (UTC)

I think we should include mention of two software packages which, as far as this amateur can tell, seem to be regarded as the canonical conversion methods:

• Naval Observatory Vector Astronomy Software
• Standards of Fundamental Astronomy by the International Astronomical Union

Jc3s5h (talk) 13:00, 21 June 2012 (UTC)

Nice. The IAU documentation has lots of references to the original papers. Tfr000 (talk) 16:45, 21 June 2012 (UTC)

Ooooh, the new changes are pretty. I have a minor quibble though, in that the equatorial part defines azimuth from the south here, but from the north in the Horizontal coordinate system article. We should pick one version and stick with it, and I personally prefer north =0 and think most other sources do as well (for example, the horizons system). I think the math can be fixed to reflect that change fairly simply, but I don't have the time to work through it at the moment. Sailsbystars (talk) 19:45, 10 July 2012 (UTC)

The problem is, there is no particular standard for azimuth. Some sources quote south, some north, some both. Meeus refers to [Chauvenet, William (1906). A Manual of Spherical and Practical Astronomy. Vol. I. J.B. Lippincott Co., Philadelphia.], quoting him as saying that astronomers use south, some others use north. Meeus also refers to [Newcomb, Simon (1906). A Compendium of Spherical Astronomy. MacMillan Co., New York.], saying he had no particular preference. The 2010 Astronomical Almanac says something like "measured eastward around the horizon from a specified point (usually north)," so even that isn't particularly set in concrete. All we can really do is make note of all of this, and be clear about what any math formulas are using. Tfr000 (talk) 21:25, 12 July 2012 (UTC)

The IMCCE has no preference either! http://www.imcce.fr/en/grandpublic/glossaire.php Tfr000 (talk) 20:42, 13 July 2012 (UTC)

Any of the four possible conventions (South over East, South over West, North over East, North over West would be find for the article. The main problem is that the article does not say whether it uses South over West or South over East. R. J. Mathar (talk) 10:13, 4 October 2012 (UTC)

Interestingly, Smart in his Textbook on Spherical Astronomy, 6th edition (1977), uses yet another system - he specifies north as the origin, and then the azimuth is in degrees west or degrees east of that origin, presumably to a maximum of 180°. I guess at some point we should write up all of this in the article. Tfr000 (talk) 17:13, 1 January 2016 (UTC)

Call me dense, but the reason why the Astronomical Almanac and Meeus differ on one of the Equatorial to horizontal equations (Celestial_coordinate_system#cite_note-7) might be because of what we're discussing above. The ${\begin{bmatrix}-1&0&0\\0&-1&0\\0&0&1\end{bmatrix}}$ matrix is a rotation around some axis by 180°. I'll have to try some sample calculations and see what happens. Tfr000 (talk) 22:43, 20 July 2012 (UTC)

I see someone went ahead and changed the equation in the article. We might want on wait on that until someone figures out the reason for the sign difference. My above comment is speculation at the moment... haven't had time to work on it. Besides that, the "notes on conversion" state that we're using south reference for horizontal coordinates. *IF* my above comment is true, changing the sign negates that statement. Tfr000 (talk) 14:05, 27 July 2012 (UTC)

Perhaps rather than embed the equations in the article, put the equations in templates. That way they can be repeated as needed without the maintenance nightmare of multiple copies. Op47 (talk) 15:55, 14 September 2012 (UTC)

This thread is 2 years old. Has anything been written about coordinate conversions? I know how to do most conversions, but I need to know how to convert between Supergalactic coordinates and Galactic coordinates in Cartesian notation. Glen Deen (talk) 19:26, 4 July 2014 (UTC)

It seems to me that there is an error in conversion between Galactic and Equatorial coordinates. Where longitude of the North Celestial Pole is used, it should be the longitude of the ascending node of the Galactic plane on the equator. I.e., l_NCP-90.

Table is a little odd now

The table in the section Celestial_coordinate_system#Coordinate_systems now has "horizontal" and "vertical" coordinates listed. Since these are all angles on a sphere, there is no horizontal or vertical. Vertical is the direction in which gravity pulls when you are standing on a planet. Yes, if you turn the celestial sphere around into some certain contrived orientation, you could then say, perhaps, that latitude is like vertical, but in the real sky nothing is oriented that way. The horizontal coordinate system, which is based where the observer is standing, is the exception. We can write it to explain all of this, if that's what we want. Tfr000 (talk) 14:24, 16 May 2015 (UTC)

Fixed it. Tfr000 (talk) 16:00, 4 July 2015 (UTC)

Vernal equinox

Maybe I'm as thick as a brick, but the explanation of the vernal equinox as the origin of longitudinal position is not clear. I think it needs a little clarification. In which direction would I be facing on the vernal equinox in order to face the origin? I understand where the Earth is located relative to its orbit around the sun on the vernal equinox, but that seems to be a location in space (the point where the Earth is located on the vernal equinox), not a direction or a line. Is the origin a line from the Earth through the sun on the equinox? If so, is the zero point where the sun is located in the sky, or is it along that same line facing the zenith at solar midnight? It seems there is one element missing still. We have the Earth's location relative to the sun at a given time, but how do we derive direction from that? The ecliptic plane and the celestial plane intersect along a line between the Earth and sun, ok, but which direction along that line is the origin of longitudinal position? (I think I'm repeating myself.) Dcs002 (talk) 23:24, 4 June 2016 (UTC)

"Celestial" latitude and longitude

See discussion at talk:celestial longitude

Requested move 25 May 2021

The following is a closed discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review after discussing it on the closer's talk page. No further edits should be made to this section.

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Celestial coordinate system → Astronomical coordinate systems – The authoritative IAU's resolutions recognize only two types of celestial reference systems: barycentric and geocentric; also notice the proposed article title was merged into the present article in the past. fgnievinski (talk) 05:46, 25 May 2021 (UTC) —Relisting. -- Calidum 03:29, 10 June 2021 (UTC)

Why not provide a citation to the authoritative IAU resolutions? Jc3s5h (talk) 02:51, 31 May 2021 (UTC)

Sorry, here it is: IAU (2000), Resolution B1.3: Definition of barycentric celestial reference system and geocentric celestial reference system fgnievinski (talk) 00:54, 6 June 2021 (UTC)

• Meh. No objection. --SmokeyJoe (talk) 01:11, 2 July 2021 (UTC)

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To editors ­fgnievinski, Calidum, Jc3s5h and SmokeyJoe: inclined to rename this article; however, since it hasn't been addressed above I would like to know if you think the plural is by the book? or would a move to Astronomical coordinate system (singular) be more appropriate? P.I. Ellsworth  ed. ^put'r there 07:47, 14 July 2021 (UTC)

Support the PLURAL exception “Articles on groups or classes of specific things”. The article covers the multiple coordinate systems, not the general concept which is quite trivial and is covered by the first paragraph. SmokeyJoe (talk) 09:58, 14 July 2021 (UTC)

Agreed with plural, as in the skew coordinates example given in WP:PLURAL#Exceptions. fgnievinski (talk) 23:38, 14 July 2021 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.