Talk:Prime meridian/Archive 1

old comments
So why did San Domingo vote against the siting of the Prime Meridian in Greenwich? Anybody know what their alternative was? Lisiate 01:23 24 Jul 2003 (UTC)

We need a public-domain map on this page, showing where this line is.

Last text version of Prime Meridian version: "The meridian (line of longitude) passing through Royal Greenwich Observatory, Greenwich, England is the prime meridian (longitude = 0 degrees).

The meridian was agreed upon in October 1884. At the behest of the President of the United States of America, 41 delegates from 25 nations met in Washington, DC, USA for the International Meridian Conference.

At the conference the following important principles were established:

1) It was desirable to adopt a single world meridian to replace the numerous ones already in existence. 2) The Meridian passing through the principal Transit Instrument at the Observatory at Greenwich was to be the 'initial meridian'. 3) That all longitude would be calculated both east and west from this meridian up to 180°. 4) All countries would adopt a universal day. 5) The universal day would be a Mean Solar Day, beginning at the mean midnight at Greenwich and counted on a 24-hour clock. 6) That nautical and astronomical days everywhere would begin at mean midnight. 7) All technical studies to regulate and extend the application of the decimal system to the division of time and space would be supported. Resolution 2, fixing the meridian at Greenwich, was passed 22-1 (San Domingo voted against), France and Brazil abstained."

The previous version declared that Rome meridian corresponds to 1&deg; 30' 28" east of Greenwich, which is obviously nonsense. I have corrected it to 12&deg; 27' 08.04", which is what I have in my sources, but I do not have any authoritative reference; if someone does and it contains some other value, please make a correction. --Mormegil 19:16, 11 Oct 2004 (UTC)

I've just thought of something. What if plate tectonics aren't entirely equal with every plate respective to every other.. wouldn't england move a bit, screwing up the prime meridian's placement??


 * You are correct. The prime meridian was defined to be where Airy's transit instrument was in 1884, but a different system is now used. Furthermore, the Royal Greenwich Observatory is now a museum, with no working instruments. The prime meridian is now defined relative to the International Terrestrial Reference System, which uses extragalactic radio and visible sources to which the rotating Earth is referenced via Very Long Baseline Interferometry and other methods. In particular, it is defined to have no net rotation regarding horizontal tectonic motions over the whole earth. &mdash; Joe Kress 19:12, Mar 17, 2005 (UTC)


 * Come on, Britain hasn't moved 100 metres since 1884!!! Lee M


 * The writer was correct when noting that "England moved a bit", a few centimeters, not 100 meters. Nevertheless, the Prime Meridian has apparently moved about 100 meters because it is no longer tied to the Royal Observatory at Greenwich nor any specific point on land—it is now tied to a geometric figure, an ellipsoid. The Prime Meridian used to be defined via the observatory's transit circle which was mounted in a vertical plane (via a plumb bob), which is perpendicular to sea level around the island of Great Britain. But sea level undulates by ±100 meters world-wide, meaning that the longitudes of other observatories were not referred to Greenwich, even though that was their intent. To correct this mess, a single ellipsoid was selected to coincide with the average world-wide sea level. A perpendicular to this world-wide ellipsoid at its own Prime Meridian now appears to intersect land about 100 meters east of the old transit circle. — Joe Kress 07:31, 16 January 2006 (UTC)


 * I still don't see why the new Prime Meridian couldn't have been adjusted to coincide with the old one. Lee M 11:27, 24 January 2006 (UTC)


 * You have a point. According to the official explanation, they tried to adjust it to match, but could not because there no working instruments at Greenwich when the single reference ellipsoid was selected. — Joe Kress 09:09, 25 January 2006 (UTC)

Which is the real prime meridian? The one marked in Greenwich or the one according to GPS recievers/Google earth?(AndrewAnorak (talk) 09:22, 12 December 2007 (UTC))

Changed Oslo to Kristiania. That was the name of the city when the local meridian was in use. Ulflarsen 18:42, 17 Mar 2005 (UTC)


 * I have copies from Norwegian maps (from U.S. Library of Congress) which say "Oslo meridian". As the stub at Kristiania says, that name applied from 1878-1924; as this article says, Greenwich was agreed on as the universal prime meridian in 1884.  So it would only have been Kristiania for about a six-year period; most of the time when this was used, it would have been Oslo.  BTW, I did find the coordinates and will add them too.  Gene Nygaard 19:31, 17 Mar 2005 (UTC)


 * Well, okay, I caught myself--I should have known better. So it was "Christiania" rather than "Kristiania" from 1624 to 1878.  Nonetheless, it is "Oslo" which I have seen on maps.  I changed it to Oslo (Kristiania) and am not going to worry about the Chr- spelling, people should be able to figure that much out themselves.  Gene Nygaard 19:41, 17 Mar 2005 (UTC)


 * Simply because the Greenwich meridian was adopted in 1884 does not mean that all local meridians suddenly ceased to exist. The Washington meridian itself was not repealed until 1912 (the borders of several western states were defined relative to it even after 1884). It was actually used for astronomical purposes until 1950 alongside the Greenwich meridian (see the American Ephemeris and Nautical Almanac). The Paris meridian has a very long history -- it was retained until 1911, and then Greenwich mean time was defined as Paris Mean Time retarded by 9 min 21 sec. Many small countries did not repeal their local meridians until much later, and some may still exist. Nepal used that of Katmandu until the 1990s and may still use it. &mdash; Joe Kress 02:42, Mar 18, 2005 (UTC)

If days start at midnight at the International Dateline, the very first hour of each day should start at the same International Dateline, not at the Prime Meridian. Even in Britain, when we consider train and airplane schedules, we should refer to a 24 hour day, starting at the first time zone of the world, located within the International Dateline (180 degrees longitude) and the 165 degrees East Meridian. To consider another way to measure 24 hour days is useless and nonsense. Even more, the Prime Meridian should not be used to divide Earth's surface into an Eastern and a Western Hemispheres. Longitude meridians should start at the International Dateline and progress westerly 360 degrees around the Equator. This way nobody should need to calculate progression of Easter Longitude backwards from the Prime Meridian, since movement of "apparent" Sun is always from East to West. Besides this, Time Zones is a mathematical concept, but English language lacks the use of another concept: Legal Time, which is in spanish-speaking countries, the time fixed by governments to rule national activities in the whole country. And these "Legal Time Zones" maybe, or maybe not, coincident with mathematical Time Zones, according to the reasons each country has to base its own decision. --Fev 17:42, 22 January 2006 (UTC)fev. And the International Dateline, or 180 degrees longitude meridian, should be included as another reference meridian since it is the line where every solar day starts. --Fev 18:53, 31 January 2006 (UTC)

Intersection of Equator and Prime Meridian
Is the intersection of the Equator and Prime Meridian marked with anything? I know it's in the ocean but surely a spot like that could be made into a pier or something and used as a tourist attraction? —Preceding unsigned comment added by 76.177.173.239 (talk) 18:19, 25 June 2008 (UTC)
 * I guess you're joking; it's 350 miles, or so, off the south coast of Ghana - don't you know? --Blake the bookbinder (talk) 17:32, 26 June 2008 (UTC)
 * I just said I know it's in the ocean. That's why I suggesting using a pier or something resembling an off-shore oil well or something to mark it.76.177.173.239 (talk) 17:08, 27 June 2008 (UTC)
 * Oops! Sorry. I wasn't meaning to be flippant - I really thought you were joking. My apologies. --Blake the bookbinder (talk) 08:30, 28 June 2008 (UTC)

International Dateline
I think Franey's contribution on reverting "gibberish" is a very poor one and lacks scientific support. My own contribution was made to improve this article, not to erase "gibberish" (and this article is full of it). At least, Franey should discuss my contribution before reverting it, in the same way I did in the page of discussion. --Fev 06:34, 19 February 2006 (UTC)
 * The International Date Line is not a meridian. It is a wiggly line that follows the 180&deg; line of longitude for maybe 60% of its length; the rest of it swings roughly between 170&deg; E and 170&deg; W. (Calling this "a few exceptions" is an interesting choice of words.)
 * No time zone starts at the Greenwich Meridian; nor is any time zone bound by the IDL and the 165&deg; E line of longitude. The Greenwich Meridian marks the theoretical centre of the GMT zone, which nominally lies between 7&deg; 30' W and 7&deg; 30' E; similarly, the +/-12 zone extends in theory from 172&deg; 30' E to 172&deg; 30' W.
 * "the first hour of each day must start at the International Dateline": please explain how a measure of time can be said to start at a particular place.
 * "Train and airplane schedules (even in Britain) follow a 24 hour a day system": what has this got to do with anything?
 * "the reference meridian can not be considered at the Greenwich Meridian, but the 180° longitude": are you suggesting that cartographers, mariners, aviators, armed forces, GPS satellites, and everyone and everything else that uses geographic co-ordinates should drop a system established internationally for over a hundred years and start measuring longitude from somewhere else?
 * &mdash; Franey 17:07, 20 February 2006 (UTC)


 * This was my reference to the International Date Line considered by Franey as gibberish and, therefore, reverted:
 * “After the International Dateline was fixed to the 180° longitude (with a few exceptions) to start every day at midnight, there is no longer need to use the Greenwich Meridian as a starting line for identifying Time Zones, since the first Time Zone must be the one lying between the International Dateline and the 165° degree meridian E of Greenwich. Of course, none of the above reference meridians has as much importance as the International Dateline and that is why this line should be considered as the most important reference meridian now: since each day starts at midnight at the 180° longitude, also the first hour of each day must start at the International Dateline. Train and airplane schedules (even in Britain) follow a 24 hour a day system, and the reference meridian can not be considered at the Greenwich Meridian, but the 180° longitude”.

Fortunately, Franey introduced some comments on the text included above and I am trying to explain now my viewpoint: •	Franey: The International Date Line is not a meridian. It is a wiggly line that follows the 180° line of longitude for maybe 60% of its length; the rest of it swings roughly between 170° E and 170° W. (Calling this "a few exceptions" is an interesting choice of words.)
 * My commentary: “The 180th meridian …., conveniently passes almost entirely through ocean” (read article, please) must be another “interesting choice of words”. And “this line (180th meridian) is used as the basis for the International Date Line”, “with some modifications” (see article again) must be another “interesting choice of words”. Even more, the International Date Line “is” at the 180th Meridian, since these “modifications” were made by governments from several islands and countries to avoid having different dates within their territories. Remember what the article says: “conveniently passes almost entirely through ocean”. Why to use the word “conveniently”?. Because if there were no land along the 180th Meridian, both lines (180th Meridian and the International Date Line) would be the same line.

•	Franey: No time zone starts at the Greenwich Meridian; nor is any time zone bound by the IDL and the 165° E line of longitude. The Greenwich Meridian marks the theoretical centre of the GMT zone, which nominally lies between 7° 30' W and 7° 30' E; similarly, the +/-12 zone extends in theory from 172° 30' E to 172° 30' W.
 * My commentary: You are right in this, except that the error is in the way of considering Time Zones both sides of the meridian of reference of every one of them, and this is not your error. Let see: the +/-12 zone extends in theory from 172° 30' E to 172° 30' W. Why to say “in theory”?. This is what actually happens. And the very problem is in naming this Time Zone  “+/-12 zone”, because it is the same thing that saying: “this zone has 7 and a half degrees of longitude both in the western and Eastern hemispheres”. So, the eastern part of this zone (at the western hemisphere) would have a certain date and the western part (at the Eastern Hemisphere, which is a kind of paradox) would be the following day. What I proposed is that measure of time must follow the apparent Sun because there is no way in thinking another way to consider time. This means that when is 12 noon at a given meridian (say, the Prime Meridian) starts the first hour in the afternoon towards the west and this very first afternoon hour will end, obviously, when the Sun reaches the 15th degree west of the Prime Meridian, just at the very same moment when it is 1 o’clock p.m. at the Prime Meridian. But the way it is wrongly established looks like if  the Sun, after reaching the Prime Meridian, “goes back” to the East 7 and a half degrees longitude. Or, the other way around, if every meridian is at the very centre of the Time Zone where it is located, there would not be a Time Zone between two contiguous time zone meridians, but two halves of two different time zones and, consequently, would be the same thing as if every meridian would indicate noon at 12:30 in the afternoon.
 * The article says: Universal Time is notionally based on the WGS84 meridian. However, the standard international time UTC can be discrepant from the observed time on the meridian by up to about one second, because of changes in the earth's rotation. This corresponds to a variation in longitude of roughly 300 metres either way at Greenwich.
 * My commentary: It is not “variation in longitude of roughly 300 metres” but variation in time. To say it the opposite way, is nonsense. This is almost the same error in saying that “a minor earthquake in Greenwich could change longitudes throughout the world” (a short text fortunately reverted) since meridians of longitude are virtual lines and consequences of a minor earthquake in Greenwich would not be virtual, but real. So, an earthquake in Greenwich would not change longitudes throughout the world even the sophisticated apparatus used to determine longitudes in Greenwich (or wherever would be sited) went crazy.

Franey states: "the first hour of each day must start at the International Dateline": please explain how a measure of time can be said to start at a particular place.
 * My commentary: “Universal Time is notionally based on the WGS84 meridian” (see article). I could not say it better.

•	Franey said: "Train and airplane schedules (even in Britain) follow a 24 hour a day system": what has this got to do with anything?
 * Elementary, my dear Franey: a 24 hour a day system is equivalent to a daily system. This means that if Universal Time is based on the WGS84 meridian (as the article says), when is Saturday midnight at this meridian, starts a new day (Sunday) throughout the world. Right?. Completely wrong, since Sunday (the very same Sunday) started 12 hours ago at the International Date Line.
 * My commentary: “the reference meridian can not be considered at the Greenwich Meridian, but the 180° longitude”

Of course, especially to measure Time, both UTC, GMT and Time Zones.

•	Franey's: “are you suggesting that cartographers, mariners, aviators, armed forces, GPS satellites, and everyone and everything else that uses geographic co-ordinates should drop a system established internationally for over a hundred years and start measuring longitude from somewhere else?”.


 * I am not suggesting it: I am saying the whole world already dropped that system since everybody uses now a 24 hour (or daily) system, which starts at the 180th meridian towards the west at 12 midnight everyday. --Fev 03:59, 22 February 2006 (UTC)

Vandalism
I deleted the word 'weenie' from "The Greenwich weenie". I presume this is mindless vandalism - I could not see any other meaning... MacAuslan 06:43, 3 October 2006 (UTC)


 * That vandalism was by 69.123.109.138. Immediately before it, the entire first paragraph and all images were deleted by Terry wonghk. If you monitor an article ("Watch this page") on a regular basis, then you can identify such vandalism via "Compare selected versions" in "Page history" by comparing the last version you know to be good with the latest version. See Vandalism. — Joe Kress 05:03, 4 October 2006 (UTC)

Distance corresponding to degree of longitude
I added the word "approximately" to this. The expression given is actually a good approximation to WGS-84, with a maximum relative error of 8.8e-6. The exact expression is a*cos(phi)/sqr(1-e^2*sin(phi)^2), where a is the equatorial radius, 6378137 m; e^2 is the square of the eccentricity; sqr is the square-root; the caret denotes exponentiation; and (1-e^2) = (1-f)^2, where f is the flattening, 1/298.257223563.

— Preceding unsigned comment added by 63.86.92.198 (talk) 18:45, 30 October 2007 (UTC)

Queen Maud Land
The article currently says that the prime meridian runs through the Norwegian part of Antarctica, called Queen Maud Land. I see two issues: And of course we should back up whatever we do with reliable sources. Joshua R. Davis (talk) 15:42, 2 March 2008 (UTC)
 * It appears that Norway's claim is not officially recognized except by a few countries. Wikipedia should not list the land as part of Norway, except perhaps with an explicit note that this is a disputed claim.
 * Do people call that region Queen Maud Land, even if they deny Norway's claim to it? If so, then we can use the name without mentioning Norway.
 * Who, besides the USA and Russia, does not recognise it? All the countries that have claims in Antarctica recognise it; the non-recognition of those who don't is just sour grapes.  The Antarctic Treaty freezes (pun intended) the status quo ante, which was that those who were actually there had claims and those who came late didn't, but were entitled to say they didn't recognise the earlier claims, so long as they didn't actually do anything about that non-recognition.  -- Zsero (talk) 16:59, 2 March 2008 (UTC)


 * The Queen Maud Land article mentions only four countries that have officially recognized Norway's claim, and the Antarctic Treaty System article says that the treaty does not recognize any claims to territory whatsoever (Article 4). Are those articles inaccurate? I'm honestly asking; I'm no expert.


 * It is obvious that the Prime Meridian runs through Antarctica. For the article to make the more specific claim that it runs through Norway, we need to cite a source, it seems to me. Joshua R. Davis (talk) 17:27, 2 March 2008 (UTC)


 * Those are the countries that have territories there, so it's their recognition that's important. The treaty simply leaves things as they were; it neither recognises nor negates the pre-existing claims, it just prevents anyone from making new claims or trying to conquer existing claims.  -- Zsero (talk) 19:05, 2 March 2008 (UTC)


 * I don't understand. What about Argentina and Chile? Do they recognize that Norway possesses that part of Antarctica? Would they agree with you that only the opinions of Australia, France, New Zealand and the UK are important?


 * When it comes to potentially contentious topics, such as biographies of living persons and territorial boundaries between countries, we need to be especially careful, to make sure that Wikipedia contains only verifiable statements.


 * How about this solution: We take out the flags (which don't serve any serious purpose anyway) and describe the antarctic part of the Prime Meridian as running through Queen Maud Land, which is claimed by Norway? Joshua R. Davis (talk) 20:59, 2 March 2008 (UTC)


 * Argentina and Chile don't recognise these claims, because nobody at all recognises their claims. But if the line went through a disputed area we'd probably want to stay neutral.  In this case, though, the area it goes through isn't disputed.  There's only one claimant, and a few countries that came too late to stake claims putting fingers in their ears and singing, while agreeing to do nothing to change the status quo ante.
 * In any case, you're right that the flags don't add anything, and we could drop them with no loss of information. No need to note that QML is claimed by Norway either.  Just put Queen Maud Land, and anyone interested in learning more about it can follow the link.  -- Zsero (talk) 21:30, 2 March 2008 (UTC)

Mecca?
When was Mecca used as a Prime Meridian? Is there a source for this? (We should probably find cites for all the meridians listed, but I'm particularly skeptical about this one, because Maimonides, writing at the height of the Arab dominance of the sciences, used a Prime Meridian 20&deg; east of Mecca. Had Mecca been the standard used by Arab geographers at the time, Maimonides might have been reluctant to follow suit, but it would have made sense for him to substitute Jerusalem, not a neutral spot far to the east.) -- Zsero (talk) 05:28, 28 April 2008 (UTC)

Precise Greenwich Meridian?
Would there be any objection to me changing the title of the section currently titled “Precise Greenwich Meridian” to something like “The WGS 84 Reference Meridian” or “The IERS Reference Meridian”?

The WGS 84 Implementation Manual does not, nor has it tried to, equate the WGS 84 Reference Meridian to the Greenwich Meridian: not George Airy’s, nor Flamsteed’s, Halley’s or Bradley’s; certainly these are the Greenwich Meridians. Page thirteen of the WGS 84 Implementation Manual (version 2.4, February 12, 1998) explicitly says its intention is “to bring its reference meridian into coincidence with the Bureau International de l'Heure (BIH)-defined zero meridian” (which is not the Greenwich Meridian). There are several Greenwich Meridians about which you can be precise, but the WGS 84 Reference Meridian is not, never has been and never will be, one of them.

Entitling the section in question “Precise Greenwich Meridian” seems to be mixing the historical with the modern in a potentially very confusing way – not too unlike the confusion resulting from the supposed interchangeability of UTC and GMT. --Blake the bookbinder (talk) 21:58, 23 June 2008 (UTC)


 * I prefer IERS Reference Meridian (I've also seen the more informal International Reference Meridian), because that is primary. Although the WGS84 meridian was earlier, these days it is dependent on the IRM, not vice-versa. But do not link IERS within the heading—do that in the text only. — Joe Kress (talk) 06:25, 29 June 2008 (UTC)

"The location of Airy's transit circle ... is on the sidewalk ..." Excuse me, I know for a fact that there are no sidewalks in Blackheath Avenue. Indeed, there are none in the entire UK. You mean pavement. Sorry. —Preceding unsigned comment added by 86.146.194.167 (talk) 11:19, 10 August 2008 (UTC)
 * Important cultural point!


 * No need to be facetious, however I've made the change. I wonder what our American friends will think to this though - "pavement" means something different in the US - however, since this article is referring to a location in London I agree that the British terminology is more appropriate. Bazonka (talk) 11:32, 10 August 2008 (UTC)


 * Thanks for the clarification. Although I was aware of some British/American vocabulary differences, like lorry/truck and bonnet/hood, pavement/sidewalk was new to me, although it is included in British/North American Vocabulary. To make the phrase understandable to an American reader, I'm adding sidewalk in parentheses like imperial units are added after metric units. — Joe Kress (talk) 02:08, 13 August 2008 (UTC)

Ordnance Survey meridian section
In a recent renaming of the IERS Reference Meridian section, I teased out the bit about the Ordnance Survey Zero Meridian (because it did not belong in the IERS Reference Meridian section) and, not knowing where else to put it, gave it its own section, but I don't think it really belongs on a page about the prime meridian (since it's the reference meridian for a single country's mapping scheme). Does anybody have an opinion about, or an objection to, it being deleted from this page? --Blake the bookbinder (talk) 20:17, 18 July 2008 (UTC)


 * The Prime Meridian is simply another name for the Greenwich Meridian, so the Ordnance Survey zero meridian (Bradley's meridian) should remain here because it was the Greenwich Meridian before 1851 and was used as the reference or prime meridian on British nautical maps covering the entire world and as the reference meridian for times within the British Nautical Almanac and Astronomical Ephemeris used by mariners around the world from 1767 to 1850. — Joe Kress (talk) 03:33, 19 July 2008 (UTC)


 * I'll work the Ordnance Survey zero meridian into the History section. Flamsteed’s, Halley’s and Bradley’s Greenwich meridians deserve a mention there too, I suppose; maybe in a history of the Greenwich Meridian as a sub-section of the History section.


 * In a somewhat different line of thought, I've been mulling over your statement: "The Prime Meridian is simply another name for the Greenwich Meridian". I think I know what you mean there, but am wondering if this is not analogous to the statement "Universal Time is simply another name for Greenwich Mean Time", in that both the Greenwich Meridian and Greenwich Mean Time have had multiple definitions over the years and to equate anything to either of those terms is to increase the likelihood of confusion (however slightly)? What is it about Greenwich, eh?


 * History has a habit of disregarding the plans of learned men, but it was the expressed intention of at least one delegate (Commander W. T. Sampson) at the 1884 International Meridian Conference that, "it is to be remembered that when the prime meridian is once adopted by all it loses its specific name and nationality, and becomes simply the Prime Meridian."


 * --Blake the bookbinder (talk) 13:13, 19 July 2008 (UTC)


 * My statement that "The Prime Meridian is simply another name for the Greenwich Meridian." did carry a lot of baggage. I should have said the modern Prime Meridian because many prime meridians at other locations have been used, and as you note there have been several Greenwich meridians. I remember that when Google first started its map service, it called the IERS meridian the Prime Meridian and was roundly criticized. — Joe Kress (talk) 19:28, 19 July 2008 (UTC)

Plate movement and longitude
Please see my query and subsequent discussion at Reference desk/Science. I think something needs to be added to the Prime Meridian article that addresses this point. --Mathew5000 (talk) 18:11, 12 August 2008 (UTC)


 * I've added some discussion of the movement of tectonic plates relative to the IRM. — Joe Kress (talk) 08:04, 13 August 2008 (UTC)

Northwest corner of the fence surrounding the Pavilion Tea House
I think the following sentence should be deleted from the article:
 * The location of Airy's transit circle if its astronomical coordinates (51° 28' 38", 0° 0' 0") are assumed to be GPS coordinates is on the pavement (sidewalk) parallel to and northeast of Blackheath Avenue near the northwest corner of the fence surrounding the Pavilion Tea House, hence the IRM passes through this point.

(1) That is not a correct usage of the phrase "astronomical coordinates". Is it? (2) The citation to a page at the Degree Confluence Project, while better than a blog or discussion forum, does not fully meet the Wikipedia definition of reliable source. (3) The wording is awkward, essentially structured to say that the location of Airy's transit circle is somewhere that it is not in fact located. (4) Is it of sufficient importance to this article to mention what exactly is at this particular point on the IERS Prime Meridian that happens to be at the same latitude as Airy's transit circle? It implies that there is something geographically significant about latitude 51° 28′ 38″ N, when in fact there is not. --Mathew5000 (talk) 22:58, 13 August 2008 (UTC)


 * Now done . I think it is sufficient to say simply that the IERS meridian is 5.31″ east of the original prime meridian that was set in 1884. I also am a bit skeptical of the following text, beginning with “This shift is a legacy of the first satellite navigation system”. If you read that part of the paragraph, you come away thinking that the difference between the GPS 0° meridian and the 1884 0° meridian was caused by something that happened in the 1960s, and may be related to the specific location of the ground station at Johns Hopkins University in Maryland. But that is not correct, is it? We should say simply that the IERS Reference Meridian is based on an American datum from the 1920s, and at that time because they didn’t have the technology to determine longitude as precisely as we do now, their longitude was off by 5 or 6 minutes. This was discovered in the 1960s, but the discrepancy dates from much earlier than that. Comments? --Mathew5000 (talk) 22:01, 14 August 2008 (UTC)


 * The pavement/sidewalk was an improvement over what was in the article before my recent edits, which used to state that the WGS84 meridian was marked by a "waste basket" (dustbin) on the path leading due east from the transit telescope.


 * The astronomical coordinates of any point on Earth are determined by observing celestial objects, especially fixed stars. But the local vertical or the direction of gravity is usually tilted relative to a line perpendicular to (normal to) an Earth-centered ellipsoid (see vertical deflection). If the local vertical used to orient Airy's transit circle is extended into the Earth, it will bypass Earth's axis by about a hundred metres. Hence the plane containing that local vertical and Airy's meridian will also bypass Earth's axis. But a nearby parallel plane will include Earth's axis, so another transit circle oriented to it will observe the same stars at the same moment in time, hence its astronomical coordinates will be the same. The 1971 Navigation article already cited and a revision in 1977 discuss this, concluding that that parallel astronomical plane differs from the geodetic plane containing the IRM by only about 0.3". So astronomical coordinates of objects on the IRM, like the pavement/sidewalk, are about the same as their GPS coordinates.


 * A 5.64" shift did indeed happen in the 1960s (now measured as 5.31") and is indeed related to the specific location of the Applied Physics Laboratory, as explicitly stated by the cited 1971 Navigation article. The shift was caused by changing the coordinates from a non Earth-centered ellipsoid (NAD27) to an Earth-centered ellipsoid (APL 4.5, later WGS84) as I stated in my edits. An example of many such shifts is shown at Position shifts from datum differences. Specifically note that the WGS84 location is quite close to the NAD27 location, but is far removed from its position on the Ordnance Survey 1936 datum (which was based on Bradley's meridian and differs from Airy's meridian by only six metres). The shift in the OS36 position is magnified because it is also non Earth-centered, whereas the WGS84 shift is not as great at Greenwich because WGS84 is Earth-centered. During the nineteenth century it was well known that positions on different non Earth-centered ellipsoids (like NAD27 and OS36) differed widely, but that was accepted in order to obtain greater accuracy within the specific region for which they were created (North America and Great Britain), so they were not 'discovered' in the 1960s. — Joe Kress (talk) 02:44, 15 August 2008 (UTC)


 * I don't think I have have the technical knowledge to understand that, at least not yet; I am going to read some more background. But the one question/issue I have, to which there might be a simple answer, is this: The particular meridian designated as zero is arbitrary; correct? Regardless of whether we use a non-Earthcentered ellipsoid or an Earthcentered ellipsoid. So even though the local vertical used to orient Airy’s transit circle does not intersect the origin used in WGS84, the centre of the transit circle still does define a particular meridian in WGS84 (just as any point on the Earth at a particular time, except the north and south poles, defines a meridian of longitude) and it could have been used as the prime meridian of WGS84. I'm not saying that it should have been, just that it could have been if the designers had wanted to remain as faithful as possible to resolution 2 of the International Meridian Conference. The result I guess would have been that preexisting printed maps of the USA would have differed by 5-6 seconds of longitude from their locations under WGS84, but other than that, the essential mathematics of it (and of GPS generally) would be unaffected. Is that wrong? --Mathew5000 (talk) 09:17, 15 August 2008 (UTC)


 * Also, do you agree with the edit I made, removing the mention of the pavement near Pavilion Tea House? --Mathew5000 (talk) 09:18, 15 August 2008 (UTC)


 * Although the choice of zero meridian is arbitrary, modern accuracy prohibits choosing a single point on the surface of the Earth as was done in 1884. Nevertheless, the point chosen could have appeared to have been much closer to Airy's transit circle than it is now. An expert at the IERS admitted that their decision not to insert a sudden jump in satellite derived coordinates developed before 1984 by the United States caused the present offset. If such a jump had been adopted, the satellite derived longitude of Airy's meridian could have been within one metre of the IRM (0.04"), even within 1 cm at present accuracy capabilities. But it could not have been exactly at Airy's transit circle because of Earth's tectonic plates, unknown in 1884. Choosing any specific point at Greenwich would have meant that the European portion of the Eurasian plate was fixed (the Asian portion is unstable due to the ongoing collision of India with Asia), giving it preferred status over all the other tectonic plates on Earth. The present decision to make the average motion of all plates zero causes any point at Greenwich to move northeast about 2.5 cm per year. This requires that measuring stations be located on all plates in order to obtain the data to average their motion—a single point at Greenwich is useless. So the IRM is the average of the reference meridians (0° longitude) of all measuring stations on Earth. This causes it to 'float' above any point at Greenwich, moving back and forth each year by about a centimetre as new data becomes available, in addition to the steady northeast motion.


 * For the above reason, an Earth-centered ellipsoid must be used, because all non Earth-centered ellipsoids give preferential status to a single point on the surface of Earth. For example, NAD27 is defined by an ellipsoid whose surface coincides with Meades Ranch, Kansas. The coordinates of all other points on Earth, especially those far away, are degraded. Similary, choosing any surface point at Greenwich would give it perfect accuracy but seriously degrade the accuracy of the coordinates of all other points on the surface of Earth, making it useless as a reference. Satellites orbit the center of mass of Earth, so Earth's center is the natural reference point—satellites in orbits with altitudes thousands of kilometres above Earth, like GPS satellites, ignore all surface irregularities. The many surface stations used by satellite systems make the entire surface of the Earth the reference—any single point, like any at Greenwich, is secondary.


 * Because astronomical coordinates depend on the tilt of the local vertical, they can never be more accurate than a few arcseconds anywhere on Earth relative to other points on Earth, including any 'point' at Greenwich. Indeed, defining such a point causes all other coordinates on Earth to shift because all other points have their own deflections, none of them equal to that at Greenwich. This also means that placing the GPS reference near Airy's meridian would only be an illusion, because astronomical coordinates are inherently different from satellite coordinates. Satellite coordinates ignore all surface deflections, so they can be accurate to a milliarcsecond relative to other points on Earth, giving them vastly superior accuracy.


 * My preference would be in keep the features near the IRM within the article, regardless of their less than perfect source, because it is much easier to visualize those features, even one as mundane as a pavement/sidewalk, than to visualize a distance of 102.5 m. For this reason a map or satellite image with both meridians drawn on it would be ideal. However, I agree that mixing GPS and astronomical coordinates was not correct—their inherent difference must be explained, although the subject is complex.


 * Several good publications can be downloaded from the National Geospatial-Intelligence Agency, including Geodesy for the layman and DoD World Geodetic System 1984. — Joe Kress (talk) 00:39, 18 August 2008 (UTC)

Four meridians?
The article mentions that four different zero meridians through Greenwich have been used: 81.236.174.122 (talk) 02:37, 8 November 2008 (UTC)
 * 1) The famous Greenwich Meridian through the Royal Observatory
 * 2) The IERS Reference Meridian (102,5 metres to the east of (1))
 * 3) United Kingdom Ordnance Survey Zero Meridian (6 metres to the west of (1))
 * 4) Which is the fourth one?
 * Possibly the meridian inferred from Airy's transit circle, at the ? Franamax (talk) 10:34, 8 November 2008 (UTC)

The four historical Greenwich meridians were/are the meridians established by four Astronomers Royal through the years.
 * John Flamsteed (1646-1719) was appointed first Astronomer Royal in 1675 and found the observatory building that had been built for him at Greenwich by Christopher Wren unsuitable for his instruments, so built his own observatory in 1676, creating the original Greenwich meridian, what would later become known as Flamsteed’s meridian.
 * Edmond Halley (1656-1742) succeeded Flamsteed to become the second Astronomer Royal in 1720 and, in 1725, he installed new observing instruments 73 inches (185.4 centimeters) to the east of Flamsteed’s meridian, establishing the second of the Greenwich meridians, today known as Halley’s Meridian.
 * When the third Astronomer Royal, James Bradley (1693-1762), was appointed in 1742, he positioned his transit instrument 36 feet, 4 inches or (11.07 meters) east of Halley’s meridian. This is known as Bradley’s Meridian. This was the meridian used in the Nautical Almanac when it was first published in 1766; before that time the meridians at Greenwich would have only been of interest to astronomers. The Bradley meridian was also used by the Ordinance Survey as its “initial meridian” and remains so to this day.
 * The seventh Astronomer Royal, George Airy, moved the observatory’s instrument (and therewith, the meridian) 19 feet (5.79 meters) east of Bradley’s meridian to it’s present position. It was first used as the observatory’s prime meridian on January 4, 1851 (four days later than planned because of three nights of cloudy weather). This meridian, sometimes referred to as Airy’s meridian, is the one marked by a brass strip along the ground at the Greenwich Observaory and was, in 1884, adopted as the International Prime Meridian at the International Meridian Conference in Washington, D.C.
 * Although the IERS passes through Greenwich, and very near the Old Royal Observatory, it is not, technically or otherwise, the Greenwich meridian. There is resistance to the IERS being called the Prime Meridian (capital P, capital M) but in light of its primacy in use as the zero meridian for global, and globally recognised, coordinate systems, it's hard to deny it is the de facto prime meridian (lowercase p, lowercase m) of the world. --Blake the bookbinder (talk) 11:57, 8 November 2008 (UTC)
 * Greenwich meridian line states that 9 meridian lines have passed through Greenwich. Although I haven't found its source, my guess is that several are near duplicates: (1) The ETRS89 meridian, which is a meridian fixed to the European plate, is now about 25 cm west of the GPS/WGS84/IERS meridian, the latter of which moves relative to that plate (and the Royal Observatory). (2) The Bradley meridian 6 metres west of the Airy meridian is really three meridians, the original meridian used for the Principal Triangulation (published in 1858 and based on observations between 1783 and 1852) was 'lost' before the Retriangulation (OSGB36) (based on observations made between 1936 and 1953 and completed 1962) recovered it, and the new 'OSGB36' meridian which is a specified amount west of the ETRS89 meridian. Another may be the meridian of ED50. — Joe Kress (talk) 20:41, 8 November 2008 (UTC)
 * Joe, did you eat a lot of alphabet soup as a kid? ;) Thanks for the thorough info, as always. Anybody interested in seeing the relative positions of Flamsteed's and Halley's meridians at the Royal Observatory can have a look at this picture and a close up of the plaque marking Bradley's meridian here. --Blake the bookbinder (talk) 16:23, 9 November 2008 (UTC)

What, no New York meridian?
How come Philadelphia gets a meridian and New York doesn't? I wouldn't mind driving a Prime Meridian through New York City Hall or Liberty Island. 68.32.48.221 (talk) 05:47, 17 January 2009 (UTC)
 * what are you talking about? *dream on*dance on* 23:17, 30 November 2009 (UTC) —Preceding unsigned comment added by Taylor Lane (talk • contribs)
 * The list of prime meridians, of course. Washington and Philadelphia have both been used as prime meridians; as far as I know New York never was.  The IP wanted to know why; I suppose the reason is that nobody ever thought New York important enough to do so.  Certainly Liberty Island would never have been considered for one, since by the time it gained any significance at all the current international convention had already been established. -- Zsero (talk) 03:22, 1 December 2009 (UTC)

Prime x Equator
Is there a name for the place where the Prime Meridian crosses the Equator? I always imagined that there must be a buoy out there and some sort of tourist trip you could take from the African coast. Jndrline (talk) 21:04, 10 September 2009 (UTC) Nope, but there are ceremonies and special awards for those who cross the point, mainly sailors and others who spend time at sea. Set Sail  For The   Seven Seas    328° 13' 30" NET    21:52, 10 September 2009 (UTC)
 * It should be called the East Pole. But it isn't. Bazonka (talk) 18:12, 11 September 2009 (UTC)
 * Um, why? It's nothing like a pole. It's a completely arbitrary point along the equator.  -- Zsero (talk) 18:19, 11 September 2009 (UTC)
 * 
 * "I expect there's an East Pole and a West Pole, though people don't like talking about them."
 * So I guess if they do exist, this and its antipode would be the best place for them.   Set Sail   For The   Seven Seas    282° 22' 15" NET    18:49, 11 September 2009 (UTC)
 * Why would the crossing point of the zero meridian and the zero latitude be called the East Pole? Why not the West Pole? Why not the May Pole? Why not the Center of the World? Why not (0,0)? I do think there ought to be some monument anchored there so people could write graffiti on it and stand next to it to have their picture taken. Dlw20070716 (talk) 03:17, 19 July 2011 (UTC)

Hemispheres
The edit that has been made to Prime Meridian is mainly for clarity, as it is easier to understand. Also, the format has been used at Equator. Another thing, if "each value of the prime meridian defines the eastern and western hemispheres" then shouldn't it use "a" rather than "the"? Since "the hemisphere" is specific, but "a hemisphere" is general. Hope this helps. Set Sail  For The   Seven Seas    279° 29' 15" NET    18:37, 21 September 2009 (UTC)
 * It is not easier to understand, it is repetitive and poor style. "The Eastern and Western Hemispheres" is perfectly grammatical and crystal clear.  They aren't just "eastern and western hemispheres" in general, they are the Eastern and Western Hemispheres.  And it doesn't matter where you actually draw the prime meridian; once you've decided that it is the prime meridian, then everything east of it, until its antipode, is the eastern hemisphere, and ditto to the west.
 * If you're aiming for grammatical correctness and clarity, you're making things worse, not better. "Its opposite" is hardly grammatical!  The 180th meridian doesn't belong to the Prime Meridian!  The previous version was perfectly grammatical and clear.  -- Zsero (talk) 21:12, 21 September 2009 (UTC)


 * Specifying a prime meridian does not necessarily divide the Earth into two hemispheres. That only happens if longitude is counted both east and west from 0° to 180°. That was the third resolution passed by the International Meridian Conference in 1884. Many previous prime meridians counted longitude only in one direction from 0° to 360°. Before the 19th century, the longitude of a ship, such as Captain Cook's, was specified from 0° to 360° from the home port, such as Portsmouth, England, not from the prime meridian on his maps. — Joe Kress (talk) 06:49, 22 September 2009 (UTC)


 * @Zsero - The main point here is that by saying "The Eastern and Western Hemispheres", there is the suggestion that the are specific Eastern and Western Hemispheres. Yet, later on the article states, "the Prime Meridian is arbitrary", which gives a contradiction. How can an arbitrary Prime Meridian give specific Eastern and Western Hemispheres? Basically, by saying a instead of the, it is stating that a particular pair of hemispheres has been picked out to go with the Prime Meridian that had been picked out beforehand. In other words, it is arbitrary but fixed, meaning that it is generally arbitrary, but once it is chosen, it is fixed. Saying the implies that it is completely fixed and unchangeable. Basically, if "Prime Meridian" were to be replaced with specific meridians, ie. "Greenwich Meridian" or "Paris Meridian" etc, then the whole thing should make sense for all the specific meridians at the same time without clashing with one another.


 * Saying "The Prime Meridian and its opposite the 180th meridian" is preferable to "The Prime Meridian and the opposite 180th meridian", because in the former, is it easily seen that the 180th meridian in question is opposite to the Prime Meridian stated earlier. It is basically saying, "The Prime Meridian and that particular Prime Meridian's opposite , which has been defined as 180th meridian", which is more precise and focused than the latter. OK, I agree that saying, "The Prime Meridian and the opposite 180th meridian" gets the point across, but it is a bit vague and it could be rewritten a lot better than that.


 * So overall, the sentence should be along the lines of "The Prime Meridian and that particular Prime Meridian's opposite , which has been defined as 180th meridian together divide the Earth into a particular Eastern Hemisphere and a particular Western Hemisphere".


 * These points may be rendered redundant per another point below.


 * @Joe Kress - In that case, I believe that the sentence saying this should probably be removed.


 * @Both - "Specifying a prime meridian does not necessarily divide the Earth into two hemispheres. This only happens if longitude is counted both east and west from 0° to 180°." I definitely agree with this, but is it necessary to have the sentence stating an east west division of the earth, since there is no real difference between the Eastern Hemisphere and the Western Hemisphere as there is with the Northern Hemisphere and the Southern Hemisphere. After all, any Prime Meridian alongside its 180th meridian opposite can divide the Earth up into an Eastern Hemisphere and a Western Hemisphere and in all possible cases with every specific Prime Meridian there is again no real difference between the Eastern Hemisphere and the Western Hemisphere as opposed to the Northern Hemisphere and the Southern Hemisphere. Hope This Helps.   Set Sail   For The   Seven Seas    194° 45' 15" NET    12:59, 22 September 2009 (UTC)
 * There are specific Eastern and Western Hemispheres, just as there is one specific Prime Meridian. The choice of that meridian, and thus of those hemispheres, is arbitrary, but it has been made.  Were the Prime Meridian elsewhere, then so would be the Eastern and Western Hemispheres; but it isn't, so they're not.
 * In any case, even if you were right about the definite v indefinite article, there is certainly no reason at all to repeat the word "hemisphere" in the sentence. The most you can argue for is deleting the definite article, and leaving it as "...into eastern and western hemispheres" (uncapitalised, since according to you they don't refer to specific entities). -- Zsero (talk) 17:51, 22 September 2009 (UTC)


 * Actually, you're probably right there, it does make sense, I'll edit the article accordingly; but I still think the sentence may be unnecessary as there is no real difference between the Eastern Hemisphere and the Western Hemisphere as there is with the Northern Hemisphere and the Southern Hemisphere. Hope this helps.   Set Sail   For The   Seven Seas    275° 8' 15" NET    18:20, 22 September 2009 (UTC)
 * Thank you. Come to think about it, how about omitting the "opposite" language altogether, and just say that the Prime Meridian and the 180th meridian divide the earth into the Eastern and Western Hemispheres?  The fact that they happen to be conveniently opposite each other isn't really relevant to that.
 * As for what real difference there is between the two hemispheres, well, for one thing, one's mostly water and the other is mostly land. For that matter, cartographers used to divide the earth, not into northern and southern or eastern and western hemispheres, but into upper and lower ones.  The lower hemisphere was imagined to be one vast ocean, far too wide to cross in the ships of the day; that's why nobody until Columbus tried it, and he only did because he was a crank who believed all the geographers were wrong, and the earth was much smaller than they thought.  He was utterly wrong, of course, and they were utterly right, but he got lucky.  -- Zsero (talk) 18:36, 22 September 2009 (UTC)
 * That suits me just fine; but I would have thought that they would have needed to be opposites in order to create two hemispheres. Also, that is an interesting fact that they used divide the world that way. Columbus may have been lucky, but Magellan wasn't, his crew bore the full brunt of underestimating the size of the Earth.   Set Sail   For The   Seven Seas    286° 24' 0" NET    19:05, 22 September 2009 (UTC)
 * You'd think, but English is flexible enough a language to accommodate unequal "hemispheres" :-) -- Zsero (talk) 23:35, 22 September 2009 (UTC)
 * "Hemi-" means half. You can't have two unequal halves. Bazonka (talk) 06:24, 23 September 2009 (UTC)
 * Actually you can. If I describe the afikoman as "the bigger half" of the matzah, you know exactly what I mean.  English is like that. -- Zsero (talk) 07:29, 23 September 2009 (UTC)
 * Well, yes, people would know what you mean but you're not strictly correct. In many contexts it doesn't matter (i.e. Jewish bread - I had to look that up), but for scientific purposes you have to be more precise: a half = 50% = the same as the other half. Geodesy is very much a science, and so it doesn't really make sense to say that the two hemispheres could be unequal. (OK I know the earth is not a perfect sphere, so they're never going to be totally identical, but any differences would be negligible.) Bazonka (talk) 09:04, 23 September 2009 (UTC)

Equator
My recent addition of the Equator to the list of features (and vice versa) has just been reverted. I think this is a significant feature, and should be included - not least so that it is shown when all the significant points are mapped. I've added its coordinates in what I hope will be an acceptable compromise. (I'd be inclined to mention the two tropics and the polar circles, too; not to mention Greenwich Observatory). Andy Mabbett (User: Pigsonthewing ); Andy's talk; Andy's edits 17:14, 22 September 2009 (UTC)
 * Andy, it was me who amended your edit (I wouldn't have described it as "reverted"). Anyway, I'm happy with your latest changes. My only minor quibble is that I'm not sure whether the South Pole really deserves a line of its own in the table, because it's still part of the Antarctic. (Note that the Greenwich Observatory coordinates are displayed at the top right of the page. I'm sure this used to be the co-ordinates of the meridian at the equator, but someone must have changed it.) Bazonka (talk) 17:33, 22 September 2009 (UTC)
 * Indeed it was once the co-ordinates of the meridian at the equator, though not for long, as seen here, but it seems that the Greenwich Observatory coordinates are more appropriate since it is the defining location of the meridian.   Set Sail   For The   Seven Seas    267° 5' 15" NET    17:48, 22 September 2009 (UTC)
 * Apologies; I thought the wording you left us with was what was there originally, but on checking I see that you merged my edit into another section. Andy Mabbett (User: Pigsonthewing ); Andy's talk; Andy's edits 18:39, 22 September 2009 (UTC)

Order of list of prime meridians
The list was once ordered by longitude, and recently changed to being lexicographically, by the name of the city etc. I have reverted it to what seems to me the natural order: by longitude.

When a category has an obvious order on its members, it is normal to use that order when listing it. For example, the article "perfect number" lists the first four perfect numbers "6, 28, 496, 8128". It does not list them in lexicographical order "28, 496, 6, 8128". And a list of monarchs of England since 1066 normally begins "William I, William II, Henry I, Stephen, ..."; I have never seen "Anne, Charles I, Charles II, Edward I, ...". Maproom (talk) 11:39, 20 December 2010 (UTC)

Which Greenwich Meridian is Prime?
Until I read this article I did not realize how "off" our time and GPS meridian are from "The Prime Meridian" defined by the 1884 international agreement (not completely arbitrary) as passing through the observatory then in operation at Greenwich. I think that the article would be improved by editing out all the occurrences of "arbitrary" and simply insist that modern GPS and atomic clocks are off because the meridian has been mis-surveyed and the mis-surveys are now grandfathered in because there are too many maps to change. What's the use of an internationally agreed standard if it can be changed by 100 yards or more just because some early GPS system got off by that much? No wonder we end up bombing the wrong targets if our GPS is off by up to 100 yards. Dlw20070716 (talk) 02:55, 19 July 2011 (UTC)


 * It isn't just a matter of mis-surveying. If the people in charge of the early efforts at space-based geodesy had tried harder, perhaps by building a radio-telescope in Greenwich, the difference might be smaller than it is now. But the current prime meridian is based on an average of several different radio-observatories, all of which are moving with respect to each other due to continental drift. So the idea of basing the prime meridian on the location of a single observatory is obsolete. Jc3s5h (talk) 03:18, 19 July 2011 (UTC)

Map
Just double-checking as to why is French Guina colored blue on the map. If it is because the Prime Meridian runs through France, then why are the Falkan Islands and South Georgia and the South Sandwich Islands not also colored-in blue, is is it because they are only territories while French Guina is a region of France? —Preceding unsigned comment added by 216.96.155.229 (talk) 23:32, 25 April 2011 (UTC)
 * Yes, that's the reason. French Guiana is as much a part of France as Paris is. But the Falkland Islands are a dependency - belonging to, but not part of, the UK. Bazonka (talk) 07:54, 19 July 2011 (UTC)

Requested move

 * The following discussion is an archived discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. No further edits should be made to this section. 

The result of the move request was: Move. Jafeluv (talk) 12:26, 9 April 2012 (UTC)

Prime Meridian → Prime meridian – We don't capitalize generic terms. Dicklyon (talk) 05:23, 31 March 2012 (UTC)
 * Comment there are several related meridian requests, see Talk:Greenwich_Meridian, Talk:Paris Meridian, talk:Washington meridian, talk:Warsaw meridian. 70.24.244.198 (talk) 04:08, 1 April 2012 (UTC)
 * Agree. If this article were just about the Earth's prime meridian, then we should capitalise. But since it also covers the prime meridians of other plenetary bodies, then it is a generic term and lower case letters are more appropriate. Bazonka (talk) 14:18, 1 April 2012 (UTC)
 * Support as it is a generic term. Pinut (talk) 16:28, 4 April 2012 (UTC)
 * Support – most dictionaries show this in lower case. ENeville (talk) 23:26, 8 April 2012 (UTC)
 * The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

Merge IERS Reference Meridian into this article
Whilst this Prime Meridian article covers prime meridians on all planetary bodies, and alternative prime meridians on Earth, it is primarily about the most widely-used Prime Meridian on Earth, 0° longitude, running through Greenwich. This is what most people will expect to find when reading the article. The WP:COMMONNAME of the IERS Reference Meridian is "Prime Meridian" (particularly for non-specialists, i.e. the vast majority of readers of Wikipedia), and I can see no good reason why it has its own separate article. Its content would be much better held in the main Prime Meridian article, in particular the non-technical List of Places section. Bazonka (talk) 07:18, 6 April 2012 (UTC)
 * OPPOSE. Per DAB. The article is primarily about the concept named "prime meridian". The article has recently been moved from "Prime Meridian" to the generic name "Prime meridian". There are even several meridians at Greenwich. The common name of any prime meridian is prime meridian. That's why there are disambiguation pages in WP, because common names often are ambiguous. HTML2011 (talk) 02:26, 16 April 2012 (UTC)

Amended proposal - move
When I originally proposed this merge, this article was entitled "Prime Meridian". I agree that "prime meridian" is not the same thing as IERS Reference Meridian, but "Prime Meridian" is the common name for it. I now suggest moving IERS Reference Meridian to "Prime Meridian" (rather than it being a redirect to "prime meridian"), and place hatnotes into the two articles to link them together. (Maybe have a Prime meridian (disambiguation) article too.) See the discussion at Talk:IERS Reference Meridian. Bazonka (talk) 17:18, 16 April 2012 (UTC)


 * Oppose. The IERS reference meridian is not the only current prime meridian, and historically other meridians were the most important meridian of their eras. Jc3s5h (talk) 17:25, 16 April 2012 (UTC)
 * Oppose. Per DAB. Prime meridian is a broad concept. HTML2011 (talk) 05:11, 17 April 2012 (UTC)


 * Prime Meridian is ambiguous, even if you try to use the upper-case M to break that ambiguity; it doesn't really help. Leave it where it is, and make sure it's well linked from a hatnote in Prime meridian (where it's not, presently) and a disambig page, too, perhaps.  Dicklyon (talk) 05:44, 17 April 2012 (UTC)
 * Hatnot may be fine. I am not a fan of these, but they do less harm than to have a topic under an ambiguous title. HTML2011 (talk) 17:47, 17 April 2012 (UTC)
 * I added a hatnote. HTML2011 (talk) 03:03, 20 April 2012 (UTC)


 * Even when referring to "the" Greenwich meridian, dictionaries use "prime meridian". The IERS uses "Reference Meridian" for the one(s) they define, and dictionaries don't mention that meridian. In the absence of any other sources commenting on whether to capitalize the one(s) defined by IERS, I would follow their usage. Jc3s5h (talk) 17:49, 14 June 2012 (UTC)

From pole to pole
I removed this section : ChemTerm (talk) 21:59, 9 October 2012 (UTC)
 * it duplicates content found at IERS Reference Meridian
 * it was badly named, since it is a "List of places", called so at IERS Reference Meridian
 * the content does not belong here, since the page is about the general concept
 * this is meant to be an encyclopedia and not a coloring book where one can plaster any information anywhere
 * I strongly disagree with the removal of this information. To answer your specific points:
 * Duplicate content. So what? The information is held in a template so there cannot be any discrepancy between the different places where it is shown. And if duplicate content is not allowed on Wikipedia, then why have templates at all? Content should be held where it is useful to the reader.
 * Badly named. OK, so rename it then. This is not a reason to delete.
 * The page is about the general concept. Yes, but think of the readers of Wikipedia, most of whom would not be experts in geodesy. What do you think they expect to find in the Prime meridian article? A schoolkid who is doing an assignment on the prime meridian, and is looking for a list of the countries through which it passes would probably look in this article before they went to IERS Reference Meridian, if they even get there at all. IERS is such a technical term that most users aren't going to look in that article, and by hiding useful information there, it does a disservice to Wikipedia. I agree that IERS Reference Meridian is a more precise name than the vague and unspecific "prime meridian" but surely WP:COMMONNAME carries a lot of weight in this regard. There are other prime meridians (including those on other planetary bodies) but the one commonly and currently used on Earth is many orders of magnitude more notable than any of the others. The inclusion of more information about this is not WP:UNDUE.
 * This is meant to be an encyclopedia. Exactly, so let's present information where people would expect to find it. This is relevant to the subject matter so it can hardly be described as just "any information". Bazonka (talk) 22:21, 9 October 2012 (UTC)

Referring to your last bullet point - and what readers expect is defined by User:Bazonka ? Please also not that you are engaging in edit warring. Your addition has be contested and removed. But you re-inserted it. ChemTerm (talk) 23:27, 9 October 2012 (UTC)
 * I don't really know how to respond to that. Your question isn't really a question so I don't understand what you're asking. I am fully aware of what edit warring is, and I know that I am within the limits defined by WP:3RR Bazonka (talk) 23:47, 9 October 2012 (UTC)
 * What is missing to make "What readers expect is defined by User:Bazonka?" a question? One of the Five Ws is there, a question mark is there, a verb is there, a subject is there. What is missing to make it a question for User:Bazonka? As for editing warring, do you understand WP:3RR "The rule is not an entitlement to revert a page a specific number of times."? ChemTerm (talk) 01:33, 10 October 2012 (UTC)
 * ChemTerm, it looks to me as though you are the one trying to change a stable article, so it is up to you to get consensus for your changes. Read WP:BRD for how the revision process works. And since it looks like it is actually you who is insisting your changes must stick, as if you have some kind of right to make them, it's quite likely that you will be the first one blocked for edit-warring. Franamax (talk) 02:16, 10 October 2012 (UTC)
 * @ChemTerm, most of your English seems good, so I'm surprised that you think that your sentence is a proper question. Having a W and a question mark doesn't necessarily make a question - you really just wrote a statement and put a question mark on the end. For example, "What ChemTerm means is unclear?" meets your criteria but it's not a question. I would answer you, but I really don't know what you're asking.
 * You also added a systemic bias template to the Earth section of the article, with the comment "why is the UK meridian first?". It seems ridiculous to query this as being biased. Why shouldn't the UK meridian be first? It is the one that is currently used (with minor variations, e.g. Greenwich and IERS) throughout the world as the prime meridian. It would seem silly to put text about the Paris Meridian (for example) at the top of the list, but not Greenwich/IERS. If you are suggesting that IERS should come ahead of Greenwich in the section, then just reorder it. Don't tag the section with silly templates. Bazonka (talk) 07:17, 10 October 2012 (UTC)

Zeroes in longitudes
I see that an editor has gone through the article removing zreoes from the longitudes, converting, for example, Washington, D.C. 77°00′32.6″ W to Washington, D.C. 77°32.6″W and Greenwich 0° 00′ 05.33″ W to Greenwich 5.33″W

I find that this makes them harder to understand, so I have reverted the whole edit. Maproom (talk) 07:39, 1 February 2014 (UTC)
 * I agree that it's more readable with the zeroes. Also, on Prime meridian (Greenwich) (for example) the zeroes are still used: in the article but also in the gps coordinates. We should have one system for noting coordinates, not two. So I reverted the change again. Sander1453 (talk) 11:42, 1 February 2014 (UTC)

Dubious - arbitrary vs defined
The following sentence was tagged "dubious" by HTML2011 during his reorganization and split of the article:
 * The Prime Meridian is ultimately arbitrary unlike the parallels of latitude, which are defined by the rotational axis of the Earth with the Poles at 90° and the Equator at 0°.[dubious – discuss]

I see nothing dubious about that statement. Which of the two parts of the sentence is dubious? — Joe Kress (talk) 00:17, 29 March 2012 (UTC)
 * I also see nothing dubious there. The whole sentence is clearly true. Maproom (talk) 09:08, 29 March 2012 (UTC)


 * Perhaps better wording could be found. "Defined" and "arbitrary" are almost synonyms. The poles are an obvious choice due to the physics of the situation. The Prime Meridian is defined. It is arbitrary from a physics point of view, but not from a social point of view, or in view of the inventory of nautical charts (the original reason for the choice) and other intellectual property. Can anyone thing of a concise word or phrase for "strongly suggested by physics"?


 * To make the situation more complicated is that there are many kinds of poles, with corresponding meridians, such as ITRS, mean, true pole of date, and position of the pole on the surface of the Earth. Jc3s5h (talk) 12:03, 29 March 2012 (UTC)


 * I have replaced "defined" by "determined" - dos this help? Maproom (talk) 18:24, 29 March 2012 (UTC)
 * The lines of latitude are kind-of arbitrary, because there is no geographical or astronomical reason why there are 90 degrees from the equator to each pole. It is human convention that drives this. Bazonka (talk) 10:47, 30 March 2012 (UTC)
 * Perhaps this paragraph should only refer to the arbitrariness of the Prime Meridian vs. the geographically-defined Equator, removing the references to other lines of latitude. Bazonka (talk) 11:00, 30 March 2012 (UTC)
 * I have simplified the sentence, removing references to all lines of latitude except the equator. Bazonka (talk) 14:23, 1 April 2012 (UTC)
 * I still think the current sentence ("A prime meridian is ultimately arbitrary, unlike an equator, which is determined by the axis of rotation ...") is misleading. A prime meridian is a meridian so it must be terminated by the north and south poles, which means that it, too, is determined by the axis of rotation. I think this sentence should just be removed. Alternatively, how about this: "The choice of the 'prime' meridian is ultimately arbitrary, unlike like an equator, which must be at a latitude midway between the poles."? — Preceding unsigned comment added by 74.66.90.14 (talk) 04:16, 14 June 2014 (UTC)
 * Sounds good to me. --Lasunncty (talk) 00:14, 18 June 2014 (UTC)

New source
New to me, that is. Read German? See: Forstner, G. (2004); Längenfehler und Ausgangsmeridiane in alten Landkarten und Positionstabelle (dissertation), München, mainly p. 21-29. Online here. Sander1453 (talk) 08:46, 27 January 2015 (UTC)

Meridian of Bern
The value given (7° 26′ 22.5″ E) is the easting of the former astronomical observatory in Bern. This meridian was used in the Swiss coordinate system to link the projection and the coordinate system of national Swiss topographic maps (and data derived from them, e.g. cadastre entries in some cantons) with coordinates based on the Greenwich meridian. I do not know of any other use; so including this in the list of meridians is actually overstating, as the meridian of Bern was never used on maps with coordinates in something like “degrees east of Bern”. Eastings in degrees are given using the meridian of Greenwich (or, on the 1840 Dufour map, of Paris). It is a principal meridian for Switzerland, not a prime meridian.

I found this exact easting (and northing) on page 5 of the document linked as “Reference System CH” on this page of the Swiss Federal Office of Topography. (Note that in the meantime, the definitions have changed slightly; this is why the article “Swiss coordinate system” cites a slightly different meridian.)

No trace of the former observatory remains. On its site, in the 1960s the building for exact sciences (Exakte Wissenschaften, abbreviated ExWi) was erected. In one of its daylight courtyards, visible from my former office, was stona and a plaque indicating the centre of the former observatory. See e.g. for an image (not suitable for inclusion in Wikipedia – written permission required). -- David N. Jansen (talk) 23:01, 29 May 2016 (UTC)

Lunar vs. Solar eclipses
The shadow cast on the moon would be seen at the same time in all places on earth. What must be meant is solar eclipses, since at different points on the earth the moon occludes sight of the sun at different times.AtomAnt (talk) 02:15, 5 January 2015 (UTC)


 * Yes, the shadow cast on the moon would be seen at the same time in all places on earth. So if one observed and recorded a full lunar eclipse at 1 AM in one country, and someone else recorded the same eclipse at 3 AM in another country, one would know the countries were separated by two hours, or 30 degrees of longitude. Jc3s5h (talk) 04:40, 5 January 2015 (UTC)


 * Actually, what the two astronomers/observers conclude first is that their clocks differ by two hours. Only if their clocks agree with their respective (true or mean) local times, measured using sun observations, they can then further conclude that they are separated by 30° of longitude. (BTW, to measure “times of lunar eclipses”, astronomers often agreed on a specific moon crater and measured the time when that crater disappeared in the shadow. This gives more exact results than measuring the time of a point of contact. Similarly, the true local time is best measured based on sunrise and sunset rather than on culmination of the sun.) -- David N. Jansen (talk) 23:16, 29 May 2016 (UTC)

External links modified
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 * The documents.wolfram.com link is now at reference.wolfram.com. --Lasunncty (talk) 01:30, 25 May 2017 (UTC)

Meridianus Posoniensis
This Meridian is not incuded in the table and should be:

Locality: Bratislava, Slovakia

Modern longitude: approximately 17°06' E based on the Google maps and description below

Meridian name: Meridianus Posoniensis

Reasoning (see Sámuel Mikoviny page on Wikipedia): In 1731, Charles III delegaed Samuel Mikoviny to construct maps for Bel’s great work, Notitia Hungariae Novae Historico-Geographica.[16] Mikoviny then made a significant contribution to the making of a new map of the Kingdom of Hungary. [...] Mikoviny used his own prime meridian for the Kingdom of Hungary, the meridianus Posoniensis, which passed through the northeast tower of Bratislava Castle.

Reference [16]: Čižmár, Jozef (2013). Samuel Mikovíni: Významný slovenský inovátor [Samuel Mikovíni: Significant Slovak innovator] (in Slovak). Bratislava: Centrum vedecko-technických informácií SR. ISBN 978-80-89354-10-8.
 * I've added it to the list. Could you please add the reference yourself? – I don't have access to it, and anyway can't read Slovak. Maproom (talk) 12:25, 21 August 2017 (UTC)

External links modified (January 2018)
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 * Added archive https://web.archive.org/web/20070529031631/http://muweb.millersville.edu/~columbus/data/geo/WASHBR04.GEO to http://muweb.millersville.edu/~columbus/data/geo/WASHBR04.GEO

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 * I found an updated link for the first one. --Lasunncty (talk) 17:47, 25 January 2018 (UTC)

what is the east to west meridian ?
what is the east to west meridian ?


 * The lines on a globe that run east to west are called parallels of latitude. The 0° parallel is the equator. Jc3s5h (talk) 03:04, 11 August 2018 (UTC)

Longitude of Cadiz
The longitude of Cadiz depends on the part of the city it passes through. Cadiz is an island in the Bay of Cadiz about one mile from the mainland that trends NNW-SSE. The modern city occupies the entire island, about 3.5 miles long. The old city occupied only the westernmost mile of it. The longitude of the nearest observatory is 6°12'19", the Real Instituto y Observatorio de la Armada in San Fernando on the Isla de León, essentially part of the mainland, about six miles southeast of the old city. The central longitude of the modern city is near 6°17', near the most recent longitude in the article, 6°17'15". The previous stable longitude was 6°17'40", passing through the eastern half of the old city. The central longitude of the old city is near 6°18'. The longitude of the Universidad de Cadiz near the western shore of the old city is near 6°18'13". The Castillo de Santa Catalina is an old fort on the western or Atlantic shore of the old city with a longitude of 6°18'29". Another old fort is Castillo de San Sebastian on a tiny island near 6°18'49" only 2000 feet southwest of the old city. Which is the better longitude? — Joe Kress (talk) 04:05, 9 November 2018 (UTC)

The original longitude of Cadiz was added 12:58, 23 December 2016‎ by 83.43.241.182 stating that it was used by the "Spanish Army and other countries during a century", at first calling it the "Cadiz (Spain) Meridian" then calling it the "San Fernando Meridian (Cádiz, Spain)" in successive edit summaries. His source was the Spanish version (cited in the article) of this English source, "In search of the lost meridian of Cadiz", El País, 2016-12-23. The referenced study is by Miguel Ramos, who said the meridian was used from 1753 to 1850. In 1753 the Navy’s Astronomical Observatory was established inside the city’s Castillo de la Villa, no longer in existence, whose longitude became this lost Cadiz meridian. It was located on the south coast of the old city, described in both cited articles in terms of landmarks. Without more research, the original editor's longitude of 6°17'40" is good enough to keep it in the article. I have no idea why he called it the San Fernando Meridian which is far to the east, 6°12'19" as stated above. — Joe Kress (talk) 15:09, 9 November 2018 (UTC)

The Castillo de la Villa (long since replaced by another building) was located north of Av. Campo del Sur (built on landfill south of the original south shore), just west of Calle San Juan de Dios, just south of Calle Meson, and just east of Calle Silencio. All of these streets are described as its former borders in the Spanish Wikipedia Castillo de la Villa. Its central latitude was 36° 17' 42", and its central longitude was 6° 17' 35". I am replacing the article's 6° 17' 40" with the latter. — Joe Kress (talk) 17:13, 9 November 2018 (UTC)

The latitude is wrong, and tenths of an arcsecond need to be added to both to properly position where they cross in the center of the building. Thus 36° 31' 42.2" N, 6° 17' 35.5" W. The northern and southern facades of the original fort had 25-meter long walls between towers, and the eastern facade's wall was 53 meters between towers. Although the western facade was the longest, no dimension is given. — Joe Kress (talk)

Changed column heading from "GPS longitude" to "Modern longitude"
I changed column heading in the table that lists historical prime meridians from "GPS longitude" to "Modern longitude". The edit that introduced the term "GPS longitude" is not accompanied by anything that would suggest that the editor researched each meridian and established that each and every one of them was a "GPS longitude". For some of these meridians it is probably impossible to determine where the meridian was with enough precision to distinguish any of the modern flavors of longitude.

Incomplete citation
The section "Prime meridian at Greenwich" contains the following citation: "Greenwich Observatory ... the story of Britain's oldest scientific institution, the Royal Observatory at Greenwich and Herstmonceux, 1675–1975 p.10. Taylor & Francis, 1975" This is incomplete. The work is in fact in three volumes by separate authors: ''Greenwich Observatory. The Royal Observatory at Greenwich and Herstmonceux'', I675-1975. Volume i: Origins and Early History (I675-1835). By Eric G. Forbes. Volume ii: Recent History (1836-1975). By A. J. Meadows. Volume iii: The Buildings and Instruments. By Derek Howse. London: Taylor & Francis, I975. It looks as though the relevant volume is ii, but it would be good to check. Does anyone have access to this work? It's not in any of the libraries I use. If not, I can find an alternative source for this. Kognos (talk) 20:11, 27 September 2020 (UTC)

List of prime meridians: can there be more than one?
The table lists many meridians but I thought that by definition there can only be one 'Prime' meridian, the one established by international convention – that's what makes it prime. So what makes the others 'prime'? Are they not just 'reference' meridians? The table needs an intro and the comments column needs some text in every row as well as a citation, surely? --John Maynard Friedman (talk) 17:53, 4 December 2020 (UTC)
 * The first definition at lexico.com is
 * "A planet's meridian adopted as the zero of longitude."


 * Before the international conference in Washington, each nation tended to adopt its own. Even today, there are different ones for different purposes, although all the modern ones tend to be within a few hundred meters of each other. Jc3s5h (talk) 17:58, 4 December 2020 (UTC)
 * Yes, I knew about the Meridian of Paris, which was the main competitor to Greenwich. I guess what I am really saying is that the information in the table is so sparse that I can't tell whether these meridians are genuinely historic or more recent (and possibly OR/spoof additions. If the table had an intro to say something like "prior to the wossname International Conference in which the Greenwich meridian was formally adopted as the worldwide standard prime meridian, many countries defined their own prime meridian as given in the table below." or something like that. I didn't realise how little I know! --John Maynard Friedman (talk) 01:16, 5 December 2020 (UTC)

Era naming consistency
The history section uses both CE (common era) and AD (anno domini). There doesn't appear to be any contextual case for using the different naming conventions. One or the other should be used. Lanceleasure (talk) 14:10, 30 July 2021 (UTC)
 * Per MOS:ERA, the one first used must be retained. You raised the question, which means you have to fix it. :-D
 * It looks like the first use was diff=532860633&oldid=532838638 on 13 January 2013 by, who used the religious notation. --John Maynard Friedman (talk) 16:47, 30 July 2021 (UTC)
 * In fact, when I put that there, there was a single use of the CE notation in the table of meridians, which was the main content of the article at the time, under Ujjain, which I probably didn't notice. The current first paragraph of the history, the only other use of the CE notation, was inserted on 21 Oct 2020 by an IP with this edit diff=984595645&oldid=979745129.
 * MOS:ERA actually states "An article's established era style should not be changed without reasons specific to its content" and it's certainly arguable that the 7 uses of the religious notation have set the established style, quite apart from the fact that the current first paragraph in the History section is out of chronological order and would be more appropriate after Eratosthenes and Ptolemy.
 * I'm open to consensus on this issue. The first paragraph should probably be moved anyway. Chris55 (talk) 17:44, 30 July 2021 (UTC)
 * Further on the Hindu calculation in the first paragraph: I've read the reference by Schmidt and it says nothing about the calculation or significance of longitude, only the use of latitude. I don't know more of the Surya Siddhanta than in the Wiki article but that article also doesn't refer to longitude. Chris55 (talk) 16:46, 31 July 2021 (UTC)
 * Sorry but sauce for the goose etc. The first use was CE, there is no reason intrinsic to the topic to say why we should use one rather than the other, so your edits (however inadvertent) changed the established era style and it is they that must be changed to use CE. I'm afraid your argument reduces to WP:ILIKEIT. The Manual of Style policy exists because this is an issue where consensus is impossible: the perspectives are irreconcilable. --John Maynard Friedman (talk) 18:57, 31 July 2021 (UTC)
 * Not clear, but I'm happy to change it. Have done. Chris55 (talk) 19:26, 1 August 2021 (UTC)
 * These things never are, so thank you for the painless resolution.
 * I haven't looked at your concern about the Hindu calculation, will you pursue that one? --John Maynard Friedman (talk) 20:01, 1 August 2021 (UTC)
 * The Surya Siddhanta (translation) certainly talks about prime meridians (I found some texts) but the citation doesn't, it's concerned with the length of the day. But the Surya doesn't go any further than Ptolemy. Chris55 (talk) 21:15, 1 August 2021 (UTC)