Talk:Prime meridian (Greenwich)

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: page moved per sources and ngram -- JHunterJ (talk) 18:17, 14 April 2012 (UTC)

---

Greenwich Meridian → Greenwich meridian – There is no basis for the capitalization. This term is usually lowercase in sources. Dicklyon (talk) 02:41, 31 March 2012 (UTC)

• Support as nom. Dicklyon (talk) 04:11, 10 April 2012 (UTC)

• Support - Five separate Greenwich meridians are recognized, those of Flamsteed, Halley, Bradley, Airy and the IRM. This article should include at least the first four that were represented by transit circles, so "meridian" should not be capitalized. Bradley's meridian is still in use as the zero meridian of the Ordnance Survey. — Joe Kress (talk) 04:35, 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) 04:39, 31 March 2012 (UTC)
  • Comment also Talk:Prime Meridian. 70.24.244.198 (talk) 04:08, 1 April 2012 (UTC)

• Oppose. Greenwich Mean Time, Greenwich Peninsula, Greenwich Peninsula Ecology Park, Greenwich Hospital, Greenwich District. Greenwich Meridian. If there are 5 people called Joe, they would still be written Joe and not joe? Would they? So what does it matter that there are four or five? This is backed by sources Ngram HTML2011 (talk) 05:31, 31 March 2012 (UTC)

Huh? Your n-gram links shows a distinct minority capitalization of "meridian" in that context. What are you claiming? I agree the argument about number is meaningless, but look at MOS:CAPS. Dicklyon (talk) 05:41, 31 March 2012 (UTC)

• Oppose. This is the proper name for the Greenwich Meridian (of which there is only one), not just a Greenwich meridian. Bazonka (talk) 17:03, 1 April 2012 (UTC)

MOS:CAPS says we decide about proper names by consulting sources. Sources don't capitalize the Greenwich meridian, as the n-gram links show. Dicklyon (talk) 23:53, 2 April 2012 (UTC)

• Support. It looks to me like it is usually not capitalized, and we can follow the sources in this regard. Good Ol’factory ^(talk) 03:36, 4 April 2012 (UTC)
• Support—go with the sources (not to mention our house style). Tony (talk) 02:50, 10 April 2012 (UTC)
• Support— per Joe Kress. --Ohconfucius ^¡digame! 04:20, 10 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.

What is that? The supporters outvote logic? People base moves on dubious Ngrams? What is "per Joe Kress" - Joe Kress said there are several Greenwich meridians, but then the article should be named Meridians in Greenwich or Greenwich meridians. Tony says "go with the sources" - which sources? The Royal Observatory which says Greenwich Meridian at this page? HTML2011 (talk) 03:36, 17 April 2012 (UTC)

Look at the ext link named "sources" in the original proposed move. Dicklyon (talk) 22:45, 18 April 2012 (UTC)

GPS anomaly

I am interested in the commentary on one of the photographs:"A GPS receiver at the prime meridian. This does not indicate a longitude of zero because the GPS reference meridian is about 100 metres to the east." I can't see any reference to this in the main body of the text nor, for that matter, in the article on GPS. Is this worth referencing within the main article, and explaining why this has occured? Perhaps naively, I would have thought that the Greenwich Meridian would be the demarcation of East and West. KPOK (talk) 17:28, 2 January 2013 (UTC)

This is explained at Prime meridian#IERS Reference Meridian. In my view, this article in its present state adds so little to what is in the "Prime meridian" that it hardly deserves to exist.

Another issue with the picture is that recreation-grade GPS receivers can be off by 100 metres, so we can't say how much of the reading displayed is to a difference between the Greenwich and IERS meridian, and how much is due to error in the GPS measurement. Jc3s5h (talk) 17:47, 2 January 2013 (UTC)

Actually, the GPS is almost spot on as its position puts the meridian line 5.4 seconds of arc west of the practical meridian. It is in fact 5.3101 seconds of arc west of the meridian, so the GPS is off by about 1.5 metres. Not bad! I B Wright (talk) 17:13, 17 August 2015 (UTC)

The ecliptic and the prime meridian

Am I mistaken, or does the ecliptic intersect the equator exactly on the prime meridian? And did this play a role in the decision?

http://en.wikipedia.org/wiki/Ecliptic_coordinate_system#mediaviewer/File:Ecliptic_vs_equator_small.gif

Werner — Preceding unsigned comment added by 178.203.185.52 (talk) 12:19, 3 October 2014 (UTC)

No, the ecliptic intersects the celestial equator (which is the projection of the terrestrial equator into space) at the equinoxes. The prime meridian is on earth and moves with the surface of the Earth. Longitude is measured on the surface of the Earth from the prime meridian, which is 0°longitude. The celestial meridian passing through the celestial poles and the equinoxes doesn't have a special name that I know of, but obviously it is nearly fixed compared to the stars; it does not move with the surface of the earth. The angle measured eastward along the celestial equator from the vernal equinox to the point of interest is the right ascension. Jc3s5h (talk) 13:24, 3 October 2014 (UTC)

Thanks! You say "the ecliptic intersects the celestial equator (which is the projection of the terrestrial equator into space) at the equinoxes." Doesn't it do so roughly at the same spot each year? I'm just wondering why old geographers included an "ecliptic" on their maps, as if it is not fixed, how was it useful for navigating? Ah, well...

Werner — Preceding unsigned comment added by 178.203.185.52 (talk) 14:27, 3 October 2014 (UTC)

Astronomers often speak of the "fixed stars". These are stars that are so far away that their motion is not detectable; they serve as fixed reference points in the sky. Due to precession of the equinoxes, the location of the equinoxes makes a complete circle around the ecliptic (which is the projection of the plane of the Earth's orbit around the Sun onto the celestial sphere) in about 26,000 years. So the location of an equinox, compared to the fixed stars, is almost fixed. When thinking of the equinoxes as reference points, they exist all the time, not just when the Sun is at the equinox. Jc3s5h (talk) 14:58, 3 October 2014 (UTC)

Upmerge

this article overlaps significantly with the merger target section. Fgnievinski (talk) 03:17, 11 February 2015 (UTC)

In its present state this article seems like a suitable merge candidate. However, it could be expanded considerably by merging in United Kingdom Ordnance Survey Zero Meridian, which really doesn't need to be a separate article. Also, a description of any better measurements between the various meridians that pass through the Royal Observatory, Greenwich, may be found or better measurements may be performed in the future. So I think this article is a good repository for any details we wish to add about these meridians rather than cluttering up the Prime meridian article, which is already fairly long. Jc3s5h (talk) 15:19, 11 February 2015 (UTC)

Consensus for Calculation of observed transits?

A London area IP editor, 86.153.131.100 (talk) * contribs), made the following change [1], [2]:

• Was: "The times for the observed transits were therefore a little earlier than they should have been causing the measurements to refer to a meridian 102 metres east of the transit circle"
• To: "The times for the observed transits were therefore 354 milliseconds earlier than they should have been causing the measurements to refer to a meridian 102 metres east of the transit circle"

(emphasis added). I reverted as unsourced [3] and explained why on the IP's talk page [4]. The IP restored, with the editsummary "Anyone can calculate this", and a response on their talk page [5] noting "The calculation is 5.3101 seconds of arc divided by 360 degrees of arc multiplied by 24 hours = 0.354+ seconds".

I am not going to re-revert, but WP:CALC states "Routine calculations do not count as original research, provided there is consensus among editors that the result of the calculation is obvious, correct, and a meaningful reflection of the sources. Basic arithmetic, such as adding numbers, converting units, or calculating a person's age are some examples of routine calculations. "

• I don't think this calculation fits the plain meaning of a routine calculation. The elements of the calculation, noted only on the IP's talk page, are not contained in the article nor is the calculation itself obvious. As such, the change provided should be backed up by a reference to a reliable source.
• What is the consensus of other editors?

Thanks, JoeSperrazza (talk) 12:56, 20 August 2015 (UTC)

There is some discussion of the extent of the math exception in the thread Wikipedia talk:No original research‎#MEDRS and primary/secondary near the end. (The math question is really a bit off-topic for that thread.) One could argue this is just a unit conversion from arcseconds to seconds of time. If this were a more technical article, where only a person who converted between angles and time in their sleep could hope to understand the article, I would let it go. Since this is a more general-interest article, I would explain the conversion in a footnote.

I also noticed the precision of the coordinates for the Airy transit were too precise and came from a self-published source, so I corrected that. Jc3s5h (talk) 13:39, 20 August 2015 (UTC)

@JoeSperrazza: The calculation is fairly straightforward, but the result needs qualification. If the real meridian is 5.3101 seconds of arc east of the marked or defined meridian, and the earth takes 23 hours and 56 minutes (a sidereal day) to go through a full revolution (360 degrees), then the calculation is a simple ratio and gives .331424 seconds (or .3456 sidereal seconds if you prefer). I believe our anonymous friend assumed that the earth turns through 360 degrees in 24 hours which put his answer in sidereal seconds. I B Wright (talk) 13:45, 20 August 2015 (UTC)

It sounds like the text entered needs clarification or correction and a footnote. Would one of you subject matter experts be able to do that? Thanks, JoeSperrazza (talk) 14:00, 20 August 2015 (UTC)

I agree with @I B Wright: except for precision. Since I've edited the article to limit precision of the longitude of the Airy transit to tenths of arcseconds, the precision of the time should be limited to 3 places after the decimal point. Indeed, since Malys et al. don't provide any obvious error bounds on the longitude coordinate, it might be safer to limit the time to 2 places after the decimal point. Jc3s5h (talk) 14:13, 20 August 2015 (UTC)

The difference is insignificant. 5.31 seconds or 5.3 seconds gives the same answer to 3 significant figures. Sourcing the true eastern shift is problematic because the Greenwich observatory itself puts the GPS referenced meridian at 5.64 seconds east of the Airy transit circle, though they don't say what type of GPS receiver they used.

The measurement should have been made using a surveying type phase carrier receiver whose error is around 20 centimetres if 12 satellites are visible overhead. The best a standard recreational one can do is about 10 metres though this can be substantially improved upon if measurements are taken over a long period of time. I B Wright (talk) 14:27, 20 August 2015 (UTC)

Checking on Google maps, 5.3" misses the Airy transit circle by about 7 metres. 5.31" is better but still misses by a metre or two. I B Wright (talk) 14:39, 20 August 2015 (UTC)

References

Edit by I B Wright

I must take issue with this edit by User:I B Wright. What I view as an incorrect approach begins with the sentence "In each case, the transit instrument was installed to be perpendicular to the the reference elipsoid...." Transit circles, even today, are installed so that the axis about which the telescope turns is parallel to local level, as indicated by a spirit level or pool of mercury. This is equivalent to the axis of the telescope, when the telescope is pointed straight up, being parallel to a plumb line. Any correction for the deflection of the vertical would have been corrected mathematically after the observations were taken. One would have to find a source from the late 19th century to determine whether a correction for deflection of the vertical was made. The people who built the observatory, and later chose the Airy transit as the prime meridian, did exactly what they intended to; the current offset is due to new discoveries and refinements since that time. Jc3s5h (talk) 14:02, 20 August 2015 (UTC)

I fail to see your point. If you are saying that transit circles today are installed perpendicular to the local level using a spirit level or pool of mercury (the later unlikely now) and that it is equavalent to using a plumb line, where is the contention? The Airy transit circle was installed just prior to 1851 which would seem to predate any measurements of deflection of the vertical (or if they had started, would appear to be not very advanced if they were still ongoing at the time of the international conference). I would also point out that part was copy editing your contributions, so if you believe I have misinterpreted them, correct them. This is the encyclopedia anybody can edit. I B Wright (talk) 14:12, 20 August 2015 (UTC)

The edit claims the transit circle was installed perpendicular to the reference ellipsoid. In fact, it was installed perpendicular to local level. These are different. Reference ellipsoids are found by making geodesic surveys over a large area and choosing an ellipsoid that is a good compromise for all the stations surveyed. See Earth ellipsoid. Jc3s5h (talk) 14:21, 20 August 2015 (UTC)

Fair enough. I B Wright (talk) 14:29, 20 August 2015 (UTC)