Talk:Greenwich Mean Time

Archive of Time Cube discussion

Archiving

This talk page is getting long. Does anyone object to me setting up archiving using MiszaBot? An example of a talk page with that setup is Talk:Gregorian calendar. --Jc3s5h (talk) 00:24, 9 July 2009 (UTC)

[From Terry0051] Sounds sensible to me. Just to make sure the preceding discussions are not in practice forgotten I took the opportunity to incorporate a few amendments to clarify/remove a couple of ambiguities/correct anachronism about UT1. Terry0051 (talk) 11:19, 9 July 2009 (UTC)

Prime Meridian

The section on the prime meridian is no longer quite correct: when the GPS system was set up, a choice of baseline had to be made as many measurements had previously been slightly inaccurate. The decision was made to preserve the 90 degree line where it was and use that as a reference (coincidentally, this is the line that runs through the US ;) and hence the meridian at Greenwich is now about 100m of its traditional location (which is great for confusing tourists clutching GPS gear...) Calum (talk) 11:12, 5 November 2009 (UTC)

[From Terry0051] That's interesting -- is there any RS for a "decision .. to preserve the 90 degree line"? Terry0051 (talk) 12:05, 5 November 2009 (UTC)

I do not believe this statement. The fundamental reference system used by the Global Positioning System is WGS84. Zero longitude in WGS84 is the IERS Reference Meridian which is also the zero longitude of the International Terrestrial Reference System (an ideal system) and the International Terrestrial Reference Frame (a practical system). I suppose a lay person's description would be a position based on an average of radio telescopes that can measure extremely distant radio sources, the moon, or special-purpose satellites. Because the radio telescopes are located on different continents, the averaging process tends to cancel out continental drift. At the time the radio techniques came into use, I'm sure there was an effort to match the Prime Meridian that was determined optically with transit circles. I am not able to give a step-by-step analysis of all the measurement tolerances and compromises that lead to the present 100 m discrepancy.

The part about the IERS Reference Meridian being about 100 m east of Airy's transit circle is true. --Jc3s5h (talk) 17:30, 5 November 2009 (UTC)

The source of Calum's claim is NADCON, which shows no change in longitude between NAD27 and NAD83 (=WGS84) just east of 90°W, roughly along 87°W. Although most of this was intentional, not all of it was. "Navigation at the Prime Meridian" (1971), cited in IERS Reference Meridian, states "The original longitude reference of the APL [world] datum was selected as the surveyed longitude of the [Applied Physics] Laboratory's site in Howard County, Maryland, a coordinate in the North American datum [1927]." The Applied Physics Laboratory is located midway between Washington, DC and Baltimore with a longitude of 76°50'43"W (and a latitude of 39°9'58"N). It developed the TRANSIT satellite Doppler navigation system beginning in 1958. The calculated shift using a standard Molodensky transformation from the non geocentric NAD27 (the center of its ellipsoid is 241 m away from Earth's center) to the geocentric NAD83 [1] yields a large 8.34" or 161 m shift at Greenwich. But actual measurement shows that the APL reference meridian was 5.64" (108.8 m) east of Airy's transit circle in June 1969, long before the BIH adopted an earth centered ellipsoid in 1984. The MERIT/COTES campaigns of 1983–84 added lunar laser ranging, satellite laser ranging, and Very Long Baseline Interferometry. These resulted in a small shift to the west of 0.806" (15.6 m) from TRANSIT's reference meridian,[2] which roughly corresponds to the current 1.09" shift at APL according to NADCON. Currently, the IERS reference meridian is 5.31" (102.5 m) east of Airy's transit instrument.

The History of the BIH (§5) has a different reason for the 100 meter shift. It states that it was caused by the continuity condition on UT1, that is, that UT1 should not exhibit any discontinuity due to the changeover in 1984 from time (and longitude) based on the meridian transits of fixed stars at several astronomical observatories to time based on observations of satellites, both artificial (SLR and Doppler) and the Moon (LLR). Indeed, there was no sudden shift in UT1 in 1984.[3] Before 1984, the BIH calculated UT1 as an average of the time determined by several national astronomical observatories, corrected for propogation delay, but not corrected for the several datums (non geocentric ellipsoids) used by those observatories. I suspect that this continuity condition on UT1 determined the reference longitude for the SLR/LLR/VLBI frame, hence caused the final 15.6 m shift in 1984. If TRANSIT is ignored, the entire shift could be attributed to UT1.

The opinion of the Royal Observatory itself, now a tourist attraction, that the 100 meter shift was the result of the accretion of several small inaccuracies, could also be included in the pre-1984 history.[4] So every one of these reasons is correct in its own way. — Joe Kress (talk) 03:51, 16 November 2009 (UTC)

Not so much inaccuracies, but since the earth is not a perfect ellipsoid each country's scientists made the best fit approximation but did so to minimise errors in their own country. Since GPS is an American system it fits best there, its not really a coincidence, if it was English there would be no error. QuentinUK (talk) 13:55, 14 April 2011 (UTC)

I would not suppose that GPS fits better in the U.S. than elsewhere. After all, it was initiated by the U.S. Dept. of Defense, and they usually fight outside North America, so I would expect equal treatment of the whole world, or at least the land areas. Jc3s5h (talk) 21:31, 14 April 2011 (UTC)

GMT date format

Can't see info about GMT date format. I've a short note to that in my head, for example: GMT date format (time format) uses the ISO 8601 standard of YYYY-MM-DD HH:MM:SS GMT. Or maybe simple internal wiki reference to ISO_8601 would be enough. Hondrej (talk) 16:08, 25 October 2010 (UTC)

The article does not mention a date format because GMT does not have any. ISO 8601 requires the use of UTC and even states that UTC is not GMT. Nautical navigation, which does use GMT, specifies that its GMT is UT1, not UTC, where UT1 is the mean solar time at the Royal Observatory, not its atomic clock approximation, UTC, which can be in error by as much as 0.9 seconds. No date format is specified in Dutton's Nautical Navigation (2004), but it uses both day/month and month/day, where the month is either spelled out or uses a three letter abbreviation. — Joe Kress (talk) 00:21, 26 October 2010 (UTC)

Format

Is it just me or does this article look grossly out of format? I see stuff that looks like a Courier fonted DOS page on my screen for this article! Ratibgreat (talk) 17:16, 1 April 2011 (UTC)

It generally looks normal to me. Was there a specific passage you had in mind? If so, please quote a bit of it so we can search for it using our browser's search feature. Jc3s5h (talk) 19:55, 1 April 2011 (UTC)

No, apparently everything's back to normal. It looked like a DOS page, I tell you!Ratibgreat (talk) 07:37, 4 April 2011 (UTC)

I have seen some Wikipedia pages, usually talk pages while I'm previewing them after adding a comment but before saving it, with monospaced font and no Wikipedia skin whatsoever, over a week ago, although not within the last week. — Joe Kress (talk) 21:21, 4 April 2011 (UTC)

I've seen this happen. It looks like the Wikipedia server is getting confused about what type of computer is connected and formats the page for a portable device. QuentinUK (talk) 13:39, 14 April 2011 (UTC)

GMT is 'atomic time', or 'earth angle time' ?

[Note: above heading was added at 13:08, 2 October 2011 by User: CorvetteZ51.

There is no answer for this. The scientific community has retired the term "GMT". Some countries, like the UK, have laws that set civil time as GMT, or an offset from GMT, but the UK parliament has declined to make any law clarifying the current meaning of GMT in the law. As far as I know, no other country with such laws has clarified the meaning of GMT since the introduction of UTC. Jc3s5h (talk) 14:27, 2 October 2011 (UTC)

OK, I give up. UTC is what I call, 'cogged - international atomic time', which is kept close to the mean-sun with 'leap seconds'. UTC is always adjusted in units of a whole second. OTOH, UT-zero is continuously adjustable and represents the angle of the earth. which of those two us closer to GMT? CorvetteZ51 (talk) 09:47, 6 October 2011 (UTC)

The only country I know of that officially bases its time scale on "Greenwich Mean Time" is the UK. Who knows if their law is even valid, it might be overridden by some European Community law. Of course, there may be other countries that base their time on GMT, I just don't know about them.

The UK House of Lords debated a bill that would have changed to UTC. One sentence from the debate is "The use of Greenwich Mean Time for scientific purposes duly died out, although the term lives on in everyday parlance. One reason the scientific community no longer uses the term GMT--as the noble Lord, Lord Tanlaw, explained to us--is that we are no longer really sure what GMT is." The bill did not pass. So my interpretation is that the UK has been asked to clarify what GMT is, or else stop using it, and they refused to act. Jc3s5h (talk) 12:00, 6 October 2011 (UTC)

Of course we refused. Greenwich is in England, and we invented the concept of standard time. --Redrose64 (talk) 12:57, 6 October 2011 (UTC)