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UTC - A Cautionary Tale
On July 4th, the International Earth Rotation Service (IERS) announced
that the first leap second in seven years will be issued at the end of December 2005. On July 5th, the head of the Earth Orientation Center of the IERS, Daniel Gambis, made the first official pronouncement that the next leap second may be the last leap second. What would this mean? This is such a drastic change in the philosophy of timekeeping that it is hard to express. This would mean that civil time worldwide - the clock on the wall, the watch on your wrist, the time provided by your cell phone and laptop and television and by the time signals on the radio - that civil time literally everywhere would cease to have any connection to the rotation of the Earth. An alternate interpretation could be that the Prime Meridian would begin to drift from the observatory at Greenwich England. For most purposes time-of-day would become just a polite fiction. For others, time-of-day would become a nightmare to calculate and correct for from tables downloaded from the internet or corrections typed in manually with all the usual ambiguity of sign and magnitude. To summarize the main points of the change proposed by the US delegation to a committee known as the ITU-R WP-7A which appears to have been indirectly delegated the authority to make this decision: 1 - Maintenance of a time scale called UTC. 2 - Suppression of the leap seconds adjustments which maintains UTC close to UT1, a time scale based on the Earth's rotation (currently UT1-UTC .9 s) 3 - The difference of UT1 from UTC should not exceed 1 hour. 4 - The change should take effect at 21 December 2007, 00:00 UTC The full text of Gambis's message is available from: http://www.ucolick.org/~sla/leapsecs/gambis.html A link to the official proposal is available at the bottom of that page, along with a list of committee members who can be contacted with comments. I have a hard time expressing how much I detest both this proposal and the process through which it has been made. Gambis is to be applauded for finally bringing it to light after six years of furtive discussions within the precision timing community. Belgian astronomer and mathematician Jean Meeus comments on the proposal: http://www.ucolick.org/~sla/leapsecs...vent2005-07-08 Steve Allen of Lick Observatory provides an excellent page of UTC and leap second resources: http://www.ucolick.org/~sla/leapsecs There would be a significant expense (software, hardware, and operations) to the astronomical community should leap seconds cease, but my visceral rejection of this proposal is rooted more deeply. We should not so blithely discard the ties between our clocks and the rotation of the Earth. As Steve Allen points out, time has always meant Earth rotation because civil time has always been a subdivision of the calendar. This proposal is simply goofy - but it is no less a real threat for that. Daniel Gambis suggests interested parties (potentially everybody on the planet) take action: "If your activity is affected by the content of the US proposal which will be discussed in November 2005 at the WP-7A, you are urged to react. This could be the last opportunity before a recommendation is issued by the WP-7A. If you wish you can express your opinion to your representative(s) at the WP-7A of ITU (for the list see the ITU website, http://www.itu.int/home/index.html) with a copy to Daniel Gambis ), IERS EOP Center." Finally, a few comments on the points of the proposal to provide context: Note the absence of any suggestion of what affected parties might do about such a change. Note the absence of any discussion of what changes or improvements might be forthcoming from WWV, NTP, GPS and other systems currently used to distribute Universal Time. The last six years would have better been spent discussing how to improve the systems we already have, not how to dismantle them. 1 - Maintenance of a time scale called UTC. Universal Time was designed to be equivalent to Greenwich Mean Time. The original wording of the standard governing the distribution of UTC via radio signals (e.g., WWV) actually strengthens this statement by reversing it: "GMT may be regarded as the general equivalent of UT" This wording has recently been removed. There are many flavors of UT - UT0, UT1, UT2 - which form progressively better approximations. The precision timing community has explicitly rejected the idea of ceasing the issuance of leap seconds by rather constructing a new civil time standard (to be called "TI" for "International Time"). They insist on continuing to call it UTC. Thus, under this proposal, one flavor of Universal Time, UTC, would diverge in meaning from all others. 2 - Suppression of the leap seconds adjustments which maintains UTC close to UT1, a time scale based on the Earth's rotation (currently UT1-UTC .9 s) The precision timing community appears to assume that all usage of UTC takes advantage of the correction signal, DUT1, which is the difference between UTC and UT1, provided to a tenth second accuracy. Most astronomical applications, however, appear to use UTC as an approximation accurate to 1 second. Software that does account for DUT1 may not be able to support a value greater than the 0.9s allowed by the current standard. Software that does not account for DUT1 will simply be wrong. The maximum excursion allowed by the current WWV correction signal is 3 seconds. Even if our WWV clocks handle DUT1, they will soon need to be replaced. The Network Time Protocol that sets the clocks on our computers provides only minimal support for leap seconds (and none for DUT1) now. It would provide none later. It is likely that any new system that emerges to provide access to UT1 (or other flavor of Universal Time) would never be implemented on the legacy computers that drive many of our telescopes and instruments. Only new hardware is likely to be responsive to our future needs. Y2K was a non-event for the astronomical community, like many other, only because the astronomical software community worked hard to make it so. Some of our telescopes tracked backward until the software was fixed. An IRAF release was required. The FITS standard was modified. Many other changes were made throughout all our systems. Even a minor change to UTC might require large - and large numbers of - changes to our software and hardware (and operating procedures). This would not be a minor change. 3 - The difference of UT1 from UTC should not exceed 1 hour. The width of a time zone is one hour. The idea of leap hours to be issued every half millennium or so is equivalent to no civil time-of-day at all. 4 - The change should take effect at 21 December 2007, 00:00 UTC The current standard is good for at least the next half millennium. The rate of leap seconds will increase quadratically. (We don't need leap seconds because the Earth is slowing down - we need leap seconds because it has already slowed down since the 1 Jan 1900 epoch.) The current standard allows one leap second per month. There is plenty of time to make this decision prudently and with public comment from all affected communities (everybody, everywhere). Law suits resulted in the 19th century from disputes over the interpretation of the precise beginning and ending times of contracts - twice. First when the world community switched from local apparent time (sundial time) to local mean time (clock time). Second when it switched from local time to standard (zone) time. This is not only (or even primarily) a technical issue. Since this is a proposal originating with the US delegation, it is important that the WP-7A understand that they don't represent a US consensus on this issue. One has to also wonder how such a narrow working group (or even the larger ITU) believes itself empowered to make a unilateral change that would affect so many other interested parties (literally everybody, everywhere). Rob Seaman NOAO |
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In sci.astro.research Rob Seaman wrote:
On July 5th, the head of the Earth Orientation Center of the IERS, Daniel Gambis, made the first official pronouncement that the next leap second may be the last leap second. [[...]] Steve Allen of Lick Observatory provides an excellent page of UTC and leap second resources: http://www.ucolick.org/~sla/leapsecs I also found the following article to be interesting: R. A. Nelson et al, "The Leap Second: Its History and Possible Future" Metrologia volume 38 (2001), pages 509-529 http://www.iop.org/EJ/article/0026-1...6/6/me1606.pdf ciao, -- -- "Jonathan Thornburg -- remove -animal to reply" Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut), Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html "Space travel is utter bilge" -- common misquote of UK Astronomer Royal Richard Woolley's remarks of 1956 "All this writing about space travel is utter bilge. To go to the moon would cost as much as a major war." -- what he actually said |
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