Re: Risks of change to UTC
M. Warner Losh wrote: UTC works for navigation, but leap seconds pose problems for other users of time. Stating absolutely that UTC is not broken ignores these other users. Those other uses, for whom leap seconds pose a problem, should be using a time scale that does not have leap seconds. They would be better served, for example, by TAI. UTC, with its leap seconds, evinces the fundamental problem that calendar designers have faced through the ages: trying to devise a compromise system that blends mutually incommensurable units. For civil use, a calendar that counts days reaonably accurately is appropriate. The Gregorian (New Style calendar) that the vast majority of the planetary population uses does this. UTC copes with the variable length of the mean solar day by inserting leap seconds as needed. The role of the IERS in decreeing when leap seconds are needed is similar to that of the Roman College of Pontifices who managed the old Roman Republican calendar (before Julius Caesar's reform) by decreeing, as needed, when to shorten the month of February and insert the intercalary month of Mercurius. Since we human creatures synch our cicadian rhythms and our daily activities to periods of light and darkness, the day is a natural unit of time for use in our calendars. Indeed, I know of no calendric system that does not count days. Unfortunately, the day is ncommensurate with the SI second, and the length of the day is changing. If your primary need is for a time scale ithat counts SI seconds, with no leap seconds to confuse the matter, then don't use UTC. Use a time scale that counts SI seconds, such as TAI or GPS time. There's no point to applying the mised radix Gregorian calendar system to such a time scale, although you can do so if you wish. Count days of 86 400 SI seconds each, or GPS weeks of 604 800 SI seconds, or just count SI seconds. If, on the other hand, you need to count solar days, or mean solar days, use a calendar and time scale that does so. In order to know which way the earth is pointing, use UT1, or a compromise scale such as UTC that is kept reasonably close to UT1. For the vast majority of the population of the planet, including celestial navigators, UTC is good enough. If you want to know the direction the earth is pointing with more precision, apply DUT1 corrections, or use other IERS products such as Bulletin A. There is no need to fix the time scale, UTC, that is used by the vast majority of the planet's population to accommodate the very small minority of precision time users who desire a time scale that has no leap seconds. Let that minority use a time scale, such as TAI, that does not have those messy leap seconds. -- James Maynard Salem, Oregon, USA
Re: Risks of change to UTC
On 21 Jan 2006 at 10:11, M. Warner Losh wrote: I maintain that for human activity, there's no need for leap seconds at all. In each person's lifetime, the accumulated error is on the order of a few minutes. Over generations, the problems with noon drifting to 1pm can trivially be solved by moving the timezones that civilian time uses. What about when that accumulated difference is over 24 hours, so the offset between solar-based time and atomic time is actually on the order of days? Will people be able to deal with a civil time standard that is based on an offset from a UTC that says it's Monday when all actual points on Earth have the local date at Saturday or Sunday? Many Web sites (including Wikipedia) use UTC as the standard for date/timestamps; will this be a reasonable thing when this causes the date of postings to be far off from what is being used locally? And when, at some future point, the Gregorian calendar itself needs adjustment to handle the fact that it doesn't get the length of the year precisely correctly (and the length of the year in terms of solar days is changing due to the lengthening of the day, anyway), will this adjustment be done to the UTC standard (why, when it doesn't follow astronomy anyway?), or as an additional offset to local times (which could result in different countries having different dates as in the Julian/Gregorian transition period)? -- == Dan == Dan's Mail Format Site: http://mailformat.dan.info/ Dan's Web Tips: http://webtips.dan.info/ Dan's Domain Site: http://domains.dan.info/
Re: Risks of change to UTC
In message: [EMAIL PROTECTED] Daniel R. Tobias [EMAIL PROTECTED] writes: : On 21 Jan 2006 at 10:11, M. Warner Losh wrote: : : I maintain that for human activity, there's no need for leap seconds : at all. In each person's lifetime, the accumulated error is on the : order of a few minutes. Over generations, the problems with noon : drifting to 1pm can trivially be solved by moving the timezones that : civilian time uses. : : What about when that accumulated difference is over 24 hours, so the : offset between solar-based time and atomic time is actually on the : order of days? From http://www.ucolick.org/~sla/leapsecs/dutc.html we know that the rate of change of the day is somewhere between 25.6 s/century^2 and 42 s/century^2. At those rates, it will be the year 6360-7633 when enough time has accumulated for there to be a day difference. Before then, however, we go through a number of events. Somewhere between 2058 and 2211, enter the realm where there's more than 2 leap seconds per year. This will be the first great UTC breakage because many devices today (ntp included) KNOW that leap seconds happen twice yearly. The next break will happen sometime between 2300 and 2600 when we'll need more than 4 leap seconds a year. The current ITU-R TG.460 standard for leap seconds defines only primary and secondary leap second times. It does not define tertiary, so no one knows when the leap seconds will happen if you need more than 4 per year, but the ITU recommendation is that they happen at the end of a month. Somewhere between 3250 and 4200 there will be more than 1 leap second a month needed. At this point the ITU scheme of having the leap second at the end of the month will need to be modified. That's at least 2000 years before 24 hours of delta have accumulated. For some perspective, we've been using UTC for only ~50 years and the gregorian calendar for only ~1500 years. I'd anticipate that something would need to be done about the slowing of the day well before 4300 years have passed. Somewhere around betwee 45,000-80,000 you'll need more than one leap second a day. : Will people be able to deal with a civil time : standard that is based on an offset from a UTC that says it's : Monday when all actual points on Earth have the local date at : Saturday or Sunday? Since that's 4k or more years into the future, who alive today will know enough about what the future is like to impose a scheme that is guranteed to work? Clearly some scheme better than leap seconds will need to be invented well in advance of these events. A new scheme will be needed well in advance of the Tuesday is really Wednesday problem. : Many Web sites (including Wikipedia) use UTC as : the standard for date/timestamps; will this be a reasonable thing : when this causes the date of postings to be far off from what is : being used locally? And when, at some future point, the Gregorian : calendar itself needs adjustment to handle the fact that it doesn't : get the length of the year precisely correctly (and the length of the : year in terms of solar days is changing due to the lengthening of the : day, anyway), will this adjustment be done to the UTC standard (why, : when it doesn't follow astronomy anyway?), or as an additional offset : to local times (which could result in different countries having : different dates as in the Julian/Gregorian transition period)? The length of the gregorian calendar is off by 23s per year. In year 5500 or so we'll have accumulated a day of error in it and we'll need to skip a leap year to correct for that problem. This is a good 2000 years before we'll have accumulated a day of DUT. As you can see from the above, leap seconds won't save you. They will run out of steap in about 1500 to 2500 years. At that point the accumulated difference will be only about 2 hours. If leap seconds are totally abolished, time zone transitions could easily continue for about 4000 years. Either way, you have a problem. The length of the SI second is fixed, and the length of the day is getting shorter. 1500 years ago, no one spoke English. Chances are the people that deal with this problem in 1000 or 2000 years won't speak any language recognizable to anybody alive today. Warner
Re: Risks of change to UTC
On Jan 20, 2006, at 10:17 PM, M. Warner Losh wrote: Any watch that is smart enough to decode those signals would be smart enough to add this minor correction as well. A viable time scale could be constructed from any periodic (or near periodic) waveform - there's nothing magic about the tick, tick, tick of delta functions (or step functions if you prefer to think of it that way). A viable watch could present a different representation - 12 hour/24 hour, sexigesimal/decimal, local/universal - every time it is consulted. It wouldn't even have to be monotonic, as long as enough temporal metadata is provided for context. That metadata (such as DUT1 and DTAI) could be provided in as circuitous and obscure a fashion as can be imagined - encrypted, proprietary, steganographically. Heck - time signals might even arrive as little blips on shortwave radio - as hard as that might be to believe. There's viable - and then there's viable... The mechanical watch might be a bit of a problem, but DUT1 doesn't change enough to introduce navigation errors similar to what we have today over the course of a year and can easily be looked up like someone would lookup what the weather was going to be like. ...and we're back to the confusion between periodic and secular effects. There seems to be some thought that mean solar time is nothing but a polite (or lately, sometimes impolite) fiction. Greenwich Mean Time is real enough to have built the British Empire. You're also working both sides of the equation. A navigator observes local apparent solar time onboard and compares it to GMT (or mean time on any other known meridian) transported via chronometer. DUT1 is a mechanism to correct mean solar time as reported by the clock. The equation of time, on the other hand, is used to convert shipboard apparent time to local mean time. Subtraction does the rest. Rob Seaman NOAO
Re: Risks of change to UTC
On Jan 21, 2006, at 12:03 AM, M. Warner Losh wrote: WWV and most of the world's time stations broadcast DUT1. I should have added in my last message that some change in the signal format would be necessary if the range of DUT1 exceeds 0.9s. Bearing in mind that the ITU proposal would cease the reporting of DUT1. I will note that the profile of high precision time users has changed since 1972 when UTC was invented. [...] Should we continue to tie our time up in knots because of a tiny minority of users? Am fascinated by the failure of the precision timekeeping community to perceive six and a half billion souls as users. Shouldn't choices related to international/civil/legal/business/historical time be based on their needs? No matter what the profile of high precision time users has become - it is that entire community who comprise a tiny minority. How many celestial navigators are there today? The Apollo astronauts relied on navigation by sextant - celestial indeed. One expects that future solar system explorers will carefully continue to carry such fundamental instrumentation - certainly for emergencies (think Shackleton, not just Magellan), but also perhaps as an agile and reliable primary resource in their toolkits. Am not arguing that this has a direct connection to the matter at hand - but rather that old doesn't mean obsolete. Over the next 50 years, these two watches will be well within the tolerance of most normal watches. This interpretation confuses systematic effects (monotonically diverging timescales) with random errors. Much (one is tempted to say, all) experimental science depends on abstracting trends from noisy data. No matter how large a tolerance one allows, the diverging meanings of clock will eventually exceed it. The approximation of civil time will be less than one minute off during that time A useful approximation captures asymptotic or otherwise limiting behavior. Where there is no limit, there may be an agreement to draw a line in the sand - but there can be no approximation. Einstein isn't right and Newton wrong, rather Newton's laws are correct in the limit - the everyday limit. High precision time users may well place stringent requirements on fundamental timescales. But civil time requires a common sense everyday compromise. What this entire début de siecle discussion has been about is whether ignoring the whole question for a few hundred years is more common sensical than continuing to issue occasional small corrections. If this is a real issue, the market will take over and produce watches that have 'navigation time' and 'civilian time' at the touch of a button The market has not proven itself creative in meeting highly technical needs. Time and again, the market has converged on significantly less than ideal solutions - Windows, VHS, internal combustion. If the magic hand of the market is the first law (conservation of energy) of modern economic theory, the second law (entropy) is the tragedy of the commons. Timekeeping is pervasive in our society, but often invisible. As with the grand environmental challenges confronting this natal century, market forces threaten to provide - oh so efficiently provide - the wrong answers to timekeeping questions. Of course, purely mechanical watches have other issues when used on a boat. And yet Harrison found a way around these when he invented the first chronometers - for the express purpose of being used on ships. Stating absolutely that UTC is not broken ignores these other users. UTC is not broken. We may agree or disagree on whether it meets various civil or technical timekeeping requirements - but broken would imply that it fails to meet its *own* requirements. UTC is eminently capable of continuing unchanged for many centuries - and for millennia more with only slight changes. After that, nothing yet proposed (except for those danged rubber seconds) is any better (see http://iraf.noao.edu/~seaman/leap). It would be the abandonment of leap seconds that would break UTC. Lobbying to base civil time on some underlying timescale distinct from UTC would be one thing. Conspiring to emasculate UTC is quite another. GLASNOS is a backup system to GPS that is not subject to DoD's selective denial of signal. Glasnost was Mikhail Gorbachev's policy of encouraging open public debate, particularly in support of perestroika - restructuring - of the Soviet economy. On the other hand, GLONASS is the Russian Global Navigation Satelllite System :-) In any event, one suspects that the Russians (or the FSU, even more so) would object to its being characterized as a GPS backup. Rob Seaman NOAO
Re: Risks of change to UTC
On 21 Jan 2006 at 15:15, M. Warner Losh wrote: For some perspective, we've been using UTC for only ~50 years and the gregorian calendar for only ~1500 years. I'd anticipate that something would need to be done about the slowing of the day well before 4300 years have passed. Actually, that's more like ~400 years for the Gregorian calendar (first instituted in 1582; adopted in different dates in different countries, as late as the 1920s in some). Its predecessor, the Julian calendar, goes back ~2000 years. -- == Dan == Dan's Mail Format Site: http://mailformat.dan.info/ Dan's Web Tips: http://webtips.dan.info/ Dan's Domain Site: http://domains.dan.info/
Re: Risks of change to UTC
On Jan 21, 2006, at 10:11 AM, M. Warner Losh wrote: Over generations, the problems with noon drifting to 1pm can trivially be solved by moving the timezones that civilian time uses. Neither trivial or a solution - quadratic disaster still looms. Keeping universal time synchronized to an arbitrary meridian is already arbitrary. The prime meridian is conventional. It is not arbitrary, rather the choice was responsive to any number of political, social, historical, etc. issues. Implementing leap seconds in software is hard to get pedantically right Pedantically right is an interesting phrase. A software design is either right or it isn't. Even many ntp servers on the net got the leap second wrong And many got it right. The world did not end. astronomers and celestial navigators are being selfish Pointing out pedantic facts of nature is unremarkable behavior for either scientists or sailors. Whether we're also selfish is immaterial. Rob Seaman NOAO
