Re: [LEAPSECS] Reliability
On Thu, 1 Jan 2009, Rob Seaman wrote: > > Mean solar time will outlast artificial clocks and the species that > built them by a factor of something like 5,000,000,000 to 50,000. Not really, because mean solar time is also artificial and can't exist without mechanical clocks and telescopes. Tony. -- f.anthony.n.finchhttp://dotat.at/ FAEROES: VARIABLE 4, BUT MAINLY SOUTHEASTERLY 4 OR 5 IN WEST, BECOMING WESTERLY OR SOUTHWESTERLY 4 OR 5, OCCASIONALLY 6 IN NORTH. MODERATE. FAIR. GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Tony Finch wrote: On Thu, 1 Jan 2009, Rob Seaman wrote: Mean solar time will outlast artificial clocks and the species that built them by a factor of something like 5,000,000,000 to 50,000. Not really, because mean solar time is also artificial and can't exist without mechanical clocks and telescopes. And I suppose the refrigerator light goes out when the door is closed :-) Once more from the top, mean solar time is just sidereal time offset by a little bit to make up for the Earth lapping the Sun once a year. Nowhere does humanity appear in the equation, just the Earth and Sun and Stars. Apparent solar time is derived from mean solar time, not the other way around. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
In message: <96c34d96-8a20-453a-b4a6-b8491287b...@noao.edu> Rob Seaman writes: : Apparent solar time is derived from mean solar time, not the other way : around. Can you explain this, since I thought it was the other way around... Warner ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
M. Warner Losh wrote: Rob Seaman writes: Apparent solar time is derived from mean solar time, not the other way around. Can you explain this, since I thought it was the other way around... We live in an empirical world. When investigating the behavior of a class of objects (or processes, in astronomy the difference isn't always clear), measurements are often combined in weird and wonderful ways through population studies. In that case, a measure of central tendency like the mean is taken as an estimator of a typical value for the class. For instance, the cosmological distance ladder is built from large numbers of measurements of classes of objects like supernovae and cepheid variables. One might also perform solar system studies on the population dynamics of different classes of asteroids or Kuiper belt objects. Studying the orbital/rotational dynamics of a single object - for instance, the Earth - is different in that a measure of central tendency would be used to refine an estimate of a characteristic intrinsic to a single object, not of a class. So the point of that preface is that the meaning of the word "mean" depends on the purpose of the exercise. In particular, ignoring relativistic issues and perturbation theory and other stuff out of my depth, the orbital dynamics of the solar system have been a solved issue since Kepler and Newton and a lot of clever French mathematicians. Like I keep saying, the mean solar day is trivial to compute from the sidereal day. Look at it this way, there are "really" 366.25 days per year. That extra day just gets sliced and diced among all the others. Rather than mean solar time being some mysterious created artifact that is assembled out of vast numbers of independent "real" measurements of a time series of apparent solar positions in the sky, the apparent position of the sun is calculable from (and dependent on) the Earth's orbital parameters (e.g., semi-major axis and eccentricity), the tilt of its axis, the corresponding rates, and latitude and longitude. To some extent it is just a point of view which are the independent and which are the dependent variables, but few are likely to choose the bizarre curlicues made by the Sun and planets on the celestial sphere as their fundamental coordinate system. Instead, a handful of parameters describe the elliptical orbit of each of the planets. The spinning planets (just angular velocity vectors) are layered on the orbits. And the apparent position of each of the other solar system objects in the Earth's sky is a function of latitude and longitude layered on top of the Earth's spin and the two orbits in question. It's the usual familiar layered architecture and the apparent position of the Sun is from a higher layer then the - so-called - mean position. Astronomers confuse the issue by using phrases like "fictitious Sun", but then astronomical terminology is always upside- down and backwards. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On 1 Jan 2009 at 20:47, Rob Seaman wrote: > So the point of that preface is that the meaning of the word "mean" > depends on the purpose of the exercise. What does "mean" mean? Don't be mean about it! :-) -- == 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/ ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Fri 2009-01-02T00:10:10 -0500, Daniel R. Tobias hath writ: > What does "mean" mean? Don't be mean about it! :-) In this particular arena, the accepted meaning of mean has been changed as it was handed along a chain of names, notably among them, but not limited to Ptolemy 150 Huygens 1665 Flamsteed 1672 Newcomb 1895 Aoki et al. 1982 Capitaine et al.2000 Curiously enough, in scant weeks the Secretary of State, who has sayso over the US position presented to ITU-R by USWP7A delegates, will be the woman whose husband's presidential deposition included the infamous words "It depends on what the meaning of the word 'is' is." http://www.youtube.com/watch?v=j4XT-l-_3y0 This whole issue is a conventional matter, and conventional reality is established in accordance with the needs of its makers. -- Steve Allen WGS-84 (GPS) UCO/Lick ObservatoryNatural Sciences II, Room 165Lat +36.99855 University of CaliforniaVoice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
> From: Rob Seaman > ... > Like I keep saying, the mean solar day is trivial to compute from the > sidereal day. Look at it this way, there are "really" 366.25 days per > year. That extra day just gets sliced and diced among all the others. Nice, now we have extra days! A "leap year" is every four years except every one hundred years except every four hundred years. Put another way, if Y is the number of the year then Y is a leap year if: (Y%4==0)&&((Y%100!=0)||(Y%400==0)) where that's the modulus operator, of course. In a four-hundred year cycle, that's 24 leap years per century except the start of the century (minus one), and then one leap year at the start of the millenium (minus one). That's 303*365+97*366=146097 days for an average of 365.2425 days per year. Woo! I guess being on break for two weeks means I haven't gotten my fill of teaching arithmetic. Brian Blackmore b...@po.cwru.edu http://home.cwru.edu/~blb8 PGP keys not at http://cheese.cwru.edu/PGP/PGP.html (ask me for them) ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Dear Brian, b...@po.cwru.edu skrev: From: Rob Seaman ... Like I keep saying, the mean solar day is trivial to compute from the sidereal day. Look at it this way, there are "really" 366.25 days per year. That extra day just gets sliced and diced among all the others. Nice, now we have extra days! A "leap year" is every four years except every one hundred years except every four hundred years. Put another way, if Y is the number of the year then Y is a leap year if: (Y%4==0)&&((Y%100!=0)||(Y%400==0)) where that's the modulus operator, of course. In a four-hundred year cycle, that's 24 leap years per century except the start of the century (minus one), and then one leap year at the start of the millenium (minus one). That's 303*365+97*366=146097 days for an average of 365.2425 days per year. Woo! I guess being on break for two weeks means I haven't gotten my fill of teaching arithmetic. I think you have mixed up your solar days with sidereal days. The sidereal day is the time it takes the earth to turn 360 degrees, and to measure that one often uses a fix-star as reference. A sidereal day is is about 23 hours and 56 minutes long. A solar day is the time it takes for the earth to turn until the sun is at the same place in the sky (i.e. using the sun as the fix-star). These are not the same thing since we have a significant movement around the sun where as a more distant fix-star has a much less angular distorsion. Your arthmetic describes solar days, but fails to describe the sidereal days. The side-real day is important. The GPS satellite orbits is 11 hour and 58 minutes long, so that their orbit around the world causes a near perfect re-tracing over the world. So yes, we have an extra day, but since the earth turns in the direction is does the solar day count is one less. Cheers, Magnus ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Rob Seaman wrote: >Apparent solar time is derived from mean solar time, not the other way >around. The way I see it, apparent solar time is, in an astronomical sense, derived from sidereal time, not from mean solar time. Apparent solar time is just sidereal time minus true anomaly. All three parts of this equation are astronomically reified angles: they could be measured directly given a protractor and a suitable viewpoint. Mean solar time, on the other hand, does not correspond to any actual geometric angle. It can't be measured directly from the instantaneous positions of the bodies. Mean solar time is sidereal time minus mean anomaly, but the mean anomaly is a mathematical construction, not a direct part of the geometry. Granted, the concept of mean solar time can be applied without requiring the kind of human activity that is inherent in timezones or TAI. In that sense it's a naturally-occurring time scale. But it's no more fundamental than apparent solar time; if anything, it is less so. Of course, for the purposes of human psychological factors, the (periodic) difference between mean solar time and apparent solar time is insignificant. An unaided human would be hard pressed to distinguish between them. -zefram ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Rob Seaman wrote: >It's the usual familiar layered architecture and the apparent position >of the Sun is from a higher layer then the - so-called - mean >position. Sidereal time isn't entirely linear in time either, as we all know. So if the mean behaviour is the more fundamental, presumably you regard UT2R as more fundamental than UT1. > few are likely to choose >the bizarre curlicues made by the Sun and planets on the celestial >sphere as their fundamental coordinate system. The mean may well make a better coordinate system, but without those bizarre curlicues the mean wouldn't exist. -zefram ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Hi Richard, Yes, it's certainly true that sundials show apparent solar time. I looked into buying or building a state of the art sundial when we moved into a new house a few years back. The cost can be staggering, so this was hard to justify, but the state of the art is pretty spiffy these days. Apparent is not more real than mean, however. Like I said, it depends on point of view. By combining multiple measurements of apparent apparitions, one attempts to recover an intrinsic parameter of the planet - mean solar time. Another point of view is gain vs sensitivity (as used in Janesick's CCD bible, for instance). Gain relates successive steps in a process in a forward direction. Sensitivity recovers them in a backward direction. Photons from astrophysical sources are more fundamental than electrons from solid-state detectors, and electrons more fundamental than the DNs from A/D converters. Image processing measures DNs to characterize photons, however. We measure apparent positions of solar system objects, comets as well as the Sun, in order to characterize the more fundamental parameters of orbital elements and the spinning bodies following those orbits (as well as weird and wonderful higher order wobbles). The Earth is spinning like a top. Its rate of spin is what we're interested in for civil time. Far from denying this fact, both the current UTC standard and the ITU proposal rely on the high regularity of mean solar time for them to be even conceptually possible. If we focus on ways to improve the logistics of the approximation scheme (without abandoning the underlying requirement), we'll reach consensus. Rob --- On Jan 2, 2009, at 6:18 AM, Richard B. Langley wrote: Rob: Just sending this offlist. Forward if you like. A conventional sundial directly shows apparent solar time. -- Richard Quoting Rob Seaman : Tony Finch wrote: On Thu, 1 Jan 2009, Rob Seaman wrote: Mean solar time will outlast artificial clocks and the species that built them by a factor of something like 5,000,000,000 to 50,000. Not really, because mean solar time is also artificial and can't exist without mechanical clocks and telescopes. And I suppose the refrigerator light goes out when the door is closed :-) Once more from the top, mean solar time is just sidereal time offset by a little bit to make up for the Earth lapping the Sun once a year. Nowhere does humanity appear in the equation, just the Earth and Sun and Stars. Apparent solar time is derived from mean solar time, not the other way around. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On 2009-01-02, at 18:14, Rob Seaman wrote: Yes, it's certainly true that sundials show apparent solar time. Not all! A Bernhardt precision sundial has a specially shaped gnomon and shows UT or local civil time to precisions well within one minute. Beautiful things. I wish I had one. N ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Zefram wrote: Rob Seaman wrote: It's the usual familiar layered architecture and the apparent position of the Sun is from a higher layer then the - so-called - mean position. Sidereal time isn't entirely linear in time either, as we all know. So if the mean behaviour is the more fundamental, presumably you regard UT2R as more fundamental than UT1. According to the IAU, sidereal time itself doesn't really exist :-) And yet the Earth spins beneath a starry sky. The mean may well make a better coordinate system, but without those bizarre curlicues the mean wouldn't exist. Mean solar time is highly regular and elegantly simple. Regression to the mean (which I think is the notion underlying this disputation of terms) from unrelated measurements would not recover such a simple result. Rather (to first order) the Earth spins at a constant angular rate. The apparent positions of the Sun from day-to-day are not unrelated, they are related precisely by the fundamental angular velocity vector of the spinning Earth (http://www.analemma.com). The fact that this results in apparent positions that vary flamboyantly reveals a number of hidden variables. The eccentricity of the Earth's orbit. The tilt of its axis. The (near) spherical coordinate system on the surface of the Earth (http://en.wikipedia.org/wiki/Eratosthenes ). For civil timekeeping, these are irrelevant. Civil timekeeping (even under the ITU proposal) is about the underlying diurnal period. The curlicues obscure underlying reality, they don't create it. Removing leap seconds (without providing an alternate mode of approximation) would just make the curlicues more bizarre. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Thu, 1 Jan 2009, Rob Seaman wrote: > > Once more from the top, mean solar time is just sidereal time offset by a > little bit to make up for the Earth lapping the Sun once a year. Nowhere does > humanity appear in the equation, just the Earth and Sun and Stars. No, since an oscillator without a counter is not a clock. Tony. -- f.anthony.n.finchhttp://dotat.at/ FAIR ISLE FAEROES: WESTERLY OR NORTHWESTERLY 4 OR 5, INCREASING 6 OR 7 FOR A TIME. MODERATE OR ROUGH. RAIN OR SHOWERS, OCCASIONAL SNOW LATER IN NORTH. MODERATE OR GOOD, OCCASIONALLY VERY POOR LATER. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Fri, 2 Jan 2009, Rob Seaman wrote: > > Mean solar time is highly regular and elegantly simple. Compared to our clocks it's too irregular. > Civil timekeeping (even under the ITU proposal) is about the underlying > diurnal period. What does atomic time have to do with the position of the Earth? I find it odd that you are arguing that the mathematical model of the earth's orbit and rotation is more real than the observations from which the model is derived. Tony. -- f.anthony.n.finchhttp://dotat.at/ SOLE LUNDY FASTNET IRISH SEA SHANNON: EASTERLY OR SOUTHEASTERLY 5 TO 7, DECREASING 4, BUT BECOMING VARIABLE 3 IN IRISH SEA. MODERATE OR ROUGH, BECOMING VERY ROUGH IN WEST SOLE AND FASTNET, MAINLY SLIGHT IN IRISH SEA. OCCASIONAL RAIN. MODERATE OR GOOD, OCCASIONALLY POOR. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Fri, 2 Jan 2009, Magnus Danielson wrote: > b...@po.cwru.edu skrev: > > > > That's 303*365+97*366=146097 days for an average of 365.2425 days per year. > > Your arthmetic describes solar days, but fails to describe the sidereal days. No, he's talking about calendar years, as opposed to the conventional astronomical year of 365.25 days that Rob mentioned. The latter is often the unit for the time co-ordinate in ephemerides and is not supposed to model the length of the earth's orbit. Tony. -- f.anthony.n.finchhttp://dotat.at/ CROMARTY FORTH TYNE: VARIABLE 3 OR 4, BECOMING WEST OR NORTHWEST 4 OR 5 , INCREASING 6 LATER IN CROMARTY. SLIGHT OR MODERATE. OCCASIONAL RAIN LATER IN CROMARTY. MODERATE OR GOOD, OCCASIONALLY POOR LATER IN CROMARTY. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Tony Finch wrote: I find it odd that you are arguing that the mathematical model of the earth's orbit and rotation is more real than the observations from which the model is derived. Clearly I failed again to make my point. There are two different uses to which one might put statistics. (Well, many more than this, but two that are being confused here.) The first is your interpretation. That multiple independent observations are combined to build a theory of some unknown process. That is not the case we have, and haven't had since Newton explained Kepler's laws that were derived from Tycho's data. Rather, we can now assume that Newtonian mechanics governs the solar system. For investigations with more precise requirements, Einstein steps in. To the level of precision needed to define civil timekeeping, we know the Earth follows an elliptical orbit around the Sun, and that the Earth spins at a constant rate on a tilted axis. There are also various wobbles and perturbations from the other objects in the solar system. Laplace is handy to explain those. We don't often need to model new theory in classical or relativistic mechanics at the modest velocities found in the solar system. So yes, I think the angular momentum of the Earth is more real than the observations that might be compiled to generate an estimate for its value. In freshman physics lab, I recall compiling a big grid of current measurements resulting from voltages applied to a wide range of resistances. Unsurprisingly, Ohm's law was confirmed. The solar system is not a mystery. In any event, this isn't some big philosophical point. I'm just looking for another way to emphasize that civil timekeeping has a diurnal cadence. How's this: 1) The ITU says an hour excursion from mean solar time is acceptable. (They appear to assume that some procedure for handling the inevitable intercalary correction will self-organize before we reach that hour.) 2) The notion is that an hour's intercalary correction might first occur about 600 years out (when the excursion ought to be around a half hour). 3) The accumulation of a one-hour error term in 600 years is one-hour in 220,000 days is one-hour in 5.3 million hours. That's equivalent to a clock that keeps mean solar time to better than 1 second in 60 days. 4) Which is to say that the ITU position - a very extreme position - depends on staying aligned to mean solar time to better than one part in several million. Civil time is solar time. The rate is the issue, not local offsets. Let's move past the fantasy that the ITU can redefine timescales willy- nilly to serve the requirements of a civilization of mole people, and rather address the actual requirements of our own civilization. The best way to build a consensus is to focus on the logistics of the approximation needed to align interval timekeepers with Earth orientation timekeepers. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Fri, 2 Jan 2009, Rob Seaman wrote: > > So yes, I think the angular momentum of the Earth is more real than the > observations that might be compiled to generate an estimate for its value. But the value is an estimate, so if you plug numbers into a model based on this estimate you are only going to get an estimate to apparent solar time. In fact, since the model has to include a value for the earth's unpredictably variable moment of inertia, the result of using the model is going to be less accurate than the estimate you started with. (Um, do we actually know the earth's angular momentum and moment of inertia to any useful accuracy? I would have thought models would be based directly on angular velocity since that can be measured more precisely.) I think it's wrong to say that a directly measurable value (such as apparent solar time) is less real when measured than when derived from a model! Perhaps the word you are looking for is "fundamental". Tony. -- f.anthony.n.finchhttp://dotat.at/ FITZROY: NORTHWESTERLY IN SOUTH AT FIRST, OTHERWISE EASTERLY BACKING NORTHEASTERLY, 5 OR 6, OCCASIONALLY 6 AT FIRST, BECOMING VARIABLE 4 IN FAR NORTHWEST LATER. ROUGH. RAIN OR SHOWERS. MODERATE OR GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Tony Finch wrote: >(Um, do we actually know the earth's angular momentum and moment of >inertia to any useful accuracy? Our knowledge of the planets' masses is limited. From watching orbits we know very precisely the product of each planet's mass with the gravitational constant. But we only know the gravitational constant itself to about five significant figures, and so we don't know the absolute planetary masses any more precisely than that. This imposes a limit on our knowledge of angular momentum, and many other quantities that have to be derived by multiplying a directly-observed quanity by mass. -zefram ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Tony Finch wrote: (Um, do we actually know the earth's angular momentum and moment of inertia to any useful accuracy? I would have thought models would be based directly on angular velocity since that can be measured more precisely.) I think it's wrong to say that a directly measurable value (such as apparent solar time) is less real when measured than when derived from a model! Perhaps the word you are looking for is "fundamental". And in several recent messages I've used the term angular velocity. I'm happy with the term fundamental. The point is that the Princes of the ITU, to borrow Steve Allen's metaphor, sit in a hushed chamber (which might extend to Polycom participants) and solemnly debate the future of time on Earth. While they are debating this, it is a mental model they have about timekeeping that guides the discussions. Their mental model clearly must include the notion that mean solar time is dispensable - else they wouldn't be trying to dispense with it. The mental model of mean solar time is, however, indispensable. What we are really debating is not how to change from one standard to another, but rather how to enable two very different conceptions of time to better coexist. Nobody here has indicated an unwillingness to haggle. It seems like we would all be delighted to see the leap second schedule extended in some fashion. It appears a two or three year lead time is possible even from a cursory look at the data. Even an extension from six months to a year would be appreciated. Other possibilities exist. Only the ITU has a completely immovable position - a position that appears to be built on a faulty mental model. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
In message , Rob Seaman writes: >While they are debating this, it is a mental model they have about >timekeeping that guides the discussions. Their mental model clearly >must include the notion that mean solar time is dispensable - else >they wouldn't be trying to dispense with it. Nobody is "dispensing with mean solar time", you will always be able to calculate it if you want to. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Our humble and long suffering moderator informs me that this message
bounced a few days back since the attachment was too big. My
apologies, since my more recent messages were predicated on folks
having seen this plot.
I've put the attachment online as I should have in the first place:
http://iraf.noao.edu/~seaman/images/HowLongIsADay.pdf
Rob
--
On Jan 2, 2009, at 2:29 AM, Rob Seaman wrote:
Let's see if an attachment will help. Here's a slide from a
conference session a few years back. I think this was in San
Lorenzo de El Escorial, Spain, which I mention since the highlight
of that conference was fittingly a solar eclipse. (Well, my
personal highlight was a day trip to see Guernica at the Reina Sofía.)
If the attachment makes it through, imagine zooming out by a factor
of 200X so the y-axis reaches one full day from 0 to 86400 seconds.
The wiggle due to the eccentricity of the Earth's orbit and the tilt
of the poles will vanish in the anti-aliasing. There will be a just
barely visible offset between the sidereal ("relative to the stars")
day length and the mean solar day length - just under 4 minutes out
of 1440 minutes per day. Add up 4 minutes per day times 365 days
and you end up with an extra day relative to the stars.
Except that this is looking at it backwards. We really spin with
respect to the stars. All the solar system action is foreground
folderol. There is the simple offset to the mean solar day from
lapping the sun once per year. And the wiggle on top of that of the
apparent solar day from the elliptical orbit and the tilt. And the
equation of time / analemma (not shown) from integrating the slight
wiggle throughout the year. (Well, maybe it makes more sense to put
the tilt into the analemma.)
My thesis is that a lot of the thrashing on this list over the years
has come from allowing the apparent solar issues to cloud the more
fundamental mean solar day. (Yes, the pun was intended, get over it.)
Rob
---
http://iraf.noao.edu/~seaman/images/HowLongIsADay.pdf
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Re: [LEAPSECS] Reliability
Poul-Henning Kamp wrote: Nobody is "dispensing with mean solar time", you will always be able to calculate it if you want to. Just as you are now able to calculate TAI from UTC :-) The issue, of course, is in details. By redefining UTC, the ITU proposal would require rewriting our extensively and remotely deployed codebase simply to maintain access to mean solar time. This is a mirror reflection of what you complain about on your part - other than that we have been coding to the standard all this time and others have apparently not. There's no need to revisit entrenched positions. Rather, let's seek a new solution that is satisfactory to all. This is not a zero-sum game. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Adi Stav wrote: On Fri, Jan 02, 2009 at 08:29:21PM -0700, Rob Seaman wrote: Civil time is solar time. The rate is the issue, not local offsets. Let's move past the fantasy that the ITU can redefine timescales willy- nilly to serve the requirements of a civilization of mole people, and rather address the actual requirements of our own civilization. I'm trying to understand this position. I have a question. I appreciate both the question and the polite way it was asked :-) If I understand correctly, you require that civil time is kept synchronized with mean solar time, and you also agree that UTC, which is synchronized to mean solar time with DUT of less than one second, complies with this requirement. I am the one expressing these opinions, yes, but this is a broadly derived requirement. UTC currently satisfies it. The ITU proposal would not. Then, what is the maximum DUT a time standard can have and still comply with this requirement? How is this maximum arrived at? Or, if compliance with the requirement cannot be decided in such terms, then in what terms can it be decided? Well stated. Requirements derive from use cases. Use cases pertain to all the stakeholders. In the case of civil timekeeping, the stakeholders are all the people on Earth. Clearly everybody can't participate in making the decision, but a little humility in the ITU's decision-making process would be appreciated. I remain flabbergasted that of all the postings I've made to this list over the years - postings like the recent one that speculated 5 billion years into the future - that of all these, the ones to generate full-throated outrage as a result are when I humbly suggest that normal system engineering protocols be followed. Which is to say that I can speculate on an acceptable maximum value for DUT1, but that misses the point. I could say, for example, that 4 seconds wouldn't make me gag too badly (even though this corresponds to a full arc-minute at the equator). If we feed this into some Bayesian simulation using historical values from Bulletin A to predict the baseline truth of Bulletin B, this seems likely to give us a decade or more lead time on announcing a leap second schedule. But speculating on a solution and then inverting the process to define the problem is not a very clever sort of engineering. Rather, characterize the problem first. With a clear set of requirements in hand, it might well be - it almost surely will be - that we would identify a consensus solution that satisfies all stakeholders much, much better. A civil timescale is not like a technical timescale. We are attempting to satisfy many different purposes at the same time. The "easiest way out" is very likely to be one of the worst choices. My suggestion for the skeleton of a process is: 1) Perform a more careful simulation as in the Arias, et. al. paper: http://www.ucolick.org/~sla/leapsecs/torino/arias_3.pdf Vary the parameters, characterize how well predictive scheduling of leap seconds can actually do. 2) Reach an interim consensus on a reasonable trade-off of the parameters. 3) Implement this under the current standard (or make a modest, non- controversial change to the standard as required). Simultaneously: 4) Seek funding for a proper long term study of the requirements of civil timekeeping. Surely some combination of national and international funding would be available to do the job right. 5) I have to believe that #4 could have resulted in findings in less than the nine years the current lack of a coherent process has burned through. 6) With actual requirements in hand, perform a broad trade-off of the very distinct options that have been mentioned here over the years. It is likely that a coherent set of requirements will suggest several options we have never even considered. 7) Further debates will then be informed by, and strengthened by, the requirements and resulting trade-off study, but also by very standard methods of system engineering such as risk analyses, sensitivity analyses, etc and so forth. 8) This isn't only the best way to do the job, it is the quickest way to finish it. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Fri, Jan 02, 2009 at 08:29:21PM -0700, Rob Seaman wrote: > > Civil time is solar time. The rate is the issue, not local offsets. > Let's move past the fantasy that the ITU can redefine timescales willy- > nilly to serve the requirements of a civilization of mole people, and > rather address the actual requirements of our own civilization. I'm trying to understand this position. I have a question. If I understand correctly, you require that civil time is kept synchronized with mean solar time, and you also agree that UTC, which is synchronized to mean solar time with DUT of less than one second, complies with this requirement. Then, what is the maximum DUT a time standard can have and still comply with this requirement? How is this maximum arrived at? Or, if compliance with the requirement cannot be decided in such terms, then in what terms can it be decided? ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Sun, Jan 04, 2009 at 08:36:31AM -0700, Rob Seaman wrote: > Adi Stav wrote: >> >> I'm trying to understand this position. I have a question. > > I appreciate both the question and the polite way it was asked :-) Thanks for that, and for your answer :) > I remain flabbergasted that of all the postings I've made to this list > over the years - postings like the recent one that speculated 5 billion > years into the future - that of all these, the ones to generate > full-throated outrage as a result are when I humbly suggest that normal > system engineering protocols be followed. Maybe because discussing possible solutions is much more interesting than talking about funding long-term requirements research, after all. > Which is to say that I can speculate on an acceptable maximum value for > DUT1, but that misses the point. But surely there's some value in exploration of the problem and solution spaces beforehand. In can help guide subsequent efforts and decisions. > I could say, for example, that 4 seconds wouldn't make me gag too badly > (even though this corresponds to a full arc-minute at the equator). If > we feed this into some Bayesian simulation using historical values from > Bulletin A to predict the baseline truth of Bulletin B, this seems likely > to give us a decade or more lead time on announcing a leap second > schedule. Then why 4 seconds? Because they could be predicted a decade in advance? Isn't that putting the cart before the horses? I think the "lead time" is a different requirement altogether. (Although, for some values you might not be able to satisfy both at the same time.) If I reckon correctly, people on this list specified 20 or 60 minutes as their guesses for the limit, based on current human tolerance as witnessed by our indifference towards the Equation of Time and our own design of the time zone system. Clearly, you think DUT should be smaller. Why? For practical reasons of astronomy? For other reasons? Or, perhaps, it's not the *magnitude* of DUT but its permanence? Maybe civil time can correct for the secular drift and ignore the decade noise? (*That* could be predicted millenia in advance.) ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
I've put the attachment online as I should have in the first place: http://iraf.noao.edu/~seaman/images/HowLongIsADay.pdf Nice. Thanks! - Jonathan ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Here's a notion I don't recall seeing before on the list: Coordinate leap seconds with leap days. Introduce an integral number of leap seconds each February 29th. Discuss. Adi Stav wrote: Then why 4 seconds? Because they could be predicted a decade in advance? Isn't that putting the cart before the horses? Yes, indeed. You asked a question. I provided a guess. Personally, I think the current standard is better than allowing celestial coordinates to slosh around by an arcminute, but it is not the astronomers here who have refused to dicker. If I reckon correctly, people on this list specified 20 or 60 minutes as their guesses for the limit Some people said such things. Why lend them greater weight than alternate opinions? Nobody has specified anything. Specifications relate to solution space. We have yet to discover the requirements describing the problem space. There is nothing to specify against. Clearly, you think DUT should be smaller. Why? For practical reasons of astronomy? For other reasons? Yes and yes. A static geographic offset is different from introducing a permanent bias in the rate. A zero-mean periodic variation as in the equation of time is different from a permanent bias. Seasonal step function jumps are different yet again. Embargoing leap seconds (or their equivalent) for periods of decades or centuries is the same as not making intercalary adjustments at all. It will introduce a tilted quadratic bias in the solar rate. The issue isn't about offsets at all, it is about preserving the correct functional form of civil timekeeping. We have heard numerous times that the Gregorian calendar is acceptable because the schedule is predictable. But why is the Gregorian calendar desirable? It is desirable because it stabilizes the civil calendar against the natural annual rhythms of life on Earth AND because it does so at a fine enough resolution to permit smoothing across the decades and centuries. When considering dates 400 years from now - or in colonial America - we don't have to wonder whether April occurs before or after the Vernal Equinox. Well - except for the damage visited through the delayed Gregorian intercalary adjustment made by the British. Why is there an English butterfly called an "April Fritillary"? Because it emerges from its chrysalis in March. The clock is a subdivision of the calendar. It needs to be stabilized against natural diurnal rhythms on Earth. And the resolution has to be fine enough to permit useful smoothing. Over what period of time? That is really the heart of this whole discussion. The current UTC leap second policy is sufficient. What is necessary? We stabilize the calendar every 4 years. Why would it be considered reasonable to forego stabilizing the clock for a thousand years? Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Sun 2009-01-04T20:58:29 -0700, Rob Seaman hath writ: > Here's a notion I don't recall seeing before on the list: > > Coordinate leap seconds with leap days. Introduce an integral number > of leap seconds each February 29th. Discuss. This ignores the existing operational systems, and in particular NTP code. Any change must be congruent with existing operational systems, and that means nothing more than one second leaps at end of June or December. It also does not address the underlying problem, which is discontinuity in the broadcast time scale. I think the "must not adversely affect existing operational systems" clause is an inviolable rule for this process. It's like the physician's "First, do no harm" rule. I'd like to think that "makes life better for some parties and not worse for any party" is also paramount in this process. -- Steve Allen WGS-84 (GPS) UCO/Lick ObservatoryNatural Sciences II, Room 165Lat +36.99855 University of CaliforniaVoice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Steve Allen wrote: On Sun 2009-01-04T20:58:29 -0700, Rob Seaman hath writ: Here's a notion I don't recall seeing before on the list: Coordinate leap seconds with leap days. Introduce an integral number of leap seconds each February 29th. Discuss. This ignores the existing operational systems, and in particular NTP code. Any change must be congruent with existing operational systems, and that means nothing more than one second leaps at end of June or December. It also does not address the underlying problem, which is discontinuity in the broadcast time scale. I think the "must not adversely affect existing operational systems" clause is an inviolable rule for this process. It's like the physician's "First, do no harm" rule. I'd like to think that "makes life better for some parties and not worse for any party" is also paramount in this process. Brainstorming comes in two phases: 1) generate ideas, and 2) winnow them down. During the first phase, ideas should breed like rabbits. During the second phase, they should be matched against clear requirements. Reexamine the requirements and repeat. Whatever problem solving process we're following here (Monte Carlo conceptualization? :-) either we're in a phase of charactering the problem space or we're in a phase of characterizing the solution space. If a problem phase, the discovery of requirements is the goal. Care should be taken to express the essential idea behind each requirement, such as (perhaps): - change must be congruent with operational systems - does "congruent" mean the existence of a viable implementation plan? - a mechanism shall exist for managing discontinuities A requirement that is worded either too specifically or too generally will bias the process. If a solution phase, a quantitative trade-off study is the ultimate goal. Before that can happen, the solution space should be extensively explored. Often, aspects of several different notional solutions are combined to form a jointly richer concept. The part I like most about the February 29th notion is that it standardizes all intercalary interactions at a very clearly stated moment. Perhaps this is an aspect that could inform other possible solutions. You'll also note that I didn't specify that this had to be applied to UTC. The first step to implementing brand new technology is to break cleanly with the past. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Zefram wrote: Rob Seaman wrote: Coordinate leap seconds with leap days. Introduce an integral number of leap seconds each February 29th. Discuss. There's also a risk that the lower frequency of leaps would exacerbate the psychology of leap seconds being an infrequent event. Countered, perhaps, by the much longer familiarity civilians have with leap days. February 29 isn't the only option for the leap day. [...] A more obvious choice is December 31: the 366th day of the year, when most years only go up to 365. It seems very unlikely that leap day will move from February. People are fond of February. Also, a leap day at the end of December would be December 32nd :-) Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Rob Seaman wrote: >Coordinate leap seconds with leap days. Introduce an integral number >of leap seconds each February 29th. Discuss. That would mean bigger leaps. I think a 62-second minute (when most minutes are of 60 seconds) is too great a disuniformity. It would also exceed the capacity of current leap second implementations, which know that there can only be one leap second at a time. The lower frequency of leaps would necessarily mean bigger excursions of DUT1. That's not a fatal problem, but you don't seem to be buying much win with this much looser tracking. There's also a risk that the lower frequency of leaps would exacerbate the psychology of leap seconds being an infrequent event. February 29 isn't the only option for the leap day. Traditionally the leap day was February 24, though you can't have that one because leap seconds are constrained to occur at the end of a month. A more obvious choice is December 31: the 366th day of the year, when most years only go up to 365. -zefram ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
In message <421fb837-f23f-4a16-b6f4-f26d1c58c...@noao.edu>, Rob Seaman writes: >It seems very unlikely that leap day will move from February. People >are fond of February. Also, a leap day at the end of December would >be December 32nd :-) Which would break incredibly badly thought out filsystem formats like FAT which encodes the day-of-month in five bits. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Rob Seaman wrote: >Also, a leap day at the end of December would >be December 32nd :-) Only if there were no February 29. My point is that the leap day appears to be at the end of the year if you don't bother with months and just use day-of-year. Just as the idea that February 29 is the leap day originates in looking at the ordinal day-of-month. Traditionally the leap day was "ante diem bis VI Kalendas Martias", located some days before the end of the month. -zefram ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Sun, Jan 04, 2009 at 08:58:29PM -0700, Rob Seaman wrote: > Adi Stav wrote: > >> Then why 4 seconds? Because they could be predicted a decade in >> advance? Isn't that putting the cart before the horses? > > Yes, indeed. You asked a question. I provided a guess. Personally, I > think the current standard is better than allowing celestial coordinates > to slosh around by an arcminute, but it is not the astronomers here who > have refused to dicker. > >> If I reckon correctly, people on this list specified 20 or 60 minutes >> as their guesses for the limit > > Some people said such things. Why lend them greater weight than > alternate opinions? They don't have greater weight, but they have their own direct justifications and so can be discussed by both those who agree and disagree. We know that human tolerance to DUT is higher than 20 minutes because we don't usually bother to compensate for apparent solar time. We know that it is probably not much higher than one or two hours because time zones generally have about that resolution. We guess that it is might be about one hour because many areas in the world choose time zones that are about one-hour offsets from their local mean time. These justifications are not necessarily valid, or maybe there are other or better justifications for smaller DUT maxima. I am just trying to find out (for myself) what these are. This is why I asked. > Nobody has specified anything. Specifications relate to solution space. > We have yet to discover the requirements describing the problem space. > There is nothing to specify against. True. I should have said instead "suggested", or "said". >> Clearly, you think DUT should be smaller. Why? For practical reasons >> of astronomy? For other reasons? > > Yes and yes. A static geographic offset is different from introducing a > permanent bias in the rate. A zero-mean periodic variation as in the > equation of time is different from a permanent bias. Seasonal step > function jumps are different yet again. > > Embargoing leap seconds (or their equivalent) for periods of decades or > centuries is the same as not making intercalary adjustments at all. Why is that? Even the Gregorian reform does not come into effect except every one or two centuries. Yet it is followed exactly. > It > will introduce a tilted quadratic bias in the solar rate. The issue > isn't about offsets at all, it is about preserving the correct functional > form of civil timekeeping. (I hope) I fully understand this requirement. So I asked about the maximal DUT instead. > We have heard numerous times that the Gregorian calendar is acceptable > because the schedule is predictable. I can think of properties that make the Gregorian calendar acceptable and that not every UTC reform will necessarily have: * It does not introduce a new intercalary mechanism, and instead modifies (slightly) the frequency of a pre-existing and accepted one. * It is not only infinitely predictable, but also easy; every schoolchild can know it. > But why is the Gregorian calendar > desirable? > > It is desirable because it stabilizes the civil calendar against the > natural annual rhythms of life on Earth AND because it does so at a fine > enough resolution to permit smoothing across the decades and centuries. > When considering dates 400 years from now - or in colonial America - we > don't have to wonder whether April occurs before or after the Vernal > Equinox. This is a property going back to the Julian calendar. But I see your meaning -- you are saying that one second is acceptable simply because it uses *our smallest standard* time unit (and four seconds likewise, with a decade of advanced warning). By the way, it can be argued that the smoothness property is not strictly necessary for calendars. Consider popular and long-used artihmetic lunisolar calendars, such as the Hebrew, Hindu, and Chinese calendars, that intercalate their years to a resolution of a month. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Adi Stav wrote: We know that human tolerance to DUT is higher than 20 minutes because we don't usually bother to compensate for apparent solar time. We know that it is probably not much higher than one or two hours because time zones generally have about that resolution. We guess that it is might be about one hour because many areas in the world choose time zones that are about one-hour offsets from their local mean time. These justifications are not necessarily valid, or maybe there are other or better justifications for smaller DUT maxima. I am just trying to find out (for myself) what these are. This is why I asked. Ok (to the second paragraph :-) Lower limits are hard to pin down. Human tolerance on a particular day is not the same thing as the tolerance over a year or a lifetime. Straining a tolerance for one human is not the same as straining it for 6 billion. Human tolerances in general need to be interpreted in terms of our infrastructure, not just personal perception as we walk from parking lot to office. The upper limit has been specified as a "statement against penal interest" by the ITU. Public enemy number one of leap seconds says an hour is the upper limit :-) Embargoing leap seconds (or their equivalent) for periods of decades or centuries is the same as not making intercalary adjustments at all. Why is that? Even the Gregorian reform does not come into effect except every one or two centuries. Yet it is followed exactly. Gregory revised the Julian calendar. The fundamental standard remains rooted in what the ancients discovered. The proper comparison is to the every four year scheduling of leap day opportunities - sometimes those opportunities remain nulled out, but they still exist. The seasonal or diurnal trends in the calendar or clock need to be sampled frequently enough to avoid significant quantization errors. Leap seconds are productive from this point of view precisely because civilians can ignore them. By the way, it can be argued that the smoothness property is not strictly necessary for calendars. Consider popular and long-used artihmetic lunisolar calendars, such as the Hebrew, Hindu, and Chinese calendars, that intercalate their years to a resolution of a month. A very interesting observation. What calendars does the world really depend on for various purposes? That is, what is the market penetration of the Gregorian/Julian calendar? I would guess nearly 100% in Europe and North America. What about the rest of the world? Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Mon, Jan 05, 2009 at 09:39:28AM -0700, Rob Seaman wrote: > > Lower limits are hard to pin down. Human tolerance on a particular day > is not the same thing as the tolerance over a year or a lifetime. > Straining a tolerance for one human is not the same as straining it for 6 > billion. Human tolerances in general need to be interpreted in terms of > our infrastructure, not just personal perception as we walk from parking > lot to office. How is that? That is to say, what problems could exceeding the tolerance(s) cause? (Especially problems that time zones far from their reference meridians, DST switches twice a year, and the difference between mean and apparent solar time don't already cause). I'm not arguing that there arent's such problems, but I don't know what they are. Only thing I can imagine that is not covered by time zones etc. is the minute of drift that will be experienced over a person's (or a system's) life time. > The upper limit has been specified as a "statement against penal > interest" by the ITU. Public enemy number one of leap seconds says an > hour is the upper limit :-) An hour makes a lot of sense from a usability point of view, because it is the primary division of a day. >>> Embargoing leap seconds (or their equivalent) for periods of decades >>> or >>> centuries is the same as not making intercalary adjustments at all. >> >> Why is that? Even the Gregorian reform does not come into effect >> except >> every one or two centuries. Yet it is followed exactly. > > Gregory revised the Julian calendar. The fundamental standard remains > rooted in what the ancients discovered. The proper comparison is to the > every four year scheduling of leap day opportunities - sometimes those > opportunities remain nulled out, but they still exist. The Gregorian reform revised the Julian calendar, but it still had to be introduced. It set a schedule that does not come into effect except once every one or two centuries, which was followed to the letter. I think this is impressive, even if it used a pre-existing mechanism. A good parallel would be adding leap hours and using the existing DST mechanism (not that I can't see other issues with it). But here's a thing -- maybe suggestions for making leaps happen very infrequently are seen as dishonest, "let it slide" in disguise. But it doesn't have to be this way. I can see good, honest, technical reasons for leaping every few centuries. You can use leaping mechanisms that are simply not available when you have to leap every year or two. (For example, you can introduce a new time scale (UTC-n) every few centuries and deprecate the old one over decades as users switch to the new one. I can think of several technical advantages of such a system over leaping an existing time scale.) Another example -- the Julian calendar did slide over a very long time, yet it did not stop people from fixing it, and that was even as its original definition did not prescribe the fix. If a definition of a time scale does explicitly require a leap minute or hour in a very long time, why assume in advance that it will not be followed? > The seasonal or diurnal trends in the calendar or clock need to be > sampled frequently enough to avoid significant quantization errors. > Leap seconds are productive from this point of view precisely because > civilians can ignore them. I'm sorry, I don't understand :) >> By the way, it can be argued that the smoothness property is not >> strictly >> necessary for calendars. Consider popular and long-used artihmetic >> lunisolar calendars, such as the Hebrew, Hindu, and Chinese calendars, >> that intercalate their years to a resolution of a month. > > A very interesting observation. What calendars does the world really > depend on for various purposes? That is, what is the market penetration > of the Gregorian/Julian calendar? I would guess nearly 100% in Europe > and North America. What about the rest of the world? I think they are used in conjuction with the Gregorian for different purposes, such as for public holidays of traditional origin. (As an anecdote, DST in Israel starts according to the Hebrew calendar but ends according to the Gregorian.) But I don't think it matters much for our purpose, because any culture which is in contact with the current global civilization will be under a lot of pressure to adopt the Gregorian calendar for many purposes, even if its traditional calendar is actually superior for its needs. Prior to contact with the Western civilization, the old Chinese calendar was used in China for a long time. I don't think we have any reason to assume that the Gregorian calendar is superior to a lunisolar calendar and that those lunisolar cultures changed calendar for this reason rather than for reasons of culture and trade influence. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/m
Re: [LEAPSECS] Reliability
Adi Stav wrote: what problems could exceeding the tolerance(s) cause? Well covered in the archive. For astronomy, 1 second of time is 15 seconds of arc on the equator. This is a large error (colossal for some purposes). It doesn't appear that any other industry has actually performed a coherent risk analysis. For some reason this is asserted to be the astronomers' responsibility. (Especially problems that time zones far from their reference meridians, DST switches twice a year, and the difference between mean and apparent solar time don't already cause). This confuses periodic with secular effects, also in the archive. A good parallel would be adding leap hours and using the existing DST mechanism Reasons why leap hours won't work are in the archive. There was a clear consensus from both sides of the aisle that the notion of leap hours is absurd. Alternately, by relying on shifting timezones, there would be no underlying stabilized civil timescale permitting commonsense timekeeping inferences by humans. By contrast, interval time is important to computers. Computers are good at computing. I don't understand :) Imagine a version of the Gregorian calendar that interpolates leap days only every 400 hundred years. That would amount to about 3 months at a time. Since this is a whole season, it is equivalent to not stabilizing the calendar at all. Leap hours or tweaking timezones can be interpreted the same way. If intercalary adjustments are the width of a timezone, no practical stabilization is occurring. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
In message: Rob Seaman writes: : Adi Stav wrote: : : > what problems could exceeding the tolerance(s) cause? : : Well covered in the archive. For astronomy, 1 second of time is 15 : seconds of arc on the equator. This is a large error (colossal for : some purposes). It doesn't appear that any other industry has : actually performed a coherent risk analysis. For some reason this is : asserted to be the astronomers' responsibility. There were three groups of people identified as caring where the earth is pointing to a high degree of accuracy. (1) Astronomers so they know where to point their telescopes. (2) Space Engineers wishing to know where their satellites or other objects of interest are. (3) Navigators that still rely on astral navigation. The first group needs time accurate to a few milliseconds to point the largest of their telescopes, less accurate for the smaller ones (with 1s being a very large error for all but the smallest scapes). Having time broadcast available, and having that time within 1s of UT allows these smaller errors to be corrected in the mid to small telescopes. The larger ones need daily updates of DUT1 to get the job done. The mid sized ones can cope well enough with the DUT1 broadcast to 100ms in things like WWVB. There's also software that's used in the astronomy world that benefits from the DUT1 < 1s rule that would need to be retooled. The second group has even tigher tolerances than the first, since their birds are moving much faster than planets and the servos have to be both fast and accurate to track them. These folks usually are in the classified world, so little can be said for sure about them. What is known is they have very high precision timing gear and software that takes the DUT1 difference into account, and may have 'back door' information to the raw measurements that go into the daily numbers that are published. This group is most likely to be able to cope with DUT1 since they have lots of $$$ to track these things. Also, 'their' might not imply titular ownership, merely interest. Most astral navigation is done with GPS these days. There are a vanishingly small number of folks that do it by hand, listen to shortwave broadcasts to get the time, etc. The US Navy doesn't even have the gear on board to do the astral navigation anymore, I'm told. Most of the folks in this forum have written them off as not being a constituency worth caring about, but they are mentioned here for completeness. All other users of time, it is widely agree, basically want everyone to agree on a time, have the sun basically overhead around noon, and do what they are told. There's debate over what each of these loosey goosey terms means, and what the boundaries are for them. : > (Especially problems that time zones far from their reference : > meridians, DST switches twice a year, and the difference between : > mean and apparent solar time don't already cause). : : This confuses periodic with secular effects, also in the archive. Well, yes and no. If you permanently shift an hour to account for the drift of time, then it is no different than permanently shifting 1s. The question is who does it and when. An interesting debating point as well. There is an important point to be made here. The reason that DUT1 matters to most people is for the sun overhead at noon feature. Sliding time zones solves that issue for many people, although there is much debate about the aesthetics of doing this. : > A good parallel would be adding leap hours and using the existing : > DST mechanism : : : Reasons why leap hours won't work are in the archive. There was a : clear consensus from both sides of the aisle that the notion of leap : hours is absurd. Alternately, by relying on shifting timezones, there : would be no underlying stabilized civil timescale permitting : commonsense timekeeping inferences by humans. Leap hours in UTC. Let's be clear what we're talking about. Also, I don't think that your assertion that there's no stabilized civil timescale causing issues has a firm foundation, let alone one to draw the conclusion that it is a problem. If DUT1 and the timezone info were known in a database (like timezones and leap seconds are today), then historians would be able determine when things happened, where the sun was etc with more or less the same precision they have today. The computations would be harder, and it has been debated as to the extent of the hardness (eg does it really matter or not). : By contrast, interval time is important to computers. Computers are : good at computing. True, but they are only as good at computing as the programmers are at writing code, and the test organizations are at validating that code. : > I don't understand :) : : Imagine a version of the Gregorian calendar that interpolates leap : days only every 400 hundred years. That would amount to about 3
Re: [LEAPSECS] Reliability
An addition to the list set forth by M. Warner is surveyors who use star or sun sightings to establish precise directions of lines (to an accuracy of less than one arcminute). Although some surveyors do this with GPS, many surveyors do not because of the expense of the equipment (about two orders of magnitude more expensive than recreation-grade GPS receivers) or because they conduct their operations in forests or other areas with poor GPS reception. On the other hand, they have the technical sophistication to compute the time even if DUT > 1 s. -Original Message- From: leapsecs-boun...@leapsecond.com [mailto:leapsecs-boun...@leapsecond.com] On Behalf Of M. Warner Losh Sent: Monday, January 05, 2009 7:26 PM To: leapsecs@leapsecond.com; sea...@noao.edu Subject: Re: [LEAPSECS] Reliability In message: Rob Seaman writes: : Adi Stav wrote: : : > what problems could exceeding the tolerance(s) cause? : : Well covered in the archive. For astronomy, 1 second of time is 15 : seconds of arc on the equator. This is a large error (colossal for : some purposes). It doesn't appear that any other industry has : actually performed a coherent risk analysis. For some reason this is : asserted to be the astronomers' responsibility. There were three groups of people identified as caring where the earth is pointing to a high degree of accuracy. (1) Astronomers so they know where to point their telescopes. (2) Space Engineers wishing to know where their satellites or other objects of interest are. (3) Navigators that still rely on astral navigation. The first group needs time accurate to a few milliseconds to point the largest of their telescopes, less accurate for the smaller ones (with 1s being a very large error for all but the smallest scapes). Having time broadcast available, and having that time within 1s of UT allows these smaller errors to be corrected in the mid to small telescopes. The larger ones need daily updates of DUT1 to get the job done. The mid sized ones can cope well enough with the DUT1 broadcast to 100ms in things like WWVB. There's also software that's used in the astronomy world that benefits from the DUT1 < 1s rule that would need to be retooled. The second group has even tigher tolerances than the first, since their birds are moving much faster than planets and the servos have to be both fast and accurate to track them. These folks usually are in the classified world, so little can be said for sure about them. What is known is they have very high precision timing gear and software that takes the DUT1 difference into account, and may have 'back door' information to the raw measurements that go into the daily numbers that are published. This group is most likely to be able to cope with DUT1 since they have lots of $$$ to track these things. Also, 'their' might not imply titular ownership, merely interest. Most astral navigation is done with GPS these days. There are a vanishingly small number of folks that do it by hand, listen to shortwave broadcasts to get the time, etc. The US Navy doesn't even have the gear on board to do the astral navigation anymore, I'm told. Most of the folks in this forum have written them off as not being a constituency worth caring about, but they are mentioned here for completeness. All other users of time, it is widely agree, basically want everyone to agree on a time, have the sun basically overhead around noon, and do what they are told. There's debate over what each of these loosey goosey terms means, and what the boundaries are for them. : > (Especially problems that time zones far from their reference : > meridians, DST switches twice a year, and the difference between : > mean and apparent solar time don't already cause). : : This confuses periodic with secular effects, also in the archive. Well, yes and no. If you permanently shift an hour to account for the drift of time, then it is no different than permanently shifting 1s. The question is who does it and when. An interesting debating point as well. There is an important point to be made here. The reason that DUT1 matters to most people is for the sun overhead at noon feature. Sliding time zones solves that issue for many people, although there is much debate about the aesthetics of doing this. : > A good parallel would be adding leap hours and using the existing : > DST mechanism : : : Reasons why leap hours won't work are in the archive. There was a : clear consensus from both sides of the aisle that the notion of leap : hours is absurd. Alternately, by relying on shifting timezones, there : would be no underlying stabilized civil timescale permitting : commonsense timekeeping inferences by humans. Leap hours in UTC. Let's be clear what we're talking about. Also, I don't think that your ass
Re: [LEAPSECS] Reliability
Rob Seaman skrev: Adi Stav wrote: We know that human tolerance to DUT is higher than 20 minutes because we don't usually bother to compensate for apparent solar time. We know that it is probably not much higher than one or two hours because time zones generally have about that resolution. We guess that it is might be about one hour because many areas in the world choose time zones that are about one-hour offsets from their local mean time. These justifications are not necessarily valid, or maybe there are other or better justifications for smaller DUT maxima. I am just trying to find out (for myself) what these are. This is why I asked. Ok (to the second paragraph :-) Lower limits are hard to pin down. Human tolerance on a particular day is not the same thing as the tolerance over a year or a lifetime. Straining a tolerance for one human is not the same as straining it for 6 billion. Human tolerances in general need to be interpreted in terms of our infrastructure, not just personal perception as we walk from parking lot to office. The upper limit has been specified as a "statement against penal interest" by the ITU. Public enemy number one of leap seconds says an hour is the upper limit :-) Embargoing leap seconds (or their equivalent) for periods of decades or centuries is the same as not making intercalary adjustments at all. Why is that? Even the Gregorian reform does not come into effect except every one or two centuries. Yet it is followed exactly. Gregory revised the Julian calendar. The fundamental standard remains rooted in what the ancients discovered. The proper comparison is to the every four year scheduling of leap day opportunities - sometimes those opportunities remain nulled out, but they still exist. One should recall that the Gregorian leap day rules where just a improvement on the static leap day scheme of the Julian calendar so that it better matched the earths spinning around the sun. They also made a correction for the accumulate error to restore phase relationships. This static scheme is not an exact mechanism, as the underlying mechanism is not exactly mirrored, but rather estimated with sufficient precission to be usefull over several centuries. Eventually we will need to revise it again, correct for the accumulated phase error and make an improved correction algorithm. Essentially, it is not entierly static, it is infact a dynamic scheme but with a very long time inbetween adjustments, which is sufficient considering the speed of events. The adjustments does take time to incoperate, infact some have still not included them. The leap second is essentially the same thing, but with a very short reshedule scheme which unfortunatly coincide with the adjustment oppertunities being used (actually there is 6 oppertunities on every decission, but a preference to only use the last). Inbetween we do have a static scheme. Part of the problem is the life-time of a decision over the static scheme provided. The actual technicalities of introducing a leap second would remain the same, but a longer schedule would cause less of a problem on the issue when they would need to be introduced. Cheers, Magnus ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Poul-Henning Kamp skrev: In message <421fb837-f23f-4a16-b6f4-f26d1c58c...@noao.edu>, Rob Seaman writes: It seems very unlikely that leap day will move from February. People are fond of February. Also, a leap day at the end of December would be December 32nd :-) Which would break incredibly badly thought out filsystem formats like FAT which encodes the day-of-month in five bits. Among many other systems... this would make leap second akes feel like a very little itch. Evil man, evil. Cheers, Magnus ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
- Original Message - From: "M. Warner Losh" To: ; Sent: Monday, January 05, 2009 4:26 PM Subject: Re: [LEAPSECS] Reliability All other users of time, it is widely agree, basically want everyone to agree on a time, have the sun basically overhead around noon, and do what they are told. There's debate over what each of these loosey goosey terms means, and what the boundaries are for them. There is an important point to be made here. The reason that DUT1 matters to most people is for the sun overhead at noon feature. Sliding time zones solves that issue for many people, although there is much debate about the aesthetics of doing this. Not being a member of the technical communities to whom this issue of leap seconds matters most, there's a limit on what I can contribute to this discussion. However, I believe I can safely say that you time lords need not worry about what the general public thinks in regard to having clock time match the sun's position in the sky, or the "noon becomes midnight" scenario. The unwarshed (sic) masses may have cared about that when society was still mostly agrarian (maybe farmers still do, but even they have to contend with DST imposed by us city folk :)), but very, very few of us get up with the chickens as a cultural necessity anymore. Trust me, whatever becomes of leap seconds and DUT1, Joe the Plumber will be just fine. For purposes of precise time and time interval, science and technology aren't merely the primary issue, they're the ONLY issue. Of course, I doubt there'd be more than a couple of these shifts before people realize that something else is needed. There may never be a shift, but instead a change to a whole new time system as well that suits the needs of future generations better. One that we cannot imagine from this vantage point in time. Can you imagine being alive at the time of Christ and thinking you'd be able to measure the length of the day so accurately that you'd detect variations at the 1e-8 level? And even if you did, would you have the skills necessary to work out all the implications of that in advance? Or that there'd be a standard written for it in a language that wasn't even around at the time? This suggests, at least to some, that predicting what people will need and want over such long periods of time is difficult at best. Both sides use this as part of their argument: the pro-leapsecond folks to say that it keeps things in sync, which gives future generations more options. The anti-leapsecond folks to say that things will be so different, it just might not matter. Well said. The recurrent discussion of what imminent changes in timekeeping might mean for our posterity 500+ years from now is irrelevant because we have no way of knowing what their timekeeping needs or preferences might be. Brian ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
M. Warner Losh wrote: It also wouldn't be a leap-hour in the UTC time scale, but rather just a DST without end once in 50 generations. Is it too much to ask that an attempt be made to describe how the logistics would work? Of course, I doubt there'd be more than a couple of these shifts before people realize that something else is needed. Design that solution instead. There may never be a shift, but instead a change to a whole new time system as well that suits the needs of future generations better. One that we cannot imagine from this vantage point in time. Huh? Astronomers toss around concepts of black holes, dark energy, microlensing and exoplanets, but humanity will evolve into some species unimaginable to a generation raised on Arthur C. Clarke and Philip K. Dick? Can you imagine being alive at the time of Christ and thinking you'd be able to measure the length of the day so accurately that you'd detect variations at the 1e-8 level? An artificial comparison. There has been more evolution of our science and culture in the past 100 years than the previous 2000. Also, nobody is suggesting civil timekeeping needs to trace solar time to better than the tenth second currently implemented. Rather, the ITU wants to degrade this by close to 5 orders of magnitude. While we shouldn't expect Bill and Ted to have many (or even any) details right, it is eminently practical for even primordial ooze such as us to speculate on implications thousands of years hence. Having a plan that later proves incomplete is better than having no plan at all. And the obvious refrain - use GPS or some other perfectly acceptable and already available option and leave Universal Time to the people who value it. We get it that you think UT is a pointless exercise. Use something else instead. And even if you did, would you have the skills necessary to work out all the implications of that in advance? Or that there'd be a standard written for it in a language that wasn't even around at the time? Do we believe that the Julian and Gregorian calendars were promulgated in English? :-) Yes, I do believe this group is very skillful. I wonder why we're so shy about putting those skills to good use designing a better timekeeping solution, rather than seeking an easy way out? Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Mon, 5 Jan 2009, Brian Garrett wrote: > > However, I believe I can safely say that you time lords need not worry > about what the general public thinks in regard to having clock time > match the sun's position in the sky, or the "noon becomes midnight" > scenario. The unwarshed (sic) masses may have cared about that when > society was still mostly agrarian (maybe farmers still do, but even they > have to contend with DST imposed by us city folk :)), but very, very few > of us get up with the chickens as a cultural necessity anymore. The reason DST exists is to more closely sync our activities to sunrise. People do care about hours of daylight, but the alignment doesn't need to be very precise. 12:00 will never drift to midnight because we'll adjust our timezones to compensate (assuming no changes in our cultural relationship to time). Tony. -- f.anthony.n.finchhttp://dotat.at/ VIKING NORTH UTSIRE SOUTH UTSIRE: SOUTHWESTERLY VEERING NORTHERLY, 6 TO GALE 8, DECREASING 5 IN VIKING LATER. MODERATE OR ROUGH, OCCASIONALLY VERY ROUGH LATER. RAIN THEN WINTRY SHOWERS. MODERATE OR POOR, BECOMING GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Mon, 5 Jan 2009, Rob Seaman wrote: > > Is it too much to ask that an attempt be made to describe how the logistics > would work? Exactly the same way that current time zones work. Every so often, jurisdictions that become dissatisfied with their current timezone offset or DST arrangements because of increasing DUT1 or because a politician is under-employed will have a discussion which may or may not result in a change. Note that there's no need for global co-ordination. Each country (or county) can change when it is convenient for them. The effect would probably be a shifting of timezone boundaries in lumps and bumps that averages out to the overall DUT1 drift. Tony. -- f.anthony.n.finchhttp://dotat.at/ FORTIES CROMARTY FORTH TYNE DOGGER: WESTERLY OR SOUTHWESTERLY, VEERING NORTHWESTERLY, 5 TO 7, DECREASING 4 OR 5 LATER. SLIGHT OR MODERATE. RAIN THEN MAINLY FAIR. MODERATE OR GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Magnus Danielson wrote: > They also made a >correction for the accumulate error to restore phase relationships. Except that this correction was faulty. By the mid 16th century, the phase relationship between the seasons and the calendar had shifted about 12.5 days since the inception of the Julian calendar (45 BCE). Applying the knowledge that went into constructing the Gregorian calendar, an attempt to correct for this shift would have amounted to skipping 12 calendar dates. (They slightly underestimated the degree of shift.) Instead they skipped 10 calendar dates. This was because they didn't aim to restore the original phase relationship of the Julian calendar. Instead, they aimed to restore the already-shifted phase relationship that had existed at the time of the Council of Nicea (325 CE). The phase shift from then until the calendar reform was about 9.8 days. So they synchronised (as best they could) to the wrong phase, locking into the calendar the very error they were supposedly fixing. People are dumb. (Sorry, I've run out of highbrow conclusions to draw from this sort of thing.) -zefram ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Zefram skrev: Magnus Danielson wrote: They also made a correction for the accumulate error to restore phase relationships. Except that this correction was faulty. By the mid 16th century, the phase relationship between the seasons and the calendar had shifted about 12.5 days since the inception of the Julian calendar (45 BCE). Applying the knowledge that went into constructing the Gregorian calendar, an attempt to correct for this shift would have amounted to skipping 12 calendar dates. (They slightly underestimated the degree of shift.) Instead they skipped 10 calendar dates. This was because they didn't aim to restore the original phase relationship of the Julian calendar. Instead, they aimed to restore the already-shifted phase relationship that had existed at the time of the Council of Nicea (325 CE). The phase shift from then until the calendar reform was about 9.8 days. So they synchronised (as best they could) to the wrong phase, locking into the calendar the very error they were supposedly fixing. People are dumb. (Sorry, I've run out of highbrow conclusions to draw from this sort of thing.) This basically suggest that making the correction points to far away also makes it a real risk of loosing the reference they where ment to adjust to. So you need something sufficiently distant not to be too annoying and sufficiently often reoccuring that it can be done correctly. Cheers, Magnus ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Tony Finch wrote: The reason DST exists is to more closely sync our activities to sunrise. The reason DST exists is because it has become a self-propagating cultural meme. Your April Fool's post on risks may be the most coherent analysis I've read on the subject. (Not trying to be ironic.) In general, this list (sad to say - now I'm being ironic :-) represents the species' hoard of knowledge on certain topics. Where I grew up in the U.S. mid-Atlantic states, the most obvious effect of DST was to extend the usable hours of daylight for Summer evenings. (Perhaps some other narrative applies at higher or lower latitudes?) Since we were off school, the morning issues were meaningless. And workers go to work when their bosses tell them to. The time they own for themselves and their families is after work. Recently, all discussions of DST are framed in turns of energy. It seems like every argument for DST (saves energy for lighting in the mornings) is countered by some argument against (increases cooling costs in the evenings). If DST were really a mechanism for managing our natural daylight resource, rather than a naive attempt at PR regarding petroleum resources, it would be applied in the Winter when the daylight is in shortest supply. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
To return to a previous point, Tony Finch wrote: Note that there's no need for global co-ordination. Each country (or county) can change when it is convenient for them. The effect would probably be a shifting of timezone boundaries in lumps and bumps that averages out to the overall DUT1 drift. Some requirements (in general, the most important ones) apply to the results of implementing a system, not to the manner of implementing it. These spiraling lumps and bumps in time are not acceptable. Perhaps we might ask a few historians or folks from other long point-of-view professions whether there is a need for global coordination? Time isn't just about what happens today. Time has permanence in our records and histories. (Also, I'm not sure saying "the politicians will fix it" is your most successful tactical point :-) Leap seconds represent authentic intercalary corrections. (So would leap hours - with the small caveat that they cannot be implemented.) On the other hand, the lumps and bumps from the timezone gimmick both accelerate with time, and pile higher and higher, one on top of the other. Real time (the solar time that drives politicians in each county or country to reluctantly deal with this issue) will get further and further from civil time. As a result, civil time will mean less and less. As with the notion of leap hours, the lumps and bumps are really more like the Himalayas. We need to stabilize civil time just like we need to stabilize the civil calendar and on a schedule not too dramatically different. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Tue, 6 Jan 2009, Rob Seaman wrote: > To return to a previous point, Tony Finch wrote: > > > Note that there's no need for global co-ordination. Each country (or > > county) can change when it is convenient for them. The effect would > > probably be a shifting of timezone boundaries in lumps and bumps that > > averages out to the overall DUT1 drift. > > These spiraling lumps and bumps in time are not acceptable. They're certainly not very tidy :-) It's how DST works today, and obviously this pisses off people like us who like technical stuff to be implemented with more care and foresight than politicians use when mucking about with clocks. > (Also, I'm not sure saying "the politicians will fix it" is your most > successful tactical point :-) :-) I don't expect them to fix it, I expect them to continue fumbling with it in much the same way as they do now - which is an inconvenience we know how to deal with, both socially and in software (unlike leap seconds). > On the other hand, the lumps and bumps from the timezone gimmick both > accelerate with time, and pile higher and higher, one on top of the > other. Yes, that's the point, but they accelerate much much slower than leap seconds, and the pile becomes unweildy ten times further in the future than the UTC rules. > Real time (the solar time that drives politicians in each county or > country to reluctantly deal with this issue) will get further and > further from civil time. As a result, civil time will mean less and > less. I think for "real time" you mean "local civil time", and for "civil time" you mean "atomic time". The way we avoid the problems caused by politicians fiddling with timezones is by using UTC instead of local time. In the future that role would be taken by atomic time. Yes it won't trivially relate to any kind of local time at any place on earth, like UTC and GMT, but that isn't what we need it for. Tony. -- f.anthony.n.finchhttp://dotat.at/ ROCKALL MALIN: MAINLY SOUTHWEST 4 OR 5, OCCASIONALLY 6 AT FIRST, BECOMING VARIABLE 3 FOR A TIME. MODERATE OR ROUGH. OCCASIONAL RAIN. MODERATE OR GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
- Original Message - From: "Rob Seaman" To: "Leap Second Discussion List" Sent: Tuesday, January 06, 2009 5:30 AM Subject: Re: [LEAPSECS] Reliability Tony Finch wrote: The reason DST exists is to more closely sync our activities to sunrise. The reason DST exists is because it has become a self-propagating cultural meme. Gotta agree on this one. I like long summer evenings as much as the next guy, but to be realistic, DST is an idea whose time has come and gone. They didn't have air conditioning, massive automobile traffic, or 24/7 business operations back in William Willet's day. Your April Fool's post on risks may be the most coherent analysis I've read on the subject. (Not trying to be ironic.) In general, this list (sad to say - now I'm being ironic :-) represents the species' hoard of knowledge on certain topics. Where I grew up in the U.S. mid-Atlantic states, the most obvious effect of DST was to extend the usable hours of daylight for Summer evenings. (Perhaps some other narrative applies at higher or lower latitudes?) Since we were off school, the morning issues were meaningless. And workers go to work when their bosses tell them to. The time they own for themselves and their families is after work. Recently, all discussions of DST are framed in turns of energy. It seems like every argument for DST (saves energy for lighting in the mornings) is countered by some argument against (increases cooling costs in the evenings). If DST were really a mechanism for managing our natural daylight resource, rather than a naive attempt at PR regarding petroleum resources, it would be applied in the Winter when the daylight is in shortest supply. Most of us have long suspected that this notion of DST saving energy is bollocks, and now we have proof: http://www2.bren.ucsb.edu/~kotchen/links/DSTpaper.pdf Lucky for Congress that this paper didn't come out until after the decision was made to extend DST. (Because, you know, that worked so well during the Ford administration...) Brian ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Tony Finch wrote: I think for "real time" you mean "local civil time", and for "civil time" you mean "atomic time". Not precisely, but that's the gist. In the future that role would be taken by atomic time. Yes it won't trivially relate to any kind of local time at any place on earth, like UTC and GMT, but that isn't what we need it for. This is the part I disagree with. "Global civil time" (the underlying timescale for the numerous local civil time variants) needs to be stationary with respect to mean solar time. The requirements for civil timekeeping are much broader than the technological issues (that we're all tediously familiar with). Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Thank you for the discussion so far. On Mon, Jan 05, 2009 at 04:31:44PM -0700, Rob Seaman wrote: > Adi Stav wrote: > >> what problems could exceeding the tolerance(s) cause? > > Well covered in the archive. For astronomy, 1 second of time is 15 > seconds of arc on the equator. This is a large error (colossal for some > purposes). It doesn't appear that any other industry has actually > performed a coherent risk analysis. For some reason this is asserted to > be the astronomers' responsibility. Right. Well, both my memory of the archives and M. Warner Losh's summary have uses that need to be aware of UT (actually, I think local sidereal time, or ET in some cases, so that have to perform conversions either way). I was referring, rather to issues with civil time having a large DUT. I am trying to identify a requirement for civil time having a low (say, below 30 minutes) DUT. So far, I can think of the common legacy of legal time being mean solar time at some longitude, but that's about it. >> (Especially problems that time zones far from their reference >> meridians, DST switches twice a year, and the difference between mean >> and apparent solar time don't already cause). > > This confuses periodic with secular effects, also in the archive. A secural effect will eventually cause infinite DUT by definition. That's why I started with a question regarding a concrete bound on DUT. Unless you mean that with any concrete bound on DUT, intercalation will become more and more frequent? Or I miss your point again? >> A good parallel would be adding leap hours and using the existing DST >> mechanism > > Reasons why leap hours won't work are in the archive. There was a clear > consensus from both sides of the aisle that the notion of leap hours is > absurd. Alternately, by relying on shifting timezones, there would be no > underlying stabilized civil timescale permitting commonsense timekeeping > inferences by humans. I said I don't think it's a good idea necessarily, only that it is the parallel of the Gregorian reform. But what do you think about my suggestion of phasing the time standard every few centuries when the standard's DUT reaches 30 minutes? Won't it make leap hours workable? >> I don't understand :) > > Imagine a version of the Gregorian calendar that interpolates leap days > only every 400 hundred years. That would amount to about 3 months at a > time. Since this is a whole season, it is equivalent to not stabilizing > the calendar at all. > > Leap hours or tweaking timezones can be interpreted the same way. If > intercalary adjustments are the width of a timezone, no practical > stabilization is occurring. Ah, I see. (Although, of course, half an hour or an hour in a day is much less harmful than a season in a year.) ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Sun, Jan 04, 2009 at 08:58:29PM -0700, Rob Seaman wrote: > Here's a notion I don't recall seeing before on the list: > > Coordinate leap seconds with leap days. Introduce an integral number of > leap seconds each February 29th. Discuss. February 29th does not start and end all over the world at the same time. Some time zones will get the leap during the 28th, others in March the 1st. Another suggestion in the same vain: standardize all the time zones of the world to two specific dates for starting and ending DST (if they use it). Add leap seconds at one of those dates only. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On 2009-01-06, at 22:35, Adi Stav wrote: I am trying to identify a requirement for civil time having a low (say, below 30 minutes) DUT. I would say that the actual requirement is for DUT to stay within a small interval. Of course this also implies a low DUT, but debating the need for a low magnitude tends obscure the real issue: the size of the interval within which DUT is allowed to wander. The assertion has been made that DUT could grow to 30 minutes or even an hour just fine because people also tolerate not living exactly on the central meridian of their time zone and also cope with DST. I believe this to be false. People's tolerance for being some fixed time offset (modulo 1 DST hour) away from their "time meridian" has nothing to do with their tolerance for this value to drift. N ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
This is the part I disagree with. "Global civil time" (the underlying timescale for the numerous local civil time variants) needs to be stationary with respect to mean solar time. The requirements for Rob, A problem is what defines your "stationary" (what bandwidth) and what defines "mean" (what averaging time). Or, another way to put it, why in your opinion, are leap seconds OK but leap tenth-seconds, or leap minutes, or leap hours not OK? Each of these preserve, to one degree or another, the notion of stationary wrt solar time. /tvb ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Adi Stav wrote: Rob Seaman wrote: Coordinate leap seconds with leap days. Introduce an integral number of leap seconds each February 29th. Discuss. February 29th does not start and end all over the world at the same time. This is no different than the end of December or June. In fact, the current standard already permits leap seconds to be issued at the end of February, and that would sometimes mean February 29th. Some time zones will get the leap during the 28th, others in March the 1st. The leap occurs at midnight UTC on 30 June or 31 December. These dates apply west of Greenwich, e.g., we saw the leap second in Tucson at 5 pm on New Years Eve. East of Greenwich it is already the morning of 1 July or 1 January when the leap second occurs. Confusion doesn't happen near the IDL since it is just before noon on the first day of the month on one side of the line or just after noon on the last day of the preceding month on the other side. So just as with all the other months (and all are currently permissible), the leap second occurs in all localities on the last day of one month or the first day of the next. It never occurs on day N-1. I don't think any of this affects the interpretation of the word "coordinate". It does, perhaps, emphasize that intercalary corrections to the calendar are made in coordination with our clocks (29 February begins at 0h local time). And it should emphasize that intercalary corrections to our clocks have to be made in coordination with our calendar (1231T235960Z or 0630T235960Z). The civil clock is a subdivision of the civil calendar. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Tue, Jan 06, 2009 at 11:31:52PM +0100, Nero Imhard wrote: > > I believe this to be false. People's tolerance for being some fixed time > offset (modulo 1 DST hour) away from their "time meridian" has nothing to > do with their tolerance for this value to drift. I see. And how would such intolerance come into effect? The little old lady who will eventally walk to church in the dark on winter Sundays has been suggested, but that was rather tongue-in-cheek :) I'd say the little old lady's tolerance for maximum drifting DUT is no less than a minute or two, because her watch isn't that accurate anyhow and she sets it from time to time. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Rob Seaman said: > The leap occurs at midnight UTC on 30 June or 31 December. These > dates apply west of Greenwich, e.g., we saw the leap second in Tucson > at 5 pm on New Years Eve. East of Greenwich it is already the morning > of 1 July or 1 January when the leap second occurs. I know what you're trying to say, but I'm several hundred metres east of Greenwich and I saw the leap second on 31 December, not 1 January. Which is one more point on the "don't care about mean solar time" side. > Confusion doesn't happen near the IDL since it is just before noon on > the first day of the month on one side of the line or just after noon > on the last day of the preceding month on the other side. Except New Zealand is, I believe, in zone +13 at this time of year. So it was 12:59:60 on 1 January. -- Clive D.W. Feather | If you lie to the compiler, cl...@davros.org | it will get its revenge. http://www.davros.org | - Henry Spencer ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On 6 Jan 2009 at 10:12, Tony Finch wrote: > Note that there's no need for global co-ordination. Each country (or > county) can change when it is convenient for them. The effect would > probably be a shifting of timezone boundaries in lumps and bumps that > averages out to the overall DUT1 drift. ...thus ending up with a time zone map even more chaotic and convoluted and ever-changing than the current one, something I wouldn't have thought to be possible. And after a few millennia, UTC will actually be a complete day or more removed from the local civil time in any place. This will be very confusing in places like Wikipedia comment signature datestamps, which use UTC. (Wikipedia edit histories also use UTC by default, but can be configured by users to display in their preferred time zone.) Also in a few millennia, when the need to alter the Gregorian calendar to correct for alignment with the seasons comes up, that will open the question of whether such calendar alterations apply to UTC, to local time systems, or both. The keepers of UTC as a strict atomic time standard will undoubtedly oppose any alteration to its calendar rules, but if the local time zones are still officially based on it (even if shifted by an offset of multiple days by then) then it wouldn't make sense to change the calendar rules for them but not UTC, so another big fight on whatever the equivalent of Internet mailing lists is in that era will be anticipated. -- == 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/ ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Tom Van Baak wrote: why in your opinion, are leap seconds OK but leap tenth-seconds, or leap minutes, or leap hours not OK? Each of these preserve, to one degree or another, the notion of stationary wrt solar time. I'll refrain from references to current practice. We often get tangled in assertion of "you can't do that!" I would say that a leap tenth-second or leap minute are within the bounds of possibility simply based on size. A leap hour is far too big a jump. The point of intercalation is to make a succession of changes which are individually of small enough amplitude to be ignored (or, at least, ignorable). The latency between intercalation events must be short enough to permit smoothing over historical periods - say, a few decades. Too short is also not good. I think tenth-second events would be needed too frequently. Clearly some here believe one-second intercalation events occur too frequently :-) On the other hand, permitting a long delay between events - or rather, between scheduling opportunities for events - risks losing the corporate knowledge to handle the events properly. Others here believe leap seconds are already demonstrating that :-) One great benefit of leap seconds is that there is a simple mechanism for introducing them into the flow of time marks. (The original design is pretty clever, actually.) A leap minute would likely be added as an additional last minute to the UTC day, basically 60 straight leap seconds. I suppose a leap tenth-second would correspond to a slightly longer final second to a day. One advantage of this is that (for the next few hundred years) a rigid schedule could be instituted. Something like add (or subtract) an integral number of tenth-seconds each and every 31 December. The schedule remains fixed, the amplitude varies. This has similarities to the various timeslicing schemes that were mentioned - oh - five years into the discussion. There are prior posts on why it would be very difficult to interpose an additional hour, basically because hours are individually tagged in each time zone, whereas each minute can cleanly add a 60th second and each hour a 60th minute. That is, in Greenwich the leap hour could be a 25th hour, but in Tucson it would fall between the end of the 17th and the beginning of the 18th hours. (Among other logistical issues.) So, with the caveat that I really do believe that we should be focusing on collecting requirements to characterize the problem, rather than speculating on possible solutions, here is a score card: leap tenth-seconds: small amplitude (too small? some might see these as rubber seconds) too frequent operationally? not so infrequent we could ignore them split-second mechanism needed to implement leap seconds small amplitude marginally too frequent (meaning people obviously disagree) marginally too infrequent (to force programmers to handle correctly :-) mechanism already deployed (obviously people disagree :-) leap minutes marginally acceptable amplitude (for some purposes, DUT1 would have to adapt) not frequent enough operationally (but not outside the bounds of reality) too infrequent to maintain corporate knowledge mechanism possible leap hours unacceptable amplitude (waayy unacceptable) not frequent enough operationally (by any interpretation) too infrequent (whole civilizations would come and go) mechanism is impossible Like I said, if the alternative is the ITU giving up on civil timekeeping entirely, we can likely reach some sort of consensus to extend scheduling based on a relaxed approximation of some sort. My personal preference is either to maintain the current status quo (although extending the schedule without relaxing DUT1 should also be possible) OR to redesign the system entirely from the ground up, eg., Steve Allen's zoneinfo concept. There clearly is resistance to admitting that there are two different underlying concepts of civil timekeeping that must both be honored. Embracing this will be the quickest way to reach a new equilibrium. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Adi Stav wrote: But what do you think about my suggestion of phasing the time standard every few centuries when the standard's DUT reaches 30 minutes? Won't it make leap hours workable? I suspect that none of the factions will welcome repeated redefinitions of a fundamental standard. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
In message: <49646f64.11204.11917...@dan.tobias.name> "Daniel R. Tobias" writes: : On 6 Jan 2009 at 10:12, Tony Finch wrote: : : > Note that there's no need for global co-ordination. Each country (or : > county) can change when it is convenient for them. The effect would : > probably be a shifting of timezone boundaries in lumps and bumps that : > averages out to the overall DUT1 drift. : : ...thus ending up with a time zone map even more chaotic and : convoluted and ever-changing than the current one, something I : wouldn't have thought to be possible. : : And after a few millennia, UTC will actually be a complete day or : more removed from the local civil time in any place. This will be : very confusing in places like Wikipedia comment signature datestamps, : which use UTC. (Wikipedia edit histories also use UTC by default, : but can be configured by users to display in their preferred time : zone.) Where a few is like 6 or 7. See http://www.ucolick.org/~sla/leapsecs/dutc.html for the timelines on that. In 5000BC mankind was a collection of hunter gathers that was just learning how to farm. The Pyramids of Giza, for example, were built around 2500BC. The earliest known hieroglyphics appear around 3200BC in pre-dynastic Egypt. It has been pointed out that we've also accelerated the rate of technology, so the next 7k years are going to see way more development than the last 7k. I don't think anybody can make any meaningful predictions out 7k years. After all, Wikipedia has only been around 7, so worrying about what it will be like in 7k years seems a bit premature or presumptuous. : Also in a few millennia, when the need to alter the Gregorian : calendar to correct for alignment with the seasons comes up, that : will open the question of whether such calendar alterations apply to : UTC, to local time systems, or both. The keepers of UTC as a strict : atomic time standard will undoubtedly oppose any alteration to its : calendar rules, but if the local time zones are still officially : based on it (even if shifted by an offset of multiple days by then) : then it wouldn't make sense to change the calendar rules for them but : not UTC, so another big fight on whatever the equivalent of Internet : mailing lists is in that era will be anticipated. The error rate for the Gregorian calendar is like 1 day in 4k years, so it will take a very long time for enough error to accumulate that people will want to do something. There have been proposals to make years ending in 4000 not be leap years to correct, but nothing official has happened on this yet. The Gregorian calendar has a tolerance of about +/- just over a day, so equinox varies between 20-dec at 20:47UT and 23-dec at 0:18 UT. Warner ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
M. Warner Losh wrote: I don't think anybody can make any meaningful predictions out 7k years. The Sun will still shine. The Earth will still spin. Lunar tides will continue their billion year trend. A solar second will be incrementally a bit longer yet than an SI second. If humans still exist, they will remain very similar anatomically and physiologically to the Cro Magnons of 50,000 years ago. Either we will have restrained our darker selves and preserved the environment - or we won't. I would suggest that either way our great^N grandchildren will care more about diurnal rhythms, rather than less. Meanwhile, computing devices in the 91st century are not likely to suffer from the ills of POSIX. Even SI units may not still exist. The simple fact of the existence of multiple kinds of timekeeping standards won't throw our big domed overlords into a tizzy: http://www.ufomystic.com/wp-content/uploads/outerlimits_os_05.jpg Work to improve the infrastructure we have, not degrade the fundamental standards underlying the infrastructure. If UTC doesn't do what you want, use GPS and leave universal time to the people who use it and like it. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Tue, 6 Jan 2009, Adi Stav wrote: > > Another suggestion in the same vain: standardize all the time zones of > the world to two specific dates for starting and ending DST (if they use > it). Add leap seconds at one of those dates only. That would require the period of DST to be exactly half the year, so that the north and south get the same amount of it. However DST is usually applied for more than half the year. Tony. -- f.anthony.n.finchhttp://dotat.at/ NORTH UTSIRE SOUTH UTSIRE: VARIABLE 3 OR 4 BECOMING SOUTHERLY 5 TO 7 THEN VEERING WESTERLY. MODERATE OR ROUGH. OCCASIONAL SNOW THEN RAIN. MODERATE OR GOOD, OCCASIONALLY POOR AT FIRST. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Tue, 6 Jan 2009, Adi Stav wrote: > > Right. Well, both my memory of the archives and M. Warner Losh's summary > have uses that need to be aware of UT (actually, I think local sidereal > time, or ET in some cases, so that have to perform conversions either > way). No-one uses ET any more. It has been replaced by TT (which is based on atomic time). Tony. -- f.anthony.n.finchhttp://dotat.at/ NORTH UTSIRE SOUTH UTSIRE: VARIABLE 3 OR 4 BECOMING SOUTHERLY 5 TO 7 THEN VEERING WESTERLY. MODERATE OR ROUGH. OCCASIONAL SNOW THEN RAIN. MODERATE OR GOOD, OCCASIONALLY POOR AT FIRST. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Mon, 5 Jan 2009, Rob Seaman wrote: > > Alternately, by relying on shifting timezones, there would be no > underlying stabilized civil timescale permitting commonsense timekeeping > inferences by humans. What do you mean by "stabilized" here? Atomic time is the basis of our most stable time scales. I don't think perturbing a timescale to follow the erratic slow-down of the earth can reasonably be called "stabilizing" it. What common-sense inferences do you have in mind? Most common sense is wrong, especially when it comes to time. > Imagine a version of the Gregorian calendar that interpolates leap days > only every 400 hundred years. That would amount to about 3 months at a > time. Since this is a whole season, it is equivalent to not stabilizing > the calendar at all. > > Leap hours or tweaking timezones can be interpreted the same way. Not really. An hour in a day is more like a couple of weeks in a year, not three months. Two weeks is less than the variability caused by intercalary months in lunisolar calendars. Tony. -- f.anthony.n.finchhttp://dotat.at/ BISCAY SOUTHEAST FITZROY: NORTHEASTERLY 4 OR 5, INCREASING 6 AT TIMES. MODERATE, OCCASIONALLY ROUGH IN SOUTHEAST FITZROY. MAINLY FAIR. MAINLY GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Wed, 7 Jan 2009, Rob Seaman wrote: > > On the other hand, permitting a long delay between events - or rather, > between scheduling opportunities for events - risks losing the corporate > knowledge to handle the events properly. The good thing about timezones is the code to implement them and alter them is exercised all the time. > One great benefit of leap seconds is that there is a simple mechanism for > introducing them into the flow of time marks. So simple it usually doesn't work! > There clearly is resistance to admitting that there are two different > underlying concepts of civil timekeeping that must both be honored. > Embracing this will be the quickest way to reach a new equilibrium. Just treat UT as another timezone offset from TI, alongside all the other earth-oriented timezones. Tony. -- f.anthony.n.finchhttp://dotat.at/ NORTHWEST FITZROY SOLE: EASTERLY OR SOUTHEASTERLY 4 OR 5, OCCASIONALLY 6 LATER IN WEST. MODERATE, OCCASIONALLY ROUGH LATER IN WEST. MAINLY FAIR. MAINLY GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
In message , Tony Fi nch writes: >On Wed, 7 Jan 2009, Rob Seaman wrote: >> >> On the other hand, permitting a long delay between events - or rather, >> between scheduling opportunities for events - risks losing the corporate >> knowledge to handle the events properly. > >The good thing about timezones is the code to implement them and alter >them is exercised all the time. ...and that politicians who are too enthusiatically mucking about with them, will face the consequences, by the (voting) hands of the affected electorate. Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Tony Finch wrote: What do you mean by "stabilized" here? Atomic time is the basis of our most stable time scales. I don't think perturbing a timescale to follow the erratic slow-down of the earth can reasonably be called "stabilizing" it. Civil timekeeping (the underlying global timescale that ties all our local timescales together into a unified whole) has requirements derived from two parent classes - interval timekeeping and Earth orientation timekeeping. In the absence of a full blown rubber second implementation, the Earth orientation part of that requires tempering to optimize (or perhaps more accurately, satisfice) its utility. (I could venture into a paean to Ernst Mach and point out that to flatlanders living on Earth it is the motions of the celestial sphere that are erratic, but I will restrain myself :-) I've been trying on different terms for this tempering, for instance, keeping civil timekeeping "stationary" with respect to diurnal trends. One could compare this (loosely) to the notion of "disciplining" a clock as in NTP. The term "stabilization" I borrowed from work I've been doing with numerical compression algorithms for scientific imaging data with a Poisson noise model, as in the "generalized Anscombe variance stabilization" transform. Precisely because the Earth's motions are erratic, they benefit from enforcing a clock discipline that keeps an arbitrary zero point (some shrubbery in the park surrounding the Greenwich Observatory) stationary in phase through an arbitrary number of cycles. What common-sense inferences do you have in mind? Simple utilitarian inferences regarding the world around us. Most common sense is wrong, especially when it comes to time. Reference to some study supporting this assertion? Humans make decisions based on mental models. Those models and the resulting decisions do not have to be vetted against quantum chromodynamics or magneto-hydrodynamics to be deemed "right". Again, I'll recommend Steven Pinker's book "The Stuff of Thought" for a discussion of the basis of an inherent model of physics contained in the structure of human language. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Wed, 7 Jan 2009, Rob Seaman wrote: > > > What common-sense inferences do you have in mind? > > Simple utilitarian inferences regarding the world around us. Such as? I can't think of anything simple enough to count as common sense which depends on the relation between UT1 and the various local times. Tony. -- f.anthony.n.finchhttp://dotat.at/ DOVER WIGHT PORTLAND PLYMOUTH: EAST VEERING SOUTHEAST 3 OR 4, OCCASIONALLY 5 EXCEPT IN DOVER. SLIGHT OR MODERATE. FAIR. MODERATE OR GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
Tony Finch wrote: Such as? I can't think of anything simple enough to count as common sense which depends on the relation between UT1 and the various local times. As with most issues discussed on this list, questions need to be framed properly before they can be addressed. The key role that UTC plays in framing the "simple utilitarian inferences regarding the world around us" that I mentioned is as a prediction of UT1. UT1 itself is only known retroactively. Similarly, UTC (and GPS) provide access to TAI, which we have been informed time and again is some mystical perfect timescale unsuitable for use by mere humans. (Or rather, it isn't UTC per se, but the various realizations of UTC that provide dual access to interval and Earth orientation timescales.) Universally (to use that word metaphorically yet again), we do not use the "various local times" to compute or intuit global assertions about our world. Which is to say, we may observe that the Sun is up or down at some particular moment, but to predict the behavior tomorrow we always tie into Universal Time in some fashion. Currently access to UTC is automatically supplied (for both simple and complex utilitarian purposes) because it is built into the system of standard time zones. The ITU proposal would sunder this. Yes, we could cheat some of the purposes some of the time, and some other purposes all of the time, but most definitely not all of the purposes forever. The real answer to "I can't think of" queries is to point out that these are explicit pleas to engage in a process of requirement discovery. Neither God nor science reside in the gaps of our imagination. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability
On Thu, 8 Jan 2009, Rob Seaman wrote: > > The key role that UTC plays in framing the "simple utilitarian inferences > regarding the world around us" that I mentioned is as a prediction of UT1. > UT1 itself is only known retroactively. I still don't know what these inferences are. Tony. -- f.anthony.n.finchhttp://dotat.at/ SOUTH UTSIRE: WEST 4 OR 5 BACKING SOUTH 5 TO 7, PERHAPS GALE 8 LATER. MODERATE OR ROUGH. MAINLY FAIR. MODERATE OR GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability (was Re: it's WP7A week in Geneva)
In message <0b8b21eb-dbea-4dec-89c5-f27557f37...@noao.edu>, Rob Seaman writes: >Tom Van Baak wrote: >It is clearly aberrant design for any system to ever lie about a >return value. Well, "lie" is such a strong word. I know for sure that both the Motorola UT+ and M12+T in certain a certain specific situation will indicate that it delivers UTC timestamps but in fact the receiver does not have sufficient information to adjust from the GPS to the UTC timescale. The exact circumstances are very specific and related to Almanac data confusions in conjunction with short interruptions of power. The timestamps delivered are often, but not always GPS timestamps in this particular case. I am not aware if Motorola has fixed this in later firmware revisions. Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability (was Re: it's WP7A week in Geneva)
Rob Seaman wrote: Tom Van Baak wrote: when all is said and done). A 12.5 minute down time means your annual reliability can only be 4 9's, not 5 9's... This is why many receivers remember the last UTC offset values and warm start with them if they have only been off a short period of time... Warner User code can usually handle being told by a GPS receiver that "UTC is not known yet; use GPS time instead". But when a GPS receiver says "sorry, by the way, all the UTC time stamps I've sent you in the past quarter hour were off by one second", that might be a little more trouble to deal with. Could somebody comment on the reliability of GPS receivers to deliver GPS time itself? On the first fix it has resolved GPS time, modulus the 1024 week wrapping. The receiver itself rarely needs GPS time as a compound unit, but it remains broken down to GPS week, Z-count (1,5 sec tick wrapping on new GPS week), and then some suitable high-rate counter. The individual channels build their own 1,023 MHz or 10,23 MHz counters (with doppler), 1 ms counter, 20 ms counter (for data bit alignment), page and frame counters etc being synchronous to the receiver. The common high-rate clock is used to sample the phase state of the channels , pseudo-ranges is built, processed and out comes the ECEF results [XYZT]. Time-errors is adjusted according to some suitable algoritm, but occasional jumps is seen in the data for some receivers. See Misra&Enge for examples of that. A lot of the processing uses various forms of issue of data to give corrections to orbit parameters for instance. So broken up GPS time is in the very core of things. But cranking out date and time isn't, it's an adaptation to the user. We have had issues where GPS time has not been reliable due to incorrect handing of the 1024 week wrapping. The produced UTC time as affected as a side consequence. If the receivers where using L2C they would be able to resolve this from the signal, as it has a 8192 week wrapping. Some early receivers used a bias scheme and in one case it processed the GPS week like this: if (GPSweek < 500) GPSweek += 1024; Works well until the actual GPS week steps from 1523 to 1524 in which the wrapped GPS week steps from 499 to 500 and the calculated GPS week after unwrapping steps from 1523 to 500. Those receivers frose. A firmware update could do this after the wrapping: if (GPSweek < 500) GPSweek += 1024; GPSweek += 1024; That would keep them running for another 19 years. Other receivers have other ways to handle it, including allowing the user to give the date as a hint. Infact, the current year is sufficient to resolve that wrapping. Regardless, the GPS time can also "lie", but it is maybe about a decimal place less likely than UTC time from a system approach. However, if one blindly expects the GPS receiver to always give "correct" time, then one has opened up a pandora-box of troubles, as you obviously is not looking at the error handling needed particular for GPS receivers. To protect yourself you need to understand how they work and what failures and mal-functions they can have. But too many times I see them being treated as this box that takes a GPS signal and cranks out "correct time". Once in place, they are mostly forgotten except on tours among the equipment. Cheers, Magnus ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability (was Re: it's WP7A week in Geneva)
Quoting Magnus Danielson : >If the receivers where using L2C they would be able to resolve this from the >signal, as it has a 8192 week >wrapping. But not quite yet. No IIR-M satellite was transmitting the CNAV messages on L2C until last week when, as a test, SVN49 started to transmit message type 0. It will be many years before the full CNAV message is implemented on IIR-M and IIF satellites. === Richard B. LangleyE-mail: l...@unb.ca Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ Dept. of Geodesy and Geomatics EngineeringPhone:+1 506 453-5142 University of New Brunswick Fax: +1 506 453-4943 Fredericton, N.B., Canada E3B 5A3 Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ === ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability (was Re: it's WP7A week in Geneva)
On 2009-10-03, at 23:56, Rob Seaman wrote: > However, is it a true assertion that currently deployed GPS receivers > return GPS time significantly more reliably (all those 9's) than they > do UTC? (Assuming a particular model supports both?) > > It's hard to see this as supporting a position that "Only UTC can be > disseminated"... It was not even clear how that phrase was meant, let alone how one would arrive at such a conclusion. Some possible interpretations: - Only the time scale currently called "UTC" can be disseminated. (why??) - It is not possible to disseminate a clean count of SI seconds (why not??) - It is only practical to disseminate a time scale that has a simple relationship to legal time (i.e. UTC). - whatever the disseminated time scale, it has to be called "UTC" (sounds like ITU's position) etc. Does anyone have a clue? N ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Reliability (was Re: it's WP7A week in Geneva)
In message , "Nero Imhard " writes: >On 2009-10-03, at 23:56, Rob Seaman wrote: >> It's hard to see this as supporting a position that "Only UTC can be >> disseminated"... > >Does anyone have a clue? I read it as: "I won't get invited to the BIPM metrological barbeque if I advocate disseminating TAI, because those guys really don't want that." :-) Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
