Re: [LEAPSECS] A leap second proposal to consider -- LSEM
On 2010-11-04 21:46, Zefram wrote: There is a recent near-renaming that shows the way: the modern form of Sidereal Time is known as Earth Rotation Angle. This name is accurate in some important ways: it's specific to Earth, and it's not time at all but an angular measure. To be precise, Earth Rotation Angle exists _in addition_ to a modern form of Greenwich mean sidereal time, whose definition uses both ERA and TT. See for example p 16 of [http://www.usno.navy.mil/USNO/astronomical-applications /publications/Circular_179.pdf]. The new name denotes a new concept, the old concept has retained its name. Michael Deckers. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
Tom Van Baak wrote: >Back when the understanding of the "universe" consisted little >more than our solar system, and back before quartz or atomic >clocks, this made sense. Now maybe it should be renamed ST >for solar time, rather than UT for universal time. Historically, we don't rename time scales in that manner. But if you are going to rename, "ST" is inadequate. There is a recent near-renaming that shows the way: the modern form of Sidereal Time is known as Earth Rotation Angle. This name is accurate in some important ways: it's specific to Earth, and it's not time at all but an angular measure. So you want Solar-origin Earth Rotation Angle, or something like that. (That sounds like it ought to refer to *apparent* solar time, and require further decoration to refer to mean solar time.) -zefram ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
On Nov 4, 2010, at 2:36 PM, Tom Van Baak wrote: > Since there are quite a few more cesium atoms than there are > planet earth's one could argue that atomic time scales are more > "universal" than an earth/sun rotation/revolution-based scale, no? No. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
All forms of UT1 have been direct measures of earth rotation. One can argue about zero points and drifts, but the underlying purpose of UT1 is to monitor rotation with a value that tracks where the sun is over the earth. In that sense UT1 tries to be a form of mean solar time, so it merits a name consistent with the 1884 IMC "universal day" and the 1928 IAU "universal time". Back when the understanding of the "universe" consisted little more than our solar system, and back before quartz or atomic clocks, this made sense. Now maybe it should be renamed ST for solar time, rather than UT for universal time. This is not the case with a purely atomic time scale uncorrected for the rotation of earth. To name that "universal time" is abuse. Since there are quite a few more cesium atoms than there are planet earth's one could argue that atomic time scales are more "universal" than an earth/sun rotation/revolution-based scale, no? /tvb ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
On Thu 2010-11-04T20:23:12 +, Michael Deckers hath writ: >Yes, you are of course right. My point is that even UT1 >does not try. Sidereal time is no longer an affine >function of UT1. All forms of UT1 have been direct measures of earth rotation. One can argue about zero points and drifts, but the underlying purpose of UT1 is to monitor rotation with a value that tracks where the sun is over the earth. In that sense UT1 tries to be a form of mean solar time, so it merits a name consistent with the 1884 IMC "universal day" and the 1928 IAU "universal time". This is not the case with a purely atomic time scale uncorrected for the rotation of earth. To name that "universal time" is abuse. -- 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] A leap second proposal to consider -- LSEM
We often get lost on this list in the details of legal or physical timescales. Astronomers need to know such details. Civilians need a simple mechanism. The current civil timescale cleverly provides access to both interval time and a measure of mean solar time in a single pragmatic realization of both. Forget all the higher mathematics - heck, professional timekeeping is often only known after the fact. Does that describe any wall clock you know? If civil time is remade without the half that provides knowledge of Earth orientation, then access to that information will be required in some other manner. It is patently obvious that the ITU does not consider this their problem. If they want to pawn this requirement off on some other group, the bare minimum price of entry is for the changes to TF-460 to instantiate a new timescale with the characteristics THEY want, such that OTHERS can build coherently and competently a new mechanism to convey mean solar time. That is - universal time - that is, something that has derived with minimal fuss and muss from the rich history of UTC, GMT, and earlier timekeeping standards. To minimize the fuss, choose a different damn name. This is what was agreed in Torino in 2003. There will still be fuss, but it will (perhaps) be manageable. Rob -- On Nov 4, 2010, at 1:23 PM, Michael Deckers wrote: > > On 2010-11-03 23:31, Steve Allen remarked: > >> I see the point of "mean solar time" not as "how accurately does the >> expression represent the sun over the earth?" but as "does the >> expression even try to represent the sun over the earth?". >> I think that the discussions and intentions surrounding the current >> draft revision of TF.460 indicate that it does not try. > > Yes, you are of course right. My point is that even UT1 > does not try. Sidereal time is no longer an affine > function of UT1. > > Michael Deckers. > ___ > LEAPSECS mailing list > LEAPSECS@leapsecond.com > http://six.pairlist.net/mailman/listinfo/leapsecs ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
On 2010-11-03 23:31, Steve Allen remarked: I see the point of "mean solar time" not as "how accurately does the expression represent the sun over the earth?" but as "does the expression even try to represent the sun over the earth?". I think that the discussions and intentions surrounding the current draft revision of TF.460 indicate that it does not try. Yes, you are of course right. My point is that even UT1 does not try. Sidereal time is no longer an affine function of UT1. Michael Deckers. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
On Wed 2010-11-03T23:06:03 +, Michael Deckers hath writ: >Since 2003, UT1 has no connection with the Sun Actually, since 1984, see the discussion in Aoki et al. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1982A%26A...105..359A In that discussion note that not even Newcomb's expression was connected with the sun because he knew that his precession value was wrong. In the archives related this subject are also the longstanding issues of the value of the constant of aberration and the problems of the "conventional longitides". But it was Newcomb's expression which was adopted by international agreement as the formula for computing mean solar time in order that everyone could agree on its value. If an expression is deficient it can be amended, but that only works if there is agreement on what it is trying to do. I see the point of "mean solar time" not as "how accurately does the expression represent the sun over the earth?" but as "does the expression even try to represent the sun over the earth?". I think that the discussions and intentions surrounding the current draft revision of TF.460 indicate that it does not try. -- 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] A leap second proposal to consider -- LSEM
On 2010-11-03 18:43, Poul-Henning Kamp observed on a remark by Rob Seaman: > "Universal Time" *means* "mean solar time". It probably did in the 1800's, in these days of Lego-toys on Mars, most people I have talked to, find it utterly strange that a timescale with "universal" in it, depends on one particular lump of rotating rock. Since 2003, UT1 has no connection with the Sun -- it measures Earth rotation independent of the revolution of the Earth around the Sun (except for geodesic precession, sigh). So "mean solar time" may well be considered a misnomer for UT1. Since sidereal time is still well-defined (based on both UT1 and TT), any definition of a mean sun could resurrect mean solar time in the original sense, if desired. For the time being, it would not deviate appreciably from UT1 (because that's how UT1 was redefined). Michael Deckers. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
In message <0859717c-0eb7-4af8-bb4d-38f657144...@noao.edu>, Rob Seaman writes: >"Universal Time" *means* "mean solar time". It probably did in the 1800's, in these days of Lego-toys on Mars, most people I have talked to, find it utterly strange that a timescale with "universal" in it, depends on one particular lump of rotating rock. -- 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] A leap second proposal to consider -- LSEM
On Wed 2010-11-03T11:06:36 -0700, Rob Seaman hath writ: > > The key point of LSEM is that 100% of the months would have leap seconds, > > I think this is a previously undiscussed option. I think this is unimplementatble silliness. Look at the plot of the history of time scales http://www.ucolick.org/~sla/leapsecs/deltat.html#POSIX Over and over during the past century (really starting with astronomers, with Ptolemy or even earlier) humans have found it necessary to define and use a time scale which is as uniform as possible. For the sake of our operational system we should broadcast the most uniform time scale that we are capable of producing. For millennia the civil authorities have found it necessary to redefine civil time in a way that suits them. Operational systems don't like that (just ask every European whose iPhone alarm went off an hour late this week). Operational time is not civil time. They are two different needs, both are valuable, and we have ways of handling the changing offsets. The ITU-R should define broadcasts to be as uniform as possible while making it clear that the purpose they are serving has no relation to civil time. -- 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] A leap second proposal to consider -- LSEM
Hi Tom, > The key point of LSEM is that 100% of the months would have leap seconds, I think this is a previously undiscussed option. When +/- dithering came up before (when the world and we were younger) the idea was that a leap second would rather amount to the omission of one of the normal monthly dithering steps. > If that means DUT1 gets closer to 1.0 instead of 0.9 so be it. So be it indeed, however note an alternative strategy (proposed in precambrian times): http://iraf.noao.edu/~seaman/leap The current standard provides a lot of leeway in implementing UTC. That leeway could be used to address our varied concerns in diverse ways. If politics is to triumph over process, even better is a political solution that requires no international agreement at all. For instance, we have the "Draft Alternate Proposal" from the final slide of the Torino Colloquium: "Evolve from the current UTC Standard by transition to Temps International (TI) [...] TI should be a continuous atomic time scale, without Leap Seconds, that is synchronized with UTC at the time of transition." (http://www.inrim.it/luc/cesio/itu/closure.pdf) "Universal Time" *means* "mean solar time". It is natural to call something that is in no way mean solar time something other than universal time. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
On 2010 Nov 3, at 00:18, Tom Van Baak wrote: > Thanks for that link (and thanks to Steve for the old USNO > archive). The particular presentation of those old files was something I created as an experiment. I don't like the presentation and I may rearrange the URLs for easier scanning by humans. -- Steve Allen WGS-84 (GPS) UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855 University of California Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
On 11/02/2010 23:15, Tom Van Baak wrote: So how about having a Leap Second Every Month (LSEM)? The definition of UTC doesn't even need to be changed. The only question for the IERS each month is if the leap will be positive or negative. But there is always a leap; with one or many months of notice. It's only 12-bits of data a year. This can be disseminated in a modern way (internet). No secular drift. No religious objections. No redefinition of UTC. No change to WWVB. No change to GPS. No change to NTP. No more silly leap second newspapers articles. Way more opportunities for software to be written correctly the first time. Wider public knowledge and understanding of leap seconds. The impetus for creating well-architected internet leap second information services. Closer tracking of UT1. Greater ability to handle unusual earth accel/deceleration episodes. The reference ntp implementation from udel currently has a filter in its leap second code that restricts leap seconds to June and December. Many older GPS steered clocks have filters as well that restrict things to Dec/June and sometimes Mar/Sept. Some software that listens to WWVB broadcasts "knows" that a leap indicator means "there will be a leap at the end of December or June." Not the end of the month. This is a small semantic change. A few years ago, there were a few of these kinds of broadcast systems knocking about. Most people will argue with you that this is a change to UTC, despite monthly leap seconds being in the definition :) I kinda like the idea, but only if you could publish the leap seconds years in advance :) Warner ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
In message <7ebbba3729a346d5b0aeb73d6d73b...@pc52>, "Tom Van Baak" writes: >What would happen if instead of getting rid of leap seconds >we had *more* of them? It really depends on only one thing: How long time in advance do you announce which way the leapsecond goes for a given month ? If we get 10 years firm notice, we can stick a table in the operating system and the majority of the worlds programmers will not have to think about it. If we still get 6 only months notice, your proposal will cost so much money that it will probably never be seriously considered. The only intrinsic benefit of your proposal, is that it will become possible to actually test the leap-second handling code in a regular project life-cycle. Either way: There's no getting around that just dropping leap seconds is a no-cost option for computers. 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] A leap second proposal to consider -- LSEM
> So how about having a Leap Second Every Month (LSEM)? I like it. I'm not sure it's the best solution, but it's an interesting idea to consider. > Astronomers should love this since it keeps UTC even closer to UT1 than what > they have today. That doesn't quite work. If I'm very close this month, then next month I'll be off by almost a second. If being close is a goal, I think you need fractional leap seconds. > The Leap Second Every Month proposal creates certainty by going to 100%. That doesn't make them predictable. I can't build the table/algorithm into some firmware and know that it will work for the next 10 years without updates. Alternatively, it requires that all systems that keep accurate time have a connection to someplace that tells them the sign of this months leap second. -- These are my opinions, not necessarily my employer's. I hate spam. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
Hi Rob, Thanks for that link (and thanks to Steve for the old USNO archive). The key point of LSEM is that 100% of the months would have leap seconds, not 80% as in Ed's email. If that means DUT1 gets closer to 1.0 instead of 0.9 so be it. In his and in today's leap second implementation there are still two questions: 1) will there be a leap second this month (or semi-year or quarter-year), and 2) what sign will it be. This results in some months of some years being special, which is what I was trying to avoid. 0% or 100% avoids special cases. A proposal to eliminate leap seconds will create certainty by going to 0%. The Leap Second Every Month proposal creates certainty by going to 100%. LSEM eliminates question 1. Everyone will know there is a leap second at the end of every month. IERS just picks the sign for us. And yes, like Ed observed, mostly it will be a string of alternating pluses and minuses. A desirable side-effect of LSEM is -- one now has a way to determine if a clock has been "armed" for the next leap second. Today, for example, in many systems there is no visible difference between not knowing if a leap second has been scheduled from knowing that no leap second has been scheduled. (re-read that sentence if necessary). This situation seems a little dangerous to me. With LSEM, if a clock has no leap second scheduled it means it still needs its monthly sign bit from the IERS. It has until the end of the month to get it from a reliable source. So you end up with three kinds of clocks: 1) Common clocks where 1 second accuracy is sufficient and leap seconds are never used. These count 86400 seconds a day, every day. The assumption is that they are less accurate than 1 second a day or even 1 second a month (approx 0.4 ppm) and are in the habit of being rate disciplined or time sync'd by a higher authority. 2a) Precision clocks, where the sign of the leap second is still not known. That is -- Warning: you have until the end of the month to fix this. 2b) Precision clocks which have been network-automatically or manually set with the sign of the next leap second and are good to go until next month. The last day of every month will always have 86399 or 86401 seconds and never 86400. I probably don't have to point out that this "dithering" method neatly reflects rather than rejects the fact that the earth is a relatively unstable clock. The newcomers to the list may enjoy: http://www.leapsecond.com/museum/earth/ /tvb - Original Message - From: "Rob Seaman" To: "Leap Second Discussion List" Sent: Tuesday, November 02, 2010 11:06 PM Subject: Re: [LEAPSECS] A leap second proposal to consider -- LSEM On Nov 2, 2010, at 10:15 PM, Tom Van Baak wrote: What would happen if instead of getting rid of leap seconds we had *more* of them? So many more that all software just had to implement them. And so often that products would have a plenty of chances to be "leap second qualified" before release; tested with both positive and negative leap seconds. By all means - something to consider... ...which we've done before, e.g., in August 2003: http://www.ucolick.org/~sla/navyls/0197.html Variations of positive/negative scheduling came up at other points, but I haven't figured out how to search the zombie Navy archives. The fundamental idea, well stated in Tom's message, is to embrace the requirement rather than run from it. This is good advice when architecting anything. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] A leap second proposal to consider -- LSEM
On Nov 2, 2010, at 10:15 PM, Tom Van Baak wrote: > What would happen if instead of getting rid of leap seconds > we had *more* of them? So many more that all software just > had to implement them. And so often that products would > have a plenty of chances to be "leap second qualified" before > release; tested with both positive and negative leap seconds. By all means - something to consider... ...which we've done before, e.g., in August 2003: http://www.ucolick.org/~sla/navyls/0197.html Variations of positive/negative scheduling came up at other points, but I haven't figured out how to search the zombie Navy archives. The fundamental idea, well stated in Tom's message, is to embrace the requirement rather than run from it. This is good advice when architecting anything. Rob ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
