Re: UT1 confidence
On Wed 2007-01-17T21:47:50 -, Robert Jones hath writ: > Does anyone know where I can find what the predicted effects of global > warming on rotation due to weight redistribution are likely to be and the > potential rate of change over the next few decades or centuries, perhaps > till all the ice has gone. In the Torino proceedings we see people who work with the IERS who did not make the best possible short term projections of rotation. It's less than a decade since the models of oceanic circulation became good enough that Richard Gross could demonstrate that the annual wobble of UT2 was mostly due to ocean currents. There are still ruminations that melting the arctic would trigger a shutdown of the north Atlantic currents that convey heat north and thus precipitate an ice age. I'd love to see this prediction, too, but I would not expect to put too much confidence in it. -- Steve Allen <[EMAIL PROTECTED]>WGS-84 (GPS) UCO/Lick ObservatoryNatural Sciences II, Room 165Lat +36.99858 University of CaliforniaVoice: +1 831 459 3046 Lng -122.06014 Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m
UT1 confidence
Does anyone know where I can find what the predicted effects of global warming on rotation due to weight redistribution are likely to be and the potential rate of change over the next few decades or centuries, perhaps till all the ice has gone. Robert
Re: UT1 confidence
In message: <[EMAIL PROTECTED]> Zefram <[EMAIL PROTECTED]> writes: : Steve Allen wrote: : >http://www.ucolick.org/~sla/leapsecs/torino/arias_3.pdf : : This is the really interesting one. They present the accuracy of : simulated predictions of UT1, and that accuracy is much poorer than : the figures we've been discussing so far. But they make it clear that : the prediction is by a naive algorithm, not even applying the well-known : periodic terms that are used in UT2. This is obviously not the algorithm : used by IERS. The good news is that even an incredibly stupid (in terms of knowledge used) algorithm can predict almost 3 years out Warner
Re: UT1 confidence
Steve Allen wrote: >http://www.ucolick.org/~sla/leapsecs/torino/arias_3.pdf This is the really interesting one. They present the accuracy of simulated predictions of UT1, and that accuracy is much poorer than the figures we've been discussing so far. But they make it clear that the prediction is by a naive algorithm, not even applying the well-known periodic terms that are used in UT2. This is obviously not the algorithm used by IERS. -zefram
Re: UT1 confidence
On Thu, 18 Jan 2007, M. Warner Losh wrote: > > : http://www.ucolick.org/~sla/leapsecs/torino/guinot.pdf > > I like figure 8, that shows that 20ms steps lead to 50ms steps lead to > 100ms steps lead to 1s steps. :-) In the late 1960s some stations broadcast "stepped atomic time" which had 200ms steps (and non-rubber seconds). Tony. -- f.a.n.finch <[EMAIL PROTECTED]> http://dotat.at/ FISHER: SOUTHWEST 6 TO GALE 8 OCCASIONALLY SEVERE GALE 9, PERHAPS STORM 10 LATER. ROUGH OR VERY ROUGH. RAIN OR SHOWERS. MODERATE OR GOOD.
Re: UT1 confidence
On Thu 2007-01-18T00:40:56 -0700, M. Warner Losh hath writ: > "Thus UT1 is not, strictly speaking, a form of solar time" This was the point made by Aoki et al. in 1982 http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1982A%26A...105..359A when they replaced Newcomb's expressions (which had been based on observations of the sun) with a new interpretation of what Newcomb had actually done. Then in 2000 Capitaine et al. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000A%26A...355..398C further removed the sun from the equation. Despite being so openly acknowledged in the literature this deviation from "solar" time eluded even Jean Meeus until around 2005; look for a pointed example in his next book on computational astronomy. The current expression for UT1 will do well enough for civil time until leap seconds would have to be happening every month. One might wonder whether, in the light of modern measurements, Newcomb would opine that his expression was supposed to be solar time. The slop of a full second in UTC made the original meaning of UT1 moot, and the geophysicists took the advantage. -- Steve Allen <[EMAIL PROTECTED]>WGS-84 (GPS) UCO/Lick ObservatoryNatural Sciences II, Room 165Lat +36.99858 University of CaliforniaVoice: +1 831 459 3046 Lng -122.06014 Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m
Re: UT1 confidence
Steve Allen wrote: The plots by Arias indicate how well UT1 could have been predicted over two and three year intervals for the 40 year interval starting around 1960. It is based on those plots that I have voiced no concerns for the pointing of our telescopes if leap seconds were published five years in advance. I'm not ready to go for ten. It appears to this observer also that a consensus for extending leap second scheduling from a six month interval to a five year interval should be straightforward to achieve. This is a factor of ten improvement, could be later lengthened as the state of the art allows, is likely already supported under the standard, and might in practice require no coarsening of the 0.9s maximum tolerance - especially if the five year lookahead were combined with the freedom to schedule each intervening leap second at the end of any month. Whether this would be the consensus - or whether some other scheduling algorithm - the first step would be to take the divisive and dangerously naive leap hour proposal off the table. It is exhausting and distracting to have to continually fend off this rabid turkey. The wisdom of taking the time and making the effort to form a robust consensus BEFORE changing civil timekeeping policies cannot be overestimated. I would, however, like to better understand what Arias means by "empirical linear prediction". This appears not to rely on any physical model of the rotating Earth. Surely the plots referenced should be taken as worst cases? One would also want to combine these predictions with the details of specific scheduling algorithms to start to understand the trade-offs and what the practical limits would be on the range of DUT1. Rob
Re: UT1 confidence
Steve, thank you for this enlightening report. In message: <[EMAIL PROTECTED]> Steve Allen <[EMAIL PROTECTED]> writes: : On Wed 2007-01-17T12:31:14 -0700, Warner Losh hath writ: : > It has been remarked that the current state of the art is that 100ms : > accuracy can be predicted about a year in advance only and that the : > models are constantly undergoing refinement. It has been estimated : > that IERS could issue leap seconds, with today's technology, about 3-5 : > years out and still be in a 95% or 99% band of certainty that the 0.9s : > margin is maintained. However, I can't find papers that show these : > models or point to any better data than hearsay... : : The best that I know of were the ones presented at the Colloquium that : the WP7A SRG held in Torino in May 2003. There was a time when the : host institution (IEN) was providing the proceedings online, but the : contents of that URL went away sometime around a year ago. (I wonder : if they may not have liked the conclusion that was reached.) : : In the spirit of promulgation I provide what they once did at : http://www.ucolick.org/~sla/leapsecs/torino/ITU.shtml : : The conclusion was originally a powerpoint drafted in real time, it is : http://www.ucolick.org/~sla/leapsecs/torino/closure.pdf : : The indications of how well predictions of UT1 might be done are found : in three presentations to which Felicitas Arias contributed. : There are two which were powerpoint : http://www.ucolick.org/~sla/leapsecs/torino/guinot.pdf : http://www.ucolick.org/~sla/leapsecs/torino/arias_2.pdf : and one which is a more verbose writeup of one of the powerpoints : http://www.ucolick.org/~sla/leapsecs/torino/arias_3.pdf I like figure 8, that shows that 20ms steps lead to 50ms steps lead to 100ms steps lead to 1s steps. :-) I also like the quotes: "Dating in UTC is ambiguous when a positive leap second occurs in systems other than those using hours, minutes and seconds; in this latter system the use of second "60" may a cause of difficulty." and "Thus UT1 is not, strictly speaking, a form of solar time" Also, did I miss figure 9 in arias_3? Proposal II has gotten much press here (the leap hour one), but Proposal I sounds a lot like what I've suggested: Use TAI time and let countries move the time zones when they feel like they no longer are close enough, but it kinda omits that last part... Warner
Re: UT1 confidence
On Wed 2007-01-17T12:31:14 -0700, Warner Losh hath writ: > It has been remarked that the current state of the art is that 100ms > accuracy can be predicted about a year in advance only and that the > models are constantly undergoing refinement. It has been estimated > that IERS could issue leap seconds, with today's technology, about 3-5 > years out and still be in a 95% or 99% band of certainty that the 0.9s > margin is maintained. However, I can't find papers that show these > models or point to any better data than hearsay... The best that I know of were the ones presented at the Colloquium that the WP7A SRG held in Torino in May 2003. There was a time when the host institution (IEN) was providing the proceedings online, but the contents of that URL went away sometime around a year ago. (I wonder if they may not have liked the conclusion that was reached.) In the spirit of promulgation I provide what they once did at http://www.ucolick.org/~sla/leapsecs/torino/ITU.shtml The conclusion was originally a powerpoint drafted in real time, it is http://www.ucolick.org/~sla/leapsecs/torino/closure.pdf The indications of how well predictions of UT1 might be done are found in three presentations to which Felicitas Arias contributed. There are two which were powerpoint http://www.ucolick.org/~sla/leapsecs/torino/guinot.pdf http://www.ucolick.org/~sla/leapsecs/torino/arias_2.pdf and one which is a more verbose writeup of one of the powerpoints http://www.ucolick.org/~sla/leapsecs/torino/arias_3.pdf The plots by Arias indicate how well UT1 could have been predicted over two and three year intervals for the 40 year interval starting around 1960. It is based on those plots that I have voiced no concerns for the pointing of our telescopes if leap seconds were published five years in advance. I'm not ready to go for ten. -- Steve Allen <[EMAIL PROTECTED]>WGS-84 (GPS) UCO/Lick ObservatoryNatural Sciences II, Room 165Lat +36.99858 University of CaliforniaVoice: +1 831 459 3046 Lng -122.06014 Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m
Re: UT1 confidence
> In message <[EMAIL PROTECTED]>, Zefram writes: > >IERS Bulletin A gives an expression for the uncertainty of its UT1-UTC > >data predictions: > > > >S t = 0.00025 (MJD-today)**0.75 > > > >where "today" is the MJD of the bulletin's publication. The Bulletin > >only predicts a year ahead. Applying that formula gives an uncertainty > >a year ahead of 21 ms. > > The question is what domain of validity the above formula has ? > > In the builletin they only apply it up to 40d. In addition, since the drift is a higher order polynomial (13th order I recall hearing), a simple linear formula (or close approximation) is likely only going to fit the curve near the prediction date with wider variances the further one gets from today. It has been remarked that the current state of the art is that 100ms accuracy can be predicted about a year in advance only and that the models are constantly undergoing refinement. It has been estimated that IERS could issue leap seconds, with today's technology, about 3-5 years out and still be in a 95% or 99% band of certainty that the 0.9s margin is maintained. However, I can't find papers that show these models or point to any better data than hearsay... Warner
Re: UT1 confidence
In message <[EMAIL PROTECTED]>, Zefram writes: >IERS Bulletin A gives an expression for the uncertainty of its UT1-UTC >data predictions: > >S t = 0.00025 (MJD-today)**0.75 > >where "today" is the MJD of the bulletin's publication. The Bulletin >only predicts a year ahead. Applying that formula gives an uncertainty >a year ahead of 21 ms. The question is what domain of validity the above formula has ? In the builletin they only apply it up to 40d. For an argument of 10 years the result is 0.12 seconds. For an argument of 100 years the result is 0.66 seconds. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
UT1 confidence
IERS Bulletin A gives an expression for the uncertainty of its UT1-UTC data predictions: S t = 0.00025 (MJD-today)**0.75 where "today" is the MJD of the bulletin's publication. The Bulletin only predicts a year ahead. Applying that formula gives an uncertainty a year ahead of 21 ms. It certainly ought to be possible, based on such a prediction, to decide with certainty whether a leap second will be required within that year. With the six-month scheduling cadence and a one-year prediction, we'd expect the kind of scheduling that we actually see: shortly after each leap opportunity they can look ahead to the next opportunity but one, and decide whether there needs to be a leap at the next opportunity. It seems to me that a switch to a monthly scheduling cadence, as Rob Seaman advocates, would have the immediate benefit of allowing a ten or eleven month scheduling lead instead of the current five or six months, without any advance in predictive ability. Immediately after each leap opportunity they could look *twelve* leap opportunities ahead, and thus decide whether the eleventh would be a good time for a leap. This is in addition to the ability to keep UT1-UTC within tighter bounds, which Rob's proposal describes. But I digress. I'm wondering how good UT1 predictions further ahead are. If the formula remains valid, it suggests that UT1 could be predicted to within 100 ms as far as eight years ahead. 100 ms is certainly a good enough prediction to schedule leap seconds on. My assumption there is highly suspect, though. Anyone know better? Does IERS publish any EOP predictions more than a year ahead? -zefram
