Re: [LEAPSECS] Degrees of Accommodating Time Based on Earth Rotation
On Tue, 2 Nov 2010, Poul-Henning Kamp wrote: Tony Finch wrote: On Tue, 2 Nov 2010, Poul-Henning Kamp wrote: I may be misremembering, I thought the longitude conference was in 1884 ? Yes, but by that time there had already been 40 years of railway time in the UK. We officially switched to a single time zone in 1880, but local mean solar time had already been given up in most places for most practical purposes by about 1855. I'm not sure I see that any one country fixes their national time as relevant, as the fact that the international community, such as it were, argreed to do so. Not even England in the age of the empire... Well if those are your criteria then I think 2000 BC as the epoch for (global) leap day politics is too early. At that time calendars were all based on local observation. (Actually I think they were all lunar or lunisolar at that time, so if there was politics it would have been arguing about when leap months fell, not leap days.) The Julian reform was an important political event but since it only affected the Roman Empire it was too local to count. So perhaps the right date is the Gregorian reform of 1582. Tony. -- f.anthony.n.finch d...@dotat.at http://dotat.at/ HUMBER THAMES DOVER WIGHT PORTLAND: NORTH BACKING WEST OR NORTHWEST, 5 TO 7, DECREASING 4 OR 5, OCCASIONALLY 6 LATER IN HUMBER AND THAMES. MODERATE OR ROUGH. RAIN THEN FAIR. GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Degrees of Accommodating Time Based on Earth Rotation
Poul-Henning Kamp said: So here is an instructive little table: Finagle Subject to FactorUnit/Resolution politics since Leap days 86400 sec. 2000 bc. The first use of leap days I'm aware of was around 46 BCE. Before that the standard approach was leap months of between 20 and 30 days. We went from random local timescales to UT[C] with 24 standardized timezones. 24? -- Clive D.W. Feather | If you lie to the compiler, Email: cl...@davros.org | it will get its revenge. Web: http://www.davros.org | - Henry Spencer Mobile: +44 7973 377646 ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
[LEAPSECS] Degrees of Accommodating Time Based on Earth Rotation
A long flight was time to really read Ken Seidelmann's book and part of Woolard and Clemence, Spherical Astronomy. Subsequent exchanges with Ken give me a new understanding. (Probably an old understanding for the rest of you.) When we solve equations approximating physical processes, we are really defining our own dynamical time scale. Even with well characterized initial conditions or observations and a well founded system of units, the clock in our analysis does not tick at the same rate as those based on real phenomena. We must correlate time as perceived in our analyses with the temporal relationships among objects in the universe. For Earth rotation, time zones are the most coarse correlation. For time measured in SI seconds, not related to external phenomena, leap seconds are the next most precise. Finally, DUT, is precise enough for most astronomical and astrodynamic applications. It is a hierarchy each level of which is sufficient for a range of applications. Every time we solve dynamical equations, we are defining a unique time scale and time interval based on things such as analytical discretization and computational architecture. Comments? Dave Finkleman Senior Scientist Center for Space Standards and Innovation Analytical Graphics, Inc. 7150 Campus Drive Colorado Springs, CO 80920 Phone: 719-510-8282 or 719-321-4780 Fax: 719-573-9079 Discover CSSI data downloads, technical webinars, publications, and outreach events at www.CenterForSpace.com. ** ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Degrees of Accommodating Time Based on Earth Rotation
On Tue 2010-11-02T13:30:42 -0400, Finkleman, Dave hath writ: We must correlate time as perceived in our analyses with the temporal relationships among objects in the universe. For the sake of operational systems I would phrase this as the requirement that all precision intercomparisons of time, i.e, the difference between any two measures of time, need a lookup table in order to communicate the information. Thus in POSIX systems we have the zoneinfo file and the leap seconds file. For the most precise of all systems we have the NIST publications of the differences between UTC(NIST) and GPS time, the publications of the same by USNO and every other national metrology agency, and the BIPM publication Circular T. For historical purposes this requires archival publications by BIH of the predecessor of Circular T, the various papers by Stephenson and Morrison, and the observing logs of various meridian circle instruments. For civil purposes this includes the records of local and national legislation and decrees by various authorities. By which I say that nobody can tell me what time it is, and neither can I say it. All I can do is to say what my clock reads, to keep records of how that agrees with other clocks, and to design systems that tolerate and/or recognize the differences. Unfortunately many systems have been designed with the presumption that there is a simple, unique answer to the question What time is it? -- Steve Allen s...@ucolick.orgWGS-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] Degrees of Accommodating Time Based on Earth Rotation
In message 3b33e89c51d2de44be2f0c757c656c88099aa...@mail02.stk.com, Finklema n, Dave writes: It is a hierarchy each level of which is sufficient for a range of applications. Every time we solve dynamical equations, we are defining a unique time scale and time interval based on things such as analytical discretization and computational architecture. That is a good summation and an interesting way to look at it. I would put leap days on top of the heap of approximations, even though that is a different physical rotation, simply so they appear on the list and people don't ask stupid questions. So here is an instructive little table: Finagle Subject to Factor Unit/Resolution politics since Leap days 86400 sec. 2000 bc. Timezones 3600 sec. 1884 Leap seconds1 sec. 1958/1972 DUT1 (Bul. A) 1 microsec. Not yet Politization of these finagle factors have always been motions toward higher predictability: We went from priests announcing Easter based on astronomical observations, to putting them on a mathematical formula that could predict the dates for future, as far as we can see it. We went from random local timescales to UT[C] with 24 standardized timezones. We went from rubber seconds to identical seconds. In that light, it seems awfully logical that we would want to nail the length of the days down also... -- 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] Degrees of Accommodating Time Based on Earth Rotation
On Tue, 2 Nov 2010, Poul-Henning Kamp wrote: Finagle Subject to FactorUnit/Resolution politics since Leap days 86400 sec. 2000 bc. Timezones 3600 sec. 1884 A better date would be 1845 (first petition to parliament in favour of a national time zone). Leap seconds 1 sec. 1958/1972 DUT1 (Bul. A) 1 microsec. Not yet Politization of these finagle factors have always been motions toward higher predictability: We went from priests announcing Easter based on astronomical observations, to putting them on a mathematical formula that could predict the dates for future, as far as we can see it. There is still an active argument along these lines about how to determine when Ramadan starts. Leap days are also an instructive example, since it took about 300 years for Europe to deploy a revision to the previous standard which had been in place for about 1600 years... We went from random local timescales to UT[C] with 24 standardized timezones. We went from rubber seconds to identical seconds. In that light, it seems awfully logical that we would want to nail the length of the days down also... Tony. -- f.anthony.n.finch d...@dotat.at http://dotat.at/ HUMBER THAMES DOVER WIGHT PORTLAND: NORTH BACKING WEST OR NORTHWEST, 5 TO 7, DECREASING 4 OR 5, OCCASIONALLY 6 LATER IN HUMBER AND THAMES. MODERATE OR ROUGH. RAIN THEN FAIR. GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Degrees of Accommodating Time Based on Earth Rotation
In message alpine.lsu.2.00.1011021839340.6...@hermes-2.csi.cam.ac.uk, Tony Fi nch writes: On Tue, 2 Nov 2010, Poul-Henning Kamp wrote: Finagle Subject to Factor Unit/Resolution politics since Leap days86400 sec. 2000 bc. Timezones3600 sec. 1884 A better date would be 1845 (first petition to parliament in favour of a national time zone). I may be misremembering, I thought the longitude conference was in 1884 ? There is still an active argument along these lines about how to determine when Ramadan starts. And a very vocal subgroup, who argues that astronomical observations is no way to run a religion it in climates like Denmark... -- 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] Degrees of Accommodating Time Based on Earth Rotation
On Tue, 2 Nov 2010, Poul-Henning Kamp wrote: I may be misremembering, I thought the longitude conference was in 1884 ? Yes, but by that time there had already been 40 years of railway time in the UK. We officially switched to a single time zone in 1880, but local mean solar time had already been given up in most places for most practical purposes by about 1855. Tony. -- f.anthony.n.finch d...@dotat.at http://dotat.at/ HUMBER THAMES DOVER WIGHT PORTLAND: NORTH BACKING WEST OR NORTHWEST, 5 TO 7, DECREASING 4 OR 5, OCCASIONALLY 6 LATER IN HUMBER AND THAMES. MODERATE OR ROUGH. RAIN THEN FAIR. GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Degrees of Accommodating Time Based on Earth Rotation
In message 211ee304-6f59-40a1-837f-3f8359f68...@noao.edu, Rob Seaman writes: Poul-Henning Kamp wrote: Except that your suggestion is that we can ignore the whole thing because the wisdom of local governance will sort it all out with kaleidoscopically shifting timezone policies. Which was exactly my point... -- 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
