Re: [tz] [PROPOSED 3/3] Vanguard form now uses subsecond precision -Brooks
On Sat 2022-07-30T13:24:01-0400 Brooks Harris hath writ:
On 2022-07-30 1:01 PM, Paul Eggert via tz wrote:
Given the other comments in this thread I'm inclined to revert the effect of this change, so that vanguard form continues to use integer multiples of a second. So I installed the attached further patches, the last of which does the reversion. I think that's best for civil time. But I had a thought. Steve Allen seemed to like the idea and I'm sympathetic to his needs as astronomer. Maybe subsecond precision could be added to "right" instead of vanguard?
No, tz does not need subsecond precision, not for anyone, not in any era. The French law of 9m 21s is an example. The legislators should have known about the funds which had been expended as part of the two century effort to establish the offset between Greenwich and Paris, and that the disagreements of many 0.1 s in their own lifetimes were from the techniques employed by the astronomers, not from the telegraphs or radios. The sub-second astronomical offsets all date from before the 1970 cutoff. 1970 was when the CCIR was persuaded to assert that one second is close enough for everyone. Historically the best time comparisons had poor precision. Before the 20th century offsets were many 0.1 s or even > 1 s. In the early 20th century offsets were still many 0.01 s or even > 1 s. Attempts to reach a precision of 0.0001 s were not widespread until the International Geophysical Year of 1957/1958. Offsets of many 0.001 s persisted after the adoption of the cesium-based coordinated time because the Soviets used their own coordination scheme and other countries did not convert from UT2 to coordinated cesium time until the late 1960s. tz has rightfully eschewed the notion of trying to track local offsets recorded in Bulletin Horaire and Circular T. The legal time of the US was offset by 0.035 seconds from the BIH at 1958-01-01. The USNO and US NBS did not attempt to bring their time scales into microsecond agreement until 1968. Over a long interval in this century the legal time of Hungary was 50 microseconds different from UTC in most other places because the time lab there declined to perform a reset of their cesium. Thinking of encoding these things is a world of pain that tz does not need. -- Steve Allen <sla@ucolick.org> WGS-84 (GPS) UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m
Historically, Amsterdam used a time zone 0h 19m 32.13s off GMT. This was from about 1909 to 1937. https://en.wikipedia.org/wiki/UTC%2B00:20 On Sat, Jul 30, 2022 at 12:41 Steve Allen via tz <tz@iana.org> wrote:
No, tz does not need subsecond precision, not for anyone, not in any era.
-- Jonathan Leffler <jonathan.leffler@gmail.com> #include <disclaimer.h> Guardian of DBD::Informix - v2018.1031 - http://dbi.perl.org "Blessed are we who can laugh at ourselves, for we shall never cease to be amused."
On Sat 2022-07-30T17:50:47-0600 Jonathan Leffler hath writ:
Historically, Amsterdam used a time zone 0h 19m 32.13s off GMT. This was from about 1909 to 1937.
This raises a different form of the "time on the ground" question that tz faces when there is a dispute about what actual time is being used to set clocks. The Netherlands did not have a radio transmitter providing precision time signals. Nearby countries Belgium, Germany, France, UK did have such transmitters, and they were providing GMT as needed for navigation. I expect it likely that the Netherlands did have wired telegraphy mechanisms for distributing time, but transferring that time from the telegraph office to the street was likely manual. In 1940 the technology of time keeping made a huge leap as the lessons learned from 20 years of BIH were codified and implemented by observatories, but before then things were not much different than the 1929 plot of offsets of the best observatories given in Bulletin Horaire volume 4 number 56 page 156 https://www.ucolick.org/~sla/leapsecs/BHv4n056p156.png where it is clear that the time of the best observatories walked more than 0.1 s during the year. The legal time of 19m 32.13s would not have been practically available, and it would have been imprecise by as much as that fractional second. So in the Netherlands a navigator who needed precise time would have used a wireless to receive signals as GMT, and then offset their pocket watch for shore leave, a person on the street could have gone to the telegraph office to see the clock there, but either way they likely set their pocket watch by hand. In the absence of contemporary sources describing how people on the street set their watches, should tz take the trouble to implement a fractional second which was not realistically accurate? -- Steve Allen <sla@ucolick.org> WGS-84 (GPS) UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m
On 7/30/22 21:50, Steve Allen via tz wrote:
should tz take the trouble to implement a fractional second which was not realistically accurate?
TZDB records idealized civil time, not actual civil time which would be a much harder task. So it doesn't much matter whether the idealized time is realistically accurate. This is true for both Amsterdam (where the idealized UT offset was not an integer number of seconds) and Paris (where it was).
participants (3)
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Jonathan Leffler -
Paul Eggert -
Steve Allen