On Fri 2019-10-25T05:12:54-0700 jimlux hath writ: > So why was 1958-01-01 chosen as an epoch time for TAI (or is it, really, > maybe TAI defines the time scale, and you use yyyy-mm-dd hh:mm:ss.sssss..... > and it's only some other standard that defines what "zero" means when you > want to represent it as a single "number" > > for example CCSDS 301.0-B-4 "Time Codes" says: > "The CCSDS-Recommended epoch is that of 1958 January 1 (TAI) and the > recommended time unit is the second, using TAI as reference time scale, for > use as a level 1 time code. This time code is not UTC-based and leap-second > corrections do not apply. " > > https://public.ccsds.org/Pubs/301x0b4e1.pdf
In the full context the exact wording of the 1971 CGPM approval to create TAI, and other recorded decisions, is important. https://www.bipm.org/en/CGPM/db/14/1/ and also resolution 2 gloss over recorded discussions that BIH was the place that could best implement TAI, but that BIH by itself did not have appropriate international standing to administer TAI, in part because despite its name 80% to 90% of BIH funding was slush money from the director of Observatoire de Paris, thus really France. Among various contributions to the meetings in the late 1960s are some that express serious concerns that the time scale which became TAI did not have a rigorous definition. I have found no recorded response to these suggestions, just silence. Everyone in the time services knew that BIH had re-set all their atomic time scales in 1964 using the epoch 1961. They knew the basis of UT2 was shifted as of 1962 https://www.ucolick.org/~sla/leapsecs/seasonal.html because of the new defintion of UT2 and the change from FK3 to FK4. They knew that the origin of A3, and thus TAI, was offset from the current notion of what the value of time should have been. My impression is that the lack of recorded responses is partly due to not having any way to fund BIH in an "appropriate" fashion, not having any way to create a replacement for BIH which had appropriate foundations, and not wanting to create yet another atomic time scale effectively the same as TAI but, by basis of a new, rigorous definition, offset from what everyone had been using in their communications and publications to that point. I think also that nobody wanted to clearly document the changes that happened at 1961 and 1962 because doing that would have emphasized that the value of atomic time scales is entirely arbitrary, not connected with any event that is observable, and not connected with calendar days of earth rotation which were and are the legal and commonly understood basis of civil time. So my impression was that they allowed the origin of TAI to be said to be about 1958 as part of trying to avoid talking about how exactly that came about. The switch from old-1960s-rubber-second UTC to new-1972-leap-second UTC was in part motivated by interpretations of a new law in Germany that old UTC was no longer legal to brodcast. So I think the lack of clear documentation was trying to avoid letting any more national bureaucrats who controlled the funding of other national time labs raise questions about whether this new no-longer-related-to-days time scale could be the basis of legal time in their country. Looking carefully at what the practitioners of time did shows that the USNO decided not to use new-UTC in its navigational broadcasts. In the USNO time service publications Winkler announced that the USNO navigational broadcasts would become an offset from TAI (and that is eventually what GPS, Galileo, and all the other satnav systems also chose). Therein lies the basis of the LORAN offset question that prompted this thread. Similarly the astronomers did not use UTC in tabulations of any almanac. They continued to use just plain UT for things on earth, and ET (now relativisticly TT, or TCB, or TDB) for things not on earth. The principal printed appearance of new-UTC was in decrees from bureaucrats who were protected from seeing that the pracitioners of time did not agree that new-UTC was the best thing to use for technical purposes. -- Steve Allen <[email protected]> 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
