[tz] [PROPOSED 3/3] Vanguard form now uses subsecond precision -Brooks

Steve Allen sla at ucolick.org
Sat Jul 30 18:40:51 UTC 2022

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 at 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

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