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

Steve Allen sla at ucolick.org
Sun Jul 31 04:50:13 UTC 2022

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
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 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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