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

Steve Allen sla at ucolick.org
Thu Jul 28 23:07:47 UTC 2022

On Thu 2022-07-28T12:02:15-0700 Paul Eggert hath writ:
> On 7/28/22 10:40, Steve Allen via tz wrote:
> > I urge that tz never attempt to encode zone offsets with precision
> > better than 1 millisecond.
> There's one instance of that in the patch, the "7:06:30.133333333" that
> comes from calculating 104 deg 17' 17" east of Paris Mean Time (00:09:21). The
> law in question gave a precision of 1 arc second for longitude, which works
> out to a precision of 66 2/3 ms for UT offset.

That comes from the tzdata asia file for Phu Lien Observatory.
That entry gives Paris Meridian as 2 deg 20' 14.03" E with no citation.

Every historical value of longitude needs an accompanying citation.

The 1911 law for France gave legal time as
Greenwich -00h 09m 21s
The 1911 law is the government of France saying we want Greenwich
without saying Greenwich, and that sub-second time is not relevant to
them -- the astronomers can figure it out at that level based on what
is obvious but not explicitly stated in the law.

The 1913/1914 radio longitude effort by USNO and Paris using 50 kW
spark gap transmitters found for Observatoire de Paris
Greenwich -00h 09m 20.932s
or 2 deg 20' 13.980" E

At the time of the first World Operation in Longitude 1926
the value used for Observatoire de Paris was
Greenwich -00h 09m 20.930s
or 2 deg 20' 13.950" E

After the 2nd World Operations in Longitude 1933 and before 1962 the
conventional longitude of Observatoire de Paris for computing time was
Greenwich -00h 09m 20.935s
or 2 deg 20' 13.815" E
That is the value of the offset which was used for France until after
radio broadcast time signals started being based on cesium atomic

Starting 1962 the conventional longitude for Observatoire de Paris was
Greenwich -00h 09m 20.921s
2 deg 20' 14.025" E
but by then that was moot for radio time signals because BIH was
coordinating the worldwide broadcasts of smeared seconds based
on cesium atomic chronometers.

The current WGS longitude of the Paris Meridian is about
2 deg 20' 11.5" E, but that is geodetic, not astronomical.

There is no way that the longitude of Indochina was known to the
accuracy of 0.01 s of time in 1906, before radio transmission and
reception were possible.

Please do not attempt sub-millisecond time offsets for astronomical
time.  When these conventional longitudes were in use the clocks in
the observatories could not easily stay stable to 0.01 s per day.
Corrections to the observatory clocks had to be computed and tabulated
based on meridian observations of stars and a conventional longitude.
Clocks did not tell what time it was.  Clocks were a way of
interpolating between the moments when it was possible to see a
catalog star and compute the next tabulated offset to the time the
clock was telling, but before radio (and even after radio because of
the fickle ionosphere) all determinations of time were intermingled
with a poorly-determined longitude that was not globally self-consistent.

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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