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 chronometers. 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@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