[tz] Fractional seconds in zic input

Steve Allen sla at ucolick.org
Tue Feb 6 03:04:12 UTC 2018

On Mon 2018-02-05T17:29:05-0800 Paul Eggert hath writ:
> Instead, for these non-integer UT offsets, we're actually recording the
> longitude of the meridians of the affected civil-time zones. (I suppose this
> should be written down in theory.html somewhere.) My impression is that such
> longitudes could be measured to a precision no worse than 0.1 s of UT offset
> (which corresponds to 1.5" of longitude), even with circa-1930 technology.

No, due to inconsistencies between the global geodetic datums which
were used as the basis of the longitudes assumed for calculations of
local standard time.  Prior to the 1980s there were offsets of
hundreds of meters between one regional geodetic datum and another.

I am going to argue that sub-second resolutions based on the
once-assumed longitudes of sites are down in the "technical details"
level that tzdb does not want to record except in comments.  Have a
look at

Publications of the USNO, second series, Vol. 4, part 4, page G16 (1906)

This has a table of the use of time in various nations constructed by
the navy and department of state.

For France (Paris) this gives a time offset of 9 minutes 20.9 seconds
from Greenwich.  In this case that offset is almost certainly correct
to that precision because
1) Greenwich had exquisite equipment and skilled astronomers
2) Paris had exquisite equipment and skilled astronomers
3) the distance between the two sites allowed for transport of a
chronometer back and forth within the span of about a day
4) the distance between the two sites allowed for telegraphic
transmission line delays to be well characterized
5) the distance between the two sites allowed for radio
transmission delays to be well characterized
6) the data from the BIH up through 1961 gave 9m 20.935s
7) the new globally adjusted BIH value from 1962 was 9m 20.921s
Pick some other place and many of these criteria will not be met.

For example, Zikawei China is given in 1906 USNO as 8h 5m 43.3s and by
BIH after 1962 as 8h 5m 42.864s, a notable systematic offset between
the once-believed longitude which had been used for local time and the
globally self-consistent longitude.

Alternatively, in the 1963 plots of time from Rio de Janiero the curve
wanders over a range of 0.6 s during the year.  Other observatories at
earlier dates did much worse at providing their local legal time.

A strong argument that the decimal seconds are technical matters,
rather than the legal or practical matters that tzdb cares to track,
is that after the 1884 International Meridian Conference there was
resistance to the notion of Greenwhich time in Paris.  Despite the
accurate decimals of the measurements known to the USNO by 1906, in
1911 the law was changed to make legal time in France 9m 21s behind
the mean solar time of Paris observatory.

If tzdb adopts sub-second resolution then it needs a very clear set of
rules about how those are to be used and interpreted, along with
caveats about how much not to believe that level of precision offset
from a globally self-consistent time.

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           http://www.ucolick.org/~sla/   Hgt +250 m

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