Timesys note review

Thu Nov 29 19:30:51 CET 2018

On Thu 2018-11-29T10:01:19+0100 Markus Demleitner hath writ:
> I'd still like to keep the number of (initial) terms as low as
> possible, and I'd expect if we let people choose between UT and UT1,
> that'd not help overall, and I'd still like to escape a separate
> "GMT" time scale if possible.

In 1928 the IAU GA resolved: "It is advisable for astronomers not to
use in any sense the letters G.M.T."
https://www.iau.org/static/resolutions/IAU1928_French.pdf

GMT is ambiguous and leads to confusions like the one in the IANA
timezone database discussion this month.  The records indicated that
when Hong Kong switched from local mean time to standard zone time in
1904 the observatory in Hong Kong dropped the time ball at 17:00 GMT,
which seemed to make no sense as it would drop invisibly in the night
at 01:00 local time.  The confusion was dispelled when it was pointed
out that in 1904 an astronomer would reckon GMT from Greenwich noon,
not Greenwich midnight.  This is seen in the thread archived at
http://mm.icann.org/pipermail/tz/2018-November/027241.html

The last meridian observation performed at Greenwich was by Gilbert
Satterthwaite on 1954-03-30.  Since that date there has been no way to
give meaning to GMT other than via historical inputs to the global
least squares solution of stations particpating with the BIH/IERS,
and the Royal Observatory admitted as much in 1957
http://adsabs.harvard.edu/full/1957Obs....77...31A

I recommend following the advice of the IAU and not allowing the use
of GMT for astronomical purposes where unambiguous meaning is important.

As Rob Seaman reiterates, very few observations have timestamps that
merit the distinction between UT1 and UT.  UT1 implies that some
special care has been taken in post-observation data reduction.  There
are no tabulations with self-consistent re-reductions of the
difference between UT1 and the available time signals from before the
change from FK3 catalog to FK4 catalog on 1962-01-01.

If you want to draw simple lines between the past and the present:

before 1962-01-01 should be UT -- unless there are specific reasons to
believe the time stamps merit a more precise time scale

from 1962-01-01 to 1972-01-01 should be UTC with the caveat that the
name UTC was not used in print before 1965 and the timestamps use the
older form of UTC where the seconds were not SI seconds

after 1972-01-01 should be the current form of UTC using SI seconds
and leap seconds with the caveat that the name UTC was neither
official nor used by the radio broadcasts until 1974

Except the middle section is still not simple because sites which were
really trying to do precise timing in North America would have favored
the use of the long-wave WWVB which was broadcasting what they called
SAB (Stepped Atomic B) and in Europe would have favored the similar
German broadcast of stepped atomic time.  These were both generically
known as SAT in 1966 when the CCIR approved the experimental broadcast
of such in Recommendation 374-1.  They provided seconds of exactly the
atomic frequency (no offset) and lots of steps of time in order to
stay near UT2.

So unless an observation was timestamped an an observatory which was
one of those participating with the BIH the use of UT1 is almost
certainly incorrect for before 1962-01-01.  And the use of UT1 is
almost certainly incorrect for after 1962-01-01 because all available
radio broadcast time signals were trying to track UT2 but actually
supplying something else based on atomic time, either UTC or SAT.

For most observations other than those taken by optical observatories
which still performed meridian astronomy (which would have had UT1),
or radio observatories which were doing earth satellite and solar
system dynamic observations (which would have reduced from the
available radio broadcast signals to ET/TT), the liklihood is that
most contemporary astronomers were ignorant of all these details and
not attempting such precision in their timestamps.

Any historic observation which actually has a precision that merits a
distinction finer than just plain "UT" is probably painstakingly
re-reduced to TT as in

Jordi et al. 1994
http://adsabs.harvard.edu/abs/1994GeoJI.117..811J
(note specifically the caveats of UT1 and UT0 near end of section 2)

and

Stephenson, Morrison, Hohenkirk 2016
http://adsabs.harvard.edu/abs/2016RSPSA.47260404S

or re-reduced to TDB/TCB as in

JPL developmental ephemerides
https://ssd.jpl.nasa.gov/

and

IMCCE/Bureau des Longitudes efforts
https://www.imcce.fr/services/ephemerides/

I recommend not allowing the use of UT1 because most people who
would use that for their data would be incorrect to do so.

--
Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
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