Last year at the Astronomical Data Analysis Software Systems (ADASS)
conference a Birds of a Feather (BoF) session resulted in an
extemporaneous group formed to focus on the subjects of leap seconds
and UTC. This year the group continued to raise awareness at ADASS
with a poster presentation.
The poster is at
http://www.ucolick.org/~sla/adass/UTCposter.pdf
It is 36 inches by 46 inches, and with the exquisite background
artwork by Pete Marenfeld it consumes 2496378 bytes.
For brevity's sake of bandwidth the 6609 bytes of text are attached.
In an attempt to harmonize with all co-authors I adopted a viewpoint
which I'll summarize as:
Yes, the sky is falling, but it has been falling for 170 years.
You should be used to it by now. Consider it as job security.
The first reaction to the poster was Get a life.
(Got one, thanks, next?)
Subsequent reactions went so far as to demonstrate that the IAU (and
other international agencies relevant to timekeeping) have produced
resolutions and decisions which are so obscure that lead scientists
for major flight project software are unaware enough of their
implications to provide self-inconsistent directives and Interface
Control Documents to their software teams.
--
Steve Allen [EMAIL PROTECTED]WGS-84 (GPS)
UCO/Lick ObservatoryNatural Sciences II, Room 165Lat +36.99858
University of CaliforniaVoice: +1 831 459 3046 Lng -122.06014
Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m
Astronomical timekeeping strategies to
accommodate possible changes to UTC
Steve Allen (UCO/Lick Observatory)
Rob Seaman (National Optical Astronomy Observatory)
Peter Bunclark (University of Cambridge/Institute of Astronomy)
Mark Calabretta (ATNF/CSIRO)
Jonathan McDowell (Smithsonian Astrophysical Observatory)
Clive Page (University of Leicester)
Arnold Rots (CfA/Harvard)
Christian Veillet (CFHT)
UTC may be redefined without leap seconds. This would significantly
affect many astronomical software systems.
No time scale has been immune to change
MNRAS vol 2, # 1, pp 11-12 (1831)
The most prominent subject of public interest, (and one that has
engaged much of the attention of your Council during the past
year,) was the proposing an amended form of the _Nautical
Almanac_. The Lords Commissioners of the Admiralty, being
desirous that the National Ephemeris should be remodelled, so as
to meet the increasing wants and intelligence of the navy, and
also the demands of astronomers, referred the consideration of the
subject to the Council of the Astronomical Society. A Committee
was formed, with the co-operation of the most distinguished
navigators and astronomers of the empire, and an elaborate Report
drawn up, recommending various alterations and additions. This
has been approved of by the proper authorities; and the _Nautical
Almanac_ of 1834 will appear in strict conformity with the plan
advised.
The most important _alteration_ is the substitution of _mean_ for
apparent solar time in all the data of the Ephemeris. It was with
considerable reluctance, and after viewing the subject in every
light, that the Committee resolved upon this material departure
from the mode adopted by Maskelyne, and rendered familiar by the
practice of more than half a century; but, as chronometers are to
be found in perhaps every ship which relies upon astronomical
means for her guidance, and as mean time must necessarily be
obtained where chronometers are used, it was deemed safer to
dismiss apparent solar time altogether as unnecessary, and as
being a source of confusion. The advantages, as to simplicity, of
exchanging a varying for a constant measure of duration, and of
assimilating the computations of the navigator and of the
astronomer, are too obvious to be dwelt upon; and, in the opinion
of all the naval officers present, no serious or lasting
inconvenience from the change is likely to be felt.
The Admiralty of 1831 understood the different time scales. They
recognized a distinction between a rarely-if-ever-reset private ship's
chronometer, measuring time intervals with its log of offsets and
rates, and a public clock, continually reset by astronomy to record an
epoch. They deemed that most navigators already had the requisite
tools for a change in time scale.
Despite objection by the IAU, the Admiralty redefined GMT by 12 hours
in 1925.
The BIH and BIPM have made adjustments to TAI since its inception,
including major changes as of 1977 and 1995.
The IAU has redefined UT (UT1) as of 1956, 1984, 1997, and 2003; TT
(TDT) in 1991 and 2000; and TDB in 1991 and 2006.
Using the language of the 1831 Admiralty, the monthly issue of BIPM
Circular T demonstrates that the best chronometers in the world are
only clocks after they compare how different they are and reset
themselves, and TT(BIPMnn)
