On Thu 2006-01-19T19:00:27 +0000, Markus Kuhn hath writ:
> Please remember that the TAI second differed noticeably from the SI
> second until about 1998, because black-body radiation shift was not
> taken into account in the definition of TAI before then. Also caesium
> fountains have improved quite a lot shortly before 2000.
True, but not really relevant.
What is important for society at large is to have a time scale upon
which we can all agree, and to have it available in as near to real
time as possible. The fact that there have been deficiencies in the
rate of TAI is only relevant to radio observatories observing pulsars
and comparing those pulse timings with their own atomic clocks.
In that sense there will always be a need for retrospective revision
of the true meaning of the time scale which was conventionally available.
If you want to have a time scale which extends far into the past, you
only have a choice of UT and TT. UT is that which people knew from
looking at the shadow of sticks in the ground. TT is that which
historians and astronomers can deduce from records of eclipses.
Starting with the IMC in 1884, UT was the time scale upon which
everyone could agree (although it did not achieve agreement on that
name until half a century later). Starting with the almanacs in 1901,
Newcomb's expression for UT was the one on which everyone could agree.
Within a few years of that, radio broadcasts became the best way to
disseminate the agreeable time. As such, it became admissable to
have only one entity which could be called the agreeable time.
As the BIH intercompared radio broadcasts it became evident that
different sources of UT disagreed because of factors both constant and
variable. Thus in 1955 came the agreement to name UT0, UT1, and UT2
and to use the most uniform one as the basis of civil time. But
agreement only existed ex post facto (by at least months) in the
publications of the BIH.
At the same time came the cesium resonator which quickly proved that
it was a better way to achieve agreement on what time it was. Adding
in the transistor and telecom revolutions made intercomparisons much
easier, and so for technical purposes the BIH's TAI became a time
upon which everyone could agree. Handing the control of UTC over to
BIH let them resolve the conventional longitude problem and finally
coordinate agreement at the sub-millisecond level. But the manner in
which TAI is agreeable is as much political as technical, for it is
maintained by a club of those who pay their dues to the BIPM and
follow the BIPM rules by buying the right equipment.
The question that the ITU-R is asking is what is the character of the
one agreeable time which will be broadcast. That is undeniably an
important concept for navigation and commerce, but it is based on a
specific notion of coordinate time. As the ability increases to
notice that my proper time progresses at a different rate than your
proper time, it becomes possible to question whether it is relevant to
admit that different applications may want to agree on different kinds
of time.
No matter whether leap seconds persist or cease, the nature of the
agreeable time scale which is most conventionally available is going
to change. The best that a computer representation can do is to be
flexible enough to admit as much in the footnotes of its
documentation. Aside from that it seems important not to over-specify
the interpretation of the underlying time scale.
To that end, examine GPS itself. The only thing its time scale
acknowledges is week number and second-of-week. The signal itself
does not presume anything about the conventional calendar scheme
to which that time will be converted, not even to the point of
pre-supposing the Gregorian calendar. Yes, there is a field
for counting leap seconds, so the signal does acknowledge that
the earth rotates and that people are interested in knowing how
far it has rotated. Until this point in history, that has been
fundamental in the notion of what we mean by the agreeable time.
--
Steve Allen <sla_at_ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99858
University of California Voice: +1 831 459 3046 Lng -122.06014
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
Received on Thu Jan 19 2006 - 12:49:15 PST