[LEAPSECS] 2 meetings on UTC and the impending ITU-R RA vote
michael.deckers at yahoo.com
Sun Jul 10 17:54:15 EDT 2011
On 2011-07-10 01:04, Steve Allen announced the meeting:
> In London UK on 2011-11-03/04
In that announcement, I find the following quote to be grossly
"After the short reign of ephemeris time as the world’s reference
time scale from 1954 until 1967, Coordinated Universal Time
(UTC), which is TAI synchronized to the rotation of the Earth by
means of leap seconds, appeared at the time as the best compromise
for satisfying the equests of all users and was adopted in 1972."
Ephemeris time (ET) _never_ has been the "world’s reference time
scale" -- since 1925, and until now, the "world’s reference time
scale" always has been some form of universal time (UT).
What actually has changed in both 1954 and in 1967 is the definition of
the second -- of course without changing its length as far as could be
ascertained at the time, and hence without any change to the "world's
reference time scale". In fact, from 1954 to 1967, ET has been some 31 s
to 37 s fast on UT, so that switching civil time to ET never was a
realistic option. There is quite a difference between changing a
time scale and changing the definition of a time unit, and I would
expect the people who decide on the future of the UTC time scale#
not to confuse these two.
Another thing that changed significantly from 1955 until 1959 is the
method of _realization_ of the "world's reference time scale" (the time
scale that is widely disseminated and that is used as the time argument
in the celestial and nautical almanacs, I assume).
Before the advent of atomic time keeping, clocks were less stable than
astronomical observations of Earth orientation, so that clock rates were
adjusted post factum to fit the astronomical observations at each site.
With atomic clocks, however, the clock rates could be estimated ante
factum, but phase offsets were an are still needed to correct for
estimation errors. Moreover, since 1961, estimated clock rates and
phase offsets have been coordinated across the world and across all
time observatories, so that disseminated time scales of different
observatories can be compared reliably.
Perhaps, this change of realization was meant -- but it happened long
before 1967 and cannot be called a "reign of ephemeris time" in any
This is all common knowledge, and can for example be looked up in the
excellent book by Dennis D McCarthy and P Kenneth Seidelmann: "TIME --
From Earth Rotation to Atomic Physics". I find it quite disconcerting
that some of the people who decide on the future of UTC apparently are
not even aware of these basic facts or, at least, are unable to
express them unambiguously in plain English.
After all, these people are the representatives of the public at large
concerning the choice of the civil time scale for the next generation.
Regardless of how they came into that position, they should at least be
knowledgable in the field in which they are about to take far-reaching
NB. Having studied several papers of Dr Felicitas Arias, I am sure that
my critique does not apply to her position and her writings.
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