Re: Fixing POSIX time

From: Steve Allen <sla_at_UCOLICK.ORG>
Date: Thu, 19 Jan 2006 11:27:48 -0800

On Thu 2006-01-19T09:54:51 -0700, Neal McBurnett hath writ:

> For TAI I'd suggest 1958-01-01, when TAI and UT were set nearly together.
> (I've seen more specific references that TAI was set according to both
> UT2 and UT1 - but they weren't the same then. Perhaps within known
> error at the time?)

As near as I can find in the literature, the intention was that all
atomic time scales (I think that was three of them) would be set equal
to the astronomical value of UT2 as of 1958-01-01. If I am not
mistaken, the formula for UT2-UT1 at that date was provisional; the
expression currently in use is distinctly different.

Worse than that, in 1958 the values of UT1 used by the various
different national observatories and broadcast time services did not
agree. They had never agreed because each observatory used its own
value for its "conventional longitude" (look for the animated
discussions of that term in the annals of the IAU general
conventions), many of which had been established early in the era of
telegraphy, and which were inconsistent with a truly global longitude
system by amounts corresponding to many milliseconds of time.

It is therefore somewhat moot to argue whether the epoch of TAI was
synchronized with UT1 or UT2, for those quantities themselves were not
well synchronized at the same level.

I also point out that the IAU has standardized on 1977-01-01 as the
epoch of synchronization for its time scales because that is the date
on which the largest intentional change was made in the rate of TAI.

If epochs are to be specified, it is incumbent on the document to
specify the full four-vector (location and time) at which
synchronization is intended, whether it is intended to be a
specification of angle, coordinate time, or proper time, whether there
is an implied conventional coordinate transformation such as the one
between TCG and TT, what other conventional coordinate systems are
presumed, and to ruminate on the meaning and likely precision of the
time scale if someone attempts to extrapolate it either into the
future or past.

(By which I do point the finger at, for example, Microsoft for
producing documents which imply that UTC can be used in the year 1601.
The term UTC can have no meaning before 1960, and even that predates
the use of the term itself by many years. Also, don't forget the
other UTC which the Soviets and Chinese were using during the 1960s.
And this is not to say that the astronomers have always got it right.
See the draft resolutions about discarding the original meaning of TDB
which are probably going to be acted upon by the IAU in Prague this

In the absence of such rigor our descendants will have to make yet
more value judgements about what they think we were thinking as they
attempt to disambiguate the mess we make by our lack of full

Steve Allen                 <>                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     Hgt +250 m
Received on Thu Jan 19 2006 - 11:28:23 PST

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