McCarthy point (was: Fixing POSIX time)

From: Markus Kuhn <Markus.Kuhn_at_CL.CAM.AC.UK>
Date: Thu, 19 Jan 2006 20:44:20 +0000

"M. Warner Losh" wrote on 2006-01-19 19:35 UTC:
> : Therefore, if people ask me for my favourite epoch for a new time scale,
> : then it is
> :
> : 2000-03-01 00:00:00 (preferably UTC, but I would not mind much
> : if it were TAI, or even "GPS time")
> :
> : This epoch has the following advantages:
> :
> : a) It is well after TAI rubber seconds were fixed in ~1998,
> : so we know the time of *that* epoch with much greater accuracy than
> : any before 1998.
> TAI and UTC have ticked at the same rate since 1972. While this rate
> has changed twice (by very small amounts, first by 1 part in 10^12 and
> then later by 2 parts in 10^14), they have been the same. Prior to
> 1972 we had both steps in time (on the order of 50ms to 100ms) as well
> as TAI and UTC having different notions of the second.

At which point we probably have reached another "McCarthy point" in the
discussion: Dennis D. McCarthy (USNO) observed at the ITU-R Torino meeting,
that "people who talk about timescale differences in the order of a few
nanoseconds and people who talk about differences in the order of a few
seconds usually do not understand each other".

All I wanted to say is that for a good choice of epoch, it would be nice
if we agreed on it not only to within a few seconds (the leap-second
problem), but also to within a few milli- or microseconds (the SI/TAI
second problem). The latter seems much easier to do for 2000 than for
1972 or even 1958. In applications such as observing planetary motion
over many years, the difference matters.


Markus Kuhn, Computer Laboratory, University of Cambridge || CB3 0FD, Great Britain
Received on Thu Jan 19 2006 - 12:44:35 PST

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