Re: [LEAPSECS] Consensus rather than compromise

From: Rob Seaman <seaman_at_noao.edu>
Date: Wed, 31 Aug 2005 15:19:20 -0700

Poul-Henning Kamp replying to Mark Calabretta:

>> If such a system were to be adopted, then in future, in order to
>> determine a historical time, the full record of timezone changes
>> would be needed.
>
> How is this different than today ? How is it different from having
> to keep a total history of leapseconds ?

1) It is different because today civil time is based on solar time
and the issue can be ignored completely for situations in which the
solar time on a given historical date was of principal importance.
Solar time is equivalent to (a simple function of) Earth orientation,
which is a reflection of a physical phenomenon. TAI is merely a
reflection of interval time. We may disagree on whether solar time
is of sufficient importance to drive public policy, but TAI in and of
itself certainly is not.

2) It is different because today timezones and related issues like
Daylight Saving are purely periodic phenomena. For most purposes -
including interval purposes - you only need to know the zone and DST
status at the beginning and end of whatever period of interest. If
the TZ system is used, on the other hand, to correct for a residual
drift in a mismatch between civil and solar time, Calabretta's
statement becomes a very strong factor.

3) It is different because today there is a single list of historical
leap seconds, and a TZ based system, as Cowan points out, would be at
the whims of local jurisdictions worldwide. One imagines a new
discipline similar to the tree ring counting of dendrochronology
whose goal would be to synchronize timestamped data from different
localities and epochs. These chronochronologists would be
responsible for building overlapping longitudinal historical clock
records from hundreds of distinct locations worldwide.

4) It is different precisely because for short periods the existence
of leap seconds can often be ignored for purposes that require
accuracy at the level of a minute or so. Presumably a TZ/DST based
system would operate in larger discontinuities, e.g., leap hours,
that might be very difficult to ignore indeed.

5) It is different because currently a longitudinal correction for
multiple leap seconds is required to recover TAI from UTC at some
historical epoch. This suggestion would require such a longitudinal
correction to recover actual UT from civil time. The current reality
is that users of TAI already have transport options such as GPS that
are at least a good as UTC. The TZ/DST system would provide no
backup mechanism for transporting universal time.

The importance of all these and similar issues is a matter of
debate. The debate should be backed up with data that can be used to
derive requirements and use cases for whatever future civil time
standard comes next.

Rob Seaman
National Optical Astronomy Observatory
Received on Wed Aug 31 2005 - 15:21:19 PDT