Re: [LEAPSECS] Wall Street Journal Article

From: Steve Allen <sla_at_UCOLICK.ORG>
Date: Sun, 31 Jul 2005 00:06:32 -0700

On Sun 2005-07-31T08:23:30 +0200, Poul-Henning Kamp hath writ:
> No, they had everything to do with computers don't liking time to
> jump around.

But the reality is that no computer system (or any system, for even
NIST and USNO don't know what the value of TAI and UTC is until next
month) can guarantee that it always knows what time it is. If the
system does not know what time it is (at boot time, or if the system
does not have radio or network connectivity) then the system clock can
be wrong. If the system clock can be wrong then the system either has
to admit that it does not care that it is wrong or the system has to
have procedures for correcting that wrongness. The system can correct
the wrongness either by changing the length of seconds or by resetting
(leaping) the system clock.

> When you start out on a long march, you don't put a stone in your
> boot deliberately, and if one is there already, you take it out.
> Leapseconds is such a stone for real-world IT installations.

In this sense leap seconds give the system designer the opportunity
and incentive to face reality instead of ignoring it. Taking away
leap seconds will not fix this.

> I'm pointing out that UTC with leap seconds is unsafe at any speed.

Presuming that the system clock is always right is delusional.

> It would have been much smarter to use TAI, wouldn't it ? I thought
> I heard some astronomer say in this discussion that all applications
> which need proper timekeeping should use TAI ?

If "proper timekeeping" means time as defined by physics then:
All applications which need proper timekeeping in the reference frame
of the solar system should use TCB.
All applications which need proper timekeeping in the terrestrial
environment should use TCG.
All applications which need proper timekeeping on the surface of the
earth should use TT.
These are the recommendations from astronomers to everyone.

But these are not practical time scales. They are Platonic ideals.
Some sort of conversion needs to be applied in order to compare them
with practical time scales. The algorithms for that conversion are
not trivial -- they involve complex numeric calculations.
This is a fact of life that cannot be defined away.

> And if anything, if astronomers switched to TAI on 2008-01-01 they
> would not run into this problem in the future.

TAI is a practical time scale, but its seconds are not of constant
length. Even if TAI were the result of perfect atomic clocks, TAI
currently ignores the diurnal GR effects of changing depth in the
gravitational potentials of solar system objects. Someday TAI will
have to incorporate those currently-too-subtle effects of the passage
of proper time for every given clock. TAI is unarguably the best time
scale for use in telecommunications, but that does not make it
perfect. And even if TAI were perfect that does not make it the best
time scale for civil time.

> I think the sneakage happened in 1972 and we're trying to evict it.

Leap seconds were proposed and instituted by the physicists who ran
the atomic clocks. They were opposed by many astronomers, but after
the shouting stopped (and in the proceedings of the IAU GAs it is
evident that there was shouting) the astronomers agreed that UTC as SI
seconds with leap seconds was the best option.

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 Sun Jul 31 2005 - 00:06:45 PDT

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