From: Rob Seaman <>
Date: Sun, 8 Jan 2006 11:20:30 -0700

On Jan 8, 2006, at 9:09 AM, Poul-Henning Kamp wrote:

> Doing so would once and for all have to divorce earth orientation
> from that unified time scale, leaving it to governments to align
> civil time with daylight as they see fit (just like today).

Without further debating the meaning of "civil time", consider the
implications of this two stage system. The first stage conveys TAI
or something related to it by a constant offset. The second stage at
any location (correct me if I misunderstand you) would be a secondary
clock disseminated at the direction of the local authorities.
Governments and technical users would subscribe to the first stage
clock. Businesses and civilians would subscribe to the second stage
clock(s). Correct so far?

For the sake of argument, let's discount the risks associated with
confusing one stage's clock with the other. One imagines, however,
that there won't be fewer safety critical, time dependent systems in
the future. We might, in fact, suspect that every party to this
conversation would both admit this and use it to argue for their own
position :-)

Those risks, however, represent only one issue falling under the
umbrella of interoperability. It is one thing to say that any random
local government can choose their own clock statutes. This is
certainly true, but in practice the future international community
will work together to reach joint decisions on evolving common clock
practices (as you say, "just like today").

I won't belabor the many worldwide systems that must interoperate for
the benefit of all. But these systems must interoperate not only
between themselves, but with natural phenomena. Forgive me (or
don't), but I am skeptical that phenomena of interest in the future
will not continue to include the rising and setting of the sun. (And
isn't claiming otherwise equivalent to saying that stage two is

The question is: how precisely does this differ from the situation
now or in the past? Whether by fiat or not, some common worldwide
"stage two" clock must exist. And some mechanism must exist for
synchronizing (to some level of tolerance that we can continue to
debate) that clock to diurnal cycles. It is this synchronization
that is ultimately of interest to us, not leap seconds, per se.

I have heard no response to my discussion of techniques for achieving
synchronization - of the difference between naive "fall back" hours
and 25 hour days. But how in practice is it envisaged that a scheme
for migrating time zones versus TAI would work, precisely? Note, for
instance, that nothing short of redefining the second can avoid the
quadratic acceleration between the stage one and stage two clocks.
Time zones (and the prime meridian?) would race more-and-more rapidly
around the globe.

Perhaps I've misunderstood, but this line of reasoning doesn't appear
to resolve anything.

Rob Seaman
National Optical Astronomy Observatory
Received on Sun Jan 08 2006 - 10:20:44 PST

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