Re: [LEAPSECS] Precise time over time

From: Rob Seaman <>
Date: Thu, 11 Aug 2005 12:37:52 -0700

On Aug 11, 2005, at 12:46 AM, Poul-Henning Kamp wrote:

> As I understood the situation last week, nobody in the gang here
> had problems with leap seconds if we got a longer warning (40-50
> years).
> So what prevents us from writing up our own proposal to ITU ?
> I'm pretty sure that we could get a rather impressive list of
> signatures from both camps if we did a bit of lobby work in our
> respective communities.

A reasonable idea. Of course, there is no reason to suppose that any
such proposal would even get a reading from the committee. They
appear to be fixated on leap hours for reasons of their own. There
would be some real value, however, in determining if there actually
is enough common ground to stand on.

No one, of course, would object to a 50 year look-ahead in the leap
second scheduling algorithm. I would prefer we cast the problem in
that fashion, not as a "warning". It may well be that alerts and
announcements of various sorts would be issued that are responsive to
the scheduling algorithm, but the underlying issue is the scheduling
of leap seconds or other clock synchronization activities. Note that
even under the ITU proposal various activities would continue for
assuring consistency between the various time scales.

The question is whether a deterministic 50 year solution exists.
Would love some feedback from the Earth rotation experts as to the
current and projected state of the art for making such predictions.
Presumably there is some trade-off between the precision of a
prediction and its latency into the future. It is likely that some
horizon exists beyond which no practical level of precision is
predictable. This horizon is certainly greater than six months. Is
it greater than 50 years? I doubt it. The LOD (Length of Day) plots
in suggest trends that
could be characterized over a few years - maybe a decade at the
outside. Note that leap seconds are a result of integrating under
the LOD curve - this smoothes out the annual signal. Other periodic
signals are undoubtedly also well characterized.

The question of this scheduling horizon is independent of the leap
second scheduling algorithm itself. I made a proposal similar to
what Kamp suggests back in April 2001:
leap. Obviously the document would have to be reworked into a form
suitable for an international standard from its current expression of
the opinion of a blowhard enthusiast, but the central notion is
simply to use the current standard fully. Permitting leap seconds to
be scheduled monthly would allow improving the tracking between UTC
and UT1. (It would also force non-conforming projects to fix their
systems to actually agree with the standard or perhaps to use a more
appropriate time scale. I consider this a feature of my proposal.
Sweeping leap seconds under the rug is not going to solve the
underlying problems.) There is no reason that my proposal could not
be combined with the 50 year predictive horizon to satisfy both of us.

As far as the maximum permitted size for DUT1 - some think 0.9
seconds is too small. There appears to be a consensus among quite
divergent thinkers here that 0.9 hours is much too big. I imagine
most astronomers would be willing to entertain intermediate values.
Personally I think such a discussion would be unwise without the
participation of folks within the ITU leap second firewall. Any
motion on the part of astronomers to unilaterally relax the 0.9s
limit would simply be taken for capitulation to the ITU proposal.
Without any transparency into the process, one might even think that
that was precisely the point of the leap hour proposal: scare folks
with an outrageous suggestion into writing a counter-proposal more to
the liking of the ITU.

It would be trivial, however, to convert the current UTC standard
based on leap seconds into a standard based on leap minutes. My
proposal suggests a scheduling algorithm that would keep DUT1 within
plus-or-minus 30s in that case. It is likely that the state of
geophysical understanding would not support a 50 year horizon for
leap-minute predictions, but it might support 10 or even 20 years.
However - are leap minutes likely to be more palatable to Kamp's
caricature of a moronic Posix programmer? Will neglectful and naive
project management really benefit from 20 year advance notice of a
leap minute? I still argue that the way to encourage compliance to a
standard is to make non-compliance a non-option. A project facing
the prospect of dealing with a potential leap second monthly is a
project that will actually expend the effort to get this right. A
project facing an event 50 years - or even 10 years - in advance will
simply ignore that event. And a culture facing a one-hour
discontinuity 500 years in the future is a culture that is facing a
massive Enron-style accounting disaster. "Creative" accounting will
always get you in trouble.

UTC is not broken. A fix is not required. The ITU proposal is
demonstrably worse than UTC over the next few centuries. That said,
UTC is not perfect and it would be delightfully refreshing to debate
ways to improve the civil time standard(s) and the infrastructure for
handling time signals, both civil and scientific. Why don't we (the
larger "we", including the folks with the power who haven't been
participating in any discussions) simply regroup, withdraw the
current silly proposal and define a process to patiently and
prudently reach a consensus.

It will take time to make time - better.

Rob Seaman
National Optical Astronomy Observatory
Received on Thu Aug 11 2005 - 12:38:16 PDT

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