Re: [LEAPSECS] Longer leap second notice

From: Rob Seaman <seaman_at_NOAO.EDU>
Date: Wed, 4 Jan 2006 12:23:51 -0700

Hi Warner,

> A more apt comparison would be to the leap year rules that we
> have. We know the rules going
> forward a thousand years or so.

Apt indeed. Leap seconds are scheduled at least six months in
advance. That's about one part in 15 million. A thousand year
horizon for scheduling leap days is one part in 365,250. So we're
already doing 50 times better than Julius Caesar and Gregory XIII.

> These too represent the fact that the oribit arount the earth is
> not exactly 365.25 days. We add leap seconds because the rotation
> of the earth isn't exactly 86400s.

As the solar day lengthens, of course, it will influence calendrical
issues as well. As has been asserted in the past, changing UTC to
eliminate leap seconds also changes the definition of the "day".

> Daylight savings time is something else entirely. It is a
> political decision that sunlight is better used in the evening than
> the morning.

These effects are all connected in the generation of a civil date and
time at any given moment in any given location on (or "near") the
Earth's surface.

> Twenty years is an example number. Ideally, as predictive science
> gets better, we can do it for longer periods of time. One would
> hope to eventually have a schedule that's published 50 or 100 years
> in advance.

Look at Steve Allen's plots (
dutc.html). The scatter is implicit in the geophysics. The real
world is inconvenient for some purposes, but that's what makes
science fun.

> If we can extend the horizon from 6 months, then that would lead to
> better predictibilty of leap seconds, and also allow for better
> testing.
> Of course, a rule that eliminates them entirely would also fit
> these needs, but appears to have little support...

Little support - and again, to a certain level of precision (easily
better than a second per day), all parties must certainly agree that
civil time (as we know it) IS mean solar time. I think we're all
willing to entertain off-the-wall suggestions - but that's what they
are - entertainment.

That said, the geophysicists have done a remarkable job improving
their predictions - look at a plot of the residuals between predicted
and observed UT1:
BminusA.gif. Would be delighted if that improvement were reflected
in better policies for UTC and civil time handling.

> Multiple sources of information about leap seconds leads to a more
> robust system. GPS can tell us about it, ntpd can tell us about
> it, and having a table of known leap seconds can inform us. These
> redundant sources of information act as a sanity check.

Can't argue with that - although ultimately a single well tied knot
is stronger than a tangle of slip knots and Grannies. And even a
well built system - a supremely constructed knot - is subject to the
Alexander factor. His sword violated the assumptions made by Midas'
father Gordius in tying the knot.

> In brief, we have a leap second table. This table can be populated
> from a number of different sources, usually via a table we've hard
> coded into our products. As the products run and discover new leap
> seconds, these are added to the table and the table is updated.
> Since we parse a number of different data formats to get leap
> second information (4 different GPS data types, peeking at the leap
> indicator on ntpd, and recovering it from time signal such as
> Loran), the system is expandable to allow any source of leap seconds.

Seems reasonable for a wide range of applications. As I've said in
the past, I'm not driven by a love of leap seconds, per se, but by
the identification of civil time with solar time. The issue of
triggering on a particular scheduled event is actually quite familiar
from other projects. One that I'm working on at the moment is a
mechanism for conveying sky transient alerts (
bin/view/IVOA/VoeventWorkshop2). Another was a way to trigger an
even sun centered cadence of CCD exposures attached to a
spectrograph. This consisted of taking >10,000 individual spectra of
a particular star (Procyon) on an even temporal grid extending over
several months. The gimmick was that the clock had to be adjusted
for the changing light travel time across the Earth's orbit. I won't
belabor the point (much), but it certainly is easier to build trust
in the correctness of such a trigger if the cadence is rapid, rather
than glacially slow.

Rob Seaman
National Optical Astronomy Observatory
Received on Wed Jan 04 2006 - 11:24:38 PST

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