The most recent leap second was introduced at the end of December 1998.
At the time leap seconds were introduced, the excess length of day (LOD)
was around 3 ms per day and it gradually dropped to around 2 ms/d by the
mid-1990s. See the plot at
<
http://hpiers.obspm.fr/eoppc/eop/eopc04/eopc04-lod.gif>. For an average
LOD of ~2 ms/d, it takes ~500 d (roughly every 1.5 years) to accumulate 1 s
and hence produce a need for a leap second. That was the situation until
the late 1990s. Since then, LOD has continued to drop to an average of
<1 ms (current average is ~0.5 ms). So lately the interval between leap
seconds has grown greatly. UT1-UTC is predicted to be -0.45 s one year from
now (see
ftp://maia.usno.navy.mil/ser7/finals.data) and therefore will still
not warrant a new leap second. There is at this moment no likelihood of
another leap second within the horizon that can be reasonably well
predicted (which is not really very long).
While the secular tidal breaking of the Earth's rotation almost certainly
dominates over geologic timescales, the dominant LOD forcing over human
timescales is the so-called "decadal LOD variations". You can clearly
see those effects in the eopc04-lod.gif cited above (at periods longer than
the seasonal LOD variations). It is really not fair to say that the
cause of the decadal variations is truly unknown, although one cannot
state that the mechanisms have been conclusively demonstrated. Direct
and indirect evidence point to the Earth's core as the driver of the
longer term LOD changes -- see the informative IERS website at
<
http://www.astro.oma.be/SBC/main.html>. Because of the very poor
observational constraints of the Earth's core, it has been difficult to
develop high-fidelity core models. But much progress has been made in
recent years and some results for correlations between LOD and core models
back to the early 1800s are shown at the website above.
Geophysically, it is rather clear that the core effects dominate over
secular braking for UT1 variations on timescales relevant for individual
people. Only if one concerns themselves with events in the very distant
future does the issue of secular braking become important. It is for this
reason that I find this entire discussion, and the proposals to change
current leap second practice, to be so utterly senseless. If somebody
(Bill K?) feels that a continuous timescale is required for some particular
application, then by all means use one of those already available (TAI,
GPS time, future Galileo time, ...). There is absolutely no justification
for making UTC into yet another such flavor while denying a timescale
closely linked to UT1. And attempting to use the predicted evolution of
leap seconds as justification for this senseless act is to ignore the
actual observational evidence -- an act approaching scientific fraud.
--Jim Ray
Ed Davies wrote:
>
> Meanwhile, this article also reminded me of a question that
> has been lurking in my mind for a while. We haven't had a leap
> second for a while even though:
>
> 1. The Earth has already slowed down enough, on average, to
> need a leap second every year or two.
>
> 2. The usual tidal effects, etc, have presumably still been
> busy slowing it down further.
>
> So, something has been speeding the Earth up for a while. As
> this article says - it's not really known what this effect is.
> My question: does anybody have a feeling as to whether this
> effect is likely to "unwind" at some point resulting in a
> faster than normal string of leap seconds? Could we get to
> leap seconds more often than once every six months any time
> soon?
>
> Ed.
>
Received on Thu Jun 26 2003 - 05:49:04 PDT
