[ntpwg] Bug: Status/Summary of slashdot leap-second crash on new years 2008-2009

[Rob Seaman]

Follow-ups to leapsecs at leapsecond.com.

Marshall Eubanks wrote:

> I disagree. In the 1970's, the excess LOD was as much as 3 msec.   
> After going down some, the mid 1990's it rose to around 2 msec.   
> Now, it is around 1 msec.
>
> Here is a plot
>
> http://www.iers.org/MainDisp.csl?pid=95-100
>
> Only the long period variations count for leap seconds - the  
> seasonal and other high frequency oscillations tend to average out.

Those plots are almost a decade out of date.  The more recent trend is  
here:

	http://tycho.usno.navy.mil/leapsec.html

Obviously we already knew it had leveled off (even risen a little) in  
the last few years since we've had two leap seconds in three years.

> In the early part of the last century (~1905), it decreased by ~ 5  
> msec in a year or so.  If that happened right now, it would go to ~  
> -4 msec negative, and we would be seeing 2 negative leap seconds or  
> more per year.

What am I missing here?  Wasn't the episode near 1905 first a big  
deceleration before it returned to "normal"?  A slower Earth leads to  
an additional accumulation of positive leap seconds.  Here's a plot  
from Steve Allen's page that shows the large positive area under the  
curve for the 1905 event:

	http://www.ucolick.org/~sla/leapsecs/lod.png

It looks like the fall took more like 2 decades than "a year or so",  
and that the initial rise was significantly steeper.

Perhaps the USNO or IERS has some other view of this event?  References?

> Even if the decrease from 1975 to 1985 happened again, it would be  
> at -1 msec, and we would have a negative leap second every two years  
> or so.

It depends what the baseline is doing before such a decrease.  Which  
is just to say, I guess, that length of day is not well modeled by a  
Markov process.

> What is a reasonable assumption is that we would likely have a year  
> or  more warning of the likelihood of a negative leap second.

Yes, but why is this left as an assumption?  As the result of  
integrating the LOD curve, leap seconds are stabilized relative to the  
variations in the curve itself.  Figure 5 from the Arias, et.al. paper  
from Torino:

	http://www.ucolick.org/~sla/leapsecs/torino/arias_3.pdf

suggests that predictions as long as three years should be possible  
even using what seems from the paper to be a fairly simple minded  
approach.  Since the Bulletin A methodology has presumably been  
optimized for six month predictions, it seems likely this could be  
extended by a year or two even without relaxing the DUT1 < 0.9s limit.

Figure 4 from Arias shows residual baseline trends extending over  
several six month windows.  A purely phenomenological detrending might  
behave better in practice than some attempt at unbiased fitting of  
physical models.

Leap seconds are an engineering issue, not science - a thumb on the  
scale isn't out of the question.

Rob Seaman
National Optical Astronomy Observatory


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