[LEAPSECS] but what does Daniel Gambis say?

[Rob Seaman]

> Serbian Astronomical Journal, including mention of negative leap  

> seconds

>

> http://arXiv.org/pdf/0808.3612


Timekeeping is one of those narrow disciplines where peer review  
implies small number statistics.  So this paper joins others of  
suspect validity such as:

	Deines & Williams (2007, AJ, 134, 64) "Time Dialation and the Length  
of the Second: Why Timescales Diverge"

That said, it doesn't appear to be devoid of content - simply rather  
"lazy".  I'm particularly taken with statements like "all the physical  
factors unambiguously recognized till nowadays cannot cause such a  
phenomenon".  Most journal editors and referees would have a hard time  
accepting a paper with such a conclusion.  Rather, the authors'  
curiosity should have driven them to further investigation of one sort  
or another.  Either tie the results to the current physical basis of  
the field, or acquire data sufficient to call current physical  
understanding into question.

One is left wondering how this paper came to be written.  PHK's  
supposition about undergrads sounds about right.  Some of the choice  
of wording seems to imply that the authors are directly or indirectly  
aware of discussions from this mailing list.  The title suggests an  
interest in abetting the ITU-R's agenda.  The shallow analysis (and  
obscure journal) should make this all rather moot.  I am pleased,  
however, to see some attempt at predicting Earth orientation, but this  
surely cannot represent the state of the attainable art :-)

I can't add much to the previous comments that a fit with so many  
degrees of freedom is immediately suspect for purposes of  
extrapolation.  It is unremarkable that they were able to track every  
tiny wiggle in the data.  FIgure 4 is quite humorous.

Modeling delta-T, a cumulative effect, doesn't seem very expressive in  
general - all the empirical insight was smoothed out of the data  
before they started.  Rather, it would be better to focus on LOD or  
other more direct indicators of Earth orientation or rotation rate.   
Integrate the models to recover UT1, etc., as a separate step.  A fit  
without residuals is like signal with no noise - begging for skepticism.

One could seek physical explanations for the more (statistically)  
significant harmonic terms.  For example, the 22 year period is twice  
the solar cycle and there could be coupling with the magnetic field  
reversals.  A few of the terms in this (apparently) unconstrained fit  
pop out near annual, fractional annual, and multi-annual periods.   
Probably not a coincidence.  The solar system is rife with harmonic  
resonances of various sorts.  Surely the Earth rotation community must  
have considered a number of such phenomena previously given that lunar  
tidal locking is recognized as the principal driver for Earth's  
rotational slowing?

On the other hand, understanding the short period seasonal effects on  
LOD (all those beautiful wiggles from the plots on Steve Allen's web  
site, for instance) benefits simply from coverage over many cycles.   
It has taken a century or two to figure out the solar cycle due to its  
long period.  Long term LOD phenomena will take similarly long to sort  
out.  A comparative study with LOD data from the other planets and  
satellites could be revealing.  (One suspects the available data  
aren't sufficiently precise for the other planets.  Sounds like a good  
argument to send astronomers to Mars :-)

The 222 year period is most likely a meaningless artifact of the  
data.  The long term trend of tidal slowing is going to pop out as the  
most significant term.  The corresponding period is truncated by the  
truncated horizon of the available data.

Would prefer to see a similar fit with fewer terms and more  
constraints.  How about forcing terms with periods of a lunar month, a  
solar year, etc?  In addition to the free harmonic terms, how about an  
explicit linear trend forcing the long term tidal slowing?  How about  
forcing the millennial term from http://www.ucolick.org/~sla/leapsecs/ancient.pdf? 
   How about estimating the magnitude of the coupling between the  
terrestrial magnetic field and the solar magnetic field?  Could this  
possibly transfer enough angular momentum to affect the Earth's  
rotation?

This particular paper is only worth attention due to the dearth of  
similar papers.

Rob Seaman
NOAO
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