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The change in LOD is 1.26 microseconds not milliseconds. See
.
-- Richard Langley


Quoting P H :

> http://www.msnbc.msn.com/id/35662192/ns/technology_and_science-space/?GT1=43001
> 
> According to this article the Chilean earthquake may shift the Earth's axis by 27
> milliarcseconds, certainly nothing relevant for CelNav.
> 
> The expected change in the length of the day warrants a bit more attention, I think. 
> If the day indeed shortens by 1.26 ms, as the article says, that would amount to a
> fractional change of:
> 
> -1.26 ms / 1 day = -1.5e-8
> 
> Now leap seconds have been added at the average rate of 1 per 1.5 years, which yields
> the fractional rate:
> 
> +1s / 1.5 years = +2.1e-8
> 
> These two rates are of the same order of magnitude then.  I am wondering whether this
> would (and should) translate into a practical effect on the deltaT and on the rate at
> which leap seconds are added.
> 
> And then, this paragraph caught my attention:
> "One Earth day is about 24 hours long. Over the course of a year, the length of a day
> normally changes gradually by one millisecond. It increases in the winter, when the
> Earth rotates more slowly, and decreases in the summer, Gross has said in the past."
> 
> I did not know about this annual variation until now.  As a physicist who does NOT
> work in the area of celestial mechanics, I would really like to understand where this
> variation comes from.
> 
> Rotation is about angular momentum, which is conserved in an isolated system.  The
> perigees and apogees of the lunar orbit do not correlate with seasons - as far as I
> know (please correct me, wherever necessary).  Leap seconds are added because the
> tidal friction gradually slows down Earth's rotation; the "lost" angular momentum is
> picked up by the Moon which as a result is receding from us at about 4cm per year. 
> So I don't think this is it.
> 
> In the (northern) winter the Earth is closer to the Sun and its orbital speed
> increases.  Is there a tiny piece of angular momentum that gets deposited into and
> withdrawn from Earth's rotation then during the course of a year?  On the other hand,
> Earth's rotation is prograde (or normal? don't have Meeus's book with me here to
> check terminology…), meaning that its orbital and rotational angular momenta are
> parallel (with the difference of 23.5 degrees) rather than antiparallel.  Yet Gross
> says that the day gets a bit LONGER in the winter…
> 
> Clearly I am missing something here.  I would really appreciate if NavList members
> would be kind enough to elaborate on the subject.
> 
> 
> Peter Hakel
> 
> 
>       


===============================================================================
 Richard B. Langley                            E-mail: lang@unb.ca
 Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
 Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
 University of New Brunswick                   Fax:      +1 506 453-4943
 Fredericton, N.B., Canada  E3B 5A3
     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
===============================================================================

   

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