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    Chile and deltaT
    From: Peter Hakel
    Date: 2010 Mar 2, 12:36 -0800
    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  Is there a tiny piece of angular momentum that gets deposited into and withdrawn from Earth's rotation then during the course of a year?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


       
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