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    Re: Almanac accuracy
    From: Paul Hirose
    Date: 2009 Jul 19, 15:46 -0700

    douglas.denny{at}btopenworld.com wrote:
    > For a full detailed explanation see the "Explanatory Supplement to the 
    Astronomical Ephemeris and the American Ephemeris and Nautical Almanac"  
    published jointly be the American and UK Nautical Almamac Offcies.
    
    That's the title of the 1961 book, now out of print. The latest version
    was published in 1992 with a different title:
    http://aa.usno.navy.mil/publications/docs/exp_supp.php
    
    A paperback version is available with a 21st century copyright (I forget
    the exact year), but its contents are identical to the 1992 book.
    
    It's a good idea to check the publisher's online errata sheets. I once
    wasted days trying to debug a complicated set of formulae in the
    Supplement. They never worked right, so I gave up. Years later I
    discovered the online errata. There was a typo!
    
    
    > They now have 'MICA' which is the same thing but with updated interfacing 
    for modern computer - and you _pay_ for it
    
    Although MICA and the almanacs must be purchased, the main resources
    needed to replicate them are free on the Web. These are the IAU SOFA
    source code, the USNO NOVAS source code, the JPL DE ephemerides (data
    files and the Fortran 77 source code to utilize them), and the Hipparcos
    star catalog.
    
    
    > Terrestrial Dynamical Time (TDT) represents the uniform precision time base 
    by definition (based on atomic time) for astronomical events and predictions, 
    whereas Universal Time is the 'apparent' (real) time as measured in practical 
    terms on Earth. The difference between the two is represented by delta-T and 
    is adjusted by leap seconds.  One leap second was added in Dec.2008 and 
    another leap second will be added in Dec 2009.
    
    "Terrestrial Dynamical Time" is the old name for something now called
    "Terrestrial Time". The difference TT-UT1 equals delta T. That value is
    not "adjusted by leap seconds", since leap seconds are applied to UTC,
    not TT or UT1.
    
    You're mistaken about a leap second at the end of 2009. The official
    bulletin from the IERS says there will be no leap second:
    http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.dat
    
    At the end of 2009, UT1 is predicted to be about .1 second ahead of UTC.
    If a leap second were inserted, that difference would increase to 1.1
    seconds, an excessive amount.
    
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