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    Re: Almanac accuracy, WAS: Re: Hughes Tables.
    From: Douglas Denny
    Date: 2009 Jul 18, 07:40 -0700

    Agreed entirely.
    
    Delta T represents the unpredictable changes in the the Earth's rotation year 
    by year, which is not fully understood.  Unpredictable changes in the angular 
    momentum of the Earth do occur though which can now be measured since atomic 
    time was introduced. 
    
    It is a curious irony that the first Astronomer Royal,  Nevil Maskelyne set 
    himself as his first task in the new Greenwich Observatory to determine if 
    the Earth's rotation was constant in rate; which he determined it was.  Until 
    this was proven there was no further way forward for the astronomical 
    determination of the longitude which is what the Greenwich Observatory was 
    estabilished for, to assist in solving the problem.
    
    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.
    
    Delta -T is uncertain for the past, and is so for the future, so 
    approximations are not possible since it appears to have no regular 
    periodicity.
    This is why almanacs cannot be used anynore than a century or so either way of today with any accuracy.
    
    Delta -T is a problem too in the 'ICE' almanac I use.   Although it used 
    official Alamac Office algorithms and data sets,  it was produced in the 
    1980s (I believe) and uses a predicted delta-T thereafter.  
    The predicted delta-T for 2009 for example is 70.7 seconds whereas it is in 
    fact 65.8025 for 2009.  So even that programme is not fully 'accurate'.
    As Humpty Dumpty said in Alice in Wonderland,  when the word 'accuracy' is 
    used, "it means just what I choose it to mean -- neither more nor less". 
    
    Almanacs always need to be used with caution regarding their sources if real accuracy is needed.
    
    Douglas Denny.
    Chichester. England.
    
    
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