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    Re: Errors in USNO celestial data
    From: George Huxtable
    Date: 2010 May 3, 10:02 +0100

    I invite Doaglas Denny to obtain the GHA of the Sun at noon (UT) today from
    ICE predictions, by the procedure he proposes, and compare his result with
    the corresponding figure taken from a current almanac.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ----- Original Message -----
    From: "Douglas Denny" 
    To: 
    Sent: Monday, May 03, 2010 12:41 AM
    Subject: [NavList] Re: Errors in USNO celestial data
    
    
    That's an interesting point; but I think not.
    
    The almanac core algorithms will be based on Dynamical Time as it is now
    called (since 1984), when Ephemeris Time which used to be defined with
    planetary dynamics, was replaced with Dynamical Time - based on atomic
    clocks.
    
    Universal Time (which is linked to rotation of the Earth and hence
    terrestrial clocks), is linked through the simple formula:
    
    UT + delta T = Dynamical Time
    
    The Dynamical Time used in the algorithm one can assume is correct and
    invariant.
    
    If the internal delta T of the programme is incorrect then the final
    solution from the algorithm is incorrect. The programme uses an internal
    delta T which is slightly 'out' after when the programme was produced in
    1988, so by by adjusting the input Universal Time accordingly, one has:
    
    (UT (+ or- correction increment for delta T)) + delta T (internal) =
    Dynamical Time
    
    ...which is now correct. Or at least it seems so to me.
    
    We have the variable UT which we can change accordingly. The internal delta
    T we cannot get at to change.
    
    Douglas Denny.
    Chichester. England.
    
    =================================
    Original Post:-
    
    Douglas Denny ended his last posting with these words-
    
    "The only difficulty in using ICE after 1988 is that Delta T becomes
    increasingly incorrect. In 2000 for example ICE uses 63.3s whereas actual
    Delta T was 63.8s. By 2010 ICE uses 71.8 but it is currently 66.5 (I
    believe).
    I have tried to find if ICE uses an algorithm for Delta T or a look-up
    table associated with the yearly files. If the latter it could be amended.
    I examined the raw data files for correlation with the listed Delta T
    values to be able to change them but have had no success. The other
    alternative if you want Delta T to be exact would be to enter a date/time
    with the slight correction between that listed by ICE and the actual value.
    It makes very little difference in practical use."
    
    =====================
    
    Comment from George-
    
    I think that would make things much worse.
    
    Our clocks have already been adjusted, by leap seconds, so compensate for
    the slowing of the Earth's rotation, so any sky position which depends on
    the rotation of the Earth is correct (within half a second). The only
    matters that are affected by changes in delta-t are those which are
    independent of the Earth's rotation, and march to a different drum, that of
    true, invariant, time (what used to be called ephemeris time).
    Particularly, this applies to the Moon's changing position in its orbit
    round the Earth, and the planets' positions in their orbit round the Sun.
    
    George.
    
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