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    Re: Errors in USNO celestial data
    From: George Huxtable
    Date: 2010 May 2, 21:40 +0100

    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.
    
    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: Sunday, May 02, 2010 6:29 PM
    Subject: [NavList] Re: Errors in USNO celestial data
    
    
    Gary,
    
    I checked the output of the programme I use as a reference, which is ICE
    Interactive Computer Ephemeris  or 1988 US Naval Observatory Nautical
    Floppy Almanac Version 2.00.88.
    
    It gives results of values of all bodies exactly as in your listing (i.e.
    to the nearest tenth minute, and for a delta T of 24.3 seconds in 1937),
    except for your anomalous Sun and Venus listings as in the data list you
    published on Navlist where your page of the Sun and Venus values do not
    have Hc and Zn.
    Nor does it give results for Mars:
    
    ICE Results are:-
    
    Mars: GHA 101-13.5  S20-02.1  HC minus18-32 Zn 108.7
    Sun: GHA and Dec as listed; but Hc 65-50.2  Zn 312.8
    Venus: GHA and Dec as listed but  Hc 23-58.5  Zn 284.1
    
    If you are not receiving these figures then it must be something wrong in
    the output from the website or an anomaly in your computer browser.
    ===============
    
    Do you want a copy of ICE? Then you will not have to use the Nautical
    Office computer each time.  It was produced by the US Nautical Office in
    the first place so should give you exactly what you want.
    
    It is very useful indeed for calibration of sextants using celestial
    objects as the references, as it is so fast to use. The local position of
    your house can be put into the startup file. Then just take a sight and
    time, punch in the date/time, and up comes HC and Zn instantly with the
    refraction, SD and HP details for your location there and then. Any
    differences observed corrected for refraction are the sextant error.
    I use it all the time for checking my sextants when having restored them.
    
    -----------
    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.
    
    
    Douglas Denny.
    Chichester. England.
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