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    Re: Errors in USNO celestial data
    From: Douglas Denny
    Date: 2010 May 2, 10:29 -0700


    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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