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    Re: Bowditch long term almanac
    From: Geoffrey Kolbe
    Date: 2012 May 18, 20:05 +0100

    At 19:23 18/05/2012, you wrote:
    >I have spent some free time updating the Bowditch Long term almanac,
    >now with base year 2008.
    How did you go about doing that? Did you use the quadrennial
    corrections given in Bowditch?
    >Any ideas how to verify that it works?
    >As for the testing I have tried several random times/dates and have
    >not been out more than 2.4' on sun and 1.9' on star (yet).
    I checked my LTA using a statistical sample testing procedure. The
    errors were typically less than 0.3' over 50 years compared to full
    blown theoretical predictions, and never more than 0.7'.
    The quadrennial corrections depend on the epoch and the period over
    which the LTA is supposed to be valid. I found that linear (single
    term) quadrennial corrections are sufficient for a period of about 50
    years, but accuracy drops off rapidly (errors greater than 1') if you
    try to try to use linear corrections for longer period of validity than that.
    One problem with with the LTA given in Bowditch (at least, in my 1981
    edition) is that the period of validity is not given. Too, simply
    transplanting the quadrennial corrections for a 1972 epoch from
    Bowditch and hoping they will work for a 2008 epoch is not a valid procedure.
    Anyway, as far as I can tell, the quadrennial corrections given in
    Bowditch did not even work very well for a 1972 epoch, which makes me
    suspect that somebody took an even older set of quadrennial
    corrections and transferred them to 1972!
    Geoffrey Kolbe

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