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    Re: Bowditch long term almanac
    From: Geoffrey Kolbe
    Date: 2012 May 20, 07:22 +0100

    Hello Hewitt
    >I'd have to dig my copies out of storage, but my recollection is
    >that Bowditch differed from you mainly around the equinoxes. My
    >recollection is that the largest Bowditch correction was 0.75' in 4
    >years for Declination. Whereas yours are about half that. Which
    >means Bowditch's max deviation would be 3.75' in 40 years.
    Actually, from a quick scan, I have Dec quadrennial corrections as
    high as 0.74' and the 1972 Bowditch LTA goes up to 0.71'.
    >In the context of small-boat navigation, I'd say the minuscule 1972
    >Bowditch table would still be useful.
    Hmm. The problem is that the Bowditch LTA corrections are reasonably
    good in places but quite wrong in other places. So one can be
    deceived into thinking the Bowditch is useful and reasonably accurate
    for any date, which is not the case. It might actually be better
    (safer) just to use the GHA and Dec for the four base years without
    any quadrennial correction than use the Bowditch corrections and
    think you are getting the claimed accuracy.
    The perception seems to be that a LTA of this sort cannot be
    accurate, and errors of a few minutes here and there are quite
    acceptable. My point is that it is possible to create a LTA with
    really quite good accuracy. The expected error of GHAs and Decs
    calculated using my LTA are around 0.25' across fifty years, which is
    only a little over twice that claimed for the NA for one year!
    Geoffrey Kolbe

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