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    Re: Bowditch long term almanac
    From: Örjan Sandström
    Date: 2012 May 18, 12:50 -0700

    Actually I had the corrections mailed to me from a man on a Astronomy website.
    My original Question was if anyone could explain to me how to do this, not one answer?
    When I "bumped" the question ataching a PDF of the 1954 Bowditch almanac he sent me a file for sun and several stars (85 in all including polaris), he stated that something like this was not very accurate and not considered good for more than few years, when I compared the star numbers with Bowditch it was more or less same numbers just differed in him using arcmin arcsec and two decimals, All I did was change them to decimal min and used 3 decimals.
    The sun he was almost rude about, pointing me to something he called JPL (which i had to search for using google) and some script to download, he sent me sample output (which is what I have to date, 505 lines of text), I have yet to find the script (RaSha.sh) only found the JPL ephemeris.

    His data was again more or less same as Bowditch, but this time just 0.x' corrections which seemed to match Bowditch so well I kept the numbers for the sun as they where one decimal better.
    let us say he was not very helpful and not even close to polite (he called me few things best left out).

    I am still waiting for his reply (5 months and several mail requests later), have not had it explained how to go about this mathematically or had explained to me how to represent the "true motion" he told me to look for in a way useful in this context nor how to find and use the script ).


    2012-05-18 21:05, Geoffrey Kolbe skrev:
    > 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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