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    Re: Bowditch long term almanac
    From: Geoffrey Kolbe
    Date: 2012 May 18, 22:31 +0100
    At 20:50 18/05/2012, you wrote:

    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?

    To be clear, your question would appear to have been addressed to the astronomy website and not this forum.

    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 annual corrections for the stars will not change much over the years.....

    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.

    But the quadrennial corrections for the sun certainly will - and they will depend on the chosen period of validity, as I have stated. If the 1954 Bowditch has the same quadrennial corrections as the 1981 Bowditch has, then it is as I suspected and the older corrections were shipped wholesale to a new epoch, which is not a valid procedure.


    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 ).

    My method was really quite simple - if time consuming. I used the Nautical Almanacs for 1996 to 1999 as my starting point for the GHA and Dec of the sun. I then used the "Stormy Weather" software program (which contains the full set of astronomical algorithms, see http://www.stormy.ca/ ) to predict what the GHA and Dec of the sun would be for each day during the years 2036 through 2039, which is ten quadrennial cycles later than my starting data set. Taking the difference for each day between the two data sets and dividing by ten, I arrived at the quadrennial corrections I use in my LTA. I used a similar approach for the stars.

    No doubt a computer program could be written to do all this. Meeus' book "Astronomical Algorithms" gives all the necessary formulae to do the job and you just need to be reasonably fluent in some suitable computer language to run up the necessary program. If I were to create my LTA again, I would do it that way. But back in 1999 I did not know of Meeus' books and the Internet was not the resource for finding out about such things as it is today. As I will be 97 when my LTA reaches the end of its valid period in 2050, I probably will not be bothering to do this job again!

    Geoffrey Kolbe
       
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