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    Re: Interesting Bowditch CelNav results was : Celestial Coordinator
    From: Antoine Couëtte
    Date: 2011 Dec 9, 20:17 -0800

    Dear all,


    Thanks to a private and detailed message with all computations received from Dave Walden, I now have a full and detailed explanation for the differences in results published in Bowditch and my former direct computation results.

    It all boils down to the fact that I had computed the intercepts from each individual DR position, which is the NOT the method carried out by Bowditch.

    As you all knew (except me ...) Bowditch publishes intercepts from different "approximate DR positions" which enable skipping some interpolation work since such individual "approximate positions" have been chosen so as to get integer numbers (too) for Local Hour Angles.

    Actually, in order to be fully "operational and usable" the Bowditch published results need to be further processed with hand drawings on a "local chart" so that from the intercepts earlier computed and from their various "approximate" DR positions, an observed FIX can be eventually derived from the "true" DR positions (I remember better this method now, after these so many years of no longer practising it).

    So, for any (still) interested Reader, and from the intercepts I am getting directly from each DR :

    Kochab 11.5 NM T / 018.7° and
    Spica 22.1 NM T / 143.3°

    I get an observed (Running) Fix in Azimut 090.6 ° from DR and at a distance of 36.6 NM from DR.
    So, starting from the SPICA DR position at GMT 06h11m26.0s , I get the following observed position :

    GMT 06h11m26.0 s N38°59'7 W156°22'8

    I can observe that given the "big" Spica intercept (22.1 NM from DR), it might be advisable to "recompute" a "last final" position from published position here-above, using - as initally - a 2.0' longitude difference between such 2 "updated" DR positions. Reprocessed data from the position found here-above give "last final" position :

    GMT 06h11m26.0 s N38°59'6 W156°22'5

    Last note : It might be interesting to compare the "last final" position here-above with Bowditch end results which actually are NOT published, since the entire and full sight reduction (the hand drawn part) is missing.

    Unfortunately this specific example is not a conclusively operational one since we are just left in the middle of the process. Given the high standards BOWDITCH have set elsewhere in a number of areas, this certainly could be regretted.

    Back on deck now ... I unfortunately carry no plotter with me in Mauritius Island (shame on me too !).

    Any taker then to draw the Observer's position at GMT 06h11m26.0s , read it from his "local chart " and then compare with the result published here-above ?

    Best Regards to all, and again

    Specials Thanks to you Dave !

    And also thanks to you Stan because you too had given me - in a more succinct way - exactly the same right explanation.


    Kermit

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