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    Re: Time of meridian passage accuracy
    From: Douglas Denny
    Date: 2009 Sep 27, 15:30 -0700

    Interesting.  I have not seen your navigation paper.
    What was the difference you found for the one minute (Bowditch) difference 
    criterion in time at Mer Passage?  
    (It will of course be different for different rates of change of declination 
    at different times of the year. The maximum rate of change of Dec being the 
    worst case).
    I note you say there was a minute and a half error. Whilst I agree this is a 
    minute and a half one does not want to be there as an error, I still find it 
    hard to believe that practical navigation better than (i.e within) plus or 
    minus a half to one minute of arc accuracy is going to be achievable by most 
    people in marine navigation;  and plus or minus say two or three minutes of 
    arc for bubble sextant observations on land.  That's all I achieve anyway 
    though I acknowledge there wil be far more skillful people out there than 
    myself who will no doubt be able to do better.
    Though the academic and theoretical side is probably my greatest interest; I 
    like to try to keep my feet on the ground (or heaving deck) too. The whole 
    business is about achieveing the best possible sight under the prevailing 
    conditions I suppose and that includes the calculation. If you can guarantee 
    a line of position to within a mile when approaching land from a long sea 
    voyage - you are doing very well thank you.
    Douglas Denny.
    Chichester.  England.
    Original post:
    You state that "the effect is very small as to be of no practical concern to practical navigators."
    I compiled a table in my Navigation paper showing differences between maximum 
    alititude and meridian passage ranging from 10 seconds to 37 minutes. My 
    trial which led me to write the paper had a difference of a minute and a 
    half, and that was at a moderate latitude and sailboat speeds.
    Jim Wilson
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