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    Re: Time of meridian passage accuracy
    From: Douglas Denny
    Date: 2009 Sep 28, 15:36 -0700

    
    Thank you Jim.
    
    A most interesting paper.
    Graphical Fig 2 being useful. 
    
    Using your graph for my April example of observations gives about 14 seconds 
    delta t correction for true LAN,  and your graph confirms making corrections 
    for North/South motion of the observer as becoming more important requiring 
    correction with increasing exaggeration and distortion of the curve.
    
    It does also show though,  that these corrections at LAN although correctable 
    (and therefore useful to do) are not very great: and that brings us back to 
    the theoretical/practicalities debate.
    
    In the final table for example, your paper  does confirm that except for the 
    last example near polar observations at 82 North where delta t becomes a 
    highly significant 23 minutes, all the other delta t corrections for LAN are 
    within eight minutes of time; and the delts h corrections to altitude are 
    within one and a half minutes of arc.
    
    As has been shown by my poor sort of graph with bubble sextant, (and also 
    demonstrated with the Admiralty Vol III curve diagrams G.H. mentioned) that 
    the rate of change of altitude is so slow at LAN that it is within about one 
    minute of arc altitude at peak for a good plus or minus eight to ten minutes, 
    so the delta t is not quite so important if the observation is 'out' for max 
    altitude - so much as the delta h correction which _is_ important for 
    accuracy of a latitude sight and what would concern me in practical terms. 
    In other words, the timing of a latitude sight at LAN is not so important as 
    the potential correction for error required in altitude (which is not obvious 
    to the casual observer).
    -----------
    
    This is not to say the Bowditch criterion should be OK for _longitude_ with a 
    laxity of one minute of time - there I agree with G.Huxtable; but your paper 
    demonstrates a latitude within a couple of minutes of arc can be obtained 
    without corrections, and hence, if as you say in your conclusions, one is 
    without GPS,  and limited resources (assuming one still has a declination 
    table available which need only occupy a single page somewhere in the 
    emergency bag) one could quite successfuly make good landfall within a 
    latitude of two miles crossing the Atlantic say from Europe with just a 
    maximum altitude observation (if you still have a sextant that is), ..... as 
    Columbus did with an astrolabe in fact. 
    ----------
    
    Perhaps a greater simplification is possible with a graph of delta h /Vs. 
    declination change rate /Vs. observer's (boat) speed /Vs. Latitude:  would 
    allow a simple correction factor of delta h to be applied to a max altitude 
    observation at transit.  Then the timing of max altitude is not important at 
    all (within reason, say +or- eight minutes), and the corrected max altitude 
    obtained for a latitude observation which is what one wants without fuss.
    
    
    Regards,
    
    Douglas Denny.
    Chichester. England.
     
    ==========
    
    Original Post:
    
    Douglas:
    George listed a link to my paper:
    Jim Wilson
    
    Linked File: imgx/v32n1-6.pdf (no thumbnail available)
    
    
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