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    Re: Lunars and Longitude
    From: George Huxtable
    Date: 2010 Mar 28, 10:07 +0100

    Douglas Denny picked up my words of 27 March-
    
    ".....knowing GMT, you look up in the almanac the predicted declination
    and GHA of the body; Sun, Moon, star, or planet. (for a star that involves
    adding GHA Aries and SHA of the star). Use its dec, with the observed alt,
    and your known lat, to find the LHA of the body, in the equation shown
    above. Subtract LHA from GHA. And the result is your (Westerly) longitude
    ....."
    
    writing-
    "Ah! but its this bit which is the problem ...  "and your KNOWN lat"."
    
    =============================
    
    That question, of how well need the latitude be known, was dealt with in
    the previous exchange with Glenn, the day before, in which he asked "Is a
    DR latitude good enough?"
    
    and I replied-
    "It depends on many factors. If the observation on which the DR was based
    was taken only a few hours ago, yes; the DR latitude should be within a
    very few miles. If the sky has been cloudy for several days, in which time
    the vessel has been beating against stormy weather, the ensuing DR latitude
    is likely to be imprecise, and therefore, so will be any time sight that
    relies on knowing that latitude. And then the resulting longitude will be
    imprecise also. But again, if the sighted body is near due East or West,
    the result time-sight will depend little on the observer's latitude."
    
    It seemed unnecessary to say all that again. But, in response to Douglas,
    now I have.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
    

       
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