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    Re: longitude by lunar distance
    From: Alexandre Eremenko
    Date: 2012 May 10, 14:04 -0400

    Paul,
    You don't need any watch to determine the local time.
    It comes from a usual altitude sight, which is called the time sight.
    It can be done almost simultaneously with your lunar distance.
    
    The usual formula sin h=sin Lat sin Dec + cos Lat cos Dec cos LHA
    gives you LHA if you know your Lat.
    Knowing LHA is essentially the same the local time.
    
    Alex.
    
    On Thu, 10 May 2012, Paul Pauliukonis wrote:
    
    >
    > How does one obtain longitude by lunar distance.
    >
    > I always thought that lunar distance measurement gives accurate GMT and then 
    one uses the GMT to calculate longitude by using sun/moon line-of-position 
    (LOP).
    >
    > In Joshua Slocum's book Sailing Alone Around The World, he states that the 
    longitude is the difference between GMT (obtained by lunar distance) and 
    Local Mean Time - LMT ("finding it is a simpler matter" p.104).
    >
    > My confusion rises from the fact that the LMT has to be accurate, since an 
    error of four minutes equals one degree.  Slocum's alarm clock did not even 
    have a minute hand, so how did he accurately calculate LMT? He claims 
    longitude accuracy of a few miles.
    >
    > Any comments on this subject will be  very appreciated - paul
    >
    >
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