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    Re: Lunars for dummies like me
    From: Jan Kalivoda
    Date: 2004 Sep 24, 07:42 +0200

    Two small addenda to George's response:
    
    The ship clock was regulated according to time sights of Sun and trusted 
    between on sailing ships, at least if there wasn't any chronometer onboard. 
    So the application of the Equation of Time wasn't important at sea without a 
    chronometer, at least before 1833 (or 1832/34?), when GAT was superseded by 
    GMT as the argument of the Nautical Almanac. This was caused by growing 
    numbers of chronometers at sea, of course. Previously even the lunars were 
    given in GAT.
    
    Theoretically, even without a previous or subsequent Sun time sight, it was 
    possible to gain the longitude by a twilight lunar distance from a star or a 
    planet without any clock. From the time sight of a star (that one used for 
    the lunar distance or another, nearer to the first vertical, if needed), the 
    local star time can be obtained easily. The GAT (before 1833) or GMT (after) 
    was needed for changing LST to LAT (to be compared for longitude  with GAT 
    obtained from the cleared lunar distance before 1833), but it can be assessed 
    very roughly - the error of one hour in it caused an error of 2.5 nm in the 
    longitude obtained on the equator, less elsewhere. (According to the changing 
    difference of LST and LAT/LMT, growing for cca 10 sec during an hour. ) A 
    trifle with lunars. This procedure was taught in manuals.
    
    Of course, the reality differed. As Frank Reed wrote here once on the basis of 
    old whalers' logbooks, the great majority of lunars was taken from the Sun 
    and probably for the great majority of the minority of lunars taken from 
    stars, the George's procedure (combining the lunar observation with a 
    previous/subsequent Sun time sight by a clock) was used as more accustomed 
    to. And it was the only option, if Moon's and/or  other body's altitudes were 
    not observed, but computed for clearing the lunar observation, as needed 
    above the indistinct night horizon.
    
    
    Jan Kalivoda
    
    
    

       
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