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    Re: Slocum's lunars (Slocum -- science and art)
    From: Jan Kalivoda
    Date: 2003 Dec 14, 12:31 +0100

    As for the "science" at Slocum, I give up this issue. The reference given by 
    Frank isn't so convincing for me as for him, but I don't want to be seen as a 
    pedant and to force my semantic opinions on native speakers and practising 
    sailors.
    
    Mischievously speaking, the last Frank's reference from Slocum proves that 
    Slocum considered the "art" of lunar observations as an obvious part of the 
    skill of a competent navigator - as late as in 1894. Why he would have 
    neglected them in the Spray?
    
    But as for Slocum's "fiddling" his lunar calculations near Nukuhiva, I take 
    this assumption very improbable. I must repeat - if he corrected himself at 
    the end, he managed to find the right table values somehow. If he had made a 
    dull mistake in the first run of calculation, why he would have concealed it 
    by deliberate pseudoexplanations or confessed it by repeating the first 
    sincere, but fuzzy manifestation of his "cabin fever" in his book? He was 
    alone in the Spray, isn' it? No witness was at hand, who would have 
    compromised him in Nav-L.
    
    I suppose that he was able to correct an isolated wrong table value (after he had realized it!):
    
    - if the error was in the last or last but one digit, the adjacent columns and 
    the rule of three were at his disposal
    - if the error was in the first or second digit, the adjacent column values 
    maybe had the same correct digits at these places
    - and as I wrote earlier, some tables gave simple auxiliary logarithmic values 
    only for transposing subtraction to addition and so on; their pertinent 
    values could be recalculated rather easily
    
    Therefore I consider his claim of correcting tables true. And in my opinion, 
    only for that he mentioned this lunar observation as the only one during his 
    voyage. Other lunars could remain without any remark.
    
    But Frank is right in my eyes, when he doubts, if Slocum used lunars every day 
    or every week. Back to Frank's whalers. The uncertainty of lunars was as high 
    as a half degree of longitude at sea - but to both sides, E/W. You should 
    accept one degree of a parallel as the set of your positions, with the center 
    at the longitude deduced from lunars. It was impossible to maintain the log 
    by everyday lunar observations, your path would be very crooked on the map. 
    And if running the constant latitude leg, you would stay on the place for one 
    or two days and then make an incredible loop forward, based on lunars. 
    Compare Bowditch (I have the digital edition from Starpath). During his 
    imaginery voyage from Boston to Madeira, he records lunar observations and 
    lunar longitudes almost every second day and the chronometer longitudes as 
    well. But for the log and for gaining the course of the day, he uses only the 
    DR longitude up to Madeira! All what he wants from the lunar and chronometer 
    lo!
    ngitudes is that they cluster around his DR longitude in a soothing manner. 
    (Norie in his Epitome proceeds another way. But I don't want to be lengthy.)
    
    Therefore lunars were excellent for guessing the landfall in the limit of some 
    eight hours after several months at sea and for timing it to daylight hours 
    or to better weather conditions, they were excellent for giving a dangerous 
    island or cape a safe berth, but they were useless for everyday deep sea 
    navigation. Slocum and other navigators (whalers) didn't need to use them 
    frequently (only for exercising them).
    
    But I wonder, if such navigator, succesfull master and former lunar expert 
    used lunars only once in his circumnavigation without a chronometer. He 
    wasn't so moth-eaten old-timer during the voyage, as it appears from some 
    Frank's words.
    
    
    Jan Kalivoda
    
    
    

       
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