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    Sumner lines: was[NAV-L] Simple celestial navigation in 1897
    From: George Huxtable
    Date: 2006 Mar 8, 00:57 -0000

    Frank Reed wrote-
    
    "With all the talk recently on Sumner's method, I  think it's worth
    remembering that Sumner's lines were considered a somewhat  exotic technique,
    one that might never be used in months at sea. Celestial lines  of position
    didn't catch on universally until almost a century after Sumner  published. And
    why that was the case is still a fascinating  question..."
    
    and again later-
    | But getting back to Sumner's method, if it was  so useful, why wasn't it
    | adopted universally? Why were navigators on merchant  vessels shooting Noon Sun
    | and time sights as late as the 1940s. I think there  are plenty of reasons, but
    | I don't think it has much to do with navigators'  failure to see.
    
    I had charged him with making a sweeping statement without justification. But perhaps those aren't
    such sweeping statements after all. All he has claimed is that "it didn't catch on universally", and
    asked asked "why wasn't it adopted universally". If there were a few diehard old-school mariners
    clinging to older methods, then indeed position-line navigation would not have become universal. Is
    that all that he is implying? In that case, perhaps we don't disagree after all.
    
    Of course, it's true that mariners kept on "Shooting Noon Sun" until modern times, whenever it was
    visible at that moment. If it was, that provided one of the position lines. And took another, later
    or earlier in the day, to provide a longitude by chronometer. The difference between us, I suggest,
    is in the way that observation would be processed. If it was by Sumner (or its later development, St
    Hilaire), that required a chart or plotting-sheet. If a noon Sun had been available, so the latitude
    was known, the longitude could be deduced without recourse to a chart, by a time-sight calculation.
    There was no difference, then, in the observations, just the procedures for data-reduction.
    
    It seems to me that one of the conditions pushing mariners to adopt Sumner techniques was the
    climate. Mariners returning to anywhere in Northern Europe had to find one of the narrow approach
    channels around the British Isles, after an ocean passage. Our climate is such that the Sun may be
    only fleetingly visible for days at a time. If and when it appeared, the sextant would be brought
    out, whether it was noon or not. The chance was too precious to be missed. There might be weeks at a
    time when no noon sights were possible at all.
    
    Sumner emphasises that very point, when he says-
    "There is no part of the seas, that is liable in a greater degree to fogs and thick weather, than
    the English Channels, North Sea, &c; and there is no part more crowded by the fleets of all nations;
    the coast, too, is dangerous; and the Westerly gales are severe, and of long duration; and ships are
    often placed in situations there, from uncertainty of their position, which render it dangerous "to
    run", and often more dangerous to "lay by", or to "stand off and on"."
    
    Indeed, reading that, nobody but an idiot would choose to sail in, or to, our part of the world.
    
    And yet, thousands of successful passages were made, by snatching a Sun sight and a position line
    whenever it was possible, transferring those lines with the ship's run, and obtaining a cross
    between them whenever it was possible.
    
    I can imagine that in areas where the climate was more conducive to celestial navigation, and the
    chances of getting a noon observation were much greater, mariners might not have been under the same
    pressures to adopt position-line navigation.
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    

       
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