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    Re: Joshua Slocum's navigational methods
    From: George Huxtable
    Date: 2005 Apr 17, 10:04 +0100

    Frank Reed revisits the question of Slocum's navigation.
    I was (somewhat reluctantly) convinced by his arguments, in that earlier
    correspondence in 2003, to the effect that there's no evidence of more than
    a single lunar-distance observation in Slocum's whole circumnavigation. And
    in describing that observation, Slocum showed such delight and pride in it,
    that if he had taken other lunars on other legs of the voyage, he would
    surely have at least mentioned them.
    So I now agree with Frank, that over Slocum's solo voyaging, there's no
    further evidence of reliance on lunars than that one example.
    Yes, he would have used dead reckoning: but the shorter and slower a vessel
    is, and the longer she takes on an oceanic passage, the more uncertain DR
    becomes. Slocum's advantage was that he was navigating at a time when (as
    well as the trade wind system) ocean currents were becoming understood,
    thanks mainly to Maury's work. And Slocum had a lifetime of ocean-going
    experience to draw on.
    When a ship's captain, his longitudes had been taken from on-board
    chronometers, at least in the later years of his career, though he had kept
    up his skill in finding longitudes by lunars.
    Well into the 19th century, though, it was common for the smaller trading
    vessels to do without that expensive chronometer, and not all shipmasters
    could manage lunars. The alternative was latitude sailing, a technique
    which had been adopted by mariners for hundreds of years. In latitude
    sailing a landfall, at the other side of an ocean, was planned, if possible
    at a spot which was benign in terms of no offlying rocks, and a coastal
    profile in which soundings would warn of a closing approach. After putting
    the vessel on that latitude, long before the expected arrival, and sailing
    East or West, a vessel could hardly fail to reach her landfall eventually:
    though without knowledge of longitude, the date of arrival would be a
    matter of guesswork. My presumption is that Slocum did much of his
    navigation that way, which didn't rely greatly on accurate dead reckoning..
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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