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    Re: Bligh's noon by chronometer
    From: George Huxtable
    Date: 2010 May 31, 16:31 +0100

    About Bligh's noon Sun observations, I had suggested that if it wasn't
    visible at noon, that could be overcome-
    
    "And then, the latitude could be readily calculated
    from the altitude at that moment, knowing the time difference from noon,
    using the Ex-meridian Tables. That was primarily what those tables were
    for."
    
    To which Frank replied-
    
    "Sure. Of course. The fact that Bligh mentioned these "exactly at noon"
    sights suggests he didn't know ex-meridian sights or didn't find them
    convenient."
    
    Not at all. All it suggests is that the Sun happened to shine at noon on
    these occasions, after all.
    
    ================================
    
    And Antoine wrote, about time-sight observations-
    
    "I thought he would have used :
    sin H = sin Lat * sin Dec + cos Lat * cos Dec * cos T , with both Lat and
    Dec being (adequately) known, then it should (might?) have been
    straightforward (and hopefully achievable with 10log tables) to reckon
    Local apparent Time " T " ...
    
    Well ... at least we need a sufficient time between time of height
    observation and Apparent Noon Time - certainly and least a couple of hours
    here - simply because we are solving for a Cosine here, and by Apparent
    Noon time, T = 0 and solving it through its Cosine is not wonderfully
    accurate.
    
    Am I missing something ?"
    
    ==================
    
    No, Antoine has it right. Indeed, the best moment for such a morning time
    sight (but only possible in Summer) would have been when the Sun was due
    East, on the Prime Vertical, in which case the vessel's latitude made no
    difference, and wasn't required. If the time interval befoire noon short,
    then cos T is changing only slowly, and the procedure becomes inaccurate,
    just as Antoine says. It's the same thing as saying that the Sun is rising
    more and more slowly, as noon approaches. The best moment is well before
    noon, provided the Sun has risen above the few degrees of dodgy refraction
    near the horizon.
    
    Of course, the same expression is used when calculating an ex-meridian
    sight, close to noon. But in this case, it's being worked in the opposite
    direction, so the slow change of cos T with time in this case is an
    advantage.
    
    The equation quoted by Antoine is fine for us to use now, with calculators
    and computers, but is awkward when using logs. Maskelyne, in the very first
    issue of Tables Requisite (1766), presented a way of doing the job using
    logs that was still being used, for time sights, 150 years later, in
    Worsley's small-boat rescue mission from Elephant Island, with Shackleton.
    If anyone asks, I'll post a copy of Maskelyne's text.
    
    =================
    
    Bligh was, at the time, in the South Atlantic, in the vicinity of Tristan
    de Cunha. He didn't need Tristan; indeed, he needed to avoid striking it.
    But he had gone out of his way, crossing the South Atlantic, from the Horn
    to the Cape, to look it out, so it could be placed better on the charts. In
    the end, he went past without seeing the island. That was the sort of
    navigator he was.
    
    =================
    
    In the light of recent Navlist comments, here is another pointer to Bligh's
    character, from the log of 24 April 1788, after a month of fruitless
    battling heavy weather in an attempt to round Cape Horn-
    
    "To enable me to keep my People as healthy as possible, I have not only
    appropriated my Cabbin to as many as could hang their Hammocks in it, but I
    have all the others bedding brought into it every day while I cannot get
    them on Deck in the bad Weather, by this means I have the Tween Decks clear
    and can be properly Aired by Fires as well as the Cabbin ..." Bligh's own
    cot had already been displaced from the great cabin into a smaller one, so
    it could be prepared to recieve the breadfruit, the object of the voyage.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
    

       
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