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Re: Bligh's noon by chronometer
From: George Huxtable
Date: 2010 May 31, 16:31 +0100
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@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.