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    Re: Beating a Dead Horse (aka Worley's Sextant)
    From: Brad Morris
    Date: 2014 Feb 9, 14:43 -0500

    Frank

    The navigator on the (modern) expedition was Paul Larsen, and not Seb Coultard.  Seb is currently north of the Arctic Circle, with limited communications capability.  In our limited contact he has indicated that he is not intimately familiar with the details of modern navigation nor the intricacies of how Worsley himself navigated.  As such, it is rather unsurprising that there may be technical errors in how Seb presents that information on his website. 

    You could write to Seb directly at his website, pointing out the inaccuracies and detailing the correct answer.  I think you will find him quite receptive to this!  I believe he wants to get the navigation correct but hasn't the tools to do so.

    People looking at Seb's website, many who are not CelNav purists, may become intrigued by the CelNav (or run from those wiggly numbers).  They may be so intrigued that they then further investigate it here on the NavList.  An easy way would be a link from there to here, something easily traded for. 

    A start would be to just send your last as a copy/paste directly to Seb.  With his limited communications, he won't have the wherewithal to peruse the NavList, in the odd chance that he will see your post.

    Brad





    I finally visited Seb Coulthard's web site http://www.sebcoulthard.com and the page specifically devoted to the navigation of the James Caird: http://www.sebcoulthard.com/navigational-instruments.html

    There are some nice photos but not much information. The caption for the Nautical Almanac reads "This is the actual Nautical Almanac used by Worsley for his Dead Reckoning (DR) calculations aboard the James Caird." Of course, that's not quite right. Dead reckoning does not require the Nautical Almanac. The almanac was used for the nautical astronomy work ("nautical astronomy", as it was known back then, is our "celestial navigation" or "astro-navigation" as it's usually called in British usage today). The same page links through to the web site of the small museum on South Georgia which has acquired that original Nautical Almanac. There they quote the Wikipedia page as follows: "The Almanac specifies for each whole hour of the year the position on the Earth's surface (in declination and Greenwich hour angle) at which the sun, moon, planets and first point of Aries is directly overhead. The positions of 57 selected stars are specified relative to the first point of Aries." This is all correct for a MODERN Nautical Almanac but not for the almanac that they have in their possession. GHA was not used until the 1930s, and the list of 57 selected stars did not become standard until 1953. Rather, the Nautical Almanac in use at that time provided the right ascension and declination of the Sun, and, most importantly for the navigation of this voyage, the equation of time. Indeed, for the type of navigation employed, all that they required was a short table of a few pages giving the Sun's declination and the equation of time. The rest of the book was dead weight. These are relatively minor issues. Again, they fall into that category of details that matter only to those who already know that they matter. But for US (NavList members), that's important stuff!

    I'm not sure from looking at the remnant volume whether this was the "Abridged Nautical Almanac" or the full "Nautical Almanac" (which was aimed more at land-based astronomers). In either case, the British Nautical Almanac, full or abridged, was rather impractical in this era, as you can see from the declination of the Moon on June 17 which is listed as S.23°7'21.6". Of course they had no use for the Moon on this voyage, but this level of precision in the position data was also found in the Sun tables, and it goes well beyond the needs of celestial navigation. The "American Nautical Almanac" at this time was already slimmed down and specifically designed for the needs of marine navigation.

    -FER

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