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    Re: Beating a Dead Horse (aka Worley's Sextant)
    From: Frank Reed
    Date: 2014 Feb 9, 10:32 -0800

    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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