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    Re: Sight reduction method history
    From: Frank Reed
    Date: 2013 Nov 7, 07:59 -0800

    Francis Upchurch, you wrote:
    "Lu also asked about slide rules? I find the cylindrical Bygrave and Fuller, with very long spiral scales are usually accurate to 1-2' for sight reduction. the Bygrave is very quick, the Fuller,using the sine/cos formula takes twice as long, so I tend to use the Bygrave."

    You may not be aware that the Bygrave slide rules have been the center of extensive discussions among NavList members over the past five years. Here's an index of a selection of NavList messages that have Bygrave in the subject:
    http://www.fer3.com/arc/sort2.aspx?y=200601&y2=201512&subject=bygrave. And as always, just because it's been discussed before does not mean we can't discuss the topic more now! (if you're interested in more discussion of the Bygrave, could I offer one suggestion: start a new subject specific to that topic).

    The Bygrave sslide rules, and other cylindrical rules, are very neat devices, but it's important to recognize that they were a brief and minor development in nautical astronomy and celestial navigation. They emerged during that period which I described previously as "Numerous methods were developed and discarded in the period from c.1910-1940 to streamline the calculations. One could spend years on the "botany" of these methods and sub-methods and their inter-relationships." Otherwise slide rules have had a limited role in the history of celestial navigation. It's definitely fun to experiment with them today. Gary LaPook developed the "Flat Bygrave" system which allows anyone to try out the Bygrave method on a few printed sheets (including one transparency). Alas, sheets of printable transparency material have become nearly as obsolete as the Bygrave itself within the past few years, and as a result, prices have skyrocketed.

    And you added:
    "Chichester famously used a Bygrave in his 1931 solo flight across the Tasman sea."

    Yes, and that's yet another of the "cultures" in the history of navigation. Aerial navigation saw some of the most important experimentation and innovation in celestial navigation during those years before the Second World War, and much of it fed back to and re-invigorated marine celestial navigation.

    And:
    "I find it much quicker and more fun than tables. I use the Fuller to clear my lunars, great fun!"

    Does the Fuller manage better than the 1' to 2' error level of the Bygrave? If not, using it for lunars is indeed "great fun" but really unacceptable for the requirements of lunars.

    You also wrote:
    "I notice Frank briefly mentioned Martelli's tables? I had never heard of them, but by amazing coincidence have just been reading the book by one of my heroes [Harry] Pidgeon, Islander 1922 circumnavigation. On page 12 he recommends the Martelli tables as his favoured method to reduce his time sights. However, by page 17, he says that going below to do the maths made him sea sick, so from then on, stayed on deck and guessed a rough LAN time for longitude from his noon meridean sight!"

    These solo circumnavigators are yet another subculture. You might even say that each qualifies as his or her own sovereign nation of navigational culture. :) Finding longitude by guessing from the noon observation is a rather poor means of getting longitude (unless more sights around noon are taken), but it will certainly give a nice rough estimate if you're sailing the wide open Pacific. A quarter century earlier, Pidgeon's circumnavigating predecessor, Joshua Slocum, only dead reckoned for longitude yet it worked well enough for him. As a general category, we might call this group "adventure sailors". From the point of view of navigation, their needs are minimalist since they can choose when to sail (often waiting for the easiest weather --wisely) and they have no schedule to keep (unless, like more recent adventure sailors, they're trying to break records) and also no one to answer to --usually no owners to whom they have to explain their navigational choices. It's a unique degree of freedom.

    Regarding those Martelli tables, the 1920s was apparently the primary period when they were popular. The "Old Navigation" was in decline, and novelty (and "viral" marketing as "mystery tables") apparently created some attraction for this idiosyncratic method. They're actually trivial, and they do not significantly improve on the usual two or three minutes to work a time sight by the standard method.

    -FER


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