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    More on lunars
    From: Frank Reed
    Date: 2007 Sep 26, 01:42 -0400

    Henry, you wrote:
    "Thanks for your interesting reply. I am of the opinion that Lunars
    generally received a bad name as far as accuracy is concerned primarily for
    two reasons."
    I should say right at the top here that lunars were, in fact, widely used by
    those navigators who needed them (navigators wo could not afford
    chronometers) in the early part of the 19th century. There are references to
    them all through the old logbooks [many of these logbooks are available
    online at mysticseaport.org]. They did not have a "bad name" regarding
    accuracy until late in the 19th century.
    You suggested:
    "1. The relative inaccuracy of early instruments. Let's remember that the
    octant, as originally produced was not fitted with a tangent screw and gear
    for easy and positve measurement of small arc increments. Although the
    vernier appeared rather early on, it was the incorporation of the tangent
    screw that allowed more positive fine adjustment. I also certainly cannot
    believe that some of the earlier instruments made of wood with ivory arcs,
    although works of art, could be relied on as respects accuracy over a wide
    range of condition to be met at sea."
    Certainly, instrument accuracy was always a big issue and still is for
    lunars. I agree with that. Regarding your specific comments on octants, I
    know of no one who shot lunars with a wooden octant in the era. A metal
    sextant --with tangent screw, telescope, and assorted shades-- were all
    recommended right from the very beginning of the lunars era. Even in 1763,
    Maskelyne spells out these requirements in detail. There's clear evidence
    that navigators carried two types of instruments: octants, like you've
    described above, for daily latitude observations (almost always Noon Sun)
    and sextants for lunars. Of course, if there WERE any navigators who
    declined the advice to get a proper sextant, and this advice was in
    practically every navigation manual and guide to shooting lunars, they would
    definitely have been disappointed with the results. Wooden octants are
    comparable in accuracy to modern plastic sextants (e.g. Davis).
    You also wrote:
    "2. The mathematical methods of clearing the distance, especially the so
    called approximate methods were a wonderment of complication, in some
    instances requiring the application of numerous corrections which in
    themselves sometimes defied reason. Most of the epitomes I have read did not
    apply to reason, but simply listed rules to follow. In my view,at least, a
    relatively simple explanation of the problem related to a trigonometrical
    format would have better served the mariner. The entire subject of spherical
    trigonometry does not appear as complicated as some of the approximate
    Here, I DO disagree. The great majority of the lunars that you will find
    worked out in 19th century logbooks are series methods (erroneously known
    sometimes as "approximate" methods). The steps involved in working them were
    no more complicated than working an ordinary time sight --something which
    nearly every navigator could do with ease. The amount of work was about
    three times greater, but the steps and mathematical skills were not greater.
    I've seen many examples of lunar observations worked out on spare pages in
    old logbooks, and not once have I seen any of the straight triangle methods,
    like Borda's, in use. They used the series methods, like Bowditch's
    Principal Method, Thompson's method (adopted into Bowditch in 1837 just
    before his death), Witchell's method, variants of Lyons' method, Bowditch's
    Original Method (known in Europe as the method of Mendoza Rios, although
    never claimed by MR himself) and so on. There's no doubt that other direct
    methods, including Dunthorne's and Borda's and Krafft's were in fact used,
    but they were less popular. The choice of method was not considered a "big
    deal" in the day. Many navigators seem to have learned a couple of different
    methods if for no other reason than for intellectual amusement.
    You also wrote:
    "With an accurate instrument and a little practice, I see no problem in,
    under favorable conditions at least, attaining an accuracy of less than 0.5
    arc minutes in measurement of the distance."
    Yes, I agree. As I've said, I find that modern navigators with good sextants
    (equipped with a 7x or better telescope, which I consider relatively
    important) can do even better than this on a regular basis.
    And you wrote:
    "I am not of the opinion that sea state has a significant effect on the
    observer, unless a vibratory effect is present, such as when a vessel be
    laboring or pounding, or when vibration is enduced by operating machinery.
    It does seem usual for a vessel to have been eased off or hove-to so as to
    provide more favorable conditions for observation. "
    There appears to have arisen lately the idea that lunars were only shot from
    large, stable vessels. In fact, many rather small vessels in the 19th
    century used them regularly.
    And you wrote:
    "You referred me to your Lunar Tables, with which I am well familiar."
    Just so there's no misunderstanding here, did you try the tool on my web
    site which lets you CLEAR a lunar online?? You put in your observations, and
    it comes back and tells you how much error you made. What do you get when
    you plug in your raw numbers? (I'm not talking about the predicted lunar
    distance tables here).
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

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