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    Lunar distances - shot clearance methods
    From: Henry Halboth
    Date: 2004 Sep 11, 13:50 -0400

    It is high time that a few of the comments/questions raised
    concerning my posting regarding Arnold's Method of clearing
    Lunar Distances be addressed. Sorry for the delay.
    Preliminarily, thanks to all those who did respond. Your comments
    were certainly insightful and conducive to the advancement of our
    discussions. Many of the older methods of navigation are fading
    into obscurity or are sometimes simply misrepresented - this forum
    provides an informative arena for there correct preservation. If you
    get it wrong, you are going to be stomped on, which often is not
    the case with published material.
    As regards Arnold's tables being somewhat restrictive by reason
    of constraints in altitude correction, please note the use criterion
    to be the bodies apparent altitude or the correction thereto; how either
    is calculated is not mandated and may be at the user's option -
    whatever refinement in the way of temperature and latitude
    corrections for refraction and parallax may certainly be applied
    at discretion. One cavalier treatment, not previously mentioned, is that
    Arnold advocates a standardized observed altitude correction to obtain
    the apparent altitude; under a Rule III, he advocates, across the board
    "To the moon's observed altitude, add 12', if the lower limb be taken,
    but if the upper limb be taken, subtract 20'. to the observed altitude of
    the sun's lower limb add 12', and from the star's observed altitude
    subtract 4', and you will have their apparent altitudes." Of course,
    we know this to be technically incorrect - perhaps it is simply a
    reflection of the often expressed opinion that an error of a few
    minutes of arch in altitude does not materially affect the result in
    clearing the distance.
    As to the longevity of Arnold's work, addressed by Frank, it is
    probably worth noting that his work is not primarily a navigational
    text. It is a wide ranging compendium of nautical knowledge, trivia
    if you will, specific to the era it which it was written; it possesses
    no ongoing values that might attract it perpetuation by a hydrographic
    office or on which subsequent authors/publishers might build on a
    continuing basis, as occurred both with Bowditch and Norie, although
    it seems Norie has pretty much run it's course, as the last edition I
    was able to obtain was that published in 1917. Subjects covered by
    Arnold in some 844 pages, exclusive of tables, include sailing
    directions, ship maneuvering instructions, sailing instructions,
    practical sea gunnery, familiar subjects in astronomy, the pepper
    trade on the West Coast of Sumatra, UIS Customs Duties, coins
    of the US, instructions to Masters of Ships, quarantine laws, laws
    of pilot and pilotage, regulation of seamen in the Merchant Service,
    marine assurance, etc. It is really a snapshot in time and contains
    priceless information, some of which has long since been forgotten,
    of a particular era in seagoing history and the United States. It bears
    a frontispiece of testimonials, albeit unsigned, by prominent marine
    figures of the day.
    There are about 103 pages devoted primarily to Lunar Distances
    and the associated astronomy and, as far as my personal interests
    were served, allowed a greater understanding of the lunar theory
    than did either Bowditch or Norie which, in my view, were more
    prone to simply providing rules of calculation rather than in appealing
    to the reader's understanding of the problem.

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