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    Re: Lunar distances - shot clearance methods
    From: Frank Reed CT
    Date: 2004 Sep 14, 16:52 EDT
    Henry H wrote:
    "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."

    Yes. Those rules were apparently standard. Bowditch recommends the same, and I have seen those rules applied successfully in practical examples of clearing lunars from the early 19th century. Note that this is the "pre-clearing" step; the actual altitude corrections are figured to the nearest 10 seconds of arc or so. The 12/20 rule amounts to 4 minutes for dip and 16 minutes for semi-diameter. Not bad...

    And wrote:
    "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"

    Interesting. I bet if Arnold had had friends in high places in Washington (as the Bowditch family did in the 1860s), then perhaps his book would be as legendary as the Navigator.

    And:
    "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"

    I'm sold! Literally... more later.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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