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    Re: Lunars.
    From: George Huxtable
    Date: 2001 Jul 03, 5:36 PM

    Nigel Gardner asked-
    >Anyone out there with a good algorithm and process for longitude by lunars?
    My answer-
    It's not as simple as that, Nigel. I suggest you need to refine your
    question a bit to explain what you are really after.
    If you are going to measure longitude by lunars, I presume you intend to
    avoid using your quartz-crystal watch, which would bypass the whole
    exercise. What else are you going to do without? Presumably, since you
    asked for an "algorithm", you intend the use of a calculator or computer.
    In that case, you aren't asking to explore the mind of a navigator of, say,
    1800, with all the complications that he faced from having to use
    logarithms for his arithmetic. Fair enough.
    The navigator of those days also had no concept of crossing any two
    position lines to get a fix, as we do now in the intercept method. Instead,
    he considered lat and long as quite separate quantities, to be determined
    independently. Given a known latitude, and a measured apparent time, and a
    measured lunar distance, he could get a longitude. To do all that he would
    normally need some sort of timekeeper, to connect the times of his latitude
    observations with the times of his lunar distance observations. The
    deck-watch to be used for this job would fall far short of a chronometer,
    it simply had to keep time to a minute or so over a few hours.
    The lunar navigator also needs to decide whether he is going to make a
    distance measurement when a horizon is visible, usually daytime Sun-Moon,
    in which case the measured altitudes of the bodies involved can provide
    additional information. Or if he is going to measure a Moon-star distance
    at night, when stars are bright. So no altitude information, just a time
    check for setting the deck-watch right.
    So there's a whole range of alternative strategies, and a whole range of
    restraints to consider. State just what you are after, Nigel, and we will
    do our best.
    Yours, George.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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