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    Re: Direction and alternate methods
    From: Bruce Stark
    Date: 2003 Jun 10, 15:31 EDT

    Guess it goes without saying that I'm reasonably happy with the drift of the
    List. But I'd like to see more posts such as Fred's recent one on timing
    sights. We ought to share more of the practical problems we've run into, and the
    solutions, if any, we've found.
    
    Mike Burks brought up the use of slide rules for interpolation. Just for fun,
    I learned to use a slide rule, but for navigation use it only to check the
    interpolation when working with the1804 and 1805 Almanacs. Although Nathaniel
    Bowditch called them slide rules, the old British navigation manuals called them
    "Sliding Gunters" after the Gunter's scale. Guess that's how that telescoping
    rig for dinghies got named (more of that history stuff).
    
    On the use of altitudes only to get Greenwich time from the moon, check back
    on the List. I weighed in on Feb. 26, '02, and said:
    
    "I just got signed on to the List, and probably shouldn't jump in so soon.
    
    The idea of comparing the altitude of the moon with that of one or more other
    bodies to get Greenwich time has been around almost continually for at least
    two centuries. It's such an appealing idea that many of the best navigation
    authorities have been blind-sided by it.
    
    First: Consider what you know about the reliable accuracy of altitudes taken
    from the sea horizon, especially from a small boat. You will be combining the
    errors of several such altitudes.
    
    Second: Consider what it is you are trying to do. You are trying to measure
    the moon's position in her orbit, NOT the rotation of the earth. If your
    latitude is less than 30? there may be times when you'll actually see the
    moon when her orbital motion is perpendicular to the horizon. At higher
    latitudes, never.
    
    The moon's is always within about 5? of the ecliptic. Her enlightened limb
    points to the sun and shows the path of her orbit. How often do you see the
    moon when her horns are pointing straight up or straight down?"
    
    I don't see why working a lunar need be so complicated, Doug. Seems to me
    anyone who can do simple addition and subtraction should be able to clear a lunar
    in ten minutes, once he's familiar with my Tables. No puzzles involved. And
    if Steve Wepster will continue his list of Precomputed Lunar Distances, and
    extend it to one-hour intervals (so we don't have second difference problems with
    the shorter ones) we won't have to calculate comparing distances.
    
    If you use my tables 7 and 8 for the three-hour interval you should multiply
    the minutes and seconds you get by three.
    
    Bruce
    
    
    

       
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