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    Re: Working a lunar
    From: Frank Reed
    Date: 2007 Sep 24, 00:01 -0400

    George H wrote:
    "Well, yes, but if it's going to be of use as an emergency tool, where are
    those precise lunar distance predictions going to come from? That was why I
    asked Henry for the source of the lunar distances he had used. And if it
    comes from some internet source, then I ask whether, at sea, in some
    undefined "emergency situation", such internet data is going to be
    available. If it's possible to print out in advance such predictions, and
    take them to sea, then that would indeed qualify as an "emergency tool" Or
    if Henry can take to sea some computing gadget that predicts such lunar
    distances on the spot from first principles, as is perfectly feasible, then
    lunars could be an emergency tool. But not, surely, if on-line web access is
    called for."
    
    First, I'm sure we all agree that the idea that there will be an emergency
    situation where you would lose GMT with no means of recovery is pretty
    outlandish! Almost any radio station will eventually give you a usable time
    signal. But let's suppose it's part of a game. You're sailing without any
    electronics (except stored away in a sealed case to be used in case of a
    REAL emergency) and it's your job to sail by traditional means. So you
    decide to shoot a lunar... Where do you get pre-calculated lunar distances?
    Well, you can pop on the Internet the day before you set sail and grab eight
    or ten weeks worth from any of the various sites that have them (Steve's,
    Ken's, mine, and others) and print them out. If you decide to stop in Hawaii
    for a month or two on your trek and your pre-printed tables expire, you hit
    an Internet cafe the night before re-starting your voyage and you print out
    a fresh set. No problem! But what do you do if the pet goat on your sailboat
    eats your lunar distance tables?? Is the game over? Well, no (so long as the
    goat didn't also eat this year's Nautical Almanac). You can calculate the
    predicted LD for the hour before and the hour after as a standard great
    circle distance problem using the GHA and Dec from the standard pages of the
    almanac. There is a slight inaccuracy introduced here, but nothing much to
    worry about. It's more work, yes, but it's only a game...
    
    Another use for lunars today, as I described last year, is to generate a
    positional fix when the horizon is obscured. Two sets of lunars, involving
    two separate astronomical bodies, gives you a complete fix in longitude and
    latitude. This is the sort of game trick that might come in handy if you're
    trying to win a sailing race using a celestial handicap --or maybe just
    trying to win a bet (I assume that the rules allow a computing device since
    this is computationally intensive, and in fact most sailing race rules
    apparently do not care how you work the sights, electronically or by-hand).
    
     -FER
    
    
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