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    Re: Celestial predictions. was:[NAV-L] Old style lunar
    From: George Huxtable
    Date: 2004 Dec 13, 15:52 +0000

    Frank Reed asked me-
    
    "Why do you have such profound antipathy for a  database?!"
    
    Frank is over-egging his argument. I have expressed no such antipathy,
    profound or otherwise, to a database. Indeed, in my last mailing, I wrote-
    
    "If anyone could offer appropriate software for installing and
    interpolating a pre-computed database of all those bodies, as Frank is
    advocating, I would certainly weigh it up seriously against the Meeus
    alternative; and perhaps implement both."
    
    No antipathy there, then.
    
    Where I have an antipathy (and strongly) is to making myself dependent on a
    series of downloads from the web.
    
    Frank put it like this- "If I were planning to spend two months  cruising
    without touching an Internet-enabled port, I would simply print out the
    pages."
    
    I really wouldn't know if my cruising has ever "touched an Internet-enabled
    port", or not. It's the last thing I'm looking for when dropping an anchor.
    Indeed, one strong reason for going cruising is to get away from such
    things. And rather than "print out the relevant pages", I would prefer to
    buy a Nautical Almanac.
    
    However, what the algorithmic approach, via Meeus, offers is this: I can
    load in, once and for all, the data that will be used compute all the
    celestial positions I will ever need in my navigating lifetime. That's what
    I did, nearly 20 years ago, and have been happily using that data, and
    those algorithms, and the resulting predictions, ever since. All I have to
    do is to keep up the batteries of my pocket calculator (and its backup). If
    I used lunars at sea then I would require higher precision, and the later
    edition of Meeus, and rather more calculating power, than I get away with
    at present.
    
    It's all my own, it's under my own control, I don't need to rely on anyone
    else maintaining a service that they might in future decide to switch-off
    or modify. That's the independence a navigator needs. I don't even need to
    replace an Almanac.
    
    If Frank can offer access to a precomputed database, on similar terms; a
    once-only download with no need for future update, and if he can suggest or
    offer a tool to implement that job, then navigators may well beat a path to
    his door. Until that day comes, his offering of downloadable predictions,
    though welcome and useful, is second-best for some seagoing navigators.
    
    Frank writes-
    
    "When memory is expensive and calculating time is unimportant, it's
    appropriate to write code based on long algorithms. When memory is cheap
    and calculating time is relevant, it's better to pre-calculate everything
    and look up the results in a database."
    
    How relevant would calculating-time be, I ask, for the Meeus algorithms on
    a modern laptop? I concede that for my digit-serial pocket calculator,
    calculating-times of several minutes are a severe embarrassment. Will a
    planet or Moon position, even with hundreds of perturbation terms, take
    more than a second or so to calculate? Is calculating-time a valid argument
    against the algorithmic approach?
    
    What I question is Frank's dismissal of the algorithm approach in these terms-
    
    "Then I realized that the whole approach was absurd for the vast majority
    of "practical" applications."
    
    George.
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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