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    Re: More on lunars
    From: George Huxtable
    Date: 2008 Jun 5, 00:34 +0100

    I asked how a navigator at sea, without internet access, would obtain
    sufficiently accurate predictions of the Moon's position, other than by
    using the Nautical Almanac."
    
    Which propted this somewhat peevish response-
    
    | First, I presume here that you are talking about predicted lunar distances
    | of sufficient accuracy, right? Well, our hypothetical navigator could
    easily
    | PRINT THEM OUT from any of a number of sources. Isn't that just obvious? I
    | mean really, REALLY obvious??
    
    No, not at all obvious, and not so obvious to Frank that he chose to offer
    that path before, when asked. Not obvious if he has no idea, at departure,
    when he will get the chance to use the method , nor which stars he will
    require lunar distances to, when that time comes. Presumably, that list is
    to be somewhat different to the standard list of zodiacal stars, as used for
    getting GMT. What time intervals between predictions will be required, to
    allow interpolation to be sufficiently precise to achieve a 0.1' OVERALL
    precision? Much less than Maskelyne's 3 hours, without a doubt. So just how
    much information will he need to precalculate, print out, and take with him,
    and what guidance will he get to produce it? Just pretending that the whole
    scheme is a practical proposition; will our erstwhile lunar-navigator need
    to invent something like a new nautical-almanac for himself beforehand? Far
    from being obvious, it's one of the practical difficlties that Frank has
    evaded, until now.
    
    I had raised no objection to the use of a computer. Indeed, my reply to Ken
    was that it is "a good way to handle the problem.". It's just that Frank,
    although asked, hasn't mentioned before that to implement this trick could
    require a computer on board (one that wouldn't fit on his key-chain). That,
    too, has been glossed over.
    
    How about this way of arguing. "Uh, how about we shoot some trial sights at
    a known locations? That's obvious, right? And then whatever standard
    deviation you get, that's what you should apply."
    
    Not at all obvious if , for example, he wishes to compare observations made
    on shipboard, with a sextant telescope appropriate for such use, with
    observations made on land, with a 10x scope and a steady platform. Is his
    6-mile precision intended to apply to measurement at sea, or not? But we
    will have to see what his claims are, when we get some hard details, if that
    day will come.
    
    Readers can hardly fail to notice that Frank has offered us a lengthy
    diatribe about errors, but once again failed to provide an error budget, or
    any explanation of how he can get all the errors, systematic and random, to
    combine together to within 0.1'. One more evasion.
    
    Two things have come out of his posting. One is that there is no reliance on
    data from the Nautical Almanac, which has been made clear for the first
    time. The other is that his predicted error of 6 miles was at one stadard
    deviation.
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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