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    Re: sight reduction tables
    From: Peter Fogg
    Date: 2007 Oct 2, 08:23 +1000


    George writes:

    Bennett's tables have least one other adherent in Alex Eremenko.
     
    Well, its more like a book, really. And there are more than us two, it seems. The book continies to sell well, and is now into its third edition.

    Users
     
    [of The Complete On-Board Celestial Navigator]

    should be aware of certain limitations, that we have discussed here ad
    nauseam in the past. Now, as there are several interested fresh members,
    perhaps it's time to cover some of that ground once again. I hope it can be
    done without the personal acrimony that was so evident last time round.
     
    You're the specialist on such matters of "acrimony", George, to judge from your somewhat paranoid reaction to having your 'clipping' of an image pointed out recently ..

    Bennett provides a perfectly good source of 5-year predictions for Solar
    System and star bodies, rather cleverly presented, but to a limited
    precision. For example, to get the GHA of a star, four quantities have to be
    added together, each being tabulated to the nearest 1' of arc. Presuming
    that these have been tabulated with a maximum error of half an arcminute
    either way, the maximum error in the result will be no more than 2', and
    usually it will be less.
     
    Not so fast, George, nor so glib. As we have gone over here before, ad nauseum if you will, this is an affront to the most elementary of statistical analysis. The errors don't simply add up, as you well know, they tend to cancel each other out. If there is a 50% chance of error with one, then a 25% of the errors accumulating, then 12.5%, 6.25%, etc.

    Each sextant correction, dip and refraction, adds a
    similar error. And so on. A prudent navigator should assume that things are
    combining against him in the worst possible way, even if, usually, they
    won't be.
     
    This is silly scaremongering nonsense, and flies in the face of a carefully detailed statistical analysis about this question presented by me some time ago. Which you failed to respond to, George. As you consistently fail to respond each time you would have to admit that you have been wrong. Don't come the raw prawn again with this half baked, ill considered claptrap. Answer when its relevant, if you can, don't just try to peddle the same silly twaddle much, much later.

    In my view, such a limited precision is perfectly appropriate for use on a
    small craft at sea, when the horizon is so hard to pin down, as long as
    those limitations are kept in mind.
     
    Which is what the book was specifically designed for

    Bennett's sight reduction tables, for obtaining calculated altitudes, seem
    to work perfectly well within those limitations, requiring only simple
    arithmetic, though they call for rather a lot of shuffling between pages.
     
    Dear dear.

    It's when we come to calculating azimuths that Bennett's tables fall down.
     
    Disengenuous again. The BOOK, its a book, by the way, George, in its latest edition, proposes 4 methods of deriving azimuth. One tabular, one graphical, one via formulas (there are several formulas proposed, even George's favourite is there) plus a rough method of estimation, to be able to quickly access other data.
     
    A long time ago George found that if he carefully chose a certain combination of input data and used that with the tabular method then he could induce error. And George has never got over it. You really should move on, George. turn the page, quite literally, to where a graphical method has, we can only assume, quite safely emerged unscathed from your most rigorous of examintions.
     
    Incidentally, the BOOK comes with warning about when the tabular method is least suitable for use. This has always been a storm in a teapot, albeit a major and enduring gale for George.
     
    The whole issue has been discussed, ad nauseaum again, in the past. If anyone is interested its all there, in our archives. I seem to remember that you failed to respond to serious dicussion, then, George ...

     

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