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    Re: sight reduction tables
    From: Lu Abel
    Date: 2007 Oct 05, 09:31 -0700

    Peter:
    
    You're both right and wrong (no personal insult intended).  There
    certainly are many ways of doing sight reductions and even today people
    are inventing new ways to do it.  Ageton (HO 211) is a result of someone
    skilled enough in math to say "there's a different/better way to reduce
    sights" (where "better" in Ageton's case meant "simpler, more compact
    tables even at the expense of more steps").  HO 229 resulted from
    computers becoming commonplace enough in the 1950s to allow vastly
    larger and more accurate sight reduction tables to be calculated.  And,
    of course, the solar-powered $15 pocket scientific calculator has made
    all tabular methods obsolete, at least in some people's opinion.
    
    But the issue extends beyond that to things like training.  All of us
    have learned to reduce sights either by reading a book or taking a
    class.  That class or book trained us in one particular sight reduction
    method.  Sociology tells us that people will not change a way of doing
    things unless they perceive some significant reward as a result of
    changing.  How many celestial navigators will want to learn a new method
    just because it might be "better" than the method they originally
    learned?  You might, I might, because we're curious about the whole art
    of navigation.  But would the average offshore voyager?  Or will he/she
    stick with sun sights and HO 249?
    
    Hand in glove with training is availability.  If I'm preparing for an
    offshore voyage and I want a set of celestial navigation tools, I'll
    probably go to a large and expert supplier such as Celestaire or
    Landfall.  What sight reduction method I use is going to be limited by
    what's in their catalog.  Again, some experts may seek out new and
    different ways of doing it, but for many it's "what's in the catalog."
    
    So, yes, you're right that there are many ways of reducing sights.  But
    as many people in the past who perceived a "better way to do it" have
    learned, what gets sold and used is determined much more by
    non-technical factors such as commonplaceness than simply by technical
    superiority.
    
    Lu Abel
    
    Peter Fogg wrote:
    >
    > Five days ago I somewhat flippantly answered Lu's:
    >
    >           It's also my
    >         understanding (at least as of a decade ago or so) that most of
    >         the long
    >         distance voyagers that still practicing celestial navigation use
    >         Ho249
    >
    >
    > with:
    >
    >     As if it was a question of one or the other, Lu!  There are other
    >     methods ...
    >
    >
    > I'm a little bit surprised that so many people, who are in other ways so
    > well informed about matters navigational, seem to think that when it
    > comes to sight reduction then the choice is limited to choosing between
    > HO 249 and HO214.
    >
    > Quoting from:* New Sight Reduction Tables*
    >
    > "The two most popular tabular techniques were using either the DR
    > position or a chosen position; the latter based on an integral degree of
    > latitude and local hour angle. This method required six volumes of
    > tables to cover all possible latitudes ...
    >
    > On the other hand, tables using the DR position require multiple
    > interpolated entry points. The Ageton Tables (H.O.211) have proved
    > popular and consist of 49 pages of log secants and log cosecants at 0.5
    > minute intervals. The solution is accomplished in ten steps, with an
    > extra four steps if an azimuth solution is required. Because the DR
    > position is used, the intercepts are usually short, whereas using a
    > chosen position the intercepts are often long and it may not be known
    > until after plotting that a mistake has been made in the sight reduction
    > procedure.
    >
    > In 1979, P F Pfab of the Honorable Cross-Staff Society of Sweden
    > published his so-called PET Tables (reviewed in the US and British
    > /Journals of Navigation/) which were based on the cosine-haversine
    > formula, as adapted by  Radler de Aquino, but only after a detailed
    > investigation of tabular methods had been made. His analysis took into
    > account such things as accuracy, rules, book openings, DR versus assumed
    > position etc. The PET Tables have been modified and adopted here. They
    > are extremely simple to use."
    >
    > In this way does the author of The Complete On-Board Celestial Navigator
    > explain the reasoning behind, and the history of, the sight reduction
    > tables contained in his book. They consist there of 20 quarto pages and
    > are tabulated for 1 minute of arc intervals, adequate for the purpose of
    > the book (practical celestial navigation from a small craft).
    >
    > They would seem to constitute a useful alternative to the HO family.
    >
    > Relatively recently George Bennett has revisited and expanded these
    > sight reduction tables, with the aim of making them suitable for use
    > with the increased requirement for precision associated with lunar
    > distances. Two versions were devised: Six figure tables (in place of the
    > original 5 digits) in half minute intervals, and then six figure tables
    > with minute intervals.
    >
    > The latter are freely available on the author's site:
    > http://gbennett.customer.netspace.net.au/
    > under:
    > *New Sight Reduction Tables
    >
    > *I find them (the six figure tables) a little more fussy to use,
    > although not much different to the original tables in the book. A table
    > of Proportional Parts and a Sum Search Values is included to assist
    > interpolation. They appear to address the lack of precision of the
    > original tables.
    >
    > They consist of (or rather, can be printed out as) 20 A4 pages, which I
    > have slid into plastic sleeves between more substantial plastic covers.
    > Hardly one volume, let alone multiple volumes that need replacing at
    > regular intervals ...
    >
    > The quotes above come from an explanatory note offered with the tables,
    > which includes a worked example.
    > *
    > *
    >
    >
    >
    > >
    
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