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This thread is discussing "The complete on-board celestial navigator" by
George G Bennett, 2003-2007 edition.

As one of the "nit-picking academic deskbound navigators" referred to by
Peter Fogg, I said-

"Bennett provides a simple way to calculate the azimuth using a lookup table"

To which Peter replied-

"WRONG !"

What's wrong with that statement, I ask? It's a simple statement of fact.
Incomplete it may be, in that it doesn't list all the possible options for
obtaining azimuth, as he does, copied below. But wrong? Please explain,
Peter.

He goes on to say-

>4 methods are provided. In order of ascending accuracy they are:
>
>1 Calculated azimuth using the prediction and indentification tables
>2 Corrected compass bearing. Accurate enough already for plotting purposes.
>3 Azimuth tables - the ones George Huxtable doesn't like.
>4 Weir diagrams, the introduction to which reads:
>'The accuracy of this method is superior to that of (Azimuth tables)
>
>The user is urged to become familiar with all methods and to use whichever
>seems appropriate.

Well, let's go through those four methods.

1. The prediction and identification tables, while useful for their
intended purpose, would be a bit of a joke for determining azimuth. Bennett
himself states "The accuracy of azimuths found by this method may not be
very high because we have chosen a latitude and LHA to the nearest 10
degrees in order to enter the tables." It's not suggested anywhere that the
user should interpolate between adjacent pages and adjacent columns, but if
he did, he might then obtain a sufficiently accurate value for azimuth.

2. Measure-it-yourself with a compass. A perfectly good standby, though of
limited accuracy in a small boat in rough weather. It's most useful as a
cross-check. But when all the data for calculation of azimuth is at hand,
why not calculate?

3. Azimuth tables, which I have criticised.

4. The Weir diagram, which I referred to, and didn't criticise.

Bennett advises that the user becomes familiar with all four methods and
uses "whichever seems appropriate". Well, on what basis does the user make
that choice? He is advised that method 3 will find azimuths "with an
accuracy of one or two degrees", which would indeed be sufficiently
accurate, so why should he choose any other? It's that misleading advice
that I object to, and which, as I understand it, Bennett has agreed to
change. If method 3 included advice to interpolate (instead of not to
interpolate) when using the azimuth table, those errors would be greatly
reduced.

Peter goes on-

>As someone who has used this book extensively I can say
>that I have never had a problem with the azimuth tables. I can check the
>tables' result with that given by my nav. calculator, accurate to a tenth of
>a degree, and if it is out by more than a degree or so I invariably find the
>fault has been mine.

I do not dispute that there are situations in which the calculated azimuth
might well be acceptably accurate, and said so in my last mailing.

However, I ask Peter if he has checked out the two examples that I
provided, and compared the azimuth result from Bennett's table with that
given by his nav. calculator, and if not, to do so, please. And I ask him
to REPORT HIS RESULTS BACK TO US. If he finds a discrepancy between the
Bennett azimuth and his own calculator (which he will), I hope he will then
tell us where he thinks the fault lies.

Example 1. dec = 55deg 29', LHA = 54deg 31', alt = 61deg 31'.

Example 2. dec = 55deg 31', LHA = 54deg 29', alt = 61deg 29' .

I chose those examples to show up where the faults in the method are near
their worst, but have no reason to think that they are in any way unique.
Any observation taken in a generally East or West direction (and my
examples are about 15 degrees away from the true East-West line) will be
susceptible to such inaccuracies.

>The thing to remember is that 'The Complete On-Board Celestial Navigator'
>has been designed to be used by navigators of small boats as a practical
>tool, rather than by nit-picking academic deskbound navigators. Who are
>welcome, of course, to play with it, and find theoretical faults. The author
>has a background as a teacher of surveying, so knows all about accuracy.
>This book attempts to make celestial navigation more easily feasible
>on-board. Don't forget that by the time you work out your fix the boat has
>moved on, ultimate accuracy in knowing where you were back then is
>irrelevant, since the next thing to do is advance your DR to give yourself a
>rough idea of what you really want to know; where you are now!

If Peter is prepared to assume that azimuths as calculated from tables in
the book are correct to "within a degree or two" (when he knows now that
errors of 15 degrees are possible), then if that gets him into trouble, he
has only himself to blame for his complacency. Other potential users,
however, might be more discerning, and might like to be made aware of this
propensity for major errors in the azimuth tables. These are not
"theoretical faults"; they are, unfortunately, all too real.

Apart from this matter of azimuths, other aspects of Bennett's book are
indeed laudable, and have not been criticised by me.

George.

================================================================
contact George Huxtable by email at george@huxtable.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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