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From: George Huxtable (no email)
Date: Sat Sep 06 2003 - 06:15:30 EDT
Peter Fogg says, in respect of Bennett's "Celestial navigator"-
>As for the Azimuth Tables, there are instructions in the book to be followed
>to deal with the potential problems when the bearing is close to 90 or 270
>degrees.
Those that own the book will realise that Peter is being very misleading
here. There are indeed instructions, which explain how to resolve the
AMBIGUITY near to 90 or 270 degrees. This is because the tables derive
azimuth from its sine, and can't tell the difference between, say, azimuths
of 70 degrees and 110 degrees, the sines of those angles being exactly the
same. It's a weakness of that method of deriving azimuth, that such a test
is necessary. That aspect I have NOT complained about, because the author
describes a procedure to resolve it. But it's an extra complication, which
wouldn't apply if an azimuth was derived, unambiguously, from its tan.
The problem with these azimuth tables, as Peter must by now be well aware,
is not in their ambiguity, but in their inaccuracy, and that inaccuracy is
exactly what I have complained about. And there is not one word, not even a
hint, in the book that major errors in azimuth can occur, for certain
observations in a VERY wide swathe around East or West.
Peter Fogg says-
"A common sense approach, remembering that the whole idea is to
>keep things simple on-board, is to give the tables a miss and use another
>method (provided) in these rare enough cases."
That's fair enough, but how does a navigator decide to do just that, with
no advice given in the book to help him? Come to that, how does Peter
himself decide when to accept what the Bennett azimuth tables tell him, and
when to give them a miss and go to another method? He refers to "rare
enough" cases, but the examples I have given below (azimuth errors of 13 to
15 degrees, and 6 degrees the other way) are for actual azimuths all of 15
degrees from the East-West line.
Remember, Peter has said-
>...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.
So, with that opinion in mind, it seems a fair question to ask: on what
basis, and for which azimuths, does Peter decide to follow his own advice,
give the azimuth tables a miss, and use another method? He can tell us, in
the same mailing as he responds to my request, repeated below (once again).
================
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.
==================
Am I being "intimidating" in this posting above? Well, maybe; but where
Peter Fogg is concerned, perhaps I now have the right, and perhaps even the
duty, to be so.
George.
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contact George Huxtable by email at , 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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