# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Slide Rule Azimuth
From: Gary LaPook
Date: 2009 Jun 02, 01:06 -0700

I am attaching the entire Table 25 (4 pages) for those who don't like the Rust Diagram.

gl

Bill Noyce wrote:
```On Sat, May 30, 2009 at 3:16 PM, George Huxtable <george{at}hux.me.uk> wrote:
```
```Gary has pointed out the ambiguity, for azimuths near East and West, which
is the serious drawback to this method of working (more serious, in its way,
that the poor precision at these angles, which Greg did recognise). But he
pointed it out, only to dismiss it, as "not a problem in real life". I
suggest he should think again.
```
```
This same azimuth formula is used in Ageton's method, where it's
especially convenient because COS declination and SIN meridian angle
(actually, the logs of their inverses) were already used in computing
the altitude.  In this application, the ambiguity as to whether the
azimuth is just north or just south of due east or west is easy to
resolve.  In Ageton's method, like the Bygrave method, one
intermediate result is the latitude at which a perpendicular from the
observed body hits the observer's meridian.  If this latitude is north
of the observer, then the body's azimuth is to the north; if south,
then south.  Of course, the problem of precision of the arc-sin of
values near 1.0 still remains, but for short intercepts that's not too
serious either.

-- Bill

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```

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