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    Re: Slide Rule Azimuth
    From: Bill Noyce
    Date: 2009 Jun 1, 14:38 -0400

    On Sat, May 30, 2009 at 3:16 PM, George Huxtable  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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