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    Re: Sight Reduction Formula
    From: George Huxtable
    Date: 2003 Oct 20, 00:37 +0100

    I said
    >> example above). Rectangular to polar conversions are conventionally
    >> intended to produce an angle which increases anticlockwise from the x-axis,
    >> whereas we require an angle that increases clockwise from the y axis, so a
    >> bit of thoughtful tinkering may be called for.
     And Paul Hirose responded-
    >One solution is to lie to the machine. Where it wants x, give it y,
    >and vice versa. Leave the signs unchanged. That will generate azimuths
    >increasing clockwise from zero at north.
    Reply from George-
    Yes, that's exactly what was done in the examples given, as copied below..
    >Many Casio calculators provide the POL (polar) function, with two inputs,
    >and when you apply
    >X = POL (( Cos Lat * Tan Dec - Sin Lat * Cos HA) , (- Sin HA))
    >then what appears in the Y variable is a quantity between -180 and +180,
    >unambiguously >defining the azimuth. (Negative azimuths can have 360 added
    >to put them into familiar >form.)
    The Casio POL function asks for the Cartesian X-coordinate first, then the
    Y coordinate, but in the example above they are provided in the opposite
    order, just as Paul suggests. The function expects the X-coordinate to be
    positive going left, and the Y-coordinate to be positive going upward.
    Because hour angle increases positively going to the right, however, the
    Sin HA term has to be given a minus sign.
    If anyone else still owns a working HP-21 that isn't in a museum, I should
    warn that the description of Polar/Rectangular coordinate conversion in its
    handbook (or at least in my 1975 version) is in error and doesn't
    correspond to the way the machine actually behaves.
    contact George Huxtable by email at george---.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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