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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.
    
    George.
    
    ================================================================
    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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