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

**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. ================================================================