# NavList:

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

**Re: Haversine formulae: a correction**

**From:**George Huxtable

**Date:**2001 Nov 18, 11:07 AM

In the mailing I sent today in the thread labelled "haversine formulae". I have discovered an error. It may not be the only one... In the section dealing with the use of the arc.tan function for obtaining azimuth, there is the following paragraph- "It's even simpler if your calculator or computer has rectangular-polar conversion (sometimes labelled the atn2 function). In that case you put into the y-coordinate the value (- sin hour angle) or (sin (L1 - L2)), and into x goes cos hour angle*sin lat - cos lat*tan dec (or its equivalent). The result emerges as a radius (which you ignore) and an angle, which is the angle you require. All you have to do (it you're bothered to) is to add 360 if it is negative.". The error is in the reference to (sin (L1 - L2)), which should read instead (sin (Lo1 - Lo2)). So that paragraph should be altered to- "It's even simpler if your calculator or computer has rectangular-polar conversion (sometimes labelled the atn2 function). In that case you put into the y-coordinate the value (- sin hour angle) or (sin (Lo1 - Lo2)), and into x goes cos hour angle*sin lat - cos lat*tan dec (or its equivalent). The result emerges as a radius (which you ignore) and an angle, which is the angle you require. All you have to do (if you're bothered to) is to add 360 if it is negative." Sorry about that. George. ------------------------------ george@huxtable.u-net.com George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. Tel. 01865 820222 or (int.) +44 1865 820222. ------------------------------