# NavList:

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

**Re: Traditional navigation by slide rule**

**From:**Paul Hirose

**Date:**2016 Nov 12, 21:55 -0800

On 2016-11-10 13:33, Bob Goethe wrote: > Attached File: http://fer3.com/arc/imgx/_Mark-2-Slide-Rule-User-Manual.pdf On the Bygrave form on page 7 I find the format of the formulas unnatural, though their meaning is perfectly clear. Isn't it usual for a variable which receives a value to be on the left? E.g., x=2 not 2=x. That's the way it works in all programming languages I've seen. But on the Bygrave form the assignment goes from left to right. On the same page, there's a problem with the rule that determines whether azimuth angle is measured from the north or south. Occasionally it's wrong, or maybe I don't understand it. Below are three examples. In each case Z itself is correct, but since declination is south of the latitude (but both are north of the equator), I put a "south" prefix on Z. That results in a wrong azimuth. LHA = 84 H = 84 L = 30 d = 29 W = arctan(tan d / cos H) = 79.3 X = 90 - L + W = 139.2 Y = 180 - X = 40.7 Z = arctan(cos W / cos Y * tan H) = S 66.7 W Hc = arctan(cos Z * tan Y) = 18.8 Correct Z = N 66.7 W, Hc = 18.8. LHA = 307 H = 53 L = 45 d = 41 W = arctan(tan d / cos H) = 55.3 X = 90 - L + W = 100.3 Y = 180 - X = 79.7 Z = arctan(cos W / cos Y * tan H) = S 76.7 E Hc = arctan(cos Z * tan Y) = 51.7 Correct Z = N 76.7 E, Hc = 51.7. LHA = 286 H = 74 L = 41 d = 30 W = arctan(tan d / cos H) = 64.5 X = 90 - L + W = 113.5 Y = 180 - X = 66.5 Z = arctan(cos W / cos Y * tan H) = S 75.1 E Zn = 104.9 Hc = arctan(cos Z * tan Y) = 30.5 Correct Z = N 75.1 E, Hc = 30.5.