# 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 Oct 25, 09:36 -0700

On 2016-10-20 12:43, Bob Goethe wrote: > > Scale ST, which currently deals in values from tangent values from 0.01 to 0.1, would - if extended - deal in values of 0.0096 to 0.105. > Not as clean for the hypothetical-slide-rule-novice for whom I am trying to design the rule and write the manual. Did you notice how it's done on the Deci-Lon? The trig scale labels ignore the extensions. They show only the nominal scale ranges. E.g., the SRT scale is marked "0.01 to 0.10," though its true range is .00960 to .1047. Also, scale extensions need not go out to the next numbered graduation as on the Deci-Lon. They could extend only to the next convenient graduation. A danger there, however, that the additional graduation could be perceived as an index. That won't happen on the Deci-Lon since the extensions are so obvious. In the Mark 1 user manual, there's an arithmetic error in Step 3 at the top of page 49. The minutes in the result are wrong. However, the next line uses the correct value. There's a mistake or omission in the Bygrave rules, or I'm doing it wrong. Please check this southern hemisphere example: L = S 27, d = S 21, LHA = 79, H = 79 approx correct az, el = 255, 19 1. W = atan( tan d / cos H ) = 63.6 2. X = (90° - L) ± W – if contrary name if meridian angle > 90, X = 90° - L - W Take absolute value. X = 126.6 3. If X < 90°, then Y = X If X > 90°, then Y = 180 - X = 53.4 4. Az = atan(cos W / cos Y * tan H) = 75.4 Use N if Dest. is N of Your DR Use E if Dest. is E of Your DR Az = N75.4W = 284.6 5. Hc = atan(cos Az * tan Y) = 18.7 That's the correct altitude, but azimuth is wrong. It should be about 15° south of west, but my value is north of west by the same angle. What do you get? The Bygrave simulation in my Monte Carlo accuracy measurement software works correctly. But its computations are arranged differently from yours. For one thing, azimuth angle is always measured from the elevated pole (south, if the assumed latitude is south of the equator) and increases east or west up to 180°. http://fer3.com/arc/m2.aspx/Bygrave-formula-accuracy-10-inch-slide-rule-Hirose-jul-2009-g8985