# NavList:

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

**Re: Cryptic Slide Rule Formulae**

**From:**Greg Rudzinski

**Date:**2016 Jan 21, 06:16 -0800

Jim,

(6.) is the prime vertical formula used with the azimuth formula (2.) when the azimuth is near 90° or 270°.

(7.) is the amplitude formula used to determine the azimuth of the Sun when the lower limb is 2/3 of a diameter above the visible horizon.

Greg Rudzinski

From:Jim RivesDate:2016 Jan 20, 12:58 -0800Hello,

My father-in-law, who sailed for years out of Huntington, LI, passed away a year ago. In one of his desk drawers I found a 10" Sterlin Slide Rule, No. 584. It is white plastic, not very eleborate. (however, still beyond my slide rule skills!) He had scratched the following 7 formulae on it. I am just wondering if anyone would recognize the text or method he was using this rule for:

In no particular order:

1. sin h = sin L sin d + cos L x cos d cos t

2. sin z = cos d sin t sec h

3. cos co-d = cos co-h cos co-L + sin co-h sin co-L cos z

4. sin t = (sin z sin co-h)/sin co-d

5. cos t = tan d cot L

6. sin h = sin d csc L

7 sin A = sin d sec L