A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Greg Rudzinski
Date: 2016 Jan 2, 12:23 -0800
Mark the D scale at 1.96 with a 0.5mm pencil and set slide to S scale index 90° (1) on the mark to start the calculation of value a. Pencil in cos degrees on rule every 10° if not marked. You will need to flip rule over to do the second formula. The a value is saved under the index mark. No intermediate values need to be recorded. Pencil mark the C scale of the slide at 2.67 which makes for quicker sets on follow-up ex-meridian calculations.
The Frederick Post 1460 10" rule has cos marked and doesn't need flipping to complete calculation making it ideal for ex-meridian calculations. These are very good value Japanese slide rules made of bamboo. The pocket 6" version may not have the cos marked. I'll have to check on that.
P.S. Just did the example ex-meridian calculation in 52 seconds time using the marked 1460 rule which equals the pocket calculator.
From: David Pike
Date: 2016 Jan 1, 13:21 -0800