A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2016 Jan 21, 20:23 -0800
You were not able to get a noon sight due to clouds but now you do get your sight, it is early afternoon. You open up your Nautical Almanac and take out the data for the sun, its declination is 13° 21.3' north. Quick! what is the declination in decimal degrees so that you can use your slide rule? And you have to make the conversion to the hundreth of a degree because if you only do it to the tenth of a degree you might end up with a 6 minute error in the final latitude (if you simply truncate.) To get it closer than that you must make the conversion to the hundreth and then either round off (which might still make a 3 minute latitude error) or use that last digit to interpolate between the tic marks on the the slide rule. It is much easier to just do this interpolation between the tic marks if the marks are minutes, place the cursor between the 20' and 25' marks, slightly above the 20' mark about 1/5th the distance to he 25' mark.