A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Lars Bergman
Date: 2023 May 28, 05:26 -0700
Frank wrote "Something I couldn't figure out: at the bottom of the calibration column, it says "R=22° 2E=69°". Any thoughts? Maybe a standardized eccentricity model??".
Yes, it seems to be an eccentricity correction, where 2·R is the graduation on the arc closest to the arm's axis. 2E is actually 69'', where E is eccentricity/radius. Then the correction to be applied to a read value A is 2·E·(sin R - sin(R - A/2)).
The parameters R and E are determined from two angle measurements that are compared with their true values, then the table for every ten degree is computed. The value for 70° is faulty.
Assuming an 8 inch radius sextant, E = 69''/2 = 1.7·10-4 radians gives an eccentricity of 0.03 mm.