A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Nov 21, 19:40 -0800
Ed Popko, you wrote:
" Thus, for D1, I subtract my off-arc reading from 60? "
Yes. There are two kinds of people in the world: people who do this in their heads without really thinking about it, and people who explicitly read the number from the micrometer and then subtract from 60 in a separate step. Neither side is "better" than the other. Just different ways of doing it. The "subtract from 60" methodology is perhaps safer when doing Sun-limb index correction tests because it's an easy mistake to make. If the IE is nearly zero, then both the on-arc and off-arc values will be close to 32 minutes of arc. The sextant reading is 32' off when the nominal value displayed by the micrometer is 28' in an off-arc case.