A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2017 Dec 8, 18:25 -0800
No, when using a bubble sextant for a moon or sun sight you use the center of the object in the center of the bubble. Then you ignore the sun correction table and use the refraction correction for a star which eliminates the semi-diameter part of the sun correction For the moon you use the star refraction table and the parallax in altitude tables. The moon correction table in the current Nautical Almanac is the most complex and confusing way to make the necessary corrections for an observation of the moon. This table combines refraction, parallax in altitude and semi-diameter into one, way overly complex table. Much better to do each correction separately. So, use the star refraction table for, DUH, refraction. For marine sextant shots apply semi-diameter which is listed at teh bottom of each moon column but with a bubble sextant you do NOT apply semi-diameter since you are measuring the altitude of the center of the moon. Now for the parallax in altitude correction, the easiest way is to use the Air Almanac which lists that correction on each page next to the moon data and is based on the horizontal parallax of the moon on that day. And yoiu can download the AA for free and it has other advantages. If you don't want to use the Air Almanac then use the Parallax in Altitude table that used to be in the Nautical Almanac which I am attaching. You'll find out that by applying corrections individually you will understand the process much better.