A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Roger W. Sinnott
Date: 2017 Dec 20, 19:29 -0500
There is one area of sextant navigation where the Earth's ellipsoidal shape has an effect: lunar distances. The "clearing" process is supposed to move the measured Moon-star angle from what it is at the Earth's surface to what it would be at its center. But all the standard corrections just move it to where the observer's vertical line, extended downward, intersects the Earth's rotational axis -- NOT the geocenter.
Chauvenet talks about this in his Manual of Spherical and Practical Astronomy (Vol. 1, page 397, of the 1891 edition reprinted by Dover). In his fully worked example for an observation made at latitude 35 N, the final adjustment to the geocenter alters the lunar distance by another 4.7 arcseconds. It appears this correction could grow two or three times larger for sights made in high latitudes.