# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Lunar Occultation continued...**

**From:**Geoffrey Kolbe

**Date:**2015 Nov 4, 09:50 +0000

On Tue, Nov 3, 2015 at 5:37 PM, Brad Morris <NoReply_Morris@fer3.com> wrote:

To the Lunatics

Herein lies a difficulty.

Assume a star directly in the path of the moon, such that the star (at immersion) bisects the northern and southern orb. The star is then immersed at the semi diameter (radius) of the moon.

Now assume that a star, upon immersion, is occluded by the Northern tip of the orb. That is, the moon occludes the star at +85° of its latitude. The immersion does NOT occur at the semi diameter of the moon along its path. Rather, it occurs at cos(moon's latitude of occultation)*SD, in this case cos(+85°)*SD.

When I calculate lunar distances, I calculate the angle subtended by the star (altitude corrected for refraction) to the centre of the Moon ( altitude corrected for parallax and refraction). To this angle is then added (or subtracted) the SD to derive the observed "lunar distance", the angle that would be observed with a sextant. It should not matter where the star is relative to the Moon.

However, at the moment of immersion or emersion, I would expect the "lunar distance" to be either zero (near) or 30.2" (far, assuming an average SD of 16.1"). Bob Crawley calculated lunar distances which were far from these values which leads me to suspect that the observatory calculated times of occultation are not correct. (Either that or Frank's program is not behaving as I would expect - which surely cannot be the case...)

Geoffrey Kolbe