A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2015 Nov 3, 12:28 -0500
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.
The difficulty is that the time of immersion is now affected by where along the edge of the moon's orb is the star occluded.
Two stars, aligned perpendicular to the moon's orbit, with precisely the same RA, will have different times of immersion, as long as they are spatially separated along that perpendicular.
I will obviously defer to our resident experts, but an occultation cannot be treated as a lunar distance of zero, unless we also consider where along the orb the occultation occurs. The time of the occultation would require adjustment as a function of where along the moon's orb.
I would suggest this is why Frank also implores us to keep the star within so many degrees of its path and to not, willy nilly, use any star we may arbitrarily choose.
I had to visualise the process and may have got it wrong but figured that both immersion and emmersion would be Near. It's a bit confusing as with a distance of zero it could be either way. This seems to agree with the reednavigation site answer however.