NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Martin Caminos
Date: 2023 Dec 23, 14:08 -0800
Considering the latest discussions about moon culmination, here are the details of my recent experience with a LAN sight but using the Moon instead of the Sun.
The procedure
I did it from my home in the Miami area, about 10 miles inland from the ocean by using an A12 bubble sextant.
I used the bubble sextant, because although I could have used the regular marine sextant for the first sight by looking NE from the beach, I could not have taken the transit sight with the marine sextant for two reasons: It would have been about two hours after nautical twilight (around 20:00 local time), but also because from the Miami coast line I cannot see directly South.
The first sight was taken about two hours before transit time. I took seven altitudes and then calculated the average of the times and the altitudes. All altitudes were higher than the prior ones and kept around the same increase rate so I did not have to discard any of them. After that, I did the regular manual sight reduction by using the 249 table and obtained the line of position and azimuth.
For the transit time, I first calculated the moon local transit time, by starting with the GMT transit time and adding the arc time to my estimated (actual) position. It was very important to correct the arc time with the moon longitude table from the nautical almanac.
After that, I took the transit sight altutude and calculated the latitude with the regular procedure and obtained the transit line of position.
Finally, I plotted both lines of positions on the universal plotting sheet and got a fix that was about 1.5 miles from my real GPS position.
You can see all of these steps in the attached file.
Some final comments
- The exercise had the advantage of being done from a static position and did not have any running fix error.
- Generally speaking, bubble sextants (sometimes called aircraft sextants) only give altitude to the rounded minute (no decimals like a marine one), so the maximum possible precision is one mile.
- Taking sight with a bubble sextant is quite tricky because it must be grasped firmly to avoid having the bubble moving all over. I did not want to use a tripod for this exercise.
In any case, this was a challenging and interesting exercise, and I am quite happy with the final results.
I hope this experience adds value to the discussion.
Martin Caminos
