A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Modris Fersters
Date: 2021 Jun 12, 07:34 -0700
I would like to offer an experiment with metal sexant. I am interested in such a problem: is your metal sextant affected by the tilt angle of the sextant. This problem is not actual for usual celestial observations where the sextant is positioned verticaly. But while taking lunar distances sextant can be tilted in any angle. I own only one sextant, therefore I have no opportunity to check other sextants. But I am sure that many NavList members own metal sextants. And an idea came to my mind: maybe you can help me by testing your instruments and by posting your results.
This experiment will not take much time. It will be the usual finding of Index Correction by the Sun. But unusual will be the position of the sextant.
Just do following steps:
1) do the usual adjustments and reduce index error to 1…2 minutes of arc ;
2)choose filters for index miror and horizon miror (or use one filter on telescope). Do not change filters during experiment!
3) tilt your sextant horizontaly with its face upwards (see picture).
4)fix index arm at position around -1 degree (off arc). Then turn micrometer only in one direction (in the direction of increasing angle). Make direct and reflected Sun images in contact. Read down micrometer. Important: for off arc angles (below 0 degrees) write down micrometer reading as it is. Dont recalculate the angle as negative value. If micrometer shows 32 minutes, then write 32 (not 60-32=28).
5) continue to turn micrometer in the same direction and again make direct and reflected images of the Sun in contact. Read the micrometer.
6) Repeat this procedur at least 5 times.
7) then do the same procedure, but with sextant facing downward.
As a result you will have 5 pairs of readings when sextant is with it face upward, and 5 pairs of readings when sextant face is oriented downward (two small tables; see picture).
From these results I will calculate two average index corrections dependent on sextant position.
My sextant SNO-M shows constant difference between these two average index corections. I am very curious to find whether your sextants have the same peculiarity.
I will be very grateful to receive your results. And please write your sextant model name and the date of observations. If I receive any results I will post the summary later.