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Hello, David

Yes, your basic assumptions about Frank’s lunar analyser are correct. You can enter sextant reading and compare cleared lunar distance from known position with actual geocentric distance and thus get the error of your observation. I am not sure you found that there are some additional entries under “DETAILS” in this online program. There you can enter index correction, temperature and preasure.

The difference of the index correction results from Star and the Sun observations you mentioned (1,5’and 2,6’) is quite large. The sources of these errors may be several. I will try to name some of them:

1) When you try to put reflected view of Star on top the direct one, it is quite hard to find the excact moment when these two images are accurately centered becauce the images are not perfect dots. The best solution, in my opinion, is to use Frank’s suggestion: to make little side error ( about 1’-2’) to your sextant and then it is much easier to find when both images are aligned horizontaly (especially if you have scope with wires).

2) Always turn micrometer drum in one direction (for example clockwise) for final adjustement. If this is ignored there may be quite large backlash errors ( 0,5’ and more). Of course, if you have an excelent instrument with very small backlash value this can be ignored (but even a very good instrument can produce about 0,1' backlash; this is not an issue for vernier sextants).  

3) The micrometer can produce additional errors. When you finde Index Correction from Star micrometer reading is about 0’. But when you measure IC by Sun your readings will be around 30’. (limb to limb observations). If your sextant’s micrometer error has large differencies at these values you will find different IC. I have one sextant with +0,5'/-0,2' errors at the range from 0'-59'(one micrometer full turn; not dependent on index arm position).

4) Sextant shades can produce additional errors. When you use Star method typically you will not use shades, but when you use the Sun method for IC then you will deffinetely use shade. Usuallly one shade does not produce more error than about 0,1’, but some shade combinations can accumulate 0,3’ error easily.

Modris Fersters

(from Latvia)