NavList:

I was addressing the refraction point which you have ignored.
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Your other point: (its a very long time since I did it last: no doubt you will correct me if wrong in method, I've had to think about it).

Calculated star distance: find RAs or SHAs from almanac or tables subtract to get an HA between them. Find co-decs from tables or almanac, then work out star distances (equivalent to zenith distance) with spherical triangle gives true distance.

Measured star distances: Corrected sextant measurements for altitudes gives co-altitudes i.e. zenith distances; use HA from almanac for stars & work out accurate azimuths. New spherical triangle with the two measured zenith distances and azimuth difference gives new included angle, find star distances.

Compare calculated with measured gives index error.

Douglas Denny.
Chcihester. England.
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