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Robin

Thankyou for sharing this information.  I have to admit I’d never really bothered with finer points you mention, because I never felt the accuracy of ancient aircraft sextants which hadn’t been factory serviced for decades warranted it.  However, it’s becoming clear that properly calibrated and chocked up in a static position such sextants with a pendulous reference including a horizon line can produce results worthy of calculating time, refraction, instrument error, semi diameter and change in declination to decimal point accuracy.  A clear sky also helps. DaveP