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    Re: Accuracy of sextant observations at sea
    From: Antoine Couëtte
    Date: 2010 Nov 25, 11:59 -0800

    Alan,

    The "weakest point" of Celnav is certainly the quality of your horizon + the uncertainties of your actual DIP correction. After that, the biggest errors would generally be the Observational ones (quality of the instrument, and regular training of the Observer).

    Everything else - except in the case of "exotic" refractions - is generally accurately "known" within 0.1' including refraction above 15°.

    Just speaking for myself, I always compute my sights without the help of "auxiliary points" which will sometimes let you derive intercepts significantly "worse" than what they actually are. Whenever standing on the beach, 50 yards from home actually, and with a clear and well defined horizon, and with a good sextant - which is certainly a most favorable environment - I am not happy when getting intercepts exceeding 1 NM on single shots on any of the SUN, MOON, STARS and PLANETS.

    If you are getting 5NM under these same conditions, something certainly can be improved.

    On the other hand, if standing on a small nutshell sailboat, I might as well be quite happy to log intercepts up to 5NM on single observations.

    I also have taken the habit - some 30 years ago and whenever achievable since that time - to take 5 successive shots (spaced about 1 minute apart) and to use their average value both in times and CORRECTED heights. AVERAGING definitely narrows errors down.

    I think that after about 100 shots - and a few week practice - you can get good and consistently reliable results. A WONDERFUL asset nowadays is given by GPS which immediateley enables you to "grade" your own shots.

    As an other example, I personnaly consider the results achieved by Jeremy as quite excellent. His Observations Standard Deviations seem to be consitently below 0.5 NM.

    Both you and I will certainly read with interest what others have to say here from their own experience, whether real world or teaching, or both.


    Best Regards


    Antoine


    Antoine M. Couëtte

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