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    Re: Accuracy of sextant observations at sea
    From: Frank Reed
    Date: 2010 Nov 25, 17:44 -0800

    Alan, you wrote:
    "I have obtained two body shots of Sun and Moon, that when reduced and plotted produced fixes within, sometimes well within 5 NM of GPS coordinates. I have also done three body shots, that when reduced and plotted produced fixes that were also within, sometimes well within, 5 NM of GPS coordinates."

    You can definitely improve on this, even using a plastic sextant with some careful steps. Since, if I remember correctly, you have a metal sextant, it should be relatively easy to get results within 1 NM most of the time. So where does the trouble lie? In the observations? In the adjustment of the sextant? In the calculations? In some aspect of using the tables? In order to figure this out, the first thing we can do is separate the observational steps from the calculation steps. So, do you have any raw observations that you recorded? Specifically, if you can give us the exact times, the observed altitudes, and the exact location you were shooting from (within a few hundred feet), then we can independently try to clear those observations and see how they compare with predictions. If they're out by more than a few minutes of arc, then we know that the problem is in the adjustment of the sextant or some aspect of the observation process. If they're within a minute of arc, then the problem has to be on the calculation side. You don't need to post all the work. Just the basic observation data for two or three sights.

    Uh-oh. Turkey time! More later... :)

    -FER
    PS: for the non-Americans who by chance have not heard, today is Thanksgiving in the US. Turkey is the traditional feast. Happy Thanksgiving to everyone. :)


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