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    Re: Accuracy of sextant observations at sea
    From: Greg Rudzinski
    Date: 2010 Nov 26, 16:27 -0800

    Alan,

    You need to include two important pieces of info- Height of Eye and Index Error.

    I went ahead and worked out the observations for a height of eye of seven feet and zero index error using the GPS position provided.

    Altair (Dec N 8° 54.0' SHA 62° 10.1' GHA Aries 16° 53.6')
    Intercept 1.5' away
    Azimuth 184.5°

    Formalhaut (Dec S 29° 33.8' SHA 15° 25.8' GHA Aries 23° 05.3')
    Intercept 5.0' away
    Azimuth 145.6°

    Jupiter (Dec S 3° 34.0' GHA 20° 14.6')
    Intercept 3.0' away
    Azimuth 113.4°

    Since all your intercepts are away it looks as if you may have about 3 minutes of index error off the arc which means that 3' can be added to each observation to improve the plot. You will also notice that this puts your fix outside the original triangle. When a round of observations differ in azimuth by less than 180° then it is possible that a constant error will place the fix outside the triangle (or cocked hat).

    Greg Rudzinski


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