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    Re: Index Error - Comparison to SD per Nautical Almanac
    From: Modris Fersters
    Date: 2020 Nov 25, 12:39 -0800

    Hi, Jim!

    As I understood, you did following steps:

    1. you turned micrometer untill the lower limb of the reflected image just touched the upper limb of the direct image of the Sun (see picture bellow).

    Then you wrote the sextant reading;

    2. then you turned micrometer while the lower limb of the direct image was in contact with upper limb of the reflected image. And again you wrote the sextant reading.

    3. Then you calculated SD: (a+b)/4; (a= reading on arc; b= 60-reading off arc).

    Example: on arc reading: 0 degrees 31 minutes;

    Off arc reading: 359 degrees 28minutes;


    And this value differed from Almanac  0,2’.


    I have some questions:

    1) Are you sure, that your sextant had no significiant side error?   

    2)Did you take some 3-5 measurments and averaged them to get SD?

    3) What sextant did you use? High quality or cheap one; old or new? What is stated accuracy of your sextant?

    4) Are you sure that your sextant had no backlach? To avoid this error allways turn micrometer drum only in one direction.

    If your method for calculating SD was different, please write. 

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