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    Re: Index error by a star-star distance
    From: Gary LaPook
    Date: 2008 Sep 22, 10:52 -0700
    O.........K.............   I guess I don't get it. Why not just sight on one star which should have a zero altitude difference between the two images of the same star. Where is the advantage in using two stars?  In addition you would not be determining "index error" since  the measured error would be the total of index error and "arc error" and you could not determine the contributions of each of these components. Star to star distance can be used to check for arc error but only after index error has been determined by some other method since this value must be subtracted from the measured star to star distance before comparing the measured distance to the computed distance thereby determining  the arc error at that point of the arc.


    Andres Ruiz wrote:

    I am writing an article about “Index error by a star-star distance”.

    Unfortunately there is not much information about this subject, or I can’t found it. Anybody has any source of information; old books, papers, … ?


    The algorithm is a simplification of the Lunar Distance method, taking into account that there is no semidiameter or parallax.

    The calculated star-star distance is compared with the observed one to obtain the sextant error, that is the index error plus the instrumental error inherent to the sextant.


    For those who want to shoot stars and obtain his IE, the program is available via e-mail under request.

    Feedback is welcoming. My two sextants have a have a negligible error; I can’t check the results for a real significant IE.

    I think the hardest part is to take the distance, is no easy to do this with two stars, and special skills and background is needed.

    A series of up to ten observations are needed in order to obtain the mean UT and Hs.





    The Sextant and Its Applications. William Simms, (free at google books)




    Andrés Ruiz



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