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    Meridian passage
    From: Andrés Ruiz
    Date: 2009 Jun 9, 12:21 +0200

    When I was a child, my first contact with celestial navigation was getting the latitude at noon. But my position was not determined, and I try to obtain the longitude also at noon, I have very good memories of that time.

    Apart of the restrictions of the method, well known today, there is no doubt about its great educational value.

    Obtaining the position at meridian passage has for me the savour of the old days, and a special affection because this method introduced me into the amazing world of the navigation.


    After this introduction I wish to share a program that I have written to calculate the fix at noon.

    The method is the following:

    1. Least squares fitting - The curve equation is: H = a0+a1*t+a2*t2

      The maximum gives:

      t max =          -a1/(2a2)

      H max =         a0-a2t2


    2. TimeOfLAN = tHmax + delta_t

      POSITION FROM OBSERVATION OF A SINGLE BODY James N. Wilson Vol. 32, No. 1, 1985 


    3. Position( TimeOfLAN ))

      Dec( TimeOfLAN ))

      GHA( TimeOfLAN ))

      Ho = yLS( TimeOfLAN )


      ( Dec, Ho, culmination ) -> B_LAN

      GHA -> L_LAN


    I have tested the results under simulated data, and the results are OK.

    The attached pdf is a real example. The source of error, compared with the GPS position, is due to the fact that the sextant observations were made under bad conditions, and an autopilot consisting in a rope attached to the tiller!!!

    I cannot do an observation on the beach near my home, because at LAN the Sun is at the south and the horizon is towards the north, maybe I try to use an artificial horizon.


    For those who wish to play with the program, I can send an add on for my software, available at my Web site (900KB).

    The input files can be generated semi-automatically or manually. I shall have to write some instructions…




    Andrés Ruiz

    Navigational Algorithms



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