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    Re: Star - Star Observations
    From: Brad Morris
    Date: 2010 Mar 12, 10:16 -0500

    Andres, you wrote:

    “ I suppose you first calculate Hc = f( B, L, Dec, GHA )”


    Yes!  And then you wrote:

    Refraction is a function of the apparent altitude: R = f(Ha)


    True.   Finally, you state that

    The solution must be obtained with precision by an iterative process.


    To help me to understand this iterative process, I must begin with the original computation

    Hc = f(B,L,Dec, GHA)

    We calculate

    R0 =  f(Hc)

    Ha0 = Hc + R0

    We can then calculate refraction as a function of apparent altitude:

                R1= f(Ha0)

    For nominal atmospheric conditions, R0 cannot not equal R1, as Ha0 does not equal Hc.  Excellent!





                            R(i+1) = f(Hai)

                            Ha(i+1) = Hai + R(i+1)

                                    If ( (Ha(i+1) - R(i+1)) = Hc) then

                                        Exit LOOP


                                        i = i+1

                                        LOOP again!

                END LOOP


    Is this what you mean?


    Apologies to all who receive email in plain text form.  I recognize that the subscripts are going to look rather odd.



    Best Regards




    From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] On Behalf Of Andres Ruiz
    Sent: Friday, March 12, 2010 8:29 AM
    To: NavList@fer3.com
    Subject: [NavList] Re: Star - Star Observations


    Brad, I suppose you first calculate Hc = f( B, L, Dec, GHA )

    Refraction is a function of the apparent altitude: R = f(Ha)

    And Ha = Hc + R = Hc + f(Ha)

    The solution must be obtained with precision by an iterative process.


    I have repeated the calculations with more precise ephemerides, and the solution is the same.

    Note the variation of R with Ha:


    Ha º













    Andrés Ruiz

    Navigational Algorithms



    De: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] En nombre de Brad Morris
    Enviado el: jueves, 11 de marzo de 2010 21:24
    Para: NavList@fer3.com
    Asunto: [NavList] Re: Star - Star Observations


    Hi Andres


    Thank you very much for sticking with me on this!  I discovered that I had inverted one of the terms in Young’s equation. 


    For the calculated distance

    Morris              40d 21m 20.92s. 

    Ruiz                 40d 21m 17.96s, a trivial difference between us of 3 seconds.


    For the observed distance, I now get

                Corner Cosines            40d 22m 4.67s

                Young’s Equation        40d 22m 4.70s

    Now there is no difference between my two results and therefore can readily retract my question about which is better. 

    Corner Cosines and Young’s Equations yield the same result.


    Your result for the observed distance is

                Ruiz                             40d 21m 55.26s

    There is a small discrepancy between our results of about 8 seconds. 


    In tracing this down, I believe it to be the altitude correction and most likely the atmospheric conditions

                USNO                          1m 36s  Sirius altitude correction

                                                    1m 36s Alphard altitude correction

                Morris                          1m 35.47s Sirius

                                                    1m 35.79s Alphard

                Ruiz                             1m 33.43 Sirius

                                                    1m 33.74 Alphard


    Not only have I corrected this deficiency in the spreadsheet, I have re-cast the interface to accept GHA Aries, SHA Object 1, SHA Object 2, Declination Object 1 & Declination Object 2.  Of course, these items will now come from the Nautical Almanac, not my Skyscout. 


    Now on to the calibration of the arc of my sextant! 


    Best Regards


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