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    Re: Equation of time question
    From: Paul Lecheler
    Date: 1995 Aug 29, 08:35 -0500

    Peter Smith said...
    >It's still useful in planning the day's work.  Given EoT and DR longitude,
    >one can quickly estimate the time of Local Apparent Noon for taking a
    >meridian altitude of the Sun.
    >
    
    I think I understand this now.  I looked at the NA for 28 Aug 1995 and found
    the following:
    Meridian Passage:  12:01Z  -  12h EoT:  1:20
    
    If I take my Long (96 38.1) and convert to time I get:   +06:02:32
    
    Therefore, corrected meridian passage time is:
    
            12:01:00Z              Meridian Passage
            06:02:32                Arc to time conversion
                  1:20                12h EoT
            ----------
            18:27:54Z               Corrected Meridian Passage time
    
    This doesn't quite match with NAVIG94 or the Celesticomp computer, however.
    Any ideas why?  I assume becuse the NA does not show the seconds for
    meridian passage.
    
    Am I on the right track?
    
    Thanks
    
    Paul Lecheler
    plecheler{at}tad.eds.com
    Electronic Data Systems Corporation
    Technology Management
    (214) 604-0546  work
    (214) 604-0280  fax
    
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    To: navigation{at}ronin.com
    From: plecheler{at}tad.eds.com (Paul Lecheler)
    Subject: Re: Equation of time question
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    Philip H. Kohl said:
    >
    >I get (using the table for conversion of arc to time in the Nautical Almanac)
    >
    >        96d          6h 24m
    >            38.1m        2m  32s
    >                     ----------
    >                     6h 26m  32s
    >
    
    Oops, I forgot to add the 6d to my calculations. (Insert red face here.)
    
    >I get
    >
    >          12h 00m 00sZ
    >           6h 26m 32s     Arc to time conversion
    >               1m 15s     18h EoT  (interpolating between the 12h value
    >          ____________    on the 28th and the 0h value on the 29th)
    >          18h 27m 47s     GMT Time of Meridian Passage
    >
    
    Why use 12h00m00s instead of the meridian passage time here?
    
    >
    >You are close. The NA does show seconds for meridian passage.
    
    At the bottom of the page in the Commercial edition it just shows 12:01 for
    the meridian passage (as I remember, but then again my memory is what I
    forget with).  Is there another place to look?
    
    
    Paul Lecheler
    plecheler{at}tad.eds.com
    Electronic Data Systems Corporation
    Technology Management
    (214) 604-0546  work
    (214) 604-0280  fax
    
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    Date: Sun, 09 Mar 1997 14:44:06 -0800
    To: navigation{at}ronin.com
    From: Gordon Talge 
    Subject: Equation of Time
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    Status: RO
    
    Thought some of you might be interested in this.
    
    I am reading a book called "Avigation" by Bradley Jones, copyright 1931
    on air navigation.=20
    
    At the time it seems that there was not a special almanac for the air and
    the nautical almanac was used. For the Sun it listed the Declination and
    the Equation of time every 2 hours, with an hourly difference for the day.
    
    The LHA was not used. What was used was called the HA, what we would now
    call the meridian angle. ( The same angle t in H.O. 214 )
    
    Example of a calculation:
    
     Requried the HA for 21h 00m 00s G.C.T.   ( G.C.T. =3D Greenwich Civil Time=
     )
     March 9, 1997                             We would call this GMT, but
                                               before 1925 GMT at 0h was what
                                               we would call 12 Noon )
    
                                               Probably to avoid confusion, the
                                               term GMT was not used since 1925
                                               was just 5 years before.
    
    Long 118degs West.
    
    or  7h 52m 00s   ( Arc to time conversion )
    
    >From the Almanac ( 1930 style ) Eq.T =3D - 10m 27s     =20
                                    Dec  =3D - 4 degs 13.3'
                                   =20
                                    For March 9, 1997 21h 00m 00s
    
    G.C.T. 9d 21h 00m 00s
    Eq.T         -10m 27s
    ---------------------------
    G.A.T. 9d 20h 49m 33s             ( G.A.T. is Greenwich Apparent Time )
    Long       7  52  00 =20
    --------------------------
    L.A.T. 9d 12h 57m  33s            (L.A.T. is Local Apparent Time)
    
    Since Local Apparent Noon is 12h 00m 33s, the sextant shot was taken=20
    57m 33s AFTER the Sun had crossed 118 degs West meridian, hence the=20
    
    H.A. is 14 degs 23.3'    ( t =3D 14 degs 23.3' West )
    
    --------------------------------------------------------------------
    1997 style calculations:
    
    at 21h 00m 00s March 9, 1997=20
    
    GHA Sun 132 degs 23.3'             Dec. S 4 degs 13.3'
    
    GHA   132 degs 23.3'
    Long  118      00.0
    ------------------------   ( Subtract since West Long )
    LHA    14 degs 23.3'
    
    Since LHA < 180, LHA =3D HA or t, t is then labeled West.
    
    If LHA were > 180, HA =3D 360 - LHA, t is then labeled East.
    =20
    
    In 1930 the GHA for the Sun is NOT listed, only the Eq. of Time. I did
    notice that in the 1937 almanac, the GHA, Dec and Eq. of Time were listed
    every 2 hours.
    
    I am not sure of the year, but sometime in the 1930s the newer method of
    using GHA that we use today came in. Eq. of Time continued to be listed
    for those who were familar with the method of using it and then it seems
    it was sometime phased out.
    
    
    
    
                              =20
                                         ,,,=20
                                        (. .)=20
             +-----------------------ooO-(_)-Ooo----------------------+
             |  Gordon Talge WB6YKK            e-mail: gtalge{at}pe.net  |
             |  Department of Mathematics      QTH: Loma Linda, CA    |=20
             |  Mt. San Jacinto College        Lat.  N  34=B0 03.1'     |=20
             |  San Jacinto, CA                Long. W 117=B0 15.2'     |
             +--------------------------------------------------------+
    
    
    

       
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