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    Re: FW: A noon sight conundrum
    From: Paul Hirose
    Date: 2003 Nov 24, 22:55 -0800

    Kieran Kelly wrote:
    >
    > Position        :       S 21d 48.3'     E 132d 40.0'
    > Date:            July 20, 2002
    >
    > The USNO computer gave me the following results:
    > GMT             Hc              Zn
    > 3h 23m 36s      47d 26.6'       357.3d
    > 3h 15m 48s      47d 29.2'       359.9d
    > 3h 15m 47s      47d 29.2        360.0d
    > 3h 15m 40s      47d 29.2'       360.0d
    > 3h 15m 38s      47d 29.2'           0.0d
    > 3h 15m 35s      47d 29.2'           0.0d
    > 3h 15m 34s      47d 29.2'           0.0d
    > 3h 15m 29s      47d 29.2'           0.0d
    > 3h 15m 28s      47d 29.2'           0.1d
    > 3h 07m 42s      47d 26.6'           2.7d
    
    I generated the following table with the USNO MICA program. Time
    scale is UT1, the same as the USNO online sight reduction program, so
    the results can be compared. Note that my table shows zenith distance,
    not altitude. The minimum zenith distance (highest altitude) seen
    below, 42 30 54.2, corresponds to altitude 47 29 05.8.
    
    
                                      Sun
    
                            Apparent Topocentric Positions
                              Local Zenith and True North
    
                     Location:  E132?40'00", S21?48'18",     0m
                     (Longitude referred to Greenwich meridian)
    
       Date        Time          Zenith          Azimuth        Distance
            (UT1)               Distance        (E of N)       to Object
    
                 h  m   s        ?  '   "        ?  '   "          AU
    2002 Jul 20 03:15:35.0      42 30 54.3       0 01 25.4     1.016087417
    2002 Jul 20 03:15:36.0      42 30 54.3       0 01 04.6     1.016087416
    2002 Jul 20 03:15:37.0      42 30 54.3       0 00 43.9     1.016087415
    2002 Jul 20 03:15:38.0      42 30 54.3       0 00 23.1     1.016087414
    2002 Jul 20 03:15:39.0      42 30 54.3       0 00 02.3     1.016087413
    2002 Jul 20 03:15:40.0      42 30 54.3     359 59 41.6     1.016087412
    2002 Jul 20 03:15:41.0      42 30 54.2     359 59 20.8     1.016087412
    2002 Jul 20 03:15:42.0      42 30 54.2     359 59 00.1     1.016087411
    2002 Jul 20 03:15:43.0      42 30 54.2     359 58 39.3     1.016087410
    2002 Jul 20 03:15:44.0      42 30 54.2     359 58 18.5     1.016087409
    2002 Jul 20 03:15:45.0      42 30 54.2     359 57 57.8     1.016087408
    2002 Jul 20 03:15:46.0      42 30 54.2     359 57 37.0     1.016087407
    2002 Jul 20 03:15:47.0      42 30 54.2     359 57 16.3     1.016087406
    2002 Jul 20 03:15:48.0      42 30 54.2     359 56 55.5     1.016087405
    2002 Jul 20 03:15:49.0      42 30 54.3     359 56 34.7     1.016087404
    2002 Jul 20 03:15:50.0      42 30 54.3     359 56 14.0     1.016087404
    
    There is a slight error in this computation: MICA assumes Terrestrial
    Time was 68.1 seconds ahead of UT1 on this date. The actual value was
    64.4, however.
    
    "Delta T", as this offset is called, is interpolated from a table
    which is hard coded into MICA. The table has delta T increasing 1
    second per year. That's too much. We haven't had a leap second since
    the end of 1998. Unfortunately, there's no way to input a value into
    MICA manually, or substitute an updated table.
    
    I obtained the true delta T of 64.4 seconds from the instructions
    here:
    
    http://maia.usno.navy.mil/
    
    (just above the section on IERS Bulletin B), and the Bulletin B from
    September 2002:
    
    http://hpiers.obspm.fr/iers/bul/bulb/bulletinb.175
    
    I'm not sure what effect the incorrect delta T has on the meridian
    passage computation.
    
    
    

       
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