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    From: Peter Blaskett
    Date: 2021 Nov 5, 13:30 -0700

    Hello Antoine,

    A simple example using ICE shows that the positions calculated are for the body centre.

    Using latitude 0.0 and computing a meridian crossing gives :-

             Celestial Navigation Data for 2021 Nov  6 at 11 43 35 UT (GMT)
                                  Delta T =  87.7 seconds
                     For Assumed Location:  Longitude       0 00.0
                                            Latitude        0 00.0

                         Almanac Data                                       Altitude Corrections
     Object       GHA        Dec            Hc           Zn          Refr    SD     PA     Sum
                        ø   '         ø   '             ø   '          ø                '        '          '         '
     SUN        359 59.3   s16 07.3   +73 52.7   180.0        -.3   16.2      .0      15.9
     ARIES      221 48.4

    Now Dec plus Hc = 90.0 deg , so Hc is for the position of the body centre.

    Also, the Documentation with ICE is explicit. Here it is. The paragraph Hc is definite. The last paragraph refers to the corrections.

    Extract from explanatory documentation

    6.8  Topocentric Altitude and Azimuth

    The altitude and azimuth of a celestial body as observed from any geographic
    location at any time can be obtained by using the "F7 Navigation" selection.
    The celestial objects which can be specified are not limited to those normally

       1988 US Naval Observatory Nautical Floppy Almanac Version 2.00.88  Page  15
    used for celestial navigation.  In fact, if you redefine the catalog in use
    (see section 6.4), it is possible to obtain "F7 Navigation" on objects such as
    the Ring Nebula or the quasar 3C 273.
    For those not familiar with navigational notation, the following definitions
    are provided:

    GHA  Greenwich hour angle.  Apparent instantaneous hour angle of object
    measured westward from the plane of the Greenwich meridian.

    Dec  Declination.  Apparent declination of object.

    Hc   Computed altitude.  Instantaneous altitude of center of object, calculated
    using its geocentric position, that is, without adjustments for topocentric
    parallax or refraction.

    Zn   Computed azimuth.  Instantaneous azimuth of object, measured eastward from
    true north.

    Refr Refraction. Atmospheric refraction correction applicable to object, if
    observed at sea level at optical wavelengths, under standard atmospheric

    SD   Semidiameter.  Half the apparent equatorial angular diameter of object.

    PA   Parallax in altitude.  Topocentric parallax correction applicable to

    Sum  Sum = Refr + SD + PA.  If Sum is subtracted from Hc, the apparent altitude
    (that comparable to observation) of the lower limb of the object is obtained.

    Your posting about using ICE to get answers corrected for the proper Delta-T might be useful for someone.

    This, and the awkwardness of the ICE interface, is what urged me to write my own software years ago.

    I hope we are all singing the same music now.



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