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    Re: Unexpected USNO height correction precepts
    From: Paul Hirose
    Date: 2018 Sep 25, 21:03 -0700

    I added an option to make my SofaJpl demonstration program give apparent
    coordinates of the center of light instead of center of mass. This
    affects only my personal version, not the source code online at the
    SofaJpl page. I may add this option to the next revision of the Lunar4
    program.
    
    The example of Antoine, worked both ways:
    
    1977-03-18 00:02:17.00 UT1
    +47.52s delta T
    
    -27°35.00' 144°53.70' N lat, E lon
    4.6 meters above ellipsoid
    
    geocentric apparent GHA, dec
      156°08.50' +16°19.01'  center of mass
      156°08.77' +16°18.83'  center of light
    
    topocentric apparent azimuth and elevation (unrefracted)
       59°49.46' 17°57.67'  center of mass
       59°49.46' 17°57.98'  center of light
    
    0.31' from center of mass to center of light
    0.42' topocentric apparent semidiameter
    
    To practical accuracy, the above two values add directly, since the
    position angle of the Sun with respect to Venus is 1° (0 = the direction
    to the zenith). Thus, the center of light is displaced almost exactly
    upward, and there is a 0.73' altitude discrepancy between the lower limb
    and center of light.
    
    Computation with JPL DE431 ephemeris, IAU 2006/00B precession/nutation.
    Delta T from the spline fit to observed data by Stephenson et al (2016).
    Since the time scale of this example is UT1 and the body is Venus (very
    slow motion compared to the Moon), any discrepancy in delta T should
    have insignificant effect.
    
    The Astronomical Almanac delta T table in section K says 47.52 at
    1977.0, identical to my value. That illustrates the convenience of the
    SofaJpl DeltaT class. It's at hand if desired, but does not interfere if
    you prefer to supply your own value.
    

       
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