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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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