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    Coordinates from IMCCE and JPL HORIZONS
    From: Paul Hirose
    Date: 2016 Mar 14, 15:17 -0700

    In the discussion on Henning Umland's almanac, Dave Walden posted
    geocentric apparent coordinates (true equator and equinox) for Mercury
    at Feb 1 1100 UT from several sources:
    19h14m15.421s   -20°38′03.89″  USNO
    19 14 15.4172   -20 38 03.896  HORIZONS
    19 14 15.42394  -20 38 03.8918 IMCCE
    The discrepancies are insignificant for navigation. Mainly they're due
    to different precession and nutation models. However, if you need high
    accuracy it's possible to adjust the coordinates to a common basis.
    HORIZONS: first apply the IAU 1980 nutation model in reverse (i.e.,
    convert from true to mean equator and equinox of date). Include the
    celestial pole offsets published by the IERS, by adding them to the
    nutation angles from the 1980 model. Then apply the IAU 1976 precession
    model in reverse to obtain the apparent place of the body in the GCRS.
     From the GCRS, apply your precession and nutation model of choice.
    IMCCE: follow the same procedure, except DO NOT apply the celestial pole
    offset corrections. In other words, simply apply IAU 1976/80 precession
    and nutation in reverse.
    I have no procedure for USNO coordinates, since I have not coded their
    precession - nutation model. (It has been published, however.)
    Here are Dave's coordinates after conversion to the GCRS. For comparison
    I include my own coordinates.
    19h13m18.3980s -20°39'54.627 HORIZONS
    19h13m18.3980s -20°39'54.625 IMCCE
    19h13m18.3979s -20°39'54.626 me
    The IAU SOFA library and my SofaJpl DLL for Windows have all the
    necessary tools for the transformations. But note that SofaJpl's
    implementation of the 1976 precession model includes frame bias. You
    must remove it from the precession matrix. Fortunately that's easy.

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