public static function Apcg13($date1, $date2, iauASTROM &$astrom) {
    $ehpv = [];
    $ebpv = [];

    /* Earth barycentric & heliocentric position/velocity (au, au/d). */
    IAU::Epv00($date1, $date2, $ehpv, $ebpv);

    /* Compute the star-independent astrometry parameters. */
    IAU::Apcg($date1, $date2, $ebpv, $ehpv[0], $astrom);

    /* Finished. */
  }