public static function Apcs13($date1, $date2, array $pv, 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::Apcs($date1, $date2, $pv, $ebpv, $ehpv[0], $astrom);

    /* Finished. */
  }