public static function Sp00($date1, $date2) {
    $t;
    $sp;

    /* Interval between fundamental epoch J2000.0 and current date (JC). */
    $t = (($date1 - DJ00) + $date2) / DJC;

    /* Approximate s'. */
    $sp = -47e-6 * $t * DAS2R;

    return $sp;
  }