code: protected static function equinoxSeptember($year, $vsop = true) {
      $month = 9;
  
      if ($vsop)
        return static::accurate($year, static::meanTerms($month, $year), $month);
      else
        return static::approx($year, static::meanTerms($month, $year));
    }
docs: Finds the date of the time of the September Equinox for a given year
  
  @param float $year The year
  @param bool  $vsop If true calculates the result using the VSOP model; If
  false approximates the result.
  
  @return float The JD and time of the September Equinox in TD