code: final int getMajorSolarTerm(long utcDays) {
          double jd = JulianDay.ofEphemerisTime(this.midnight(utcDays)).getValue();
          int index = (2 + (int) Math.floor(SolarTerm.solarLongitude(jd) / 30)) % 12;
          return ((index == 0) ? 12 : index);
      }
docs: index of major solar term (used in test cases only)