code: private double eclipticObliquity() {
          if (eclipObliquity == INVALID) {
              final double epoch = 2451545.0;     // 2000 AD, January 1.5
  
              double T = (getJulianDay() - epoch) / 36525;
  
              eclipObliquity = 23.439292
                             - 46.815/3600 * T
                             - 0.0006/3600 * T*T
                             + 0.00181/3600 * T*T*T;
  
              eclipObliquity *= DEG_RAD;
          }
          return eclipObliquity;
      }
docs: Return the obliquity of the ecliptic (the angle between the ecliptic
  and the earth's equator) at the current time.  This varies due to
  the precession of the earth's axis.
  
  @return  the obliquity of the ecliptic relative to the equator,
  measured in radians.