def obliquity2000(jd):
    """
----------------------------------------------------------------------
Purpose:   What is the obliquity of the ecliptic at this Julian date?
           IAU2000
Input:     Julian date
Returns:   Mean obliquity in degrees
Reference: Fukushima, T. 2003, AJ, 126,1
           Kaplan, H., 2005, , The IAU Resolutions
           on Astronomical Reference Systems,
           Time Scales, and Earth Rotation Models, 
           United States Naval Observatory circular no. 179, 
           http://aa.usno.navy.mil/publications/docs/Circular_179.pdf
           (page 44)
notes:     The epoch is entered in Julian date and the time is calculated 
           w.r.t. J2000
           The obliquity is the angle between the mean equator and
           ecliptic, or, between the ecliptic pole and mean celestial 
           pole of date.
----------------------------------------------------------------------
   """
    T = (jd - 2451545.0) / 36525.0
    eps = (84381.406 + (-46.836769 + (-0.0001831 + (0.0020034 + (-5.76e-07 +
        -4.34e-08 * T) * T) * T) * T) * T) / 3600.0
    return eps
