def mean_obliquity_of_ecliptic(t='now'):
"""
Returns the mean obliquity of the ecliptic, using the IAU 2006 definition. No correction for
nutation is included.
Parameters
----------
t : {parse_time_types}
Time to use in a parse-time-compatible format
"""
time = parse_time(t)
jd1, jd2 = get_jd12(time, 'tt')
obl = erfa.obl06(jd1, jd2) * u.radian
return Angle(obl, u.arcsec)
def true_obliquity_of_ecliptic(t='now'):
"""
Returns the true obliquity of the ecliptic, using the IAU 2006 definition. Correction for
nutation is included.
Parameters
----------
t : {parse_time_types}
Time to use in a parse-time-compatible format
Notes
-----
The nutation model is IAU 2000A nutation with adjustments to match IAU 2006 precession.
"""
time = parse_time(t)
jd1, jd2 = get_jd12(time, 'tt')
obl = erfa.obl06(jd1, jd2) * u.radian
_, nut_obl = erfa.nut06a(jd1, jd2) * u.radian
obl += nut_obl
return Angle(obl, u.arcsec)
def _rotation_matrix_obliquity(time):
"""
Return the rotation matrix from Earth equatorial to ecliptic coordinates
"""
return rotation_matrix(erfa.obl06(*get_jd12(time, 'tt'))*u.radian, 'x')
