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 apparent_longitude(t='now'):
"""
Returns the Sun's apparent longitude, referred to the true equinox of date. Corrections for
nutation and aberration (for Earth motion) are 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)
sun = SkyCoord(0 * u.deg, 0 * u.deg, 0 * u.AU, frame='hcrs', obstime=time)
coord = sun.transform_to(GeocentricMeanEcliptic(equinox=time))
# Astropy's GeocentricMeanEcliptic already includes aberration, so only add nutation
jd1, jd2 = get_jd12(time, 'tt')
nut_lon, _ = erfa.nut06a(jd1, jd2) * u.radian
lon = coord.lon + nut_lon
return Longitude(lon)