def _obliquity_only_rotation_matrix(obl=erfa.obl80(EQUINOX_J2000.jd1, EQUINOX_J2000.jd2) * u.radian):
# This code only accounts for the obliquity,
# which can be passed explicitly.
# The default value is the IAU 1980 value for J2000,
# which is computed using obl80 from ERFA:
#
# obl = erfa.obl80(EQUINOX_J2000.jd1, EQUINOX_J2000.jd2) * u.radian
return rotation_matrix(obl, "x")
IAU 1976/1980/1982/1994, equinox based
======================================
''')
# IAU 1976 precession matrix, J2000.0 to date.
RP = erfa.pmat76(DJMJD0, TT)
# IAU 1980 nutation
DP80, DE80 = erfa.nut80(DJMJD0, TT)
# Add adjustments: frame bias, precession-rates, geophysical
DPSI = DP80 + DDP80
DEPS = DE80 + DDE80
# Mean obliquity
EPSA = erfa.obl80(DJMJD0, TT)
# Build the rotation matrix
RN = erfa.numat(EPSA, DPSI, DEPS)
# Combine the matrices: PN = N x P
RNPB = erfa.rxr(RN, RP)
print("NPB matrix, equinox based:")
pprint(RNPB)
# Equation of the equinoxes, including nutation correction
EE = erfa.eqeq94(DJMJD0, TT) + DDP80 * math.cos(EPSA)
# Greenwich apparent sidereal time (IAU 1982/1994).
GST = erfa.anp(erfa.gmst82(DJMJD0 + DATE, TUT) + EE)
print("GST = %.17f radians" % GST)
IAU 1976/1980/1982/1994, equinox based
======================================
''')
# IAU 1976 precession matrix, J2000.0 to date.
RP = erfa.pmat76(DJMJD0, TT)
# IAU 1980 nutation
DP80, DE80 = erfa.nut80(DJMJD0, TT)
# Add adjustments: frame bias, precession-rates, geophysical
DPSI = DP80 + DDP80
DEPS = DE80 + DDE80
# Mean obliquity
EPSA = erfa.obl80(DJMJD0, TT)
# Build the rotation matrix
RN = erfa.numat(EPSA, DPSI, DEPS)
# Combine the matrices: PN = N x P
RNPB = erfa.rxr(RN, RP)
print("NPB matrix, equinox based:")
pprint(RNPB)
# Equation of the equinoxes, including nutation correction
EE = erfa.eqeq94(DJMJD0, TT) + DDP80 * math.cos(EPSA)
# Greenwich apparent sidereal time (IAU 1982/1994).
GST = erfa.anp(erfa.gmst82(DJMJD0+DATE, TUT) + EE)
print("GST = %.17f radians"%GST)