def get_cip(jd1, jd2):
"""
Find the X, Y coordinates of the CIP and the CIO locator, s.
Parameters
----------
jd1 : float or `np.ndarray`
First part of two part Julian date (TDB)
jd2 : float or `np.ndarray`
Second part of two part Julian date (TDB)
Returns
--------
x : float or `np.ndarray`
x coordinate of the CIP
y : float or `np.ndarray`
y coordinate of the CIP
s : float or `np.ndarray`
CIO locator, s
"""
# classical NPB matrix, IAU 2006/2000A
rpnb = erfa.pnm06a(jd1, jd2)
# CIP X, Y coordinates from array
x, y = erfa.bpn2xy(rpnb)
# CIO locator, s
s = erfa.s06(jd1, jd2, x, y)
return x, y, s
def get_cip(jd1, jd2):
"""
Find the X, Y coordinates of the CIP and the CIO locator, s.
Parameters
----------
jd1 : float or `np.ndarray`
First part of two part Julian date (TDB)
jd2 : float or `np.ndarray`
Second part of two part Julian date (TDB)
Returns
--------
x : float or `np.ndarray`
x coordinate of the CIP
y : float or `np.ndarray`
y coordinate of the CIP
s : float or `np.ndarray`
CIO locator, s
"""
# classical NPB matrix, IAU 2006/2000A
rpnb = erfa.pnm06a(jd1, jd2)
# CIP X, Y coordinates from array
x, y = erfa.bpn2xy(rpnb)
# CIO locator, s
s = erfa.s06(jd1, jd2, x, y)
return x, y, s
def _true_ecliptic_rotation_matrix(equinox):
# This code calls pnm06a from ERFA, which retrieves the precession
# matrix (including frame bias) according to the IAU 2006 model, and
# including the nutation. This family of systems is less popular
# (see https://github.com/astropy/astropy/pull/6508).
jd1, jd2 = get_jd12(equinox, 'tt')
rnpb = erfa.pnm06a(jd1, jd2)
obl = erfa.obl06(jd1, jd2)*u.radian
return matrix_product(rotation_matrix(obl, 'x'), rnpb)
def _apparent_position_in_true_coordinates(skycoord):
"""
Convert Skycoord in GCRS frame into one in which RA and Dec
are defined w.r.t to the true equinox and poles of the Earth
"""
jd1, jd2 = get_jd12(skycoord.obstime, 'tt')
# Classical NPB matrix, IAU 2006/2000A
# (same as in builtin_frames.utils.get_cip).
rbpn = erfa.pnm06a(jd1, jd2)
return SkyCoord(skycoord.frame.realize_frame(
skycoord.cartesian.transform(rbpn)))