def unix_to_era(unix_time):
"""Calculate the Earth Rotation Angle for a given time.
The Earth Rotation Angle is the angle between the Celetial and Terrestrial
Intermediate origins, and is a modern replacement for the Greenwich Sidereal
Time.
Parameters
----------
unix_time : float or array of.
Unix/POSIX time.
Returns
-------
era : float or array of
The Earth Rotation Angle in degrees.
"""
from skyfield import earthlib
t = unix_to_skyfield_time(unix_time)
era = earthlib.earth_rotation_angle(t.ut1) # in cycles
return 360.0 * era
def unix_to_era(unix_time):
"""Calculate the Earth Rotation Angle for a given time.
The Earth Rotation Angle is the angle between the Celetial and Terrestrial
Intermediate origins, and is a modern replacement for the Greenwich Sidereal
Time.
Parameters
----------
unix_time : float or array of.
Unix/POSIX time.
Returns
-------
era : float or array of
The Earth Rotation Angle in degrees.
"""
from skyfield import earthlib
t = unix_to_skyfield_time(unix_time)
era = earthlib.earth_rotation_angle(t.ut1) # in cycles
return (360.0 * era)
def test_earth_rotation_angle():
epsilon = 1e-12
a0 = c.era(T0)
aA = c.era(TA)
aB = c.era(TB)
t = array([T0, TA, TB])
v = earthlib.earth_rotation_angle(t)
eq(v, [a0, aA, aB], epsilon)
def test_earth_rotation_angle(self):
self.delta = 1e-12
a0 = c.era(T0)
aA = c.era(TA)
aB = c.era(TB)
t = array([T0, TA, TB])
v = earthlib.earth_rotation_angle(t)
self.eq(v, [a0, aA, aB])
def test_lsa_skyfield(self):
# Check an lsa calculated by caput.time against one calculated by PyEphem
from skyfield import earthlib, api
dt = datetime(2014, 10, 2, 13, 4, 5)
dt_utc = dt.replace(tzinfo=api.utc)
t1 = ctime.datetime_to_unix(dt)
obs = ctime.Observer(42.8, 4.7)
lsa1 = obs.unix_to_lsa(t1)
t = ctime.skyfield_wrapper.timescale.utc(dt_utc)
lsa2 = (earthlib.earth_rotation_angle(t.ut1) * 360.0 + obs.longitude) % 360.0
self.assertAlmostEqual(lsa1, lsa2, 4)
def test_lsa_skyfield(self):
# Check an lsa calculated by caput.time against one calculated by PyEphem
from skyfield import earthlib, api
dt = datetime(2014, 10, 2, 13, 4, 5)
dt_utc = dt.replace(tzinfo=api.utc)
t1 = ctime.datetime_to_unix(dt)
obs = ctime.Observer(42.8, 4.7)
lsa1 = obs.unix_to_lsa(t1)
t = ctime.skyfield_wrapper.timescale.utc(dt_utc)
lsa2 = (earthlib.earth_rotation_angle(t.ut1) * 360.0 +
obs.longitude) % 360.0
self.assertAlmostEqual(lsa1, lsa2, 4)
def test_cirs_era():
ts = api.load.timescale()
st = ts.utc(year=np.arange(1951, 2051))
planets = api.load('de421.bsp')
pos = planets['earth'] + api.Topos(longitude_degrees=0.0, latitude_degrees=0.0)
# Get the TIO
tio = pos.at(st).from_altaz(alt_degrees=90, az_degrees=180)
# Get the TIOs RA in CIRS coordinates, and the Earth Rotation Angle
tio_ra, tio_dec, _ = tio.cirs_radec(st)
era = 360.0 * earth_rotation_angle(st.ut1)
tol = (1e-8 / 3600.0) # 10 nano arc-second precision
assert np.allclose(tio_ra._degrees, era, rtol=0.0, atol=tol)
assert np.allclose(tio_dec._degrees, 0.0, rtol=0.0, atol=tol)
def test_cirs_era():
ts = api.load.timescale()
st = ts.utc(year=np.arange(1951, 2051))
planets = api.load('de421.bsp')
pos = planets['earth'] + api.Topos(longitude_degrees=0.0, latitude_degrees=0.0)
# Get the TIO
tio = pos.at(st).from_altaz(alt_degrees=90, az_degrees=180)
# Get the TIOs RA in CIRS coordinates, and the Earth Rotation Angle
tio_ra, tio_dec, _ = tio.cirs_radec(st)
era = 360.0 * earth_rotation_angle(st.ut1)
tol = (1e-8 / 3600.0) # 10 nano arc-second precision
assert np.allclose(tio_ra._degrees, era, rtol=0.0, atol=tol)
assert np.allclose(tio_dec._degrees, 0.0, rtol=0.0, atol=tol)
def test_cirs_meridian():
ts = api.load.timescale()
st = ts.utc(year=2051)
planets = api.load('de421.bsp')
pos = planets['earth'] + api.Topos(longitude_degrees=0.0, latitude_degrees=0.0)
# Get a series of points along the meridian
alt = np.arange(1, 90)
meridian = pos.at(st).from_altaz(alt_degrees=alt, az_degrees=0.0)
# Get the TIOs RA in CIRS coordinates, and the Earth Rotation Angle
md_ra, md_dec, _ = meridian.cirs_radec(st)
era = 360.0 * earth_rotation_angle(st.ut1)
tol = (1e-7 / 3600.0) # 100 nano arc-second precision
assert np.allclose(md_ra._degrees, era, rtol=0.0, atol=tol)
assert np.allclose(md_dec._degrees, 90 - alt, rtol=0.0, atol=tol)
def test_cirs_meridian():
ts = api.load.timescale()
st = ts.utc(year=2051)
planets = api.load('de421.bsp')
pos = planets['earth'] + api.Topos(longitude_degrees=0.0, latitude_degrees=0.0)
# Get a series of points along the meridian
alt = np.arange(1, 90)
meridian = pos.at(st).from_altaz(alt_degrees=alt, az_degrees=0.0)
# Get the TIOs RA in CIRS coordinates, and the Earth Rotation Angle
md_ra, md_dec, _ = meridian.cirs_radec(st)
era = 360.0 * earth_rotation_angle(st.ut1)
tol = (1e-7 / 3600.0) # 100 nano arc-second precision
assert np.allclose(md_ra._degrees, era, rtol=0.0, atol=tol)
assert np.allclose(md_dec._degrees, 90 - alt, rtol=0.0, atol=tol)
def test_earth_rotation_angle(jd_float_or_vector):
jd_ut1 = jd_float_or_vector
u = c.era(jd_ut1)
v = earthlib.earth_rotation_angle(jd_ut1)
epsilon = 1e-12 # degrees; 14 to 15 digits of agreement
eq(u, v, epsilon)
