Hide Time._nutation_angles_radians for now

I have still not settled on the API for offering alternative nutation
routines, so hiding this gives me the freedom to release later today
without making a public API commitment yet.

skyfield/almanac.py

@@ -81,7 +81,7 @@ def seasons(ephemeris):
 
     def season_at(t):
         """Return season 0 (Spring) through 3 (Winter) at time `t`."""
-        t.nutation_angles_radians = iau2000b_radians(t)
+        t._nutation_angles_radians = iau2000b_radians(t)
         e = earth.at(t)
         _, slon, _ = e.observe(sun).apparent().ecliptic_latlon('date')
         return (slon.radians // (tau / 4) % 4).astype(int)
@@ -111,7 +111,7 @@ def moon_phases(ephemeris):
 
     def moon_phase_at(t):
         """Return the phase of the moon 0 through 3 at time `t`."""
-        t.nutation_angles_radians = iau2000b_radians(t)
+        t._nutation_angles_radians = iau2000b_radians(t)
         e = earth.at(t)
         _, mlon, _ = e.observe(moon).apparent().ecliptic_latlon('date')
         _, slon, _ = e.observe(sun).apparent().ecliptic_latlon('date')
@@ -200,7 +200,7 @@ def sunrise_sunset(ephemeris, topos):
         than -0.8333 degrees.
 
         """
-        t.nutation_angles_radians = iau2000b_radians(t)
+        t._nutation_angles_radians = iau2000b_radians(t)
         return topos_at(t).observe(sun).apparent().altaz()[0].degrees >= -0.8333
 
     is_sun_up_at.rough_period = 0.5  # twice a day
@@ -232,7 +232,7 @@ def dark_twilight_day(ephemeris, topos):
 
     def is_it_dark_twilight_day_at(t):
         """Return whether the sun is up, down, or whether there is twilight."""
-        t.nutation_angles_radians = iau2000b_radians(t)
+        t._nutation_angles_radians = iau2000b_radians(t)
         degrees = topos_at(t).observe(sun).apparent().altaz()[0].degrees
         r = zeros_like(degrees, int)
         r[degrees >= -18.0] = 1
@@ -261,7 +261,7 @@ def risings_and_settings(ephemeris, target, topos,
 
     def is_body_up_at(t):
         """Return `True` if the target has risen by time `t`."""
-        t.nutation_angles_radians = iau2000b_radians(t)
+        t._nutation_angles_radians = iau2000b_radians(t)
         return topos_at(t).observe(target).apparent().altaz()[0].degrees > h
 
     is_body_up_at.rough_period = 0.5  # twice a day
@@ -269,7 +269,7 @@ def risings_and_settings(ephemeris, target, topos,
 
 def _distance_to(center, target):
     def distance_at(t):
-        t.nutation_angles_radians = iau2000b_radians(t)
+        t._nutation_angles_radians = iau2000b_radians(t)
         distance = center.at(t).observe(target).distance().au
         return distance
     return distance_at

skyfield/almanac_east_asia.py

@@ -137,7 +137,7 @@ def solar_terms(ephemeris):
 
     def solar_term_at(t):
         """Return season 0 through 23 at time `t`."""
-        t.nutation_angles_radians = iau2000b_radians(t)
+        t._nutation_angles_radians = iau2000b_radians(t)
         e = earth.at(t)
         _, slon, _ = e.observe(sun).apparent().ecliptic_latlon('date')
         return (slon.radians // (tau / 24) % 24).astype(int)

skyfield/nutationlib.py

@@ -355,10 +355,12 @@ se1_1 = +0.00e-6
 # of our tests also still call them, to help keep them working.
 
 def compute_nutation(t):
+    """Deprecated: this is now a method on the Time object."""
     return t.N
 
 def earth_tilt(t):
-    d_psi, d_eps = t.nutation_angles_radians
+    """Deprecated: these are now computed separately on the Time object."""
+    d_psi, d_eps = t._nutation_angles_radians
     mean_ob = t._mean_obliquity_radians
     true_ob = mean_ob + d_eps
     c_terms = equation_of_the_equinoxes_complimentary_terms(t.tt)

skyfield/positionlib.py

@@ -315,7 +315,7 @@ class ICRF(object):
                              ' a floating point Terrestrial Time (TT),'
                              ' or the string "date" for epoch-of-date')
 
-        _, d_eps = epoch.nutation_angles_radians
+        _, d_eps = epoch._nutation_angles_radians
         true_obliquity = epoch._mean_obliquity_radians + d_eps
         rotation = _mxm(rot_x(- true_obliquity), epoch.M)
         position_au = _mxv(rotation, position_au)

skyfield/timelib.py

@@ -618,7 +618,7 @@ class Time(object):
         return rollaxis(self.C, 1)
 
     @reify
-    def nutation_angles_radians(self):
+    def _nutation_angles_radians(self):
         # TODO: add psi and eps corrections support back in here, rather
         # than at points of use.
         return iau2000a_radians(self)
@@ -632,7 +632,7 @@ class Time(object):
         # continues to support.
 
         d_psi, d_eps = angles
-        self.nutation_angles_radians = (
+        self._nutation_angles_radians = (
             d_psi / 1e7 * ASEC2RAD,
             d_eps / 1e7 * ASEC2RAD,
         )
@@ -687,7 +687,7 @@ class Time(object):
     @reify
     def gast(self):
         """Greenwich Apparent Sidereal Time as decimal hours."""
-        d_psi, _ = self.nutation_angles_radians
+        d_psi, _ = self._nutation_angles_radians
         tt = self.tt
         # TODO: move this into an eqeq function?
         c_terms = equation_of_the_equinoxes_complimentary_terms(tt)
@@ -708,7 +708,7 @@ class Time(object):
 
     def nutation_matrix(self):
         """Compute the 3×3 nutation matrix N for this date."""
-        d_psi, d_eps = self.nutation_angles_radians
+        d_psi, d_eps = self._nutation_angles_radians
         mean_obliquity = self._mean_obliquity_radians
         true_obliquity = mean_obliquity + d_eps
         return build_nutation_matrix(mean_obliquity, true_obliquity, d_psi)