FIX: Add clip limits to Nightshade floating point calculations

Depending on the values input to Nightshade, the floating point
precision could put the range for arccos outside of [-1, 1], which
in turn numpy returns as nan's, yielding bad geometries further
downstream. This patch clips the arccos calculations to [-1, 1] to
guarantee we aren't out of the valid floating point bounds.

lib/cartopy/feature/nightshade.py

@@ -77,8 +77,12 @@ def __init__(self, date=None, delta=0.1, refraction=-0.83,
 
         # Solve the generalized equation for omega0, which is the
         # angle of sunrise/sunset from solar noon
-        omega0 = np.rad2deg(np.arccos(np.sin(np.deg2rad(refraction)) /
-                                      np.cos(np.deg2rad(y))))
+        # We need to clip the input to arccos to [-1, 1] due to floating
+        # point precision and arccos creating nans for values outside
+        # of the domain
+        arccos_tmp = np.clip(np.sin(np.deg2rad(refraction)) /
+                             np.cos(np.deg2rad(y)), -1, 1)
+        omega0 = np.rad2deg(np.arccos(arccos_tmp))
 
         # Fill the longitude values from the offset for midnight.
         # This needs to be a closed loop to fill the polygon.
@@ -89,7 +93,6 @@ def __init__(self, date=None, delta=0.1, refraction=-0.83,
 
         kwargs.setdefault('facecolor', color)
         kwargs.setdefault('alpha', alpha)
-
         geom = sgeom.Polygon(np.column_stack((x, y)))
         return super().__init__(
             [geom], rotated_pole, **kwargs)

lib/cartopy/tests/feature/test_nightshade.py

@@ -8,7 +8,7 @@
 
 import pytest
 
-from cartopy.feature.nightshade import _julian_day, _solar_position
+from cartopy.feature.nightshade import _julian_day, _solar_position, Nightshade
 
 
 def test_julian_day():
@@ -44,3 +44,12 @@ def test_solar_position(dt, true_lat, true_lon):
     lat, lon = _solar_position(dt)
     assert pytest.approx(true_lat, 0.1) == lat
     assert pytest.approx(true_lon, 0.1) == lon
+
+
+def test_nightshade_floating_point():
+    # Smoke test for clipping nightshade floating point values
+    date = datetime(1999, 12, 31, 12)
+
+    # This can cause an error with floating point precision if it is
+    # set to exactly -6 and arccos input is not clipped to [-1, 1]
+    Nightshade(date, refraction=-6.0, color='none')