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public/py/vendor_linux/skyfield/trigonometry.py

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+"""Routines whose currency is Angle objects."""
+
+from numpy import arctan2, sin, cos, tan
+from skyfield.constants import tau
+from skyfield.units import Angle
+
+def position_angle_of(anglepair1, anglepair2):
+    """Return the position angle of one position with respect to another.
+
+    Each argument should be a tuple whose first two items are
+    :class:`~skyfield.units.Angle` objects, like the tuples returned by
+    :meth:`~skyfield.positionlib.ICRF.radec()`,
+    :meth:`~skyfield.positionlib.ICRF.frame_latlon()`, and
+    :meth:`~skyfield.positionlib.ICRF.altaz()`.
+
+    If one of the two angle items is signed (if its ``.signed``
+    attribute is true), then that angle is used as the latitude and the
+    other as the longitude; otherwise the first argument is assumed to
+    be the latitude.
+
+    If the longitude has a ``.preference`` attribute of ``'hours'``, it
+    is assumed to be a right ascension which is positive towards the
+    east.  The position angle returned will be 0 degrees if position #2
+    is directly north of position #1 on the celestial sphere, 90 degrees
+    if east, 180 if south, and 270 if west.
+
+    Otherwise, the longitude is assumed to be azimuth, which measures
+    north to east around the horizon.  The position angle returned will
+    be 0 degrees if position #2 is directly above position #1 in the
+    sky, 90 degrees to its left, 180 if below, and 270 if to the right.
+
+    >>> from skyfield.trigonometry import position_angle_of
+    >>> from skyfield.units import Angle
+    >>> a = Angle(degrees=0), Angle(degrees=0)
+    >>> b = Angle(degrees=1), Angle(degrees=1)
+    >>> position_angle_of(a, b)
+    <Angle 315deg 00' 15.7">
+
+    """
+    lat1, lon1 = anglepair1[0], anglepair1[1]
+    if lon1.signed:
+        lat1, lon1 = lon1, lat1
+
+    lat2, lon2 = anglepair2[0], anglepair2[1]
+    if lon2.signed:
+        lat2, lon2 = lon2, lat2
+
+    lat1 = lat1.radians
+    lon1 = lon1.radians if lon1.preference == 'hours' else -lon1.radians
+    lat2 = lat2.radians
+    lon2 = lon2.radians if lon2.preference == 'hours' else -lon2.radians
+
+    return Angle(radians=arctan2(
+        sin(lon2 - lon1),
+        cos(lat1) * tan(lat2) - sin(lat1) * cos(lon2 - lon1),
+    ) % tau)