API Reference — Trigonometry¶

skyfield.trigonometry.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 Angle objects, like the tuples returned by radec(), frame_latlon(), and 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">