Position angle, usually abbreviated PA, is the convention for measuring angles on the sky in astronomy. The International Astronomical Union defines it as the angle measured counterclockwise relative to the north celestial pole.
As the example illustrates, if one were observing a hypothetical binary star with a PA of 135 degrees, that means an imaginary line in the eyepiece drawn from the north celestial pole (NCP) to the primary (P) would be offset from the secondary (S) such that the NCP-P-S angle would be 135 degrees.
When graphing visual binary orbits, the NCP line is traditionally drawn downward—that is, with north at bottom—and PA is measured counterclockwise.
Also the proper motion angle (see proper motion) is sometimes called the position angle.
The definition of position angle is also extended to apply to extended objects like galaxies, where it refers to the angle made by the major axis of the object with the NCP line.
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