Position angle

The International Astronomical Union defines it as the angle measured relative to the north celestial pole (NCP), turning positive into the direction of the right ascension.

In the standard (non-flipped) images, this is a counterclockwise measure relative to the axis into the direction of positive declination.

As the example illustrates, if one were observing a hypothetical binary star with a PA of 30°, that means an imaginary line in the eyepiece drawn from the north celestial pole to the primary (P) would be offset from the secondary (S) such that the NCP-P-S angle would be 30°.

When graphing visual binaries, the NCP is, as in the illustration, normally drawn from the center point (origin) that is the Primary downward–that is, with north at bottom–and PA is measured counterclockwise.

This article considers only the abstraction of minimizing the distance between s and t traveling on the surface of a sphere with some radius R: In which direction angle p relative to North should the ship steer to reach the target position?

An illustration of how position angle is estimated through a telescope eyepiece; the primary star is at center.