Position angle

Position angle, usually abbreviated PA, is a measurement derived from observing visual binary stars. It is defined as the angular offset in degrees of the secondary star to the primary, relative to the north celestial pole.

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

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, from 0 to 359 degrees.

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.

Further reading

D. Scott Birney, Guillermo Gonzalez, David Oesper (2007). Observational Astronomy. Cambridge University Press. p. 75. ISBN 0521853702. http://books.google.com/books?id=cc9L8QWcZWsC&pg=RA1-PA73#PRA1-PA75,M1. 

External links

• The Orbits of 150 Visual Binary Stars, by Dibon Smith (Accessed 2/26/06)

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