Position angle
McGraw-Hill Science & Technology Dictionary:
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position angle
(pə′zish·ən ′aŋ·gəl)
(astronomy) The angle formed by the great circle running through two celestial objects and the hour circle running through one of the objects. In measuring double stars, the angle formed between the great circle running through both components and the hour circle going through the primary measured from the north through the east from 0 to 360°.
(navigation) That angle of the navigational triangle at the celestial body having the hour circle and the vertical circle at its sides. Also known as parallactic angle.
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Position angle
Wiley Book of Astronomy:
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position angle
position angle (P.A.)
The direction of an imaginary arrow in the sky, measured in degrees from north through east. Applied to a binary system, it is the angle between the primary and secondary components. Applied to a galaxy, it measures the object's tilt—that is, the angle between the galaxy's long axis and a line from its center headed north.
The direction of an imaginary arrow in the sky, measured in degrees from north through east. Applied to a binary system, it is the angle between the primary and secondary components. Applied to a galaxy, it measures the object's tilt—that is, the angle between the galaxy's long axis and a line from its center headed north.
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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.
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 0-521-85370-2. 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)
- Position Angle Scales for 1.25" Telescope Eyepieces
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