When the position angle measurement is made it is made with respect to the north celestial pole at the time of the observation. The pole, however, is not a fixed point in the sky by varies due to the effects of precession. This means that the position angle can change over time without any orbital motion of the companion star. Furthermore, the proper motion of the star causes the star to change its position with respect to the pole and consequently induces changes in the position angle.
It is important, then, to reduce the observed position angle to some standard epoch so that any changes observed in the position angle will be due to orbital motion and not to the variability of the reference frame.
Likewise, when calculating the position angle and separation from the orbital elements (see Chapter 7) it is important to allow for these effects. The table of orbital elements should give the epoch of the position angle of the ascending node (Q), but unfortunately not all tables of orbital elements give it. When it is given there are two ways to carry out the calculation. First calculate the position angle and separation and then reduce the position angle to the date of observation, or first reduce the position angle of the ascending node to the date of observation and then calculate the position angle and separation. The result is the same either way.
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