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I am trying to find the position angle of a star whose Ra and dec are known. I have already looked at the 'Measuring misalignment between two positions on sky' question and answer, but the answer gives an undefined solution if one of the points is the North Celestial Pole (NCP). I would appreciate if anyone could tell me how to find an object's position angle given its equatorial coordinates.

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  • $\begingroup$ The position angle is the angle between three points: two objects (such as two stars in a double star) and the north celestial pole (NCP). It sounds like you only have one star, so position angle is not applicable. Are you asking for a different angle? Perhaps the parallactic angle? (Parallactic angle is the angle between NCP and the zenith.) $\endgroup$
    – JohnHoltz
    Jun 2 '20 at 14:12
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    $\begingroup$ You've identified a degenerate case, and the formula in JohnHoltz's answer to the linked question reflects that. The direction from NCP to itself is undefined. $\endgroup$
    – Mike G
    Jun 2 '20 at 15:18
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    $\begingroup$ @JohnHoltz Ah, thank you. The question gave information about two stars but asked 'what is the position angle of the secondary star with respect to north', which means I need to include the other star as well. Thanks again. $\endgroup$
    – Physics
    Jun 3 '20 at 9:29

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