HA, Dec to Parallactic Angle. (double precision)
ha d hour angle in radians (geocentric apparent) dec d declination in radians (geocentric apparent) phi d observatory latitude in radians (geodetic) The result is in the range -pi to +pi
1) The parallactic angle at a point in the sky is the position angle of the vertical, i.e. the angle between the direction to the pole and to the zenith. In precise applications care must be taken only to use geocentric apparent HA,Dec and to consider separately the effects of atmospheric refraction and telescope mount errors. 2) At the pole a zero result is returned. P.T.Wallace Starlink 1 April 1994