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      DOUBLE PRECISION FUNCTION slPA (HA, DEC, PHI)

     - - -
      P A
     - - -

  HA, Dec to Parallactic Angle (double precision)

  Given:
     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

  Notes:

  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   16 August 1994

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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