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      DOUBLE PRECISION FUNCTION slZD (HA, DEC, PHI)

     - - -
      Z D
     - - -

  HA, Dec to Zenith Distance (double precision)

  Given:
     HA     d     Hour Angle in radians
     DEC    d     declination in radians
     PHI    d     observatory latitude in radians

  The result is in the range 0 to pi.

  Notes:

  1)  The latitude must be geodetic.  In critical applications,
      corrections for polar motion should be applied.

  2)  In some applications it will be important to specify the
      correct type of hour angle and declination in order to
      produce the required type of zenith distance.  In particular,
      it may be important to distinguish between the zenith distance
      as affected by refraction, which would require the "observed"
      HA,Dec, and the zenith distance in vacuo, which would require
      the "topocentric" HA,Dec.  If the effects of diurnal aberration
      can be neglected, the "apparent" HA,Dec may be used instead of
      the topocentric HA,Dec.

  3)  No range checking of arguments is done.

  4)  In applications which involve many zenith distance calculations,
      rather than calling the present routine it will be more efficient
      to use inline code, having previously computed fixed terms such
      as sine and cosine of latitude, and perhaps sine and cosine of
      declination.

  P.T.Wallace   Starlink   3 April 1994

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

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