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+.help zd Jun99 "Slalib Package"
+.nf
+
+      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.
+
+.fi
+.endhelp