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+      DOUBLE PRECISION FUNCTION sla_ZD (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
+*-
+
+      IMPLICIT NONE
+
+      DOUBLE PRECISION HA,DEC,PHI
+
+      DOUBLE PRECISION SH,CH,SD,CD,SP,CP,X,Y,Z
+
+
+      SH=SIN(HA)
+      CH=COS(HA)
+      SD=SIN(DEC)
+      CD=COS(DEC)
+      SP=SIN(PHI)
+      CP=COS(PHI)
+      X=CH*CD*SP-SD*CP
+      Y=SH*CD
+      Z=CH*CD*CP+SD*SP
+      sla_ZD=ATAN2(SQRT(X*X+Y*Y),Z)
+
+      END