Convert rectangular coordinates to range, right ascension, and
   declination.

   None. 

   VARIABLE  I/O  DESCRIPTION 
   --------  ---  -------------------------------------------------- 
   rectan     I   Rectangular coordinates of a point. 
   range      O   Distance of the point from the origin. 
   ra         O   Right ascension in radians. 
   dec        O   Declination in radians. 

   rectan     The rectangular coordinates of a point. 

   range      is the distance of the point `rectan' from the origin. 

              The units associated with `range' are those associated
              with the input `rectan'.

   ra         is the right ascension of `rectan'.  This is the angular
              distance measured toward the east from the prime meridian
              to the meridian containing the input point. The direction
              of increasing right ascension is from the +X axis towards
              the +Y axis.

              `ra' is output in radians.  The range of `ra' is [0, 2*pi]. 


   dec        is the declination of `rectan'.  This is the angle from
              the XY plane of the ray from the origin through the
              point.

              `dec' is output in radians.  The range of `dec' is
              [-pi/2, pi/2].

   None. 

   None.

   The following code fragment converts right ascension and 
   declination from the B1950 reference frame to the J2000 frame.

      #include "SpiceUsr.h"

      SpiceDouble      ra;
      SpiceDouble      dec; 
      SpiceDouble      r;
      SpiceDouble      mtrans  [ 3 ][ 3 ]; 
      SpiceDouble      v1950   [ 3 ];
      SpiceDouble      v2000   [ 3 ];

      /.
      Convert RA and DEC to a 3-vector expressed in the B1950 frame.
      ./
      radrec_c ( 1.0, ra, dec, v1950 );

      /.
      We use the CSPICE routine pxform_c to obtain the transformation
      matrix for converting vectors between the B1950 and J2000
      reference frames.  Since both frames are inertial, the input time
      value we supply to pxform_c is arbitrary.  We choose zero seconds
      past the J2000 epoch as the input value.
      ./
      pxform_c ( "B1950", "J2000", 0.0, mtrans );

      /.
      Transform the vector to the J2000 frame.
      ./
      mxv_c    ( mtrans, v1950, v2000 );

      /.
      Find the RA and DEC of the J2000-relative vector.
      ./
      recrad_c ( v2000, &r, &ra, &dec ); 

   None. 

   Error free. 

   1) If the X and Y components of `rectan' are both zero, the
      right ascension is set to zero.

   2) If `rectan' is the zero vector, right ascension and declination
      are both set to zero.

   None. 

   N.J. Bachman    (JPL)
   H.A. Neilan     (JPL) 
   E.D. Wright     (JPL)

   None. 

   -CSPICE Version 1.1.2, 30-JUL-2003 (NJB)

       Various header corrections were made.

   -CSPICE Version 1.1.0, 22-OCT-1998 (NJB)

      Made input vector const.

   -CSPICE Version 1.0.0, 08-FEB-1998 (EDW)

   rectangular coordinates to ra and dec 
   rectangular to right_ascension and declination