Procedure

   recrad_c ( Rectangular coordinates to RA and DEC ) 

   void recrad_c ( ConstSpiceDouble    rectan[3],
                   SpiceDouble       * range,
                   SpiceDouble       * ra,
                   SpiceDouble       * dec      )

Abstract

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

Required_Reading

   None.

Keywords

   CONVERSION
   COORDINATES

Brief_I/O

   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.

Detailed_Input

   rectan      are the rectangular coordinates of a point.

Detailed_Output

   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].

Parameters

   None.

Exceptions

   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.

Files

   None.

Particulars

   None.

Examples

   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 );

Restrictions

   None.

Literature_References

   None.

Author_and_Institution

   N.J. Bachman        (JPL)
   J. Diaz del Rio     (ODC Space)
   E.D. Wright         (JPL)

Version

   -CSPICE Version 1.1.3, 04-JUL-2021 (JDR)

       Edited the header to comply with NAIF standard.

   -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)

Index_Entries

   rectangular coordinates to ra and dec
   rectangular to right_ascension and declination