Return half the value of pi (the ratio of the circumference of 
   a circle to its diameter). 

   None. 

   The function returns half the value of pi. 

   None. 

   The function returns half the value of pi (the ratio of 
   a circle's circumference to its diameter), determined by 
   the ACOS function. That is, 

         halfpi_c = acos ( -1.0 ) * 0.50 

   None. 

   The first time the function is referenced, the value is computed 
   as shown above. The value is saved, and returned directly upon 
   subsequent reference. 

   The subroutine shown below illustrates the use of halfpi_c. 

            void bftran ( ra, dec, w, tipm )
               {

                /. 
                Compute the transformation from inertial to body 
                fixed coordinates, given the directions of the north 
                pole and prime meridian of the body. 
                ./

                SpiceDouble   ra;
                SpiceDouble   dec; 
                SpiceDouble   w;
                SpiceDouble   tipm [3][3];


                /.
                The transformation is defined by the compund 
                rotation 

                  [W] [pi/2 - Dec] [RA + pi/2] 
                     3            1           3 
                ./


                rotate_c (        ra + halfpi_c(), 3, tipm );
                rotmat_c ( tipm, halfpi_c() - dec, 1, tipm );
                rotmat_c ( tipm, w,                3, tipm );

               } 

   None. 

   Error free. 

   None. 

   W.L. Taber      (JPL) 
   I.M. Underwood  (JPL) 
   E.D. Wright     (JPL)

   None. 

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

   half the value of pi