Compute L_s, the planetocentric longitude of the sun, as seen 
   from a specified body. 

   NAIF_IDS 
   PCK 
   TIME 
   SPK 

   Variable  I/O  Description 
   --------  ---  -------------------------------------------------- 
   body       I   Name of central body. 
   et         I   Epoch in seconds past J2000 TDB. 
   abcorr     I   Aberration correction. 

   The function returns the value of L_s for the specified body 
   at the specified time. 

   body        is the name of the central body, typically a planet. 

   et          is the epoch at which the longitude of the sun (L_s) is
               to be computed. `et' is expressed as seconds past J2000
               TDB (Barycentric Dynamical Time).

   abcorr      indicates the aberration corrections to be applied 
               when computing the longitude of the sun.  `abcorr' 
               may be any of the following. 

                  "NONE"     Apply no correction. 

                  "LT"       Correct the position of the sun, 
                             relative to the central body, for 
                             planetary (light time) aberration. 

                  "LT+S"     Correct the position of the sun, 
                             relative to the central body, for 
                             planetary and stellar aberrations. 

   The function returns the planetocentric longitude of the sun, 
   often called "L_s," for the specified body at the specified time. 
   This is the longitude of the body-sun vector in a right-handed 
   frame whose basis vectors are defined as follows: 

      - The positive Z direction is given by the instantaneous 
        angular velocity vector of the orbit of the body about 
        the sun. 

      - The positive X direction is that of the cross product of the 
        instantaneous north spin axis of the body with the
        positive Z direction.

      - The positive Y direction is Z x X. 

   Units are radians; the range is 0 to 2*pi.  Longitudes are 
   positive to the east. 

   None. 

   The direction of the vernal equinox for the central body is 
   determined from the instantaneous equatorial and orbital planes 
   of the central body.  This equinox definition is specified in 
   reference [1].  The "instantaneous orbital plane" is interpreted 
   in this routine as the plane normal to the cross product of the 
   position and velocity of the central body relative to the sun. 
   The geometric state of the central body relative to the sun is 
   used for this normal vector computation. The "instantaneous 
   equatorial plane" is normal to the central body's north pole 
   at the requested epoch.  The pole direction is determined from 
   rotational elements loaded via a PCK file. 

   The result returned by this routine will depend on the 
   ephemeris data and rotational elements used.  The result may 
   differ from that given in any particular version of the 
   Astronomical Almanac, due to differences in these input data, 
   and due to differences in precision of the computations. 

   1) A simple program that computes L_s for a body and time 
      supplied interactively.  The geometric state of the sun is 
      used. 

         #include <stdio.h>
         #include "SpiceUsr.h"

         int main()
         {
            #define  ABCORR         "NONE" 
            #define  FILSIZ         256
            #define  NAMLEN         37
            #define  TIMLEN         41
            #define  ABCORR         "NONE" 

            SpiceChar               body   [ NAMLEN ];
            SpiceChar               lsk    [ FILSIZ ];
            SpiceChar               pck    [ FILSIZ ];
            SpiceChar               spk    [ FILSIZ ];
            SpiceChar               timstr [ TIMLEN ];

            SpiceDouble             et;
            SpiceDouble             lon;

            prompt_c ( "Enter name of leapseconds kernel > ", FILSIZ, lsk ); 
            prompt_c ( "Enter name of PCK file           > ", FILSIZ, pck ); 
            prompt_c ( "Enter name of SPK file           > ", FILSIZ, spk ); 

            furnsh_c ( spk );
            furnsh_c ( lsk ); 
            furnsh_c ( pck ); 

            printf ( "\n"
                     "Kernels have been loaded.\n"
                     "\n"                           );

            while ( SPICETRUE )
            { 
               prompt_c ( "Enter name of central body       > ",  
                          NAMLEN, 
                          body                                   ); 
               prompt_c ( "Enter calendar, JD, or DOY time  > ",  
                          TIMLEN, 
                          timstr                                 );

               str2et_c ( timstr, &et ); 

               /.
               Convert longitude to degrees. 
               ./
               lon = dpr_c() * lspcn_c ( body, et, ABCORR );

               printf ( "\n"
                        "Central body              = %s\n"
                        "Time                      = %s\n"
                        "Planetocentric L_s (deg.) = %f\n"
                        "\n",
                        body,
                        timstr,
                        lon                               );
            }
            return ( 0 );
         }

   None. 

   1) If the input body name cannot be translated to an ID code, 
      and if the name is not a string representation of an integer 
      (for example, "399"), the error SPICE(NOTRANSLATION) is 
      signaled. 

   2) If no SPK (ephemeris) file has been loaded prior to calling 
      this routine, or if the SPK data has insufficient coverage, an 
      error will be diagnosed and signaled by a routine in the call 
      tree of this routine. 

   3) If a PCK file containing rotational elements for the central 
      body has not been loaded prior to calling this routine, an 
      error will be diagnosed and signaled by a routine called by a 
      routine in the call tree of this routine. 

   4) If the instantaneous angular velocity and spin axis of `body' 
      are parallel, the error will be diagnosed and signaled by a 
      routine in the call tree of this routine. 

   5) The error SPICE(EMPTYSTRING) is signaled if the input
      string `body' does not contain at least one character, since the
      input string cannot be converted to a Fortran-style string in
      this case.

   6) The error SPICE(NULLPOINTER) is signaled if the input string
      pointer `body' is null.

   1) An SPK file (or file) containing ephemeris data sufficient to 
      compute the geometric state of the central body relative to 
      the sun at `et' must be loaded before this routine is called. If 
      light time correction is used, data must be available that 
      enable computation of the state the sun relative to the solar 
      system barycenter at the light-time corrected epoch.  If 
      stellar aberration correction is used, data must be available 
      that enable computation of the state the central body relative 
      to the solar system barycenter at `et'. 

   2) A PCK file containing rotational elements for the central body 
      must be loaded before this routine is called. 

   N.J. Bachman       (JPL) 

   [1] "The Astronomical Almanac for the Year 2005." U.S. Government 
       Printing Office, Washington, D.C., 1984, page L9. 

   -CSPICE Version 1.0.0, 06-JAN-2005 (NJB)

   planetocentric longitude of sun 
   compute L_s 
   compute Ls 
   compute L_sub_s