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Index Page lspcn_c
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Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries
#### Procedure
       lspcn_c ( Longitude of the sun, planetocentric )

       SpiceDouble lspcn_c ( ConstSpiceChar   * body,
                             SpiceDouble        et,
                             ConstSpiceChar   * abcorr )
#### Abstract
       Compute L_s, the planetocentric longitude of the sun, as seen
       from a specified body.
#### Required_Reading
       NAIF_IDS
       PCK
       TIME
       SPK
#### Keywords
       GEOMETRY
       TIME
#### Brief_I/O
       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.
#### Detailed_Input
       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.
#### Detailed_Output
       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.
#### Parameters
       None.
#### Exceptions
       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 by a routine in the call tree of this routine.

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

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

       5)  If the `body' input string pointer is null, the error
           SPICE(NULLPOINTER) is signaled. The function returns the value
           0.0.

       6)  If the `body' input string has zero length, the error
           SPICE(EMPTYSTRING) is signaled. The function returns the value
           0.0.
#### Files
       Appropriate SPICE kernels must be loaded by the calling program
       before this routine is called.

       The following data are required:

       -  An SPK file (or files) 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'.

       -  A PCK file containing rotational elements for the central body
          must be loaded before this routine is called.
#### Particulars
       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.
#### Examples
       The numerical results shown for this example may differ across
       platforms. The results depend on the SPICE kernels used as
       input, the compiler and supporting libraries, and the machine
       specific arithmetic implementation.

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

          Example code begins here.

          /.
             Program lspcn_ex1
          ./
          #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"                           );

             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 );
          }

          When this program was executed on a Mac/Intel/cc/64-bit
          platform, using the LSK file named naif0012.tls, the PCK file
          named pck00010.tpc, the SPK file named de421.bsp; the "EARTH"
          as central body and the time string "2018 Mar 8  17:59 UTC",
          the output was:

          Enter name of leapseconds kernel > naif0012.tls
          Enter name of PCK file           > pck00010.tpc
          Enter name of SPK file           > de421.bsp

          Kernels have been loaded.

          Enter name of central body       > EARTH
          Enter calendar, JD, or DOY time  > 2018 Mar 8  17:59 UTC

          Central body              = EARTH
          Time                      = 2018 Mar 8  17:59 UTC
          Planetocentric L_s (deg.) = 348.115940
#### Restrictions
       None.
#### Literature_References
       [1]  "The Astronomical Almanac for the Year 2005," page L9,
            United States Naval Observatory, U.S. Government Printing
            Office, Washington, D.C., 2004.
#### Author_and_Institution
       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
#### Version
       -CSPICE Version 1.0.1, 25-AUG-2021 (JDR)

           Edited the header to comply to comply with NAIF standard. Removed while
           loop from example code, and added solution.

       -CSPICE Version 1.0.0, 06-JAN-2005 (NJB)
#### Index_Entries
       planetocentric longitude of Sun
       compute L_s
       compute Ls
       compute L_sub_s
#### Link to routine lspcn_c source file lspcn_c.c

    Fri Dec 31 18:41:09 2021