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

       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.

       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.

#### Files

       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.

#### 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

       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 );
             }

#### Restrictions

       None.

#### Literature_References

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

#### Author_and_Institution

       N.J. Bachman       (JPL)

#### Version

       -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

    Wed Apr  5 17:54:38 2017