SpiceDouble lspcn_c ( ConstSpiceChar * body,
SpiceDouble et,
ConstSpiceChar * abcorr )
Compute L_s, the planetocentric longitude of the sun, as seen
from a specified body.
NAIF_IDS
PCK
TIME
SPK
GEOMETRY
TIME
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.
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.
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] "The Astronomical Almanac for the Year 2005." U.S. Government
Printing Office, Washington, D.C., 1984, page L9.
N.J. Bachman (JPL)
-CSPICE Version 1.0.0, 06-JAN-2005 (NJB)
planetocentric longitude of sun
compute L_s
compute Ls
compute L_sub_s
Link to routine lspcn_c source file lspcn_c.c
|