| lspcn_c |
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Table of contents
Procedure
lspcn_c ( Longitude of the sun, planetocentric )
SpiceDouble lspcn_c ( ConstSpiceChar * body,
SpiceDouble et,
ConstSpiceChar * abcorr )
AbstractCompute L_s, the planetocentric longitude of the sun, as seen from a specified body. Required_ReadingNAIF_IDS PCK TIME SPK KeywordsGEOMETRY TIME Brief_I/OVARIABLE 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.
ParametersNone. 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.
ParticularsThe 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
RestrictionsNone. 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_InstitutionN.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_Entriesplanetocentric longitude of Sun compute L_s compute Ls compute L_sub_s |
Fri Dec 31 18:41:09 2021