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Дата изменения: Sat Dec 17 06:09:15 2005 Дата индексирования: Sun Apr 10 23:26:19 2016 Кодировка: Поисковые слова: annular solar eclipse |
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