source: Sophya/trunk/SophyaExt/XephemAstroLib/plshadow.c@ 3654

#include <math.h>

#include "astro.h"

#undef sqr
#define sqr(x)          ((x)*(x))

/* given a planet, the sun, the planet's eq pole position and a
 * position of a satellite (as eq x=+e y=+s z=front in planet radii) find x,y
 * position of shadow.
 * return 0 if ok else -1 if shadow not on planet
 */
int
plshadow (Obj *op, Obj *sop, double polera, double poledec, double x,
double y, double z, float *sxp, float *syp)
{
        /* equatorial to ecliptic sky-plane rotation */
        double sa = cos(op->s_dec) * cos(poledec) *
                        (cos(op->s_ra)*sin(polera) - sin(op->s_ra)*cos(polera));
        double ca = sqrt (1.0 - sa*sa);

        /* rotate moon from equatorial to ecliptic */
        double ex =  x*ca + y*sa;
        double ey = -x*sa + y*ca;

        /* find angle subtended by earth-sun from planet */
        double a = asin (sin(op->s_hlong - sop->s_hlong)/op->s_edist);
        double b = asin (-sin(op->s_hlat)/op->s_edist);

        /* find displacement in sky plane */
        double x0 = ex - z*tan(a);
        double y0 = ey - z*tan(b);

        /* projection onto unit sphere */
        double x1 = x0 + (ex-x0)/sqrt(sqr(ex-x0)+sqr(z));
        double y1 = y0 + (ey-y0)/sqrt(sqr(ey-y0)+sqr(z));

        /* check behind or off edge */
        if (z < 0 || sqr(x1) + sqr(y1) > 1)
            return (-1);

        /* rotate back to equatorial */
        *sxp = x1*ca - y1*sa;
        *syp = x1*sa + y1*ca;

        return (0);
}

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: plshadow.c,v $ $Date: 2009-07-16 10:34:38 $ $Revision: 1.6 $ $Name: not supported by cvs2svn $"};