mirror of https://github.com/XEphem/XEphem.git
49 lines
1.3 KiB
C
49 lines
1.3 KiB
C
#include <math.h>
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#include "astro.h"
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#undef sqr
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#define sqr(x) ((x)*(x))
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/* given a planet, the sun, the planet's eq pole position and a
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* position of a satellite (as eq x=+e y=+s z=front in planet radii) find x,y
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* position of shadow.
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* return 0 if ok else -1 if shadow not on planet
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*/
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int
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plshadow (Obj *op, Obj *sop, double polera, double poledec, double x,
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double y, double z, float *sxp, float *syp)
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{
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/* equatorial to ecliptic sky-plane rotation */
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double sa = cos(op->s_dec) * cos(poledec) *
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(cos(op->s_ra)*sin(polera) - sin(op->s_ra)*cos(polera));
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double ca = sqrt (1.0 - sa*sa);
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/* rotate moon from equatorial to ecliptic */
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double ex = x*ca + y*sa;
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double ey = -x*sa + y*ca;
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/* find angle subtended by earth-sun from planet */
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double a = asin (sin(op->s_hlong - sop->s_hlong)/op->s_edist);
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double b = asin (-sin(op->s_hlat)/op->s_edist);
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/* find displacement in sky plane */
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double x0 = ex - z*tan(a);
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double y0 = ey - z*tan(b);
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/* projection onto unit sphere */
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double x1 = x0 + (ex-x0)/sqrt(sqr(ex-x0)+sqr(z));
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double y1 = y0 + (ey-y0)/sqrt(sqr(ey-y0)+sqr(z));
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/* check behind or off edge */
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if (z < 0 || sqr(x1) + sqr(y1) > 1)
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return (-1);
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/* rotate back to equatorial */
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*sxp = x1*ca - y1*sa;
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*syp = x1*sa + y1*ca;
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return (0);
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}
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