source: Sophya/trunk/SophyaExt/XephemAstroLib/rings.c@ 1719

#include <stdio.h>
#include <math.h>
#if defined(__STDC__)
#include <stdlib.h>
#endif

#include "P_.h"
#include "astro.h"
#include "circum.h"

/*  RINGS OF SATURN by Olson, et al, BASIC Code from Sky & Telescope, May 1995.
 *  As converted from BASIC to C by pmartz@dsd.es.com (Paul Martz)
 *  Adapted to xephem by Elwood Charles Downey.
 */
void
satrings (sb, sl, sr, el, er, JD, etiltp, stiltp)
double sb, sl, sr;              /* Saturn hlat, hlong, sun dist */
double el, er;                  /* Earth hlong, sun dist */
double JD;                      /* Julian date */
double *etiltp, *stiltp;        /* tilt from earth and sun, rads southward */
{
        double t, i, om;
        double x, y, z;
        double la, be;
        double s, b, sp, bp;

        t = (JD-2451545.)/365250.;
        i = degrad(28.04922-.13*t+.0004*t*t);
        om = degrad(169.53+13.826*t+.04*t*t);

        x = sr*cos(sb)*cos(sl)-er*cos(el);
        y = sr*cos(sb)*sin(sl)-er*sin(el);
        z = sr*sin(sb);

        la = atan(y/x);
        if (x<0) la+=PI;
        be = atan(z/sqrt(x*x+y*y));

        s = sin(i)*cos(be)*sin(la-om)-cos(i)*sin(be);
        b = atan(s/sqrt(1.-s*s));
        sp = sin(i)*cos(sb)*sin(sl-om)-cos(i)*sin(sb);
        bp = atan(sp/sqrt(1.-sp*sp));

        *etiltp = b;
        *stiltp = bp;
}