source: Sophya/trunk/SophyaExt/XephemAstroLib/comet.c@ 2643

#include <math.h>

#include "astro.h"

/* given a modified Julian date, mj, and a set of heliocentric parabolic
 * orbital elements referred to the epoch of date (mj):
 *   ep:   epoch of perihelion,
 *   inc:  inclination,
 *   ap:   argument of perihelion (equals the longitude of perihelion minus the
 *         longitude of ascending node)
 *   qp:   perihelion distance,
 *   om:   longitude of ascending node;
 * find:
 *   lpd:  heliocentric longitude, 
 *   psi:  heliocentric latitude,
 *   rp:   distance from the sun to the planet, 
 *   rho:  distance from the Earth to the planet,
 *   lam:  geocentric ecliptic longitude, 
 *   bet:  geocentric ecliptic latitude,
 *         none are corrected for light time, ie, they are the true values for
 *         the given instant.
 *
 * all angles are in radians, all distances in AU.
 * mutual perturbation corrections with other solar system objects are not
 * applied. corrections for nutation and abberation must be made by the caller.
 * The RA and DEC calculated from the fully-corrected ecliptic coordinates are
 * then the apparent geocentric coordinates. Further corrections can be made,
 * if required, for atmospheric refraction and geocentric parallax.
 */
void
comet (double mj, double ep, double inc, double ap, double qp, double om,
double *lpd, double *psi, double *rp, double *rho, double *lam, double *bet)
{
        double w, s, s2;
        double l, sl, cl, y;
        double spsi, cpsi;
        double rd, lsn, rsn;
        double lg, re, ll;
        double cll, sll;
        double nu;

#define ERRLMT  0.0001
        w = ((mj-ep)*3.649116e-02)/(qp*sqrt(qp));
        s = w/3;
        for (;;) {
            double d;
            s2 = s*s;
            d = (s2+3)*s-w;
            if (fabs(d) <= ERRLMT)
                break;
            s = ((2*s*s2)+w)/(3*(s2+1));
        }

        nu = 2*atan(s);
        *rp = qp*(1+s2);
        l = nu+ap;
        sl = sin(l);
        cl = cos(l);
        spsi = sl*sin(inc);
        *psi = asin(spsi);
        y = sl*cos(inc);
        *lpd = atan(y/cl)+om;
        cpsi = cos(*psi);
        if (cl<0) *lpd += PI;
        range (lpd, 2*PI);
        rd = *rp * cpsi;
        sunpos (mj, &lsn, &rsn, 0);
        lg = lsn+PI;
        re = rsn;
        ll = *lpd - lg;
        cll = cos(ll);
        sll = sin(ll);
        *rho = sqrt((re * re)+(*rp * *rp)-(2*re*rd*cll));
        if (rd<re) 
            *lam = atan((-1*rd*sll)/(re-(rd*cll)))+lg+PI;
        else
            *lam = atan((re*sll)/(rd-(re*cll)))+*lpd;
        range (lam, 2*PI);
        *bet = atan((rd*spsi*sin(*lam-*lpd))/(cpsi*re*sll));
}

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: comet.c,v $ $Date: 2005-01-17 10:13:04 $ $Revision: 1.4 $ $Name: not supported by cvs2svn $"};