mirror of https://github.com/XEphem/XEphem.git
82 lines
2.3 KiB
C
82 lines
2.3 KiB
C
#include <math.h>
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#include "astro.h"
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/* given a modified Julian date, mj, and a set of heliocentric parabolic
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* orbital elements referred to the epoch of date (mj):
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* ep: epoch of perihelion,
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* inc: inclination,
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* ap: argument of perihelion (equals the longitude of perihelion minus the
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* longitude of ascending node)
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* qp: perihelion distance,
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* om: longitude of ascending node;
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* find:
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* lpd: heliocentric longitude,
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* psi: heliocentric latitude,
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* rp: distance from the sun to the planet,
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* rho: distance from the Earth to the planet,
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* lam: geocentric ecliptic longitude,
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* bet: geocentric ecliptic latitude,
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* none are corrected for light time, ie, they are the true values for
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* the given instant.
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*
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* all angles are in radians, all distances in AU.
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* mutual perturbation corrections with other solar system objects are not
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* applied. corrections for nutation and abberation must be made by the caller.
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* The RA and DEC calculated from the fully-corrected ecliptic coordinates are
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* then the apparent geocentric coordinates. Further corrections can be made,
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* if required, for atmospheric refraction and geocentric parallax.
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*/
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void
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comet (double mj, double ep, double inc, double ap, double qp, double om,
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double *lpd, double *psi, double *rp, double *rho, double *lam, double *bet)
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{
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double w, s, s2;
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double l, sl, cl, y;
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double spsi, cpsi;
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double rd, lsn, rsn;
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double lg, re, ll;
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double cll, sll;
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double nu;
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#define ERRLMT 0.0001
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w = ((mj-ep)*3.649116e-02)/(qp*sqrt(qp));
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s = w/3;
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for (;;) {
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double d;
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s2 = s*s;
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d = (s2+3)*s-w;
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if (fabs(d) <= ERRLMT)
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break;
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s = ((2*s*s2)+w)/(3*(s2+1));
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}
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nu = 2*atan(s);
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*rp = qp*(1+s2);
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l = nu+ap;
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sl = sin(l);
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cl = cos(l);
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spsi = sl*sin(inc);
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*psi = asin(spsi);
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y = sl*cos(inc);
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*lpd = atan(y/cl)+om;
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cpsi = cos(*psi);
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if (cl<0) *lpd += PI;
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range (lpd, 2*PI);
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rd = *rp * cpsi;
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sunpos (mj, &lsn, &rsn, 0);
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lg = lsn+PI;
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re = rsn;
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ll = *lpd - lg;
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cll = cos(ll);
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sll = sin(ll);
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*rho = sqrt((re * re)+(*rp * *rp)-(2*re*rd*cll));
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if (rd<re)
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*lam = atan((-1*rd*sll)/(re-(rd*cll)))+lg+PI;
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else
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*lam = atan((re*sll)/(rd-(re*cll)))+*lpd;
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range (lam, 2*PI);
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*bet = atan((rd*spsi*sin(*lam-*lpd))/(cpsi*re*sll));
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}
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