source: Sophya/trunk/SophyaExt/XephemAstroLib/aberration.c@ 2897

/* aberration, Jean Meeus, "Astronomical Algorithms", Willman-Bell, 1995;
 * based on secular unperturbed Kepler orbit
 *
 * the corrections should be applied to ra/dec and lam/beta at the 
 * epoch of date.
 */

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

#include "astro.h"

#define ABERR_CONST     (20.49552/3600./180.*PI)  /* aberr const in rad */
#define AB_ECL_EOD      0
#define AB_EQ_EOD       1

static void ab_aux (double mj, double *x, double *y, double lsn, int mode);

/* apply aberration correction to ecliptical coordinates *lam and *bet
 * (in radians) for a given time m and handily supplied longitude of sun,
 * lsn (in radians)
 */
void
ab_ecl (double mj, double lsn, double *lam, double *bet)
{
        ab_aux(mj, lam, bet, lsn, AB_ECL_EOD);
}

/* apply aberration correction to equatoreal coordinates *ra and *dec
 * (in radians) for a given time m and handily supplied longitude of sun,
 * lsn (in radians)
 */
void
ab_eq (double mj, double lsn, double *ra, double *dec)
{
        ab_aux(mj, ra, dec, lsn, AB_EQ_EOD);
}

/* because the e-terms are secular, keep the real transformation for both
 * coordinate systems in here with the secular variables cached.
 * mode == AB_ECL_EOD:  x = lam, y = bet        (ecliptical)
 * mode == AB_EQ_EOD:   x = ra,  y = dec        (equatoreal)
 */
static void
ab_aux (double mj, double *x, double *y, double lsn, int mode)
{
        static double lastmj = -10000;
        static double eexc;     /* earth orbit excentricity */
        static double leperi;   /* ... and longitude of perihelion */
        static char dirty = 1;  /* flag for cached trig terms */

        if (mj != lastmj) {
            double T;           /* centuries since J2000 */

            T = (mj - J2000)/36525.;
            eexc = 0.016708617 - (42.037e-6 + 0.1236e-6 * T) * T;
            leperi = degrad(102.93735 + (0.71953 + 0.00046 * T) * T);
            lastmj = mj;
            dirty = 1;
        }

        switch (mode) {
        case AB_ECL_EOD:                /* ecliptical coords */
            {
                double *lam = x, *bet = y;
                double dlsun, dlperi;

                dlsun = lsn - *lam;
                dlperi = leperi - *lam;

                /* valid only for *bet != +-PI/2 */
                *lam -= ABERR_CONST/cos(*bet) * (cos(dlsun) -
                                eexc*cos(dlperi));
                *bet -= ABERR_CONST*sin(*bet) * (sin(dlsun) -
                                eexc*sin(dlperi));
            }
            break;

        case AB_EQ_EOD:                 /* equatoreal coords */
            {
                double *ra = x, *dec = y;
                double sr, cr, sd, cd, sls, cls;/* trig values coords */
                static double cp, sp, ce, se;   /* .. and perihel/eclipic */
                double dra, ddec;               /* changes in ra and dec */

                if (dirty) {
                    double eps;

                    cp = cos(leperi);
                    sp = sin(leperi);
                    obliquity(mj, &eps);
                    se = sin(eps);
                    ce = cos(eps);
                    dirty = 0;
                }

                sr = sin(*ra);
                cr = cos(*ra);
                sd = sin(*dec);
                cd = cos(*dec);
                sls = sin(lsn);
                cls = cos(lsn);

                dra = ABERR_CONST/cd * ( -(cr * cls * ce + sr * sls) +
                            eexc * (cr * cp * ce + sr * sp));

                ddec = se/ce * cd - sr * sd;    /* tmp use */
                ddec = ABERR_CONST * ( -(cls * ce * ddec + cr * sd * sls) +
                            eexc * (cp * ce * ddec + cr * sd * sp) );
                
                *ra += dra;
                range (ra, 2*PI);
                *dec += ddec;
            }
            break;

        default:
            printf ("ab_aux: bad mode: %d\n", mode);
            abort();
            break;

        } /* switch (mode) */
}

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: aberration.c,v $ $Date: 2005-08-21 10:02:36 $ $Revision: 1.5 $ $Name: not supported by cvs2svn $"};