source: Sophya/trunk/SophyaExt/XephemAstroLib/eq_gal.c@ 3891

/* code to convert between equitorial and galactic coordinates */

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

#include "astro.h"

static void galeq_aux (int sw, double x, double y, double *p, double *q);
static void galeq_init (void);

#define EQtoGAL 1
#define GALtoEQ (-1)
#define SMALL   (1e-20)

static double an = degrad(32.93192);    /* G lng of asc node on equator */
static double gpr = degrad(192.85948);  /* RA of North Gal Pole, 2000 */
static double gpd = degrad(27.12825);   /* Dec of  " */
static double cgpd, sgpd;               /* cos() and sin() of gpd */
static double mj2000;                   /* mj of 2000 */
static int before;                      /* whether these have been set yet */

/* given ra and dec, each in radians, for the given epoch, find the
 * corresponding galactic latitude, *lt, and longititude, *lg, also each in
 * radians.
 */
void
eq_gal (double mj, double ra, double dec, double *lt, double *lg)
{
        galeq_init();
        precess (mj, mj2000, &ra, &dec);
        galeq_aux (EQtoGAL, ra, dec, lg, lt);
}

/* given galactic latitude, lt, and longititude, lg, each in radians, find
 * the corresponding equitorial ra and dec, also each in radians, at the 
 * given epoch.
 */
void
gal_eq (double mj, double lt, double lg, double *ra, double *dec)
{
        galeq_init();
        galeq_aux (GALtoEQ, lg, lt, ra, dec);
        precess (mj2000, mj, ra, dec);
}

static void
galeq_aux (
int sw,                 /* +1 for eq to gal, -1 for vv. */
double x, double y,     /* sw==1: x==ra, y==dec.  sw==-1: x==lg, y==lt. */
double *p, double *q)   /* sw==1: p==lg, q==lt. sw==-1: p==ra, q==dec. */
{
        double sy, cy, a, ca, sa, b, sq, c, d; 

        cy = cos(y);
        sy = sin(y);
        a = x - an;
        if (sw == EQtoGAL)
            a = x - gpr;
        ca = cos(a);
        sa = sin(a);
        b = sa;
        if (sw == EQtoGAL)
            b = ca;
        sq = (cy*cgpd*b) + (sy*sgpd);
        *q = asin (sq);

        if (sw == GALtoEQ) {
            c = cy*ca;
            d = (sy*cgpd) - (cy*sgpd*sa);
            if (fabs(d) < SMALL)
                d = SMALL;
            *p = atan (c/d) + gpr;
        } else {
            c = sy - (sq*sgpd);
            d = cy*sa*cgpd;
            if (fabs(d) < SMALL)
                d = SMALL;
            *p = atan (c/d) + an;
        }

        if (d < 0) *p += PI;
        if (*p < 0) *p += 2*PI;
        if (*p > 2*PI) *p -= 2*PI;
}

/* set up the definitions */
static void
galeq_init()
{
        if (!before) {
            cgpd = cos (gpd);
            sgpd = sin (gpd);
            mj2000 = J2000;
            before = 1;
        }
}

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: eq_gal.c,v $ $Date: 2009-07-16 10:34:37 $ $Revision: 1.8 $ $Name: not supported by cvs2svn $"};