source: Sophya/trunk/SophyaExt/XephemAstroLib/eq_gal.c@ 1826

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

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

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

static void galeq_aux P_((int sw, double x, double y, double *p, double *q));
static void galeq_init P_((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 mjd2000;                  /* mjd 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, *lat, and longititude, *lng, also each in
 * radians.
 */
void
eq_gal (mjd, ra, dec, lat, lng)
double mjd, ra, dec;
double *lat, *lng;
{
        galeq_init();
        precess (mjd, mjd2000, &ra, &dec);
        galeq_aux (EQtoGAL, ra, dec, lng, lat);
}

/* given galactic latitude, lat, and longititude, lng, each in radians, find
 * the corresponding equitorial ra and dec, also each in radians, at the 
 * given epoch.
 */
void
gal_eq (mjd, lat, lng, ra, dec)
double mjd, lat, lng;
double *ra, *dec;
{
        galeq_init();
        galeq_aux (GALtoEQ, lng, lat, ra, dec);
        precess (mjd2000, mjd, ra, dec);
}

static void
galeq_aux (sw, x, y, p, q)
int sw;                 /* +1 for eq to gal, -1 for vv. */
double x, y;            /* sw==1: x==ra, y==dec.  sw==-1: x==lng, y==lat. */
double *p, *q;          /* sw==1: p==lng, q==lat. 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);
            mjd2000 = J2000;
            before = 1;
        }
}

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: eq_gal.c,v $ $Date: 2001-10-22 12:08:27 $ $Revision: 1.2 $ $Name: not supported by cvs2svn $"};