mirror of https://github.com/XEphem/XEphem.git
98 lines
2.2 KiB
C
98 lines
2.2 KiB
C
/* code to convert between equitorial and galactic coordinates */
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#include <stdio.h>
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#include <math.h>
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#include "astro.h"
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static void galeq_aux (int sw, double x, double y, double *p, double *q);
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static void galeq_init (void);
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#define EQtoGAL 1
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#define GALtoEQ (-1)
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#define SMALL (1e-20)
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static double an = degrad(32.93192); /* G lng of asc node on equator */
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static double gpr = degrad(192.85948); /* RA of North Gal Pole, 2000 */
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static double gpd = degrad(27.12825); /* Dec of " */
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static double cgpd, sgpd; /* cos() and sin() of gpd */
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static double mj2000; /* mj of 2000 */
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static int before; /* whether these have been set yet */
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/* given ra and dec, each in radians, for the given epoch, find the
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* corresponding galactic latitude, *lt, and longititude, *lg, also each in
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* radians.
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*/
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void
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eq_gal (double mj, double ra, double dec, double *lt, double *lg)
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{
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galeq_init();
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precess (mj, mj2000, &ra, &dec);
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galeq_aux (EQtoGAL, ra, dec, lg, lt);
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}
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/* given galactic latitude, lt, and longititude, lg, each in radians, find
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* the corresponding equitorial ra and dec, also each in radians, at the
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* given epoch.
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*/
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void
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gal_eq (double mj, double lt, double lg, double *ra, double *dec)
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{
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galeq_init();
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galeq_aux (GALtoEQ, lg, lt, ra, dec);
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precess (mj2000, mj, ra, dec);
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}
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static void
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galeq_aux (
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int sw, /* +1 for eq to gal, -1 for vv. */
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double x, double y, /* sw==1: x==ra, y==dec. sw==-1: x==lg, y==lt. */
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double *p, double *q) /* sw==1: p==lg, q==lt. sw==-1: p==ra, q==dec. */
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{
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double sy, cy, a, ca, sa, b, sq, c, d;
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cy = cos(y);
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sy = sin(y);
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a = x - an;
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if (sw == EQtoGAL)
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a = x - gpr;
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ca = cos(a);
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sa = sin(a);
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b = sa;
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if (sw == EQtoGAL)
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b = ca;
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sq = (cy*cgpd*b) + (sy*sgpd);
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*q = asin (sq);
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if (sw == GALtoEQ) {
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c = cy*ca;
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d = (sy*cgpd) - (cy*sgpd*sa);
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if (fabs(d) < SMALL)
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d = SMALL;
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*p = atan (c/d) + gpr;
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} else {
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c = sy - (sq*sgpd);
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d = cy*sa*cgpd;
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if (fabs(d) < SMALL)
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d = SMALL;
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*p = atan (c/d) + an;
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}
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if (d < 0) *p += PI;
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if (*p < 0) *p += 2*PI;
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if (*p > 2*PI) *p -= 2*PI;
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}
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/* set up the definitions */
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static void
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galeq_init()
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{
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if (!before) {
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cgpd = cos (gpd);
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sgpd = sin (gpd);
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mj2000 = J2000;
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before = 1;
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}
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}
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