geo.c (1080B)
1 #include "astro.h" 2 3 void 4 geo(void) 5 { 6 7 /* 8 * uses alpha, delta, rp 9 */ 10 11 /* 12 * sets ra, decl, lha, decl2, az, el 13 */ 14 15 /* 16 * geo converts geocentric equatorial coordinates 17 * to topocentric equatorial and topocentric horizon 18 * coordinates. 19 * All are (usually) referred to the true equator. 20 */ 21 22 double sel, saz, caz; 23 double f; 24 double sa, ca, sd; 25 26 /* 27 * convert to local hour angle and declination 28 */ 29 30 lha = gst - alpha - wlong; 31 decl = delta; 32 33 /* 34 * compute diurnal parallax (requires geocentric latitude) 35 */ 36 37 sa = cos(decl)*sin(lha); 38 ca = cos(decl)*cos(lha) - erad*cos(glat)*sin(hp); 39 sd = sin(decl) - erad*sin(glat)*sin(hp); 40 41 lha = atan2(sa, ca); 42 decl2 = atan2(sd, sqrt(sa*sa+ca*ca)); 43 f = sqrt(sa*sa+ca*ca+sd*sd); 44 semi2 = semi/f; 45 ra = gst - lha - wlong; 46 ra = pinorm(ra); 47 48 /* 49 * convert to horizon coordinates 50 */ 51 52 sel = sin(nlat)*sin(decl2) + cos(nlat)*cos(decl2)*cos(lha); 53 el = atan2(sel, pyth(sel)); 54 saz = sin(lha)*cos(decl2); 55 caz = cos(nlat)*sin(decl2) - sin(nlat)*cos(decl2)*cos(lha); 56 az = pi + atan2(saz, -caz); 57 58 az /= radian; 59 el /= radian; 60 }