ref: e55cb65c247a9cdfa0528d34dec70d435898d098
dir: /sys/src/cmd/astro/geo.c/
#include "astro.h" void geo(void) { /* * uses alpha, delta, rp */ /* * sets ra, decl, lha, decl2, az, el */ /* * geo converts geocentric equatorial coordinates * to topocentric equatorial and topocentric horizon * coordinates. * All are (usually) referred to the true equator. */ double sel, saz, caz; double f; double sa, ca, sd; /* * convert to local hour angle and declination */ lha = gst - alpha - wlong; decl = delta; /* * compute diurnal parallax (requires geocentric latitude) */ sa = cos(decl)*sin(lha); ca = cos(decl)*cos(lha) - erad*cos(glat)*sin(hp); sd = sin(decl) - erad*sin(glat)*sin(hp); lha = atan2(sa, ca); decl2 = atan2(sd, sqrt(sa*sa+ca*ca)); f = sqrt(sa*sa+ca*ca+sd*sd); semi2 = semi/f; ra = gst - lha - wlong; ra = pinorm(ra); /* * convert to horizon coordinates */ sel = sin(nlat)*sin(decl2) + cos(nlat)*cos(decl2)*cos(lha); el = atan2(sel, pyth(sel)); saz = sin(lha)*cos(decl2); caz = cos(nlat)*sin(decl2) - sin(nlat)*cos(decl2)*cos(lha); az = pi + atan2(saz, -caz); az /= radian; el /= radian; }