8 * uses alpha, delta, rp
12 * sets ra, decl, lha, decl2, az, el
16 * geo converts geocentric equatorial coordinates
17 * to topocentric equatorial and topocentric horizon
19 * All are (usually) referred to the true equator.
27 * convert to local hour angle and declination
30 lha = gst - alpha - wlong;
34 * compute diurnal parallax (requires geocentric latitude)
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);
42 decl2 = atan2(sd, sqrt(sa*sa+ca*ca));
43 f = sqrt(sa*sa+ca*ca+sd*sd);
45 ra = gst - lha - wlong;
49 * convert to horizon coordinates
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);