1 #include "astro.h"
2
3 void
geo(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 }
61