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 }