tgeo.c - plan9port - [fork] Plan 9 from user space
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       ---
       tgeo.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 }