source: Sophya/trunk/SophyaExt/XephemAstroLib/riset.c@ 3005

#include <stdio.h>
#include <math.h>

#include "astro.h"

/* given the true geocentric ra and dec of an object, the observer's latitude,
 *   lt, and a horizon displacement correction, dis, all in radians, find the
 *   local sidereal times and azimuths of rising and setting, lstr/s
 *   and azr/s, also all in radians, respectively.
 * dis is the vertical displacement from the true position of the horizon. it
 *   is positive if the apparent position is higher than the true position.
 *   said another way, it is positive if the shift causes the object to spend
 *   longer above the horizon. for example, atmospheric refraction is typically
 *   assumed to produce a vertical shift of 34 arc minutes at the horizon; dis
 *   would then take on the value +9.89e-3 (radians). On the other hand, if
 *   your horizon has hills such that your apparent horizon is, say, 1 degree
 *   above sea level, you would allow for this by setting dis to -1.75e-2
 *   (radians).
 *
 * This version contributed by Konrad Bernloehr, Nov. 1996
 *
 * status: 0=normal; 1=never rises; -1=circumpolar.
 * In case of non-zero status, all other returned variables are undefined.
 */
void
riset (double ra, double dec, double lt, double dis, double *lstr,
double *lsts, double *azr, double *azs, int *status)
{
#define EPS     (1e-9)  /* math rounding fudge - always the way, eh? */
        double h;               /* hour angle */
        double cos_h;           /* cos h */
        double z;               /* zenith angle */
        double zmin, zmax;      /* Minimum and maximum zenith angles */
        double xaz, yaz;        /* components of az */
        int shemi;              /* flag for southern hemisphere reflection */

        /* reflect lt and dec if in southern hemisphere, then az back later */
        if ((shemi= (lt < 0.)) != 0) {
            lt = -lt;
            dec = -dec;
        }

        /* establish zenith angle, and its extrema */
        z = (PI/2.) + dis;
        zmin = fabs (dec - lt);
        zmax = PI - fabs(dec + lt);

        /* first consider special cases.
         * these also avoid any boundary problems in subsequent computations.
         */
        if (zmax <= z + EPS) {
            *status = -1;       /* never sets */
            return;
        }
        if (zmin >= z - EPS) {
            *status = 1;        /* never rises */
            return;
        }

        /* compute rising hour angle -- beware found off */
        cos_h = (cos(z)-sin(lt)*sin(dec))/(cos(lt)*cos(dec));
        if (cos_h >= 1.)
            h =  0.;
        else if (cos_h <= -1.)
            h = PI;
        else
            h = acos (cos_h);

        /* compute setting azimuth -- beware found off */
        xaz = sin(dec)*cos(lt)-cos(dec)*cos(h)*sin(lt);
        yaz = -1.*cos(dec)*sin(h);
        if (xaz == 0.) {
            if (yaz > 0)
                *azs = PI/2;
            else
                *azs = -PI/2;
        } else
            *azs = atan2 (yaz, xaz);

        /* reflect az back if southern */
        if (shemi)
            *azs = PI - *azs;
        range(azs, 2.*PI);

        /* rising is just the opposite side */
        *azr = 2.*PI - *azs;
        range(azr, 2.*PI);

        /* rise and set are just ha either side of ra */
        *lstr = radhr(ra-h);
        range(lstr,24.0);
        *lsts = radhr(ra+h);
        range(lsts,24.0);

        /* OK */
        *status = 0;
}

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: riset.c,v $ $Date: 2005-08-21 10:02:39 $ $Revision: 1.5 $ $Name: not supported by cvs2svn $"};