source: Sophya/trunk/SophyaExt/XephemAstroLib/parallactic.c@ 4017

/* compure parallactic angle: angle formed by N pole - Object - Zenith
 */

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

#include "astro.h"

/* compute parallactic angle given latitude, object dec and alt.
 * all angles in rads.
 * N.B. always return >= 0, caller must determine sign and degenerate cases at
 *   pole or zenith.
 */
double
parallacticLDA (double lt, double dec, double alt)
{
        double ca = sin(lt);
        double cb = sin(dec);
        double sb = cos(dec);
        double cc = sin(alt);
        double sc = cos(alt);
        double cpa;

        /* given three sides find an angle */
        if (sb==0 || sc==0)
            return (0);
        cpa = (ca - cb*cc)/(sb*sc);
        if (cpa < -1) cpa = -1;
        if (cpa >  1) cpa =  1;
        return (acos (cpa));
}

/* compute parallactic angle given latitude, object HA and Dec.
 * all angles in rads.
 * return value is between -PI and PI, sign is like HA, ie +west
 */
double
parallacticLHD (double lt, double ha, double dec)
{
        double A, b, cc, sc, B;

        A = ha;
        b = PI/2 - lt;
        cc = sin(dec);
        sc = cos(dec);
        solve_sphere (A, b, cc, sc, NULL, &B);

        if (B > PI)
            B -= 2*PI;
        return (B);
}

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: parallactic.c,v $ $Date: 2011-09-21 16:17:50 $ $Revision: 1.7 $ $Name: not supported by cvs2svn $"};