source: Sophya/trunk/SophyaExt/XephemAstroLib/parallax.c@ 1719

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

#include "P_.h"
#include "astro.h"


/* given true ha and dec, tha and tdec, the geographical latitude, phi, the
 * height above sea-level (as a fraction of the earths radius, 6378.16km),
 * ht, and the geocentric distance rho in Earth radii(!), find the apparent
 * ha and dec, aha and adec allowing for parallax.
 * all angles in radians. ehp is the angle subtended at the body by the
 * earth's equator.
 */
void
ta_par (tha, tdec, phi, ht, rho, aha, adec)
double tha, tdec, phi, ht, *rho;
double *aha, *adec;
{
        static double last_phi = 1000.0, last_ht = -1000.0, xobs, zobs;
        double x, y, z; /* obj cartesian coord, in Earth radii */

        /* avoid calcs involving the same phi and ht */
        if (phi != last_phi || ht != last_ht) {
            double cphi, sphi, robs, e2 = (2 - 1/298.257)/298.257;
            cphi = cos(phi);
            sphi = sin(phi);
            robs = 1/sqrt(1 - e2 * sphi * sphi);

            /* observer coordinates: x to meridian, y east, z north */
            xobs = (robs + ht) * cphi;
            zobs = (robs*(1-e2) + ht) * sphi;
            last_phi  =  phi;
            last_ht  =  ht;
        }

        sphcart(-tha, tdec, *rho, &x, &y, &z);
        cartsph(x - xobs, y, z - zobs, aha, adec, rho);
        *aha *= -1;
        range (aha, 2*PI);
}

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: parallax.c,v $ $Date: 2001-10-22 12:08:27 $ $Revision: 1.2 $ $Name: not supported by cvs2svn $"};