source: Sophya/trunk/SophyaExt/XephemAstroLib/precess.c@ 1681

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

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

static void precess_hiprec P_((double mjd1, double mjd2, double *ra,
    double *dec));


#define DCOS(x)         cos(degrad(x))
#define DSIN(x)         sin(degrad(x))
#define DASIN(x)        raddeg(asin(x))
#define DATAN2(y,x)     raddeg(atan2((y),(x)))

/* corrects ra and dec, both in radians, for precession from epoch 1 to epoch 2.
 * the epochs are given by their modified JDs, mjd1 and mjd2, respectively.
 * N.B. ra and dec are modifed IN PLACE.
 */
void
precess (mjd1, mjd2, ra, dec)
double mjd1, mjd2;      /* initial and final epoch modified JDs */
double *ra, *dec;       /* ra/dec for mjd1 in, for mjd2 out */
{
        precess_hiprec (mjd1, mjd2, ra, dec);
}

/*
 * Copyright (c) 1990 by Craig Counterman. All rights reserved.
 *
 * This software may be redistributed freely, not sold.
 * This copyright notice and disclaimer of warranty must remain
 *    unchanged. 
 *
 * No representation is made about the suitability of this
 * software for any purpose.  It is provided "as is" without express or
 * implied warranty, to the extent permitted by applicable law.
 *
 * Rigorous precession. From Astronomical Ephemeris 1989, p. B18
 *
 * 96-06-20 Hayo Hase <hase@wettzell.ifag.de>: theta_a corrected
 */
static void
precess_hiprec (mjd1, mjd2, ra, dec)
double mjd1, mjd2;      /* initial and final epoch modified JDs */
double *ra, *dec;       /* ra/dec for mjd1 in, for mjd2 out */
{
        static double last_mjd1 = -213.432, last_from;
        static double last_mjd2 = -213.432, last_to;
        double zeta_A, z_A, theta_A;
        double T;
        double A, B, C;
        double alpha, delta;
        double alpha_in, delta_in;
        double from_equinox, to_equinox;
        double alpha2000, delta2000;

        /* convert mjds to years;
         * avoid the remarkably expensive calls to mjd_year()
         */
        if (last_mjd1 == mjd1)
            from_equinox = last_from;
        else {
            mjd_year (mjd1, &from_equinox);
            last_mjd1 = mjd1;
            last_from = from_equinox;
        }
        if (last_mjd2 == mjd2)
            to_equinox = last_to;
        else {
            mjd_year (mjd2, &to_equinox);
            last_mjd2 = mjd2;
            last_to = to_equinox;
        }

        /* convert coords in rads to degs */
        alpha_in = raddeg(*ra);
        delta_in = raddeg(*dec);

        /* precession progresses about 1 arc second in .047 years */
        /* From from_equinox to 2000.0 */
        if (fabs (from_equinox-2000.0) > .02) {
            T = (from_equinox - 2000.0)/100.0;
            zeta_A  = 0.6406161* T + 0.0000839* T*T + 0.0000050* T*T*T;
            z_A     = 0.6406161* T + 0.0003041* T*T + 0.0000051* T*T*T;
            theta_A = 0.5567530* T - 0.0001185* T*T - 0.0000116* T*T*T;

            A = DSIN(alpha_in - z_A) * DCOS(delta_in);
            B = DCOS(alpha_in - z_A) * DCOS(theta_A) * DCOS(delta_in)
              + DSIN(theta_A) * DSIN(delta_in);
            C = -DCOS(alpha_in - z_A) * DSIN(theta_A) * DCOS(delta_in)
              + DCOS(theta_A) * DSIN(delta_in);

            alpha2000 = DATAN2(A,B) - zeta_A;
            range (&alpha2000, 360.0);
            delta2000 = DASIN(C);
        } else {
            /* should get the same answer, but this could improve accruacy */
            alpha2000 = alpha_in;
            delta2000 = delta_in;
        };


        /* From 2000.0 to to_equinox */
        if (fabs (to_equinox - 2000.0) > .02) {
            T = (to_equinox - 2000.0)/100.0;
            zeta_A  = 0.6406161* T + 0.0000839* T*T + 0.0000050* T*T*T;
            z_A     = 0.6406161* T + 0.0003041* T*T + 0.0000051* T*T*T;
            theta_A = 0.5567530* T - 0.0001185* T*T - 0.0000116* T*T*T;

            A = DSIN(alpha2000 + zeta_A) * DCOS(delta2000);
            B = DCOS(alpha2000 + zeta_A) * DCOS(theta_A) * DCOS(delta2000)
              - DSIN(theta_A) * DSIN(delta2000);
            C = DCOS(alpha2000 + zeta_A) * DSIN(theta_A) * DCOS(delta2000)
              + DCOS(theta_A) * DSIN(delta2000);

            alpha = DATAN2(A,B) + z_A;
            range(&alpha, 360.0);
            delta = DASIN(C);
        } else {
            /* should get the same answer, but this could improve accruacy */
            alpha = alpha2000;
            delta = delta2000;
        };

        *ra = degrad(alpha);
        *dec = degrad(delta);
}

#if 0
static void
precess_fast (mjd1, mjd2, ra, dec)
double mjd1, mjd2;      /* initial and final epoch modified JDs */
double *ra, *dec;       /* ra/dec for mjd1 in, for mjd2 out */
{
#define N       degrad (20.0468/3600.0)
#define M       hrrad (3.07234/3600.0)
        double nyrs;

        nyrs = (mjd2 - mjd1)/365.2425;
        *dec += N * cos(*ra) * nyrs;
        *ra += (M + (N * sin(*ra) * tan(*dec))) * nyrs;
        range (ra, 2.0*PI);
}
#endif

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: precess.c,v $ $Date: 2001-04-10 14:40:47 $ $Revision: 1.1.1.1 $ $Name: not supported by cvs2svn $"};