mirror of https://github.com/XEphem/XEphem.git
70 lines
2.3 KiB
C
70 lines
2.3 KiB
C
#include <stdio.h>
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#include <math.h>
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#include "astro.h"
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/* Precession computation to or from J2000
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* If dir < 0 go from equinox to J2000
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* If dir > 0 go from J2000 to equinox
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*/
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static void
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do_precess (
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double equinox, double *alpha, double *delta, int dir
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)
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{
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double T;
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double zeta_A, z_A, theta_A;
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double A, B, C;
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/* Precession progresses about 1 arc second in .047 years = 17 days
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* Threshold must be at most 1.7 days for 0.1 arcsec accuracy */
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double threshhold=1;
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if (fabs (equinox - J2000) < threshhold)
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return;
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T = (equinox - J2000)/36525.0;
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/* Compute coefficients in arcseconds from the Naval
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* Observatory, Her Majesty's Nautical Amanac Office;
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* Rutherford Appleton Laboratory, ISSN 0737-6421. */
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zeta_A = arcsecrad(2.650545 + T*(2306.083227+T*(0.2988499 + T*(0.01801828 +T*(-5.971e-6-3.173e-7*T)))));
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z_A = arcsecrad(-2.650545+T*(2306.077181+T*(1.0927348+T*(0.01826837 +T*(-28.596e-6-2.904e-7*T)))));
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theta_A = arcsecrad(T*(2004.191903-T*(0.4294934+T*(0.04182264+T*(7.089e-6+1.274e-7*T)))));
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if (dir<0){
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/* If dir is negative, go from equinox to 2000.0 */
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A = sin(*alpha - z_A) * cos(*delta);
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B = cos(*alpha - z_A) * cos(theta_A) * cos(*delta)
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+ sin(theta_A) * sin(*delta);
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C = -cos(*alpha - z_A) * sin(theta_A) * cos(*delta)
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+ cos(theta_A) * sin(*delta);
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*alpha = atan2(A,B) - zeta_A;
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} else {
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/* If dir is positive, go from 2000.0 to equinox */
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A = sin(*alpha + zeta_A) * cos(*delta);
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B = cos(*alpha + zeta_A) * cos(theta_A) * cos(*delta)
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- sin(theta_A) * sin(*delta);
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C = cos(*alpha + zeta_A) * sin(theta_A) * cos(*delta)
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+ cos(theta_A) * sin(*delta);
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*alpha = atan2(A,B) + z_A;
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}
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range (alpha, TWOPI);
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*delta = asin(C);
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}
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/* Corrects ra and dec, both in radians, for precession from epoch 1 to
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* epoch 2. The epochs are given by their modified JDs, from_mjd and
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* to_mjd, respectively. N.B. ra and dec are modifed IN PLACE.
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*/
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void
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precess (
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double from_mjd, double to_mjd, /* initial and final epoch modified JDs */
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double *ra, double *dec) /* ra/dec for from_mjd in, for to_mjd out */
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{
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/* From mjd1 to J2000, "backwards" inplace transformation : */
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do_precess(from_mjd, ra, dec, -1);
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/* From J2000 to mjd2, "forward" inplace transformation : */
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do_precess(to_mjd, ra, dec, 1);
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}
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