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/*
* Copyright (C) 2007 Red Hat, Inc.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License as
* published by the Free Software Foundation; either version 2 of the
* License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
* 02110-1301, USA.
*
* Authors:
* Jonathan Blandford <jrb@redhat.com>
* Matthias Clasen <mclasen@redhat.com>
*/
#include <time.h>
#include <gtk/gtk.h>
#include <math.h>
#include "clock-sunpos.h"
/* Calculated with the methods and figures from "Practical Astronomy With Your
* Calculator, version 3" by Peter Duffet-Smith.
*/
/* Table 6. Details of the Sun's apparent orbit at epoch 1990.0 */
#define EPOCH 2447891.5 /* days */ /* epoch 1990 */
#define UNIX_EPOCH 2440586.5 /* days */ /* epoch 1970 */
#define EPSILON_G 279.403303 /* degrees */ /* ecliptic longitude at epoch 1990.0 */
#define MU_G 282.768422 /* degrees */ /* ecliptic longitude at perigee */
#define ECCENTRICITY 0.016713 /* eccentricity of orbit */
#define R_0 149598500 /* km */ /* semi-major axis */
#define THETA_0 0.533128 /* degrees */ /* angular diameter at r = r_0 */
#define MEAN_OBLIQUITY 23.440592 /* degrees */ /* mean obliquity of earth's axis at epoch 1990.0 */
#define NORMALIZE(x) \
while (x>360) x-=360; while (x<0) x+= 360;
#define DEG_TO_RADS(x) \
(x * G_PI/180.0)
#define RADS_TO_DEG(x) \
(x * 180.0/G_PI)
/* Calculate number of days since 4713BC.
*/
static gdouble
unix_time_to_julian_date (gint unix_time)
{
return UNIX_EPOCH + (double) unix_time / (60 * 60 * 24);
}
/* Finds an iterative solution for [ E - e sin (E) = M ] for values of e less
than 0.1. Page 90 */
#define ERROR_ACCURACY 1e-6 /* radians */
static gdouble
solve_keplers_equation (gdouble e,
gdouble M)
{
gdouble d, E;
/* start with an initial estimate */
E = M;
d = E - e * sin (E) - M;
while (ABS (d) > ERROR_ACCURACY)
{
E = E - (d / (1 - e * cos (E)));
d = E - e * sin (E) - M;
}
return E;
}
/* convert the ecliptic longitude to right ascension and declination. Section 27. */
static void
ecliptic_to_equatorial (gdouble lambda,
gdouble beta,
gdouble *ra,
gdouble *dec)
{
gdouble cos_mo;
gdouble sin_mo;
g_assert (ra != NULL);
g_assert (dec != NULL);
sin_mo = sin (DEG_TO_RADS (MEAN_OBLIQUITY));
cos_mo = cos (DEG_TO_RADS (MEAN_OBLIQUITY));
*ra = atan2 (sin (lambda) * cos_mo - tan (beta) * sin_mo, cos (lambda));
*dec = asin (sin (beta) * cos_mo + cos (beta) * sin_mo * sin (lambda));
}
/* calculate GST. Section 12 */
static gdouble
greenwich_sidereal_time (gdouble unix_time)
{
gdouble u, JD, T, T0, UT;
u = fmod (unix_time, 24 * 60 * 60);
JD = unix_time_to_julian_date (unix_time - u);
T = (JD - 2451545) / 36525;
T0 = 6.697374558 + (2400.051336 * T) + (0.000025862 * T * T);
T0 = fmod (T0, 24);
UT = u / (60 * 60);
T0 = T0 + UT * 1.002737909;
T0 = fmod (T0, 24);
return T0;
}
/* Calculate the position of the sun at a given time. pages 89-91 */
void
sun_position (time_t unix_time, gdouble *lat, gdouble *lon)
{
gdouble jd, D, N, M, E, x, v, lambda;
gdouble ra, dec;
jd = unix_time_to_julian_date (unix_time);
/* Calculate number of days since the epoch */
D = jd - EPOCH;
N = D*360/365.242191;
/* normalize to 0 - 360 degrees */
NORMALIZE (N);
/* Step 4: */
M = N + EPSILON_G - MU_G;
NORMALIZE (M);
/* Step 5: convert to radians */
M = DEG_TO_RADS (M);
/* Step 6: */
E = solve_keplers_equation (ECCENTRICITY, M);
/* Step 7: */
x = sqrt ((1 + ECCENTRICITY)/(1 - ECCENTRICITY)) * tan (E/2);
/* Step 8, 9 */
v = 2 * RADS_TO_DEG (atan (x));
NORMALIZE (v);
/* Step 10 */
lambda = v + MU_G;
NORMALIZE (lambda);
/* convert the ecliptic longitude to right ascension and declination */
ecliptic_to_equatorial (DEG_TO_RADS (lambda), 0.0, &ra, &dec);
ra = ra - (G_PI/12) * greenwich_sidereal_time (unix_time);
ra = RADS_TO_DEG (ra);
dec = RADS_TO_DEG (dec);
NORMALIZE (ra);
NORMALIZE (dec);
*lat = dec;
*lon = ra;
}
#if 0
int
main (int argc, char *argv[])
{
gint i;
gint now;
GTimeVal timeval;
gdouble lat, lon;
gtk_init (&argc, &argv);
g_get_current_time (&timeval);
now = timeval.tv_sec;
for (i = 0; i < now; i += 15 * 60)
{
sun_position (i, &lat, &lon);
g_print ("%d: %f %f\n", lat, lon);
}
return 0;
}
#endif
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