6 * code for calculating the position on the earth's surface for which
7 * the sun is directly overhead (adapted from _practical astronomy
8 * with your calculator, third edition_, peter duffett-smith,
9 * cambridge university press, 1988.)
13 * Copyright (C) 1989, 1990, 1993, 1994, 1995 Kirk Lauritz Johnson
15 * Parts of the source code (as marked) are:
16 * Copyright (C) 1989, 1990, 1991 by Jim Frost
17 * Copyright (C) 1992 by Jamie Zawinski <jwz@lucid.com>
19 * Permission to use, copy, modify and freely distribute xearth for
20 * non-commercial and not-for-profit purposes is hereby granted
21 * without fee, provided that both the above copyright notice and this
22 * permission notice appear in all copies and in supporting
25 * The author makes no representations about the suitability of this
26 * software for any purpose. It is provided "as is" without express or
29 * THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
30 * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS,
31 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT
32 * OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
33 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
34 * NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
35 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
38 * (Log is kept at end of this file)
48 #include "../Include/constants.h"
49 #include "../Main/views.h"
50 #include "../Math/fg_geodesy.h"
51 #include "../Math/mat3.h"
52 #include "../Math/polar.h"
59 * the epoch upon which these astronomical calculations are based is
60 * 1990 january 0.0, 631065600 seconds since the beginning of the
61 * "unix epoch" (00:00:00 GMT, Jan. 1, 1970)
63 * given a number of seconds since the start of the unix epoch,
64 * DaysSinceEpoch() computes the number of days since the start of the
65 * astronomical epoch (1990 january 0.0)
68 #define EpochStart (631065600)
69 #define DaysSinceEpoch(secs) (((secs)-EpochStart)*(1.0/(24*3600)))
72 * assuming the apparent orbit of the sun about the earth is circular,
73 * the rate at which the orbit progresses is given by RadsPerDay --
74 * FG_2PI radians per orbit divided by 365.242191 days per year:
77 #define RadsPerDay (FG_2PI/365.242191)
80 * details of sun's apparent orbit at epoch 1990.0 (after
81 * duffett-smith, table 6, section 46)
83 * Epsilon_g (ecliptic longitude at epoch 1990.0) 279.403303 degrees
84 * OmegaBar_g (ecliptic longitude of perigee) 282.768422 degrees
85 * Eccentricity (eccentricity of orbit) 0.016713
88 #define Epsilon_g (279.403303*(FG_2PI/360))
89 #define OmegaBar_g (282.768422*(FG_2PI/360))
90 #define Eccentricity (0.016713)
93 * MeanObliquity gives the mean obliquity of the earth's axis at epoch
94 * 1990.0 (computed as 23.440592 degrees according to the method given
95 * in duffett-smith, section 27)
97 #define MeanObliquity (23.440592*(FG_2PI/360))
99 static double solve_keplers_equation(double);
100 static double sun_ecliptic_longitude(time_t);
101 static void ecliptic_to_equatorial(double, double, double *, double *);
102 static double julian_date(int, int, int);
103 static double GST(time_t);
106 * solve Kepler's equation via Newton's method
107 * (after duffett-smith, section 47)
109 static double solve_keplers_equation(double M) {
115 delta = E - Eccentricity*sin(E) - M;
116 if (fabs(delta) <= 1e-10) break;
117 E -= delta / (1 - Eccentricity*cos(E));
124 /* compute ecliptic longitude of sun (in radians) (after
125 * duffett-smith, section 47) */
127 static double sun_ecliptic_longitude(time_t ssue) {
128 /* time_t ssue; seconds since unix epoch */
133 D = DaysSinceEpoch(ssue);
137 if (N < 0) N += FG_2PI;
139 M_sun = N + Epsilon_g - OmegaBar_g;
140 if (M_sun < 0) M_sun += FG_2PI;
142 E = solve_keplers_equation(M_sun);
143 v = 2 * atan(sqrt((1+Eccentricity)/(1-Eccentricity)) * tan(E/2));
145 return (v + OmegaBar_g);
149 /* convert from ecliptic to equatorial coordinates (after
150 * duffett-smith, section 27) */
152 static void ecliptic_to_equatorial(double lambda, double beta,
153 double *alpha, double *delta) {
154 /* double lambda; ecliptic longitude */
155 /* double beta; ecliptic latitude */
156 /* double *alpha; (return) right ascension */
157 /* double *delta; (return) declination */
161 sin_e = sin(MeanObliquity);
162 cos_e = cos(MeanObliquity);
164 *alpha = atan2(sin(lambda)*cos_e - tan(beta)*sin_e, cos(lambda));
165 *delta = asin(sin(beta)*cos_e + cos(beta)*sin_e*sin(lambda));
169 /* computing julian dates (assuming gregorian calendar, thus this is
170 * only valid for dates of 1582 oct 15 or later) (after duffett-smith,
173 static double julian_date(int y, int m, int d) {
174 /* int y; year (e.g. 19xx) */
175 /* int m; month (jan=1, feb=2, ...) */
176 /* int d; day of month */
181 /* lazy test to ensure gregorian calendar */
183 printf("WHOOPS! Julian dates only valid for 1582 oct 15 or later\n");
186 if ((m == 1) || (m == 2)) {
194 D = 30.6001 * (m + 1);
196 JD = B + C + D + d + 1720994.5;
202 /* compute greenwich mean sidereal time (GST) corresponding to a given
203 * number of seconds since the unix epoch (after duffett-smith,
205 static double GST(time_t ssue) {
206 /* time_t ssue; seconds since unix epoch */
215 JD = julian_date(tm->tm_year+1900, tm->tm_mon+1, tm->tm_mday);
216 T = (JD - 2451545) / 36525;
218 T0 = ((T + 2.5862e-5) * T + 2400.051336) * T + 6.697374558;
221 if (T0 < 0) T0 += 24;
223 UT = tm->tm_hour + (tm->tm_min + tm->tm_sec / 60.0) / 60.0;
225 T0 += UT * 1.002737909;
227 if (T0 < 0) T0 += 24;
233 /* given a particular time (expressed in seconds since the unix
234 * epoch), compute position on the earth (lat, lon) such that sun is
235 * directly overhead. (lat, lon are reported in radians */
237 void fgSunPosition(time_t ssue, double *lon, double *lat) {
238 /* time_t ssue; seconds since unix epoch */
239 /* double *lat; (return) latitude */
240 /* double *lon; (return) longitude */
246 lambda = sun_ecliptic_longitude(ssue);
247 ecliptic_to_equatorial(lambda, 0.0, &alpha, &delta);
249 tmp = alpha - (FG_2PI/24)*GST(ssue);
252 while (tmp < -FG_PI);
253 } else if (tmp > FG_PI) {
255 while (tmp < -FG_PI);
263 /* update the cur_time_params structure with the current sun position */
264 void fgUpdateSunPos() {
269 double sun_gd_lat, sl_radius, temp;
270 static int time_warp = 0;
272 l = &cur_light_params;
273 t = &cur_time_params;
276 printf(" Updating Sun position\n");
278 time_warp += 0; /* increase this to make the world spin real fast */
280 fgSunPosition(t->cur_time + time_warp, &l->sun_lon, &sun_gd_lat);
282 fgGeodToGeoc(sun_gd_lat, 0.0, &sl_radius, &l->sun_gc_lat);
284 l->fg_sunpos = fgPolarToCart(l->sun_lon, l->sun_gc_lat, sl_radius);
286 /* printf(" Geodetic lat = %.5f Geocentric lat = %.5f\n", sun_gd_lat,
289 /* FALSE! (?> the sun position has to be translated just like
291 /* l->sun_vec_inv[0] = l->fg_sunpos.x - scenery_center.x; */
292 /* l->sun_vec_inv[1] = l->fg_sunpos.y - scenery_center.y; */
293 /* l->sun_vec_inv[2] = l->fg_sunpos.z - scenery_center.z; */
294 /* MAT3_SCALE_VEC(l->sun_vec, l->sun_vec_inv, -1.0); */
296 /* I think this will work better for generating the sun light vector */
297 l->sun_vec[0] = l->fg_sunpos.x;
298 l->sun_vec[1] = l->fg_sunpos.y;
299 l->sun_vec[2] = l->fg_sunpos.z;
300 MAT3_NORMALIZE_VEC(l->sun_vec, temp);
301 MAT3_SCALE_VEC(l->sun_vec_inv, l->sun_vec, -1.0);
303 /* make these are directional light sources only */
305 l->sun_vec_inv[3] = 0.0;
307 /* calculate the sun's relative angle to local up */
308 MAT3_COPY_VEC(nup, v->local_up);
309 nsun[0] = l->fg_sunpos.x;
310 nsun[1] = l->fg_sunpos.y;
311 nsun[2] = l->fg_sunpos.z;
312 MAT3_NORMALIZE_VEC(nup, temp);
313 MAT3_NORMALIZE_VEC(nsun, temp);
315 l->sun_angle = acos(MAT3_DOT_PRODUCT(nup, nsun));
316 printf(" SUN ANGLE relative to current location = %.3f rads.\n",
322 /* Revision 1.19 1997/12/30 20:47:59 curt
323 /* Integrated new event manager with subsystem initializations.
325 * Revision 1.18 1997/12/23 04:58:40 curt
326 * Tweaked the sky coloring a bit to build in structures to allow finer rgb
329 * Revision 1.17 1997/12/15 23:55:08 curt
330 * Add xgl wrappers for debugging.
331 * Generate terrain normals on the fly.
333 * Revision 1.16 1997/12/11 04:43:57 curt
334 * Fixed sun vector and lighting problems. I thing the moon is now lit
337 * Revision 1.15 1997/12/10 22:37:55 curt
338 * Prepended "fg" on the name of all global structures that didn't have it yet.
339 * i.e. "struct WEATHER {}" became "struct fgWEATHER {}"
341 * Revision 1.14 1997/12/09 04:25:39 curt
342 * Working on adding a global lighting params structure.
344 * Revision 1.13 1997/11/25 19:25:42 curt
345 * Changes to integrate Durk's moon/sun code updates + clean up.
347 * Revision 1.12 1997/11/15 18:15:39 curt
348 * Reverse direction of sun vector, so object normals can be more normal.
350 * Revision 1.11 1997/10/28 21:07:21 curt
351 * Changed GLUT/ -> Main/
353 * Revision 1.10 1997/09/13 02:00:09 curt
354 * Mostly working on stars and generating sidereal time for accurate star
357 * Revision 1.9 1997/09/05 14:17:31 curt
358 * More tweaking with stars.
360 * Revision 1.8 1997/09/05 01:36:04 curt
361 * Working on getting stars right.
363 * Revision 1.7 1997/09/04 02:17:40 curt
366 * Revision 1.6 1997/08/27 03:30:37 curt
367 * Changed naming scheme of basic shared structures.
369 * Revision 1.5 1997/08/22 21:34:41 curt
370 * Doing a bit of reorganizing and house cleaning.
372 * Revision 1.4 1997/08/19 23:55:09 curt
373 * Worked on better simulating real lighting.
375 * Revision 1.3 1997/08/13 20:23:49 curt
376 * The interface to sunpos now updates a global structure rather than returning
377 * current sun position.
379 * Revision 1.2 1997/08/06 00:24:32 curt
380 * Working on correct real time sun lighting.
382 * Revision 1.1 1997/08/01 15:27:56 curt