1 // moonpos.cxx (basically, this is a slightly modified version of the
2 // 'sunpos.cxx' file, adapted from XEarth)
7 // code for calculating the position on the earth's surface for which
8 // the moon is directly overhead (adapted from _practical astronomy
9 // with your calculator, third edition_, peter duffett-smith,
10 // cambridge university press, 1988.)
12 // Copyright (C) 1989, 1990, 1993, 1994, 1995 Kirk Lauritz Johnson
14 // Parts of the source code (as marked) are:
15 // Copyright (C) 1989, 1990, 1991 by Jim Frost
16 // Copyright (C) 1992 by Jamie Zawinski <jwz@lucid.com>
18 // Permission to use, copy, modify and freely distribute xearth for
19 // non-commercial and not-for-profit purposes is hereby granted
20 // without fee, provided that both the above copyright notice and this
21 // permission notice appear in all copies and in supporting
24 // The author makes no representations about the suitability of this
25 // software for any purpose. It is provided "as is" without express or
28 // THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
29 // INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS,
30 // IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT
31 // OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
32 // LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
33 // NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
34 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
43 #include <simgear/compiler.h>
45 #ifdef FG_HAVE_STD_INCLUDES
55 #include <simgear/constants.h>
56 #include <simgear/debug/logstream.hxx>
57 #include <simgear/ephemeris/ephemeris.hxx>
58 #include <simgear/math/point3d.hxx>
59 #include <simgear/math/polar3d.hxx>
60 #include <simgear/math/sg_geodesy.hxx>
61 #include <simgear/math/vector.hxx>
63 #include <Main/globals.hxx>
64 #include <Scenery/scenery.hxx>
65 #include <Time/light.hxx>
67 #include "moonpos.hxx"
73 * the epoch upon which these astronomical calculations are based is
74 * 1990 january 0.0, 631065600 seconds since the beginning of the
75 * "unix epoch" (00:00:00 GMT, Jan. 1, 1970)
77 * given a number of seconds since the start of the unix epoch,
78 * DaysSinceEpoch() computes the number of days since the start of the
79 * astronomical epoch (1990 january 0.0)
82 #define EpochStart (631065600)
83 #define DaysSinceEpoch(secs) (((secs)-EpochStart)*(1.0/(24*3600)))
86 * assuming the apparent orbit of the moon about the earth is circular,
87 * the rate at which the orbit progresses is given by RadsPerDay --
88 * FG_2PI radians per orbit divided by 365.242191 days per year:
91 #define RadsPerDay (FG_2PI/365.242191)
94 * details of moon's apparent orbit at epoch 1990.0 (after
95 * duffett-smith, table 6, section 46)
97 * Epsilon_g (ecliptic longitude at epoch 1990.0) 279.403303 degrees
98 * OmegaBar_g (ecliptic longitude of perigee) 282.768422 degrees
99 * Eccentricity (eccentricity of orbit) 0.016713
102 #define Epsilon_g (279.403303*(FG_2PI/360))
103 #define OmegaBar_g (282.768422*(FG_2PI/360))
104 #define Eccentricity (0.016713)
107 * MeanObliquity gives the mean obliquity of the earth's axis at epoch
108 * 1990.0 (computed as 23.440592 degrees according to the method given
109 * in duffett-smith, section 27)
111 #define MeanObliquity (23.440592*(FG_2PI/360))
113 /* static double solve_keplers_equation(double); */
114 /* static double moon_ecliptic_longitude(time_t); */
115 static void ecliptic_to_equatorial(double, double, double *, double *);
116 static double julian_date(int, int, int);
117 static double GST(time_t);
120 * solve Kepler's equation via Newton's method
121 * (after duffett-smith, section 47)
124 static double solve_keplers_equation(double M) {
130 delta = E - Eccentricity*sin(E) - M;
131 if (fabs(delta) <= 1e-10) break;
132 E -= delta / (1 - Eccentricity*cos(E));
140 /* compute ecliptic longitude of moon (in radians) (after
141 * duffett-smith, section 47) */
143 static double moon_ecliptic_longitude(time_t ssue) {
144 // time_t ssue; // seconds since unix epoch
149 D = DaysSinceEpoch(ssue);
153 if (N < 0) N += FG_2PI;
155 M_moon = N + Epsilon_g - OmegaBar_g;
156 if (M_moon < 0) M_moon += FG_2PI;
158 E = solve_keplers_equation(M_moon);
159 v = 2 * atan(sqrt((1+Eccentricity)/(1-Eccentricity)) * tan(E/2));
161 return (v + OmegaBar_g);
166 /* convert from ecliptic to equatorial coordinates (after
167 * duffett-smith, section 27) */
169 static void ecliptic_to_equatorial(double lambda, double beta,
170 double *alpha, double *delta) {
171 /* double lambda; ecliptic longitude */
172 /* double beta; ecliptic latitude */
173 /* double *alpha; (return) right ascension */
174 /* double *delta; (return) declination */
179 sin_e = sin(MeanObliquity);
180 cos_e = cos(MeanObliquity);
184 *alpha = atan2(sin_l*cos_e - tan(beta)*sin_e, cos_l);
185 *delta = asin(sin(beta)*cos_e + cos(beta)*sin_e*sin_l);
189 /* computing julian dates (assuming gregorian calendar, thus this is
190 * only valid for dates of 1582 oct 15 or later) (after duffett-smith,
193 static double julian_date(int y, int m, int d) {
194 /* int y; year (e.g. 19xx) */
195 /* int m; month (jan=1, feb=2, ...) */
196 /* int d; day of month */
201 /* lazy test to ensure gregorian calendar */
203 FG_LOG( FG_EVENT, FG_ALERT,
204 "WHOOPS! Julian dates only valid for 1582 oct 15 or later" );
207 if ((m == 1) || (m == 2)) {
214 C = (int)(365.25 * y);
215 D = (int)(30.6001 * (m + 1));
217 JD = B + C + D + d + 1720994.5;
223 /* compute greenwich mean sidereal time (GST) corresponding to a given
224 * number of seconds since the unix epoch (after duffett-smith,
226 static double GST(time_t ssue) {
227 /* time_t ssue; seconds since unix epoch */
236 JD = julian_date(tm->tm_year+1900, tm->tm_mon+1, tm->tm_mday);
237 T = (JD - 2451545) / 36525;
239 T0 = ((T + 2.5862e-5) * T + 2400.051336) * T + 6.697374558;
242 if (T0 < 0) T0 += 24;
244 UT = tm->tm_hour + (tm->tm_min + tm->tm_sec / 60.0) / 60.0;
246 T0 += UT * 1.002737909;
248 if (T0 < 0) T0 += 24;
254 /* given a particular time (expressed in seconds since the unix
255 * epoch), compute position on the earth (lat, lon) such that moon is
256 * directly overhead. (lat, lon are reported in radians */
258 void fgMoonPosition(time_t ssue, double *lon, double *lat) {
259 /* time_t ssue; seconds since unix epoch */
260 /* double *lat; (return) latitude */
261 /* double *lon; (return) longitude */
267 /* lambda = moon_ecliptic_longitude(ssue); */
268 /* ecliptic_to_equatorial(lambda, 0.0, &alpha, &delta); */
269 //ecliptic_to_equatorial (solarPosition.lonMoon, 0.0, &alpha, &delta);
271 /* **********************************************************************
272 * NOTE: in the next function, each time the moon's position is updated, the
273 * the moon's longitude is returned from solarSystem->moon. Note that the
274 * moon's position is updated at a much higher frequency than the rate at
275 * which the solar system's rebuilds occur. This is not a problem, however,
276 * because the fgMoonPosition we're talking about here concerns the changing
277 * position of the moon due to the daily rotation of the earth.
278 * The ecliptic longitude, however, represents the position of the moon with
279 * respect to the stars, and completes just one cycle over the course of a
280 * year. Its therefore pretty safe to update the moon's longitude only once
281 * every ten minutes. (Comment added by Durk Talsma).
282 ************************************************************************/
284 ecliptic_to_equatorial( globals->get_ephem()->get_moon()->getLon(),
285 0.0, &alpha, &delta );
286 tmp = alpha - (FG_2PI/24)*GST(ssue);
289 while (tmp < -FG_PI);
290 } else if (tmp > FG_PI) {
292 while (tmp < -FG_PI);
300 /* given a particular time expressed in side real time at prime
301 * meridian (GST), compute position on the earth (lat, lon) such that
302 * moon is directly overhead. (lat, lon are reported in radians */
304 static void fgMoonPositionGST(double gst, double *lon, double *lat) {
305 /* time_t ssue; seconds since unix epoch */
306 /* double *lat; (return) latitude */
307 /* double *lon; (return) longitude */
313 /* lambda = moon_ecliptic_longitude(ssue); */
314 /* ecliptic_to_equatorial(lambda, 0.0, &alpha, &delta); */
315 //ecliptic_to_equatorial (solarPosition.lonMoon, 0.0, &alpha, &delta);
316 ecliptic_to_equatorial( globals->get_ephem()->get_moon()->getLon(),
317 globals->get_ephem()->get_moon()->getLat(),
320 // tmp = alpha - (FG_2PI/24)*GST(ssue);
321 tmp = alpha - (FG_2PI/24)*gst;
324 while (tmp < -FG_PI);
325 } else if (tmp > FG_PI) {
327 while (tmp < -FG_PI);
335 // update the cur_time_params structure with the current moon position
336 void fgUpdateMoonPos( void ) {
340 Point3D p, rel_moonpos;
341 double dot, east_dot;
342 double moon_gd_lat, sl_radius;
344 // vector in cartesian coordinates from current position to the
345 // postion on the earth's surface the moon is directly over
348 // surface direction to go to head towards moon
349 sgVec3 surface_to_moon;
351 l = &cur_light_params;
352 SGTime *t = globals->get_time_params();
353 v = (FGViewerRPH *)globals->get_current_view();
355 FG_LOG( FG_EVENT, FG_INFO, " Updating Moon position" );
357 // (not sure why there was two)
358 // fgMoonPosition(t->cur_time, &l->moon_lon, &moon_gd_lat);
359 fgMoonPositionGST(t->getGst(), &l->moon_lon, &moon_gd_lat);
361 sgGeodToGeoc(moon_gd_lat, 0.0, &sl_radius, &l->moon_gc_lat);
363 p = Point3D( l->moon_lon, l->moon_gc_lat, sl_radius );
364 l->fg_moonpos = sgPolarToCart3d(p);
366 FG_LOG( FG_EVENT, FG_INFO, " t->cur_time = " << t->get_cur_time() );
367 FG_LOG( FG_EVENT, FG_INFO,
368 " Moon Geodetic lat = " << moon_gd_lat
369 << " Geocentric lat = " << l->moon_gc_lat );
371 // update the sun light vector
372 sgSetVec4( l->moon_vec,
373 l->fg_moonpos.x(), l->fg_moonpos.y(), l->fg_moonpos.z(), 0.0 );
374 sgNormalizeVec4( l->moon_vec );
375 sgCopyVec4( l->moon_vec_inv, l->moon_vec );
376 sgNegateVec4( l->moon_vec_inv );
378 // make sure these are directional light sources only
379 l->moon_vec[3] = l->moon_vec_inv[3] = 0.0;
380 // cout << " l->moon_vec = " << l->moon_vec[0] << "," << l->moon_vec[1]
381 // << ","<< l->moon_vec[2] << endl;
383 // calculate the moon's relative angle to local up
384 sgCopyVec3( nup, v->get_world_up() );
385 sgSetVec3( nmoon, l->fg_moonpos.x(), l->fg_moonpos.y(), l->fg_moonpos.z() );
386 sgNormalizeVec3(nup);
387 sgNormalizeVec3(nmoon);
388 // cout << "nup = " << nup[0] << "," << nup[1] << ","
389 // << nup[2] << endl;
390 // cout << "nmoon = " << nmoon[0] << "," << nmoon[1] << ","
391 // << nmoon[2] << endl;
393 l->moon_angle = acos( sgScalarProductVec3( nup, nmoon ) );
394 FG_LOG( FG_EVENT, FG_INFO, "moon angle relative to current location = "
397 // calculate vector to moon's position on the earth's surface
398 Point3D vp( v->get_view_pos()[0],
399 v->get_view_pos()[1],
400 v->get_view_pos()[2] );
401 rel_moonpos = l->fg_moonpos - ( vp + scenery.center );
402 sgSetVec3( to_moon, rel_moonpos.x(), rel_moonpos.y(), rel_moonpos.z() );
403 // printf( "Vector to moon = %.2f %.2f %.2f\n",
404 // to_moon[0], to_moon[1], to_moon[2]);
406 // Given a vector from the view position to the point on the
407 // earth's surface the moon is directly over, map into onto the
408 // local plane representing "horizontal".
410 sgmap_vec_onto_cur_surface_plane( v->get_world_up(), v->get_view_pos(),
411 to_moon, surface_to_moon );
412 sgNormalizeVec3(surface_to_moon);
413 // cout << "(sg) Surface direction to moon is "
414 // << surface_to_moon[0] << ","
415 // << surface_to_moon[1] << ","
416 // << surface_to_moon[2] << endl;
417 // cout << "Should be close to zero = "
418 // << sgScalarProductVec3(nup, surface_to_moon) << endl;
420 // calculate the angle between v->surface_to_moon and
421 // v->surface_east. We do this so we can sort out the acos()
422 // ambiguity. I wish I could think of a more efficient way ... :-(
423 east_dot = sgScalarProductVec3( surface_to_moon, v->get_surface_east() );
424 // cout << " East dot product = " << east_dot << endl;
426 // calculate the angle between v->surface_to_moon and
427 // v->surface_south. this is how much we have to rotate the sky
428 // for it to align with the moon
429 dot = sgScalarProductVec3( surface_to_moon, v->get_surface_south() );
430 // cout << " Dot product = " << dot << endl;
432 if ( east_dot >= 0 ) {
433 l->moon_rotation = acos(dot);
435 l->moon_rotation = -acos(dot);
437 // cout << " Sky needs to rotate = " << angle << " rads = "
438 // << angle * RAD_TO_DEG << " degrees." << endl;