2 * sunsolver.cxx - given a location on earth and a time of day/date,
3 * find the number of seconds to various sun positions.
5 * Written by Curtis Olson, started September 2003.
7 * Copyright (C) 2003 Curtis L. Olson - http://www.flightgear.org/~curt
9 * This program is free software; you can redistribute it and/or
10 * modify it under the terms of the GNU General Public License as
11 * published by the Free Software Foundation; either version 2 of the
12 * License, or (at your option) any later version.
14 * This program is distributed in the hope that it will be useful, but
15 * WITHOUT ANY WARRANTY; without even the implied warranty of
16 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
17 * General Public License for more details.
19 * You should have received a copy of the GNU General Public License
20 * along with this program; if not, write to the Free Software
21 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
30 #ifdef SG_HAVE_STD_INCLUDES
41 #include <simgear/math/SGMath.hxx>
42 #include <simgear/ephemeris/ephemeris.hxx>
43 #include <simgear/timing/sg_time.hxx>
45 #include <Main/globals.hxx>
48 #include "sunsolver.hxx"
51 static const time_t day_secs = 86400;
52 static const time_t half_day_secs = day_secs / 2;
53 static const time_t step_secs = 60;
55 /* given a particular time expressed in side real time at prime
56 * meridian (GST), compute position on the earth (lat, lon) such that
57 * sun is directly overhead. (lat, lon are reported in radians */
59 void fgSunPositionGST(double gst, double *lon, double *lat) {
60 /* time_t ssue; seconds since unix epoch */
61 /* double *lat; (return) latitude */
62 /* double *lon; (return) longitude */
67 double beta = globals->get_ephem()->get_sun()->getLat();
68 double r = globals->get_ephem()->get_sun()->getDistance();
69 double xs = globals->get_ephem()->get_sun()->getxs();
70 double ys = globals->get_ephem()->get_sun()->getys();
71 double ye = globals->get_ephem()->get_sun()->getye();
72 double ze = globals->get_ephem()->get_sun()->getze();
73 alpha = atan2(ys - tan(beta)*ze/ys, xs);
74 delta = asin(sin(beta)*ye/ys + cos(beta)*ze);
76 tmp = alpha - (SGD_2PI/24)*gst;
79 while (tmp < -SGD_PI);
80 } else if (tmp > SGD_PI) {
82 while (tmp < -SGD_PI);
89 static double sun_angle( const SGTime &t, const SGVec3d& world_up,
90 double lon_rad, double lat_rad ) {
91 SG_LOG( SG_EVENT, SG_DEBUG, " Updating Sun position" );
92 SG_LOG( SG_EVENT, SG_DEBUG, " Gst = " << t.getGst() );
94 double sun_lon, sun_gd_lat;
95 fgSunPositionGST( t.getGst(), &sun_lon, &sun_gd_lat );
96 SGVec3d sunpos = SGVec3d::fromGeod(SGGeod::fromRad(sun_lon, sun_gd_lat));
98 SG_LOG( SG_EVENT, SG_DEBUG, " t.cur_time = " << t.get_cur_time() );
99 SG_LOG( SG_EVENT, SG_DEBUG,
100 " Sun Geodetic lat = " << sun_gd_lat );
102 // calculate the sun's relative angle to local up
103 SGVec3f nup = normalize(toVec3f(world_up));
104 SGVec3f nsun = normalize(toVec3f(sunpos));
105 // cout << "nup = " << nup[0] << "," << nup[1] << ","
106 // << nup[2] << endl;
107 // cout << "nsun = " << nsun[0] << "," << nsun[1] << ","
108 // << nsun[2] << endl;
110 double sun_angle = acos( dot( nup, nsun ) );
111 double sun_angle_deg = sun_angle * SG_RADIANS_TO_DEGREES;
112 while ( sun_angle_deg < -180 ) { sun_angle += 360; }
113 SG_LOG( SG_EVENT, SG_DEBUG, "sun angle relative to current location = "
116 return sun_angle_deg;
121 * Given the current unix time in seconds, calculate seconds to the
122 * specified sun angle (relative to straight up.) Also specify if we
123 * want the angle while the sun is ascending or descending. For
124 * instance noon is when the sun angle is 0 (or the closest it can
125 * get.) Dusk is when the sun angle is 90 and descending. Dawn is
126 * when the sun angle is 90 and ascending.
128 time_t fgTimeSecondsUntilSunAngle( time_t cur_time,
131 double target_angle_deg,
134 // cout << "location = " << lon_rad * SG_RADIANS_TO_DEGREES << ", "
135 // << lat_rad * SG_RADIANS_TO_DEGREES << endl;
136 SGVec3d world_up = SGVec3d::fromGeod(SGGeod::fromRad(lon_rad, lat_rad));
137 SGTime t = SGTime( lon_rad, lat_rad, "", 0 );
139 double best_diff = 180.0;
140 double last_angle = -99999.0;
141 time_t best_time = cur_time;
143 for ( time_t secs = cur_time - half_day_secs;
144 secs < cur_time + half_day_secs;
147 t.update( lon_rad, lat_rad, secs, 0 );
148 double angle_deg = sun_angle( t, world_up, lon_rad, lat_rad );
149 double diff = fabs( angle_deg - target_angle_deg );
150 if ( diff < best_diff ) {
151 if ( last_angle <= 180.0 && ascending
152 && ( last_angle > angle_deg ) ) {
153 // cout << "best angle = " << angle << " offset = "
154 // << secs - cur_time << endl;
157 } else if ( last_angle <= 180.0 && !ascending
158 && ( last_angle < angle_deg ) ) {
159 // cout << "best angle = " << angle << " offset = "
160 // << secs - cur_time << endl;
166 last_angle = angle_deg;
169 return best_time - cur_time;