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.
33 #include <simgear/math/SGMath.hxx>
34 #include <simgear/timing/sg_time.hxx>
36 #include <Main/globals.hxx>
37 #include <Main/fg_props.hxx>
40 #include "sunsolver.hxx"
43 static const time_t day_secs = 86400;
44 static const time_t half_day_secs = day_secs / 2;
45 static const time_t step_secs = 60;
47 /* given a particular time expressed in side real time at prime
48 * meridian (GST), compute position on the earth (lat, lon) such that
49 * sun is directly overhead. (lat, lon are reported in radians */
51 void fgSunPositionGST(double gst, double *lon, double *lat) {
52 /* time_t ssue; seconds since unix epoch */
53 /* double *lat; (return) latitude */
54 /* double *lon; (return) longitude */
59 SGPropertyNode* sun = fgGetNode("/ephemeris/sun");
61 double beta = sun->getDoubleValue("lat-deg");
62 // double r = globals->get_ephem()->get_sun()->getDistance();
63 double xs = sun->getDoubleValue("xs");
64 double ys = sun->getDoubleValue("ys");
65 double ye = sun->getDoubleValue("ye");
66 double ze = sun->getDoubleValue("ze");
67 alpha = atan2(ys - tan(beta)*ze/ys, xs);
68 delta = asin(sin(beta)*ye/ys + cos(beta)*ze);
70 tmp = alpha - (SGD_2PI/24)*gst;
73 while (tmp < -SGD_PI);
74 } else if (tmp > SGD_PI) {
76 while (tmp < -SGD_PI);
83 static double sun_angle( const SGTime &t, const SGVec3d& world_up,
84 double lon_rad, double lat_rad ) {
85 SG_LOG( SG_EVENT, SG_DEBUG, " Updating Sun position" );
86 SG_LOG( SG_EVENT, SG_DEBUG, " Gst = " << t.getGst() );
88 double sun_lon, sun_gd_lat;
89 fgSunPositionGST( t.getGst(), &sun_lon, &sun_gd_lat );
90 SGVec3d sunpos = SGVec3d::fromGeod(SGGeod::fromRad(sun_lon, sun_gd_lat));
92 SG_LOG( SG_EVENT, SG_DEBUG, " t.cur_time = " << t.get_cur_time() );
93 SG_LOG( SG_EVENT, SG_DEBUG,
94 " Sun Geodetic lat = " << sun_gd_lat );
96 // calculate the sun's relative angle to local up
97 SGVec3f nup = normalize(toVec3f(world_up));
98 SGVec3f nsun = normalize(toVec3f(sunpos));
99 // cout << "nup = " << nup[0] << "," << nup[1] << ","
100 // << nup[2] << endl;
101 // cout << "nsun = " << nsun[0] << "," << nsun[1] << ","
102 // << nsun[2] << endl;
104 double sun_angle = acos( dot( nup, nsun ) );
105 double sun_angle_deg = sun_angle * SG_RADIANS_TO_DEGREES;
106 while ( sun_angle_deg < -180 ) { sun_angle += 360; }
107 SG_LOG( SG_EVENT, SG_DEBUG, "sun angle relative to current location = "
110 return sun_angle_deg;
115 * Given the current unix time in seconds, calculate seconds to the
116 * specified sun angle (relative to straight up.) Also specify if we
117 * want the angle while the sun is ascending or descending. For
118 * instance noon is when the sun angle is 0 (or the closest it can
119 * get.) Dusk is when the sun angle is 90 and descending. Dawn is
120 * when the sun angle is 90 and ascending.
122 time_t fgTimeSecondsUntilSunAngle( time_t cur_time,
125 double target_angle_deg,
128 // cout << "location = " << lon_rad * SG_RADIANS_TO_DEGREES << ", "
129 // << lat_rad * SG_RADIANS_TO_DEGREES << endl;
130 SGVec3d world_up = SGVec3d::fromGeod(SGGeod::fromRad(lon_rad, lat_rad));
131 SGTime t = SGTime( lon_rad, lat_rad, "", 0 );
133 double best_diff = 180.0;
134 double last_angle = -99999.0;
135 time_t best_time = cur_time;
137 for ( time_t secs = cur_time - half_day_secs;
138 secs < cur_time + half_day_secs;
141 t.update( lon_rad, lat_rad, secs, 0 );
142 double angle_deg = sun_angle( t, world_up, lon_rad, lat_rad );
143 double diff = fabs( angle_deg - target_angle_deg );
144 if ( diff < best_diff ) {
145 if ( last_angle <= 180.0 && ascending
146 && ( last_angle > angle_deg ) ) {
147 // cout << "best angle = " << angle << " offset = "
148 // << secs - cur_time << endl;
151 } else if ( last_angle <= 180.0 && !ascending
152 && ( last_angle < angle_deg ) ) {
153 // cout << "best angle = " << angle << " offset = "
154 // << secs - cur_time << endl;
160 last_angle = angle_deg;
163 return best_time - cur_time;