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., 675 Mass Ave, Cambridge, MA 02139, USA.
30 #ifdef SG_HAVE_STD_INCLUDES
39 // # include <stdio.h>
43 #include <simgear/math/point3d.hxx>
44 #include <simgear/math/sg_geodesy.hxx>
45 #include <simgear/ephemeris/ephemeris.hxx>
46 #include <simgear/timing/sg_time.hxx>
48 #include <Main/globals.hxx>
51 #include "sunsolver.hxx"
54 static const time_t day_secs = 86400;
55 static const time_t half_day_secs = day_secs / 2;
56 static const time_t step_secs = 60;
58 /* given a particular time expressed in side real time at prime
59 * meridian (GST), compute position on the earth (lat, lon) such that
60 * sun is directly overhead. (lat, lon are reported in radians */
62 void fgSunPositionGST(double gst, double *lon, double *lat) {
63 /* time_t ssue; seconds since unix epoch */
64 /* double *lat; (return) latitude */
65 /* double *lon; (return) longitude */
71 double beta = globals->get_ephem()->get_sun()->getLat();
72 double r = globals->get_ephem()->get_sun()->getDistance();
73 double xs = globals->get_ephem()->get_sun()->getxs();
74 double ys = globals->get_ephem()->get_sun()->getys();
75 double ye = globals->get_ephem()->get_sun()->getye();
76 double ze = globals->get_ephem()->get_sun()->getze();
77 alpha = atan2(ys - tan(beta)*ze/ys, xs);
78 delta = asin(sin(beta)*ye/ys + cos(beta)*ze);
80 // tmp = alpha - (SGD_2PI/24)*GST(ssue);
81 tmp = alpha - (SGD_2PI/24)*gst;
84 while (tmp < -SGD_PI);
85 } else if (tmp > SGD_PI) {
87 while (tmp < -SGD_PI);
94 static double sun_angle( const SGTime &t, sgVec3 world_up,
95 double lon_rad, double lat_rad ) {
97 Point3D p, rel_sunpos;
99 SG_LOG( SG_EVENT, SG_DEBUG, " Updating Sun position" );
100 SG_LOG( SG_EVENT, SG_DEBUG, " Gst = " << t.getGst() );
102 double sun_lon, sun_gd_lat;
103 fgSunPositionGST( t.getGst(), &sun_lon, &sun_gd_lat );
104 Point3D sunpos = sgGeodToCart(Point3D(sun_lon, sun_gd_lat, 0));
106 SG_LOG( SG_EVENT, SG_DEBUG, " t.cur_time = " << t.get_cur_time() );
107 SG_LOG( SG_EVENT, SG_DEBUG,
108 " Sun Geodetic lat = " << sun_gd_lat );
110 // calculate the sun's relative angle to local up
111 sgCopyVec3( nup, world_up );
112 sgSetVec3( nsun, sunpos.x(), sunpos.y(), sunpos.z() );
113 sgNormalizeVec3(nup);
114 sgNormalizeVec3(nsun);
115 // cout << "nup = " << nup[0] << "," << nup[1] << ","
116 // << nup[2] << endl;
117 // cout << "nsun = " << nsun[0] << "," << nsun[1] << ","
118 // << nsun[2] << endl;
120 double sun_angle = acos( sgScalarProductVec3 ( nup, nsun ) );
121 double sun_angle_deg = sun_angle * SG_RADIANS_TO_DEGREES;
122 while ( sun_angle_deg < -180 ) { sun_angle += 360; }
123 SG_LOG( SG_EVENT, SG_DEBUG, "sun angle relative to current location = "
126 return sun_angle_deg;
131 * Given the current unix time in seconds, calculate seconds to the
132 * specified sun angle (relative to straight up.) Also specify if we
133 * want the angle while the sun is ascending or descending. For
134 * instance noon is when the sun angle is 0 (or the closest it can
135 * get.) Dusk is when the sun angle is 90 and descending. Dawn is
136 * when the sun angle is 90 and ascending.
138 time_t fgTimeSecondsUntilSunAngle( time_t cur_time,
141 double target_angle_deg,
144 // cout << "location = " << lon_rad * SG_RADIANS_TO_DEGREES << ", "
145 // << lat_rad * SG_RADIANS_TO_DEGREES << endl;
146 Point3D geod( lon_rad, lat_rad, 0 );
147 Point3D tmp = sgGeodToCart( geod );
149 sgSetVec3( world_up, tmp.x(), tmp.y(), tmp.z() );
150 SGTime t = SGTime( lon_rad, lat_rad, "", 0 );
152 double best_diff = 180.0;
153 double last_angle = -99999.0;
154 time_t best_time = cur_time;
156 for ( time_t secs = cur_time - half_day_secs;
157 secs < cur_time + half_day_secs;
160 t.update( lon_rad, lat_rad, secs, 0 );
161 double angle_deg = sun_angle( t, world_up, lon_rad, lat_rad );
162 double diff = fabs( angle_deg - target_angle_deg );
163 if ( diff < best_diff ) {
164 if ( last_angle <= 180.0 && ascending
165 && ( last_angle > angle_deg ) ) {
166 // cout << "best angle = " << angle << " offset = "
167 // << secs - cur_time << endl;
170 } else if ( last_angle <= 180.0 && !ascending
171 && ( last_angle < angle_deg ) ) {
172 // cout << "best angle = " << angle << " offset = "
173 // << secs - cur_time << endl;
179 last_angle = angle_deg;
182 return best_time - cur_time;