+// update the cur_time_params structure with the current sun position
+void FGLight::updateSunPos()
+{
+ SGTime *t = globals->get_time_params();
+ SG_LOG( SG_EVENT, SG_DEBUG, " Updating Sun position" );
+ SG_LOG( SG_EVENT, SG_DEBUG, " Gst = " << t->getGst() );
+
+ fgSunPositionGST(t->getGst(), &_sun_lon, &_sun_lat);
+
+ // It might seem that sun_gc_lat needs to be converted to geodetic
+ // latitude here, but it doesn't. The sun latitude is the latitude
+ // of the point on the earth where the up vector has the same
+ // angle from geocentric Z as the sun direction. But geodetic
+ // latitude is defined as 90 - angle of up vector from Z!
+ SGVec3d sunpos = SGVec3d::fromGeoc(SGGeoc::fromRadM(_sun_lon, _sun_lat,
+ SGGeodesy::EQURAD));
+
+ SG_LOG( SG_EVENT, SG_DEBUG, " t->cur_time = " << t->get_cur_time() );
+ SG_LOG( SG_EVENT, SG_DEBUG,
+ " Sun Geocentric lat = " << _sun_lat
+ << " Geodcentric lat = " << _sun_lat );
+
+ // update the sun light vector
+ _sun_vec = SGVec4f(toVec3f(normalize(sunpos)), 0);
+ _sun_vec_inv = - _sun_vec;
+
+ // calculate the sun's relative angle to local up
+ FGViewer *v = globals->get_current_view();
+ SGQuatd hlOr = SGQuatd::fromLonLat( v->getPosition() );
+ SGVec3d world_up = hlOr.backTransform( -SGVec3d::e3() );
+ // cout << "nup = " << nup[0] << "," << nup[1] << ","
+ // << nup[2] << endl;
+ // cout << "nsun = " << nsun[0] << "," << nsun[1] << ","
+ // << nsun[2] << endl;
+
+ SGVec3d nsun = normalize(sunpos);
+ SGVec3d nup = normalize(world_up);
+ _sun_angle = acos( dot( nup, nsun ) );
+
+ double signedPI = (_sun_angle < 0.0) ? -SGD_PI : SGD_PI;
+ _sun_angle = fmod(_sun_angle+signedPI, SGD_2PI) - signedPI;
+
+ SG_LOG( SG_EVENT, SG_DEBUG, "sun angle relative to current location = "
+ << get_sun_angle() );
+
+ // Get direction to the sun in the local frame.
+ SGVec3d local_sun_vec = hlOr.transform(nsun);
+
+ // Angle from south.
+ _sun_rotation = 2.0 * atan2(local_sun_vec.x(), -local_sun_vec.y());
+
+ // cout << " Sky needs to rotate = " << _sun_rotation << " rads = "
+ // << _sun_rotation * SGD_RADIANS_TO_DEGREES << " degrees." << endl;
+
+ _sunAngleRad->setDoubleValue(_sun_angle);
+}