Transform::rotation(Angle::half_turn()-time_of_day, Vector(0, 0, 1))*
Transform::rotation(axial_tilt, Vector(-1, 0, 0));
Vector sun_dir = trans.transform_linear(Vector(0, -1, 0));
Transform::rotation(Angle::half_turn()-time_of_day, Vector(0, 0, 1))*
Transform::rotation(axial_tilt, Vector(-1, 0, 0));
Vector sun_dir = trans.transform_linear(Vector(0, -1, 0));