/// <summary>
        /// Set the sun's position.
        /// </summary>
        private void SetSun()
        {
            SunCalculation.GetSunCalculation(-9, 48, out double azimut, out double h);
            Vector3D v       = SphericalCoordinatesToCartesic(1, 9, 48);
            Vector3D sun_vec = v + SphericalCoordinatesToCartesic(23481.4, -azimut, h);

            MainModel3Dgroup.Children.Add(new GeometryModel3D(AddSmoothSphere(new Point3D(sun_vec.X, sun_vec.Y, sun_vec.Z), 109.16, 180, 360), new DiffuseMaterial(Brushes.Yellow)));

            PointLight pointLight = new PointLight(Colors.LightYellow, new Point3D(sun_vec.X, sun_vec.Y, sun_vec.Z))
            {
                Range = 50000
            };

            MainModel3Dgroup.Children.Add(pointLight);
            SpotLight spotLight = new SpotLight(Colors.LightYellow, new Point3D(v.X, v.Y, v.Z), SphericalCoordinatesToCartesic(23481.4, -azimut, h), 50, 50);

            MainModel3Dgroup.Children.Add(spotLight);
        }
        /// <summary>
        /// Set the sun's position.
        /// </summary>
        /// <param name="longitude"></param>
        /// <param name="latitude"></param>
        private void SetSun(double longitude, double latitude)
        {
            SunCalculation.GetSunCalculation(longitude, latitude, out double azimut, out double h);
            Vector3D v       = SphericalCoordinatesToCartesic(r_e, -longitude, latitude);
            Vector3D sun_vec = v + SphericalCoordinatesToCartesic(23481.4 * r_e, -azimut, h);

            MainModel3Dgroup.Children.Add(new GeometryModel3D(AddSmoothSphere(new Point3D(sun_vec.X, sun_vec.Y, sun_vec.Z), 109.16 * r_e, 180, 360), new EmissiveMaterial(Brushes.LightYellow)));

            PointLight pointLight = new PointLight(Colors.LightYellow, new Point3D(sun_vec.X, sun_vec.Y, sun_vec.Z))
            {
                Range = 50000 * r_e
            };

            MainModel3Dgroup.Children.Add(pointLight);

            SpotLight spotLight = new SpotLight(Colors.LightYellow, new Point3D(v.X, v.Y, v.Z), SphericalCoordinatesToCartesic(23481.4, -azimut, h), 50, 50)
            {
                Range = r_e * 50000
            };

            MainModel3Dgroup.Children.Add(spotLight);
        }