请教怎么让圆内一条半径旋转

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<Canvas Name="canvas" Margin="3" Grid.Row="1" Background="LightBlue" >
<Ellipse Width="100" Height="100" Stroke="Green" Name="circle"
Canvas.Top="100" Canvas.Left="100" ></Ellipse>
</Canvas>
private void Window_Loaded(object sender, RoutedEventArgs e)
{
var top = Canvas.GetTop(circle);
var left = Canvas.GetLeft(circle);
var w = circle.ActualWidth;
var h = circle.ActualHeight;
var top2 = top + h / 2;
var left2 = left + w / 2;
Line line = new Line()
{
Stroke = circle.Stroke,
X1 = left2,
Y1 = top2,
X2 = left2,
Y2 = top2 - w / 2
};
canvas.Children.Add(line);
var timer = new System.Windows.Threading.DispatcherTimer();
timer.Interval = TimeSpan.FromSeconds(1);
timer.Tag = line;
timer.Tick += new EventHandler(timer_Tick);
timer.Start();
}
int angle = 30;
void timer_Tick(object sender, EventArgs e)
{
var timer = sender as System.Windows.Threading.DispatcherTimer;
var line = timer.Tag as Line;
line.RenderTransform = new RotateTransform(angle, line.X1, line.Y1);
if (angle >= 360)
angle = 30;
else
angle = angle + 30;
}