Code
<Window x:Class="SLTest.test"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="test" Height="600" Width="800">
    <Window.Resources>
        <Storyboard x:Key="Storyboard1">
            <PointAnimationUsingPath Storyboard.TargetName="MyAnimatedEllipseGeometry" Storyboard.TargetProperty="Center" Duration="0:0:5" RepeatBehavior="Forever">
                <PointAnimationUsingPath.PathGeometry>
                    <PathGeometry Figures="M 299.9 300 A 100,100 360 1 1 300.1,300 Z"/>
                </PointAnimationUsingPath.PathGeometry>
            </PointAnimationUsingPath>
        </Storyboard>
    </Window.Resources>
    <Window.Triggers>
        <EventTrigger RoutedEvent="FrameworkElement.Loaded">
            <BeginStoryboard Storyboard="{StaticResource Storyboard1}"/>
        </EventTrigger>
    </Window.Triggers>
    <Grid>
        <Path x:Name="pt" Stroke="Black" Fill="Gray" Data="M 299.9 300 A 100,100 360 1 1 300.1,300 Z" />
        <Path Fill="Blue">
            <Path.Data>
                <!-- Describes an ellipse. -->
                <EllipseGeometry x:Name="MyAnimatedEllipseGeometry"
             Center="300,300" RadiusX="15" RadiusY="15" />
            </Path.Data>
        </Path>
    </Grid>
</Window>