A circle is positioned with the cx and cy attributes. These indicate the x and y coordinates of the center of the circle. The radius is determined by the r attribute. The following is an example you can experiment with ( http://localhost:8080/chapter-2/circle.html):
<circle cx="62" cy="62" r="50"></circle>
Now type in the following code:
circle { stroke-width: 5; stroke:steelblue; fill:#888; fill-opacity: .5; }
This will create a circle with the familiar blue outline, a gray interior, and half-way opaque.