Make sure that Snap to Objects is turned on and draw a circle. Also draw a vertical line that is much taller than the circle.
Double-click the center of the circle to select the line and the fill and then click and drag from the center of the circle so that you get the solid ring near your cursor. Snap the center of the circle to the line.
Your drawing should now look like the following figure:
Click to select just the vertical line at the bottom. Using the arrow keys, nudge the vertical line up.
Click away from the line to deselect it. With the Selection tool, bring your cursor near the top of the line you just moved until you see the cursor type that indicates you'll be dragging the end point. Click and start dragging.
Don't snap the end point to the circle. Snap it to the end of the line, on top.
Click one semicircle to select it. Then delete the fill.
Click and delete the line portion that corresponds to the fill you just deleted.