Make an ellipse 72 wide by 16 high and position it as shown on the larger outer ellipse. (The inner ellipse has been pulled away and set aside, remember?) Select the larger ellipse and the new small ellipse. Open the Align palette (Window > Align) and click the Horizontal Align Center button.
Select the small ellipse and go to Edit > Copy (cmd/ctrl + C), then Edit > Paste in Front (cmd/ctrl + F).
Select the front small ellipse and the larger ellipse. In the Pathfinder palette (Window > Pathfinder), opt/alt + click on the Subtract from Shape Area button. When you hold opt/alt and click, the shapes are expanded without having to click the expand button. Through this whole step you won't see any change in the appearance of the shapes.
