How to draw an ellipse.
Another mystic secret revealed
An ellipse has 2 radii (radiuses). In this case the long radius is 26 inches, or half the width of my ceiling. The small radius is 16 inchs, or the height of my ceiling. I found by trial and error that putting my nails 7 inches in from my end point gave me a 16 height. This method can also be used for a full 360o ellipse.
A loop of string goes around the two nails 7 inches from the end, and the pencil that is held even with the end point. You can see how the small radius is defined by limiting the distance the pencil can travel.