This animation is a merging of the Ellipse Device and the
Ellipse Foci animations, illustrating that the ellipse device
generates a perfect ellipse.
Actually, the Ellipse Device is easier to use when
constructing a particular sized ellipse than is the Ellipse
Foci method. You always need to know the semi major and semi
minor axis for a given ellipse for either construction.
To use the Ellipse Device, the tracing point is set to the
dimension of the semi major axis, as measured from the red
shuttle. Then the blue shuttle is set to the dimension of the
semi minor axis, as measured from the tracing point. Now, draw
the ellipse. Easy.
To use the Ellipse Foci method, the Pin and String method,
the foci have to be found using the following formula: d =
sqrt(a2 - b2). The foci, F1 and F2, are
located on either side of the center of the ellipse, the
distance "d", along the semi major axis. "a" and "b" are the
semi major and semi minor axis dimensions respectively. A
third point has to be found, the first marking point, so that
the string can be tied and used to trace the ellipse. See