I first made a circle, with the center of the circle being the first foci. I put another point outside of the circle which will be the second foci. I put a third point on the circle, then put a line from that point to the first foci. I then connected the point on the circle to the second foci with a line segment. I used the midpoint of the segment and made a line perpendicular to the segment. I traced the point where the two lines intersect and turned on the animation. You can use the slider to change the radius of the circle.