# Director circle

An ellipse, its minimum bounding box, and its director circle.

In geometry, the director circle of an ellipse or hyperbola (also called the orthoptic circle or Fermat–Apollonius circle) is a circle formed by the points where two perpendicular tangent lines to the curve cross.

## Properties

The director circle of an ellipse circumscribes the minimum bounding box of the ellipse. It has the same center as the ellipse, with radius $\sqrt{a^2+b^2}$, where $a$ and $b$ are the semi-major axis and semi-minor axis of the ellipse. Additionally, it has the property that, when viewed from any point on the circle, the ellipse spans a right angle.

## Generalization

More generally, for any collection of points Pi, weights wi, and constant C, one can define a circle as the locus of points X such that

$\sum w_i \, d^2(X,P_i) = C.$

The director circle of an ellipse is a special case of this more general construction with two points P1 and P2 at the foci of the ellipse, weights w1 = w2 = 1, and C equal to the square of the major axis of the ellipse. The Apollonius circle, the locus of points X such that the ratio of distances of X to two foci P1 and P1 is a fixed constant r, is another special case, with w1 = 1, w2 = −r2, and C = 0.

## Related constructions

In the case of a parabola the director circle degenerates to a straight line, the directrix of the parabola.