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An ellipse may also be defined in terms of one focal point and a line outside the ellipse called the directrix: for all points on the ellipse, the ratio between the distance to the focus and the distance to the directrix is a constant.
The linear eccentricity of an ellipse or hyperbola, denoted c (or sometimes f or e), is the distance between its center and either of its two foci. The eccentricity can be defined as the ratio of the linear eccentricity to the semimajor axis a : that is, e = c a {\displaystyle e={\frac {c}{a}}} (lacking a center, the linear eccentricity for ...
A conic is defined as the locus of points for each of which the distance to the focus divided by the distance to the directrix is a fixed positive constant, called the eccentricity e. If 0 < e < 1 the conic is an ellipse, if e = 1 the conic is a parabola, and if e > 1 the conic is a hyperbola.
In an ellipse, the semi-major axis is the geometric mean of the distance from the center to either focus and the distance from the center to either directrix. The semi-minor axis of an ellipse runs from the center of the ellipse (a point halfway between and on the line running between the foci) to the edge of the ellipse. The semi-minor axis is ...
In addition to the eccentricity (e), foci, and directrix, various geometric features and lengths are associated with a conic section. The principal axis is the line joining the foci of an ellipse or hyperbola, and its midpoint is the curve's center. A parabola has no center. The linear eccentricity (c) is the distance between the center and a ...
A parabola has only one focus, and can be considered as a limit curve of a set of ellipses (or a set of hyperbolas), where one focus and one vertex are kept fixed, while the second focus is moved to infinity. If this transformation is performed on each conic in an orthogonal net of confocal ellipses and hyperbolas, the limit is an orthogonal ...
Their combined length P 1 P + PP 2 = P 1 P 2 = L is the distance between circles k 1 and k 2, and is independent of the choice of P; thus any point on the ellipse has PF 1 + PF 2 = L. This construction shows how the focal points of an ellipse can be found using the Dandelin spheres.
The coordinates (,) have a simple relation to the distances to the foci and . For any point in the plane, the sum d 1 + d 2 {\displaystyle d_{1}+d_{2}} of its distances to the foci equals 2 a σ {\displaystyle 2a\sigma } , whereas their difference d 1 − d 2 {\displaystyle d_{1}-d_{2}} equals 2 a τ {\displaystyle 2a\tau } .