A Directrix is a line outside the elliptical curve that is perpendicular to the major axis. This line, or directrix, is used to define the ellipse in this way: The elliptical curve is created such that any point maintains a ratio of two segments; The first segment is a line from the point on the curve to the directrix where it is perpendicular to the directrix, and the second segment is a line form the point on the curve to the nearest focus (F1). As shown in Figure 1 below, the first segment is designated L1 and the second segment is designated L2.
The eccentricity of an ellipse is basically an indicator of its shape, or how "oval" it is. The value ranges from 0 to 1 and when based on the directrix definition, it is the ratio of 'L1' to 'L2'. The example shown in Figure 1 has an eccentricity of 0.7.
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Figure 1 |