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Astro*Dictionary by Michael Erlewine





1 article for "Conic Section"

Conic Section [Astro*Index]

Originally, the plane surface produced by passing a plane through a cone.

Today, the term applies to the curved line bounding these surfaces; the conic section is defined by one of its geometrical properties, most commonly that it is the locus of a point which moves so that the ratio of its distance from a fixed point (called the focus) to its distance from a fixed line (called the directrix) is constant. This ratio is called the eccentricity, which is usually denoted by e. When e is equal to 0, the conic is a circle; when less than 1.0, an ellipse; when equal to 1.0, a parabola; when greater than 1.0, an hyperbola. A degenerate conic (a point, a straight line, or a pair of straight lines) is formed as a limiting case of the above curves. The orbital motion of a celestial body follows one of such curves.

See also:
♦ Orbit ♦ Eccentricity


Astro*Index Copyright © 1997 Michael Erlewine