# conic sections

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## conic sections

(**kon**-ik) (conics) A family of curves that are the locus of a point that moves so that its distance from a fixed point (the focus) is a constant fraction of its distance from a fixed line (the directrix). The fraction,

*e*, is the eccentricity of the conic. The value of the eccentricity determines the form of the conic. If

*e*is less than 1 the conic is an ellipse. A circle is a special case of this with

*e*= 0. If

*e*= 1 the conic is a parabola and if

*e*exceeds 1 the conic is a hyperbola.

These curves are known as conic sections because they can be obtained by taking sections of a right circular cone at different angles: a horizontal section gives a circle, an inclined one an ellipse, one parallel to the slope of the cone is a parabola, and one with an even greater inclination is a hyperbola (see illustration). Conics are important in astronomy since they represent the paths of bodies that move in a gravitational field. See also orbit.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

## Conic sections

Circle, ellipse, parabola, and hyperbola; all produced by cutting a plane through a cone at different angles.

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