| eccentricity (Symbol e) |
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One of the parameters used to describe the shape of curves belonging to the family known as conic sections: circles, ellipses, parabolas and hyperbolas. The orbit of a body moving under the influence of a central gravitational force, such as a planet travelling round the Sun, is necessarily one of the conic section curves. Thus eccentricity is one of the important elements used to describe an orbit. Circles and ellipses are closed curves. A circle is defined as having e = 0. The eccentricity of an ellipse is a measure of how much it deviates from being a circle. If c is the distance from the centre of an ellipse to one of its focal points, and a the semimajor axis of the ellipse, the eccentricity is given by the ratio c/a. The eccentricity of an ellipse must be less than unity. Parabolas and hyperbolas are open curves. The observed orbits of non-periodic comets are typically parabolic. A parabola has e = 1 and a hyperbola e > 1. See also: orbital elements. |