Astronomy Answers: From the Astronomical Dictionary

Astronomy Answers
From the Astronomical Dictionary

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the eccentricity

The eccentricity of an orbit is one of the orbital elements. It is a number that indicates how much the orbit deviates from a circle. A circular orbit has an eccentricity equal to zero. An elliptical orbit has an eccentricity between zero and one. In this case, the eccentricity is equal to the difference between the lengths of the long and short axes of the ellipse, divided by the sum of those lengths. A parabolic orbit has an eccentricity of one, and a hyperbolic orbit has an eccentricity larger than one.

Orbits with eccentricities less than one are closed, so the objects in such orbits return to the same position regularly. Orbits with eccentricities greater than one are open, which means that objects in such orbits never return to the same position.