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This animation shows the shapes of some elliptical orbits. These orbits have different eccentricities. If an ellipse has a high eccentricity, is it round like a circle or long like an oval?
Click on image for full size
Original animation by Windows to the Universe staff (Randy Russell).

Eccentricity of an Orbit

Do you think Earth moves around the Sun in a circle? That is almost true, but not quite. The shape of Earth's orbit isn't quite a perfect circle. It is more like a "stretched out" circle or an oval. Mathematicians and astronomers call this shape an "ellipse".

Astronomers use a special word to describe how round, or how long and skinny, and ellipse is. The word they use is "eccentricity". If the eccentricity of an orbit is close to zero, the ellipse is nearly a circle. If it is close to one, the ellipse is long and skinny.

Earth's orbit has a small eccentricity, close to zero. Earth's orbit is almost a perfect circle. Can you guess which planet has the highest eccentricity? It's Pluto! Pluto's orbit is not as much like a nice, round circle as Earth's. Most comets have even more stretched out orbits than planets. Comet orbits usually have high eccentricities.

Last modified December 16, 2005 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF