# eccentricity

/ˌɛksənˈtɹɪsəti/ (US) · /ˌɛksɪntɹˈɪsɪti/ (UK) · CEFR C1 · Noun

Source: https://englishreference.com/word/eccentricity
Updated: 2026-05-20

## Definition

the shape of an orbit that is not a perfect circle. It measures how much an object's path around another is stretched out.

For teachers: a measure of the deviation of an orbit from a perfect circle. In astronomy, it is defined as the ratio of the distance between the foci of the ellipse to the length of its major axis.

## Examples

- The planet has a high eccentricity, so its orbit is very oval.
- Astronomers use the eccentricity of a comet's orbit to predict how close it will come to the sun during its next visit.
- While most planets in the solar system have nearly circular orbits with low eccentricity, the highly elliptical path of Halley's Comet is a classic example of significant orbital eccentricity.

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