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Eccentricity
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A measure of how elongated an orbit is.
e = c/ae = c/a measures how stretched an ellipse is: 0 for a circle, near 1 for long thin ellipses; e = 1 is a parabola (escape), >1 hyperbola.
Halley's comet e = 0.97.
e ≥ 1 means unbound.
In more detail
Eccentricity measures how much an ellipse deviates from a circle. A circle has eccentricity zero, and values approaching one give very elongated orbits.
Real-world uses
- Halley's comet has a high eccentricity, so it spends most of its orbit far from the Sun.
- Mars has a modest eccentricity that affects seasonal temperatures.
- Comets have very high eccentricities, so their orbits are long and thin compared with the nearly circular orbit of Earth.
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Frequently asked questions
What eccentricity is a circle?
Zero.
Can eccentricity be above one?
Yes, for open orbits such as hyperbolas, which escape the central body.
What eccentricity is a circle?
Zero. Eccentricity between zero and one describes ellipses, and a value of one gives a parabola.
Related topics in Gravitation & Orbits
Kepler's second law · Kepler's third law · Elliptical orbit energy · Perihelion and aphelion · Orbital transfers · Gravity assist