eccentricity
Quick Summary
The eccentricity of a conic section completely characterizes its shape. For example, all circles have zero eccentricity, and all parabolas have unit eccentricity; hence, all circles (and all parabolas) have the same shape, only varying in size. (Under appropriate magnification they are indistinguishable.) In contrast, ellipses and hyperbolas vary greatly in shape.
In mathematics, eccentricity is a measure of a conic section's deviation from being circular. For a given conic section, it's the ratio of the distance from any point on the curve to a focus, divided by the distance from that point to a corresponding directrix.1
- A circle has an eccentricity of 0.
- An ellipse has an eccentricity between 0 and 1.
- A parabola has an eccentricity of 1.
- A hyperbola has an eccentricity greater than 1.2
The eccentricity of an astronomical orbit, like that of the Moon, indicates how much it deviates from a perfect circle. For instance, Earth's orbit has an eccentricity of 0.0167, making it nearly circular. The term "eccentricity" can also refer to behavior that deviates from the norm, often in a whimsical or peculiar way.1345
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