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ellipticity
A measure of the amount by which an object, such as a planet or a galaxy, deviates from a perfect sphere; also known as oblateness. It is found by subtracting the polar diameter from the equatorial diameter, then dividing by the equatorial diameter. Ellipticity is typically an indication of how fast a body is rotating. The ellipticity of Earth is about 1/299. The ellipticity of elliptical galaxies increases in going from type E0 to E7.
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