Ellipticity is 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.