ABSTRACT
Compared with a sphere, an oblate ellipsoid is flattened at the poles. It is often expressed as a ratio. This ratio, and sometimes its reciprocal, is used as one specification for reference ellipsoids used to model the Earth. It cannot fulfill the definition alone, however, and another parameter is always needed. That is frequently the semi-major axis, i.e., the long equatorial axis. An ellipsoid can also be fully determined with the statement of the semi-major axis and the semi-minor axis, i.e., the short or polar axis. Eccentricity, or first eccentricity, is often used in conjunction with the semi-major axis as the definition of a reference ellipsoid, but the reciprocal of the eccentricity is not.
