The ellipticity of a rapidly rotating planet, expressed in terms of the reduction in distance between the poles, compared to the distance through the equator. Thus, if a planet's diameter is 3% less through the poles, than through the equator, it could be said to have a flattening of 3%. Numerically, identical to the oblateness, and only differing from the flattening in terms of the viewpoint. In the case of flattening, the planet is thought of as being squashed through the poles. In the case of oblateness, it is thought of as bulging out at the equator.