(where a = semimajor axis, b = semiminor axis). The contribution of centrifugal force is approximately − − G e m cos 2 ⁡ ⁡ φ φ , whereas the gravitational attraction itself varies approximately as G e ( 3 2 m − − f ) sin 2 ⁡ ⁡ φ φ . This formula holds when the surface is perpendicular to the direction of gravity (including centrifugal force), even if (as usually) the density is not constant (in which case the gravitational attraction can be calculated at any point from the shape alone, without reference to m ). For the earth, m ≈ ≈ 1 / 289 , and 5 2 m ≈ ≈ 1 / 116 , while f ≈ ≈ 1 / 300 , so g is greater at the poles than on the equator.