An approximate formula for gravity at the earth's surface, assuming that the earth is an ellipsoid; states that the gravity is equal to g_e [1+(⁵/₂ m' - f ) sin² θ], where θ is the latitude, g_e is the gravity at the equator, m' is the ratio of centrifugal acceleration to gravity at the equatorial surface, and f is the earth's flattening, equal to (a - b )/a, where a is the semimajor axis and b is the semiminor axis.