The equation (a b)/a + (τ_p - τ_e)/ τ_e =(5ω²a/2τ_e) - f(f + ω²a/τ_e) + aC4/4, in which τ_p and τ_e are the values of gravity acceleration at the poles and at the equator, respectively, of an equipotential, rotational ellipsoidal solid having equatorial diameter 2a and polar diameter 2b and rotating at the rate ω. f is the flattening, (a b)/a and C4 is the coefficient of the fourth-degree term in the representation of the geopotential by a Legendre series.