 The determination of the figure of the earth became a problem of the highest importance in astronomy, inasmuch as the diameter of the earth was the unit to which all celestial distances had to be referred.British physicist Isaac Newton, who used Picard's Earth measurement for establishing his law of universal gravity, explained this variation of the seconds pendulum's length in his "Principia Mathematica" (1687) in which he outlined his theory and calculations on the shape of the Earth.Newton theorized correctly that the Earth was not precisely a sphere but had an oblate ellipsoidal shape, slightly flattened at the poles due to the centrifugal force of its rotation.