External luni-solar torque exerted on the difference (B-A) of the Earth's two equatorial principal moments of inertia gives rise to two types of librational motions in the Earth's rotation: the semidiurnal libration in spin and the prograde diurnal libration in polar motion. Formulas for the librations considering a realistic Earth model and their tidal decompositions are derived and evaluated. The spin libration has a maximum peak-to-peak amplitude of 0.09 milliarcseconds, that of the polar libration is 0.06 milliarcseconds. Implications concerning their detectability and role in the tidal variation of Earth rotation are discussed.