In addition to a change in the direction of the Earth's rotation axis described so far, there is also a small movement of the location of the intersection of the rotation axis with the Earth's surface...In addition to a change in the direction of the Earth's rotation axis described so far, there is also a small movement of the location of the intersection of the rotation axis with the Earth's surface; this movement is called polar motion. The two oscillations "beat" together, so that the pole moves in a circle that expands and contracts in diameter with a frequency equal to the beat frequency (the difference in frequencies of the two motions); therefore the period of the change in diameter is
