POLAR MOTION

a periodic rotation of the Earth's spin axis about a mean axis, somewhat like the wobble of a spinning top. Slight variations in latitude and longitude result from this wobble because the poles are displaced from their mean positions. The north pole of rotation rotates counterclockwise around its mean position. Polar motion is primarily made up of two discrete periodic oscillations: one, called the Chandler Wobble, has about a 14-month period, and the other has a 12-month period. The combination of these two wobbles causes the poles to trace spiral paths out of, around, and eventually back into their mean positions over a period of about 6.5 years. The separation between the actual and mean poles was exceptionally large in about 1952, when they were separated by 12 m (37 feet), or 0.37 arc second (0.37). Their maximum separation during the 6.5-year period averages about 0.25. Polar motion was first predicted by the Swiss physicist Leonhard Euler in 1765 using dynamical theory and a rigid model of the Earth; he predicted a 10-month oscillation period for the phenomenon. Observational proof for the postulated latitude variations was obtained in the mid-1880s, and about that time the American astronomer S.C. Chandler analyzed these data and obtained both the 14-month and the 12-month periods. The four-month difference between Euler's predicted period and the actual duration of the Chandler Wobble is due to the elasticity of the Earth's mantle and the mobility of the oceans, which together subtly affect the Earth's response to rotation and for which Euler had not provided in his calculations. Astronomical observations of the Earth's rotational position in space, which are used in determining Universal Time, must be corrected for slight variations in longitude caused by polar motion.