The Earth’s Rotation

The Earth's rotation is not constant. Instead, both the rate of rotation and the position of the rotation axis vary with time. Changes in the rotation rate are directly proportional to changes in the length of a day (LOD). In addition, the time integral of the LOD variability is proportional to fluctuations in Universal Time, the measure of time as determined by the overhead transits of celestial objects. Variations in the position of the rotation axis are usually classified either as "polar motion" or as "nutation," where "polar motion" describes motion of the axis with respect to the Earth's surface, and "nutation" denotes motion of the axis with respect to inertial space. The distinction between polar motion and nutation is somewhat artificial, since, in general, nutation cannot occur without some accompanying polar motion, and vice versa. Tn practice, though, axis motion caused by an individual excitation process is mostly either one or the other, depending on the time scale. Excitation at periods much longer than one day as seen by an observer on the Earth causes mostly polar motion: The rotation axis does not move much with respect to inertial space compared with its motion with respect to the Earth. Thus, since processes originating within the Earth capable of affecting rotation generally have long time scales, they cause polar motion. Conversely, excitation with a nearly diurnal (retrograde) period as seen from the Earth causes axis motion that is mostly nutation. For example, the gravitational attraction of the Sun and Moon causes nutational motion, since the Sun and Moon have nearly diurnal periods as seen from the Earth. This article is a survey of rotation observations and, especially, of the geophysical implications of those observations for all three types of