Abstract

The Erath’s rotation is not steady; it varies slightly with time as have been measured over a century by astronomical means and nowadays by space geodetic techniques. There are two types of variation per different physical reasons. The astronomical variations are due to external torques, notably by the luni-solar tides with characteristic tidal periods, including the familiar precession and nutations. Other planets also exert forces and torques that slowly change the Sun-Earth orbital and orientational relations, known as the three Milankovitch cycles that have grave influences on the Earth climate on long timescales. The geophysical variations, on the other hand, happen under the conservation of angular momentum, where the solid Earth (w.r.t. which the “Earth’s rotation” is defined) exchanges angular momentum with the “fluid” parts including the atmosphere, oceans, hydrology, cores, and even mantle convections and earthquakes. These variations come in two forms: length-of-day change and polar motion, the latter represents the excited rotational normal mode known as the Chandler wobble. There are other, yet elusive, rotational normal modes supposedly associated with the cores. Measuring how and how much the Earth’s rotation varies tells a lot about the overall properties and internal workings of the planet. The story goes on.