Earth Rotation Variations – Long Period

The solid Earth is subject to a wide variety of forces including external forces due to the gravitational attraction of the Sun, Moon, and planets; surficial forces due to the action of the atmosphere, oceans, and water stored on land; and internal forces due to earthquakes and tectonic motions, mantle convection, and coupling between the mantle and both the fluid outer core and the solid inner core. The solid Earth responds to these forces by displacing its mass, deforming its shape, and changing its rotation. Geodetic observing systems can measure the change in the Earth's gravity caused by mass displacement, the change in the Earth's shape, and the change in the Earth's rotation. Consequently, geodetic observing systems can be used to study both the mechanisms causing the Earth's shape, rotation, and gravity to change and the response of the solid Earth to these forcing mechanisms. As a result, geodetic observing systems can be used to gain greater understanding of the Earth's interior structure and of the nature of the forcing mechanisms including their temporal evolution. In this chapter, the variations in the Earth's rotation that occur on timescales greater than a day are discussed. The standard theory used to study the variations is reviewed, the techniques by which the variations are observed are described, and the causes of the observed variations are discussed.

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