The study of Earth's rotation would not be of much interest if the Earth rotated uniformly about a fixed axis. Variations in the speed of rotation and changes in the orientation of the rotation axis, both within the body of the Earth and in space, make the subject deeply fascinating and rewarding. The subject has a long and interesting history that is well reviewed in the classic treatise The Rotation of the Earth (Munk and MacDonald, 1960), which did much to revitalise modern interest. More recently, a second authoritative treatise entitled The Earth's Variable Rotation, by Kurt Lambeck, has appeared (Lambeck, 1980), giving a modern overview of the subject.

Observations of the rotation are generally made by observatories attached to the Earth, measuring motions with reference to stars and other celestial objects. Thus, both a terrestrial reference frame and a celestial reference frame need to be defined to make such observations.

Reference frames

To the lowest order of approximation, observations are made in a rigid, uniformly rotating frame. Of course, the subject is of interest because the actual frame is neither perfectly rigid nor perfectly uniform in its rotation. The observer's frame is usually defined by a prescribed method of adjusting the frame to the mean motion of a set of observatories, in such a way as to approximately minimise the variance of the relative motions over all the observatories. For example, the Bureau International de l'Heure (BIH) defines the 1968 BIH (Guinot and Feissel, 1969) reference system in terms of the latitudes and longitudes assigned to 68 observatories, with each having an assigned weight in latitude and time.