Geology Reference
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4
Earth's rotation: observations and theory
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 Rota-
tion
, 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.
4.1 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 Interna-
tional 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.
The modern definition of the terrestrial reference frame is given by the Interna-
tional Earth Rotation and Reference Systems Service (IERS) (www.iers.org) and is
called the International Terrestrial Reference System (ITRS). This is geocentric, in
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