Earth’s rotation: observations and theory - Earth Dynamics: Deformations and Oscillations of the Rotating Earth - page 274

the sense that it has its origin at the centre of mass of the whole Earth, including the
oceans and the atmosphere. Its initial orientation is defined to coincide with that of
the BIH at year 1984.0. The time evolution of the orientation is defined by a no net
rotation condition with respect to horizontal tectonic motions over the whole Earth.
Similarly, the modern definition of the celestial reference frame is given by the
IERS and is called the International Celestial Reference System (ICRS). This is
based on the co-ordinates of extragalactic radio sources. Its origin is at the bary-
centre of the solar system. It replaced the previous system, based on the FK5 star
catalogue (Fricke et al ., 1988), as of 1 January 1998, to provide a reference system
with greater accuracy (Feissel and Mignard, 1998).

4.2 Polar motion and wobble
Classically, the Earth's wobble, or the motion of the rotation axis within the Earth,
has been measured by observations of the associated latitude variation. Euler had
observed in 1765 that a rigid Earth would have a free wobble with period, in sider-
eal days, near the reciprocal of the dynamical ellipticity, or close to 10 months.
The associated latitude variation was confirmed by simultaneous observations at
Berlin and Waikiki in 1891. Since these two locations are close to 180 ◦ apart in
longitude, the variations, as expected, were found to be opposite in phase. Also in
1891, S. C. Chandler, an amateur astronomer, produced his analysis of a series of
latitude observations showing that there were two principal components, an annual
term and a 14-month variation some 40% longer than Euler's period for a rigid
Earth. The lengthened period was quickly shown to be due to the fluidity of the
oceans and outer core and the elastic yielding of the solid Earth. This motion is
now called the Chandler wobble.
These discoveries led to the establishment of the International Latitude Service
(ILS) in 1895, with five stations all operating at latitude 39 ◦ 08 N and beginning
regular observations in 1899. Details of the early history are given by Munk and
MacDonald (1960).
The original ILS stations used visual zenith telescopes (VZTs). The inclusion
of additional stations and instruments followed the formation of the International
Polar Motion Service (IPMS) as the successor to the ILS in 1962. In the meantime,
the Bureau International de l'Heure had established an independent set of observat-
ories to monitor polar motion to be applied to correct for the e ect of polar motion

on Universal Time (UT). These incorporated instruments such as the photographic
zenith tube (PZT), having improved stability and measurement accuracy. These
developments are described in more detail by Lambeck (1980).
The co-ordination of polar motion observations using modern space measure-
ment techniques, such as very long baseline interferometry (VLBI), lunar laser

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