involved. The tidal displacement of a site will include the solid tide, possibly a
contribution from the deformation of the crust by variations in the oceanload,
and by the loading of the crust by atmospheric tides. To reach a better
understanding of the solid Earth therefore also requires improved understanding
of some of these "extraneous" factors. But these factors actually expand the very
range of forces available and they too are worthy of study.
Some examples
Numerous examples can be found that illustrate the hand-in-glove link between
geodesy and the other disciplines of the Earth Sciences. The Earth's irregular
rotation is one such example.Perhaps a rather esoteric subject but one that is not
without importance. Modern geodetic techniques have provided highly accurate
and high resolution observations of the changes in the Earth's rotation and fills
an important need in space navigation; in guiding spacecraft to their planetary
encounters or in the very high accuracy orbit analysis of terrestrial satellites for
other geodetic purposes. The major challenge however lies not so much in the
measurement and data analysis as in providing a physical framework for the
observations of both the regular and irregular rotational motions. It requires
evaluation of the torques acting on the planet and of the redistributions of mass
within it. It requires evaluation of atmospheric, oceanographic, seismic, magnetic
forces that excite the rotation. Yet the geodetic observations with all the
supporting information will not contribute to one of the major outstanding
problems, namely, the secular acceleration of the Earth and the evolution of the
Earth-Moon system. To develop useful models the record needs to be extended
back in time. This requires inter aha delving into the mysteries of ancient
astronomical observations and into the fossil record of corals and brachiopods.
Where geodesy ends and other disciplines begin is a moot point but a systematic
approach across the spectrum is essential if we wish to understand the orbital and
spin evolution of the Earth-Moon system.
A second example is provided by the Earth's gravity field. A, perhaps the, major
contribution of modern geodesy to science has been the measurement of the
Earth's gravity field and no other property of the Earth other than possibly its
topography is now known with better accuracy and spatial resolution. The spatial
variations point to the planet that is out of hydrostatic equilibrium and, while this
is in itself a profound conclusion, little else could be said if this was all that was
available. The evidence points to the occurrence of lateral variations in density
in the crust and mantle but no inverse procedure will tell with certainty where
these anomalies occur. Does it mean that the Earth has a finite strength and has
supported these anomalies since early days? Does it mean that the Earth is a
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