The most ancient and fundamental concern of astronomy is the orientation and motion of a terrestrial observer relative to the stars. Its geophysical aspects date from the time of Newton and Halley, and its mathematical foundations were laid by Euler 200 years ago. Despite this honorable antiquity, the subject is far from moribund and today presents a rich and fascinating array of challenges to observation, experiment, data analysis, and theory. The many-faceted problems of the three-dimensional rotation of the earth about its center of mass now attract astronomers and paleontologists, solid earth geophysicists and electrical engineers, general relativists and oceanographers, and applied mathematicians and scholars of classical texts.
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