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Published online by Cambridge University Press: 14 August 2015
Two fundamental frames of reference are used in the study of the rotation of the Earth: the nonrotating celestial coordinate system XYZ attached to the directions to stars and/or extragalactic sources, and the terrestrial coordinate system xyz attached in “a prescribed way” to several points (observatories) on the surface of the Earth or to the pencil of unit vectors drawn from an arbitrary origin parallel to the local verticals at these points.