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Encyclopedia entry: Earth orientation
- Article from:
- The Oxford Companion to the Earth
- Author:
Copyright© The Oxford Companion to the Earth 2000, originally published by Oxford University Press 2000. (Hide copyright information)
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Earth orientation
Earth orientation is defined as the instantaneous angular relationship between an Earth-fixed reference frame and an inertial, external reference frame. Intimately linked with this concept is that of Earth rotation or rate of change of orientation over timescales of from thousands of years to a few hours. To first approximation, the Earth in its annual orbit about the Sun is inclined such that the angle, the obliquity, between the Earth's equator and the orbital plane, the ecliptic, is approximately 23.4 degrees, and the Earth's rotational axis is fixed in inertial space. The Earth spins about this axis in a period of 24 hours, or 86400 seconds. The study and observation ...