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What is Earth Orientation?

The term "Earth orientation" refers to the direction in space of axes which have been defined on the Earth. It is usually measured using five quantities: two angles which identify the direction of the Earth's rotation axis within the Earth (polar motion), an angle describing the rotational motion of the Earth (UT1-UTC), and two angles which characterize the direction of the Earth's rotation axis in space (celestial pole). With these coordinates, the orientation of the Earth in space is fully described. The Rapid Service/Prediction Center of the International Earth Rotation and Reference Systems Service (IERS) monitors the Earth's orientation and disseminates this information to many organizations on a continuous basis.
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