U.S. Naval Observatory Earth Orientation Department

Question: What is Earth Orientation?

Answer: The term "Earth Orientation" refers to the orientation of the Earth in three dimensional space. It is usually measured with respect to coordinate axes defined on the Earth using five angles: two angles called the polar coordinates which identify the direction of the Earth's rotation axis within the Earth (polar coordinates measure the position of the Earth's instantaneous pole of rotation in a reference frame which is defined by the adopted locations of terrestrial observatories); an angle describing the rotational motion of the Earth (called UT1; this coordinate measures the angle through which the Earth has turned in a given period of time); and two angles which characterize the direction of the Earth's rotation axis in space. With these coordinates, the orientation of the Earth in three dimensional space is fully described.

A more detailed explanation is also available.

Question: What is a leap second?

Answer: The term "leap second" refers to a one second adjustment made to Civil time in order to minimize the cumulative difference between a uniform time scale defined by atomic clocks (the basis for Civil time) and a time scale defined by the Earth's rotation. This occasional adjustment is necessary because the rotation of the Earth is constantly undergoing a non-uniform deceleration primarily caused by the braking action of the tides. A leap second can be either positive or negative depending on the Earth's rotation. However, since their introduction in 1972, all leap seconds have been positive. This reflects the general slowing of the Earth's rotation.

A more detailed explanation is also available.

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Last modified: 23 June 2003 Approved by EO Dept. Head, USNO