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.
more detailed explanation is also
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
A more detailed explanation is also available.
This is an official U.S. Navy web site.
Last modified: 23 June 2003
Approved by EO Dept. Head, USNO