|U.S. Naval Observatory||Earth Orientation Department| In 1956,
following several years of work, two astronomers at the U. S.
Naval Observatory (USNO) and two astronomers at the National
Physical Laboratory (Teddington, England) determined the
relationship between the frequency of the Cesium atom (the
standard of time) and the rotation of the Earth at a particular
epoch. As a result, they defined the second of atomic time as
the length of time required for 9 192 631 770 cycles of the
Cesium atom at zero magnetic field. The second thus defined was
equivalent to the second defined by the fraction 1 / 31 556
925.9747 of the year 1900. The atomic second was set equal,
then, to an average second of Earth rotation time near the end
of the 19th century. The Rapid Service/Prediction Center of the
International Earth Rotation Service (IERS), located at the U.S.
Naval Observatory, monitors the Earth's rotation. Part of its
mission involves the determination of a time scale based on the
current rate of the rotation of t