Civil time is occasionally adjusted by one second increments to insure that the difference between a uniform time scale defined by atomic clocks does not differ from the Earth's rotational time by more than 0.9 seconds. Coordinated Universal Time (UTC), an atomic time, is the basis for our civil time.

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 radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium 133 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 turn of the 20th century.

The Sub-bureau for Rapid Service and Predictions of the International Earth Rotation Service (IERS), located at the USNO, monitors the Earth's rotation. Part of its mission involves the dermination of a time scale based on the current rate of the rotation of the Earth. UT1 is the non-uniform time based on the Earth's rotation.

The Earth is constantly undergoing a deceleration caused by the braking action of the tides. Through the use of ancient observations of eclipses, it is possible to determine the deceleration of the Earth to be roughly 1-3 milliseconds per day per century. This is an effect which causes the Earth's rotational time to slow with respect to the atomic clock time. Since it has been nearly 1 century since the defining epoch (i.e. the ninety year difference between 1990 and 1900), the difference is roughly 2 miliseconds per day. Other factors also affect the Earth, some in unpredictable ways, so that it is necessary to monitor the Earth's rotation continuously.

In order to keep the cumulative difference in UT1-UTC less than 0.9 seconds, a leap second is added to the atomic time to decrease the difference between the two. This leap second can be either positive or negative depending on the Earth's rotation. Since the first leap second in 1972, all leap seconds have been positive. This reflects the general slowing trend of the Earth due to tidal braking.

Confusion sometimes arises over the misconception that the regular insertion of leap seconds every few years indicates that the Earth should stop rotating within a few millennia. The confusion arises because some mistake leap seconds for a measure of the rate at which the Earth is slowing. The one-second increments are, however, indications of the accumulated difference in time between the two systems. As an example, the situation is similar to what would happen if a person owned a watch that lost two seconds per day. If it were set to a perfect clock today, the watch would be found to be slow by two seconds tomorrow. At the end of a month, the watch will be roughly a minute in error (thirty days of two second error accumulated each day). The person would then find it convenient to reset the watch by one minute to have the correct time again.

This scenario is analogous to that encountered with the leap second. The difference is that instead of setting the clock that is running slow, we choose to set the clock that is keeping a uniform, precise time. The reason for this is that we can change the time on an atomic clock while it is not possible to alter the Earth's rotational speed to match the atomic clocks! Currently the Earth runs slow at roughly 2 milliseconds per day. After 500 days, the difference between the Earth rotation time and the atomic time would be one second. Instead of allowing this to happen, a leap second is inserted to bring the two times closer together.

International Atomic Time (TAI) is the atomic time scale of the BIPM; its unit interval is exactly one SI second at sea level. The origin of TAI is such that UT1-TAI is approximately 0 (zero) on January 1, 1958. TAI is not adjusted for leap seconds. UTC is defined by the CCIR Recommendation 460-4 (1986). It differs from TAI by an integral number of seconds, in such a way that UT1-UTC stays smaller than 0.9s in absolute value.

The decision to introduce a leap second in UTC is the responsibility of the International Earth Rotation Service (IERS). According to the CCIR Recommendation, first preference is given to the opportunities at the end of December and June, and second preference to those at the end of March and September. Since the system was introduced in 1972, only dates in June and December have been used.

The first leap second was introduced on June 30, 1972. Information on the most recent leap second can be found here. The historical list of leap seconds lists all leap seconds.

The OMEGA epoch is January 1, 1972 and OMEGA is synchronized to UTC. OMEGA is NOT adjusted for leap seconds.

The Global Positioning System (GPS) epoch is January 6, 1980 and synchronized to UTC. GPS is NOT adjusted for leap seconds.

As of 1 January 1996,
TAI is ahead of UTC   by 30 seconds.
TAI is ahead of GPS   by 19 seconds.
TAI is ahead of OMEGA by 10 seconds.

OMEGA is ahead of UTC by 20 seconds.
GPS   is ahead of UTC by 11 seconds.

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