1972

U.S. Naval Observatory Public Affairs Office, August 17, 1998
U. S. Naval Observatory to Add Leap Second to Clocks

In 1972, by international agreement, it was decided to let atomic clocks run independently of the Earth, keep two separate times, and coordinate the two. In order to keep the difference between Earth time and atomic time within nine tenths of a second as the two times get out of sync, leap seconds are added to the atomic time scale. The International Earth Rotation Service (for which the U. S. Naval Observatory provides the Rapid Service) is the organization that monitors the difference between the two time scales and calls for leap seconds to be inserted when necessary. Since 1972, leap seconds have been added at intervals varying from six months to two-and-one-half years -- this leap second is eighteen months since the last one. Leap seconds are added because the Earth's rotation is tending to slow down. If the Earth were to speed up, a leap second would be removed.

Leap Second on UTC on June 30, 1997
On 30 June 1997, the last minute of the day will last 61 seconds. Why?

The Coordinated Universal Time (UTC, replacing GMT) is the reference time scale derived from The Temps Atomique International (TAI) calculated by the Bureau International des Poids et Mesures (BIPM) using a worldwide network of atomic clocks. UTC differs from TAI by an integer number of seconds; it is the basis of all activities in the world. UT1 is the time scale based on the observation of the Earth's rotation. It is now derived from Very Long Baseline Interferometry (VLBI). The various irregular fluctuations progressively detected in the rotation rate of the Earth lead in 1972 to the replacement of UT1 as the reference time scale . However, it was desired by the scientific community to maintain the difference UT1-UTC smaller than 0.9 second to ensure agreement between the physical and astronomical time scales.

Since the adoption of this system in 1972, firstly due to the initial choice of the value of the second (1/86400 mean solar day of the year 1900) and secondly to the general slowing down of the Earth's rotation, it has been necessary to add 20 s to UTC. The next additionnal second will be introduced on 1 July 1997. The decision to introduce a leap second in UTC is the responsibility of the International Earth Rotation Service (IERS). According to international agreements, 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.