The International Rotation and Reference Systems Service in Paris, France keeps track of time by measuring the Earth's rotation as well as by an atomic clock. The atomic clock is an absolute measurement of time, where the earth's rotation varies, so it is occasionally necessary to re-synchronize the atomic clock with the earth's rotation. When a difference in the two clocks shows up, the IERS adds a second (or subtracts a second in theory, although this has not yet happened) to a day, usually either June 30 or December 31.
Leap seconds have been added to the clock on the following days:
- June 30, 1972
- December 31, 1972
- December 31, 1973
- December 31, 1974
- December 31, 1975
- December 31, 1976
- December 31, 1977
- December 31, 1978
- December 31, 1979
- June 30, 1981
- June 30, 1982
- June 30, 1983
- June 30, 1985
- December 31, 1987
- December 31, 1989
- December 31, 1990
- June 30, 1992
- June 30, 1993
- June 30, 1994
- December 31, 1995
- June 30, 1997
- December 31, 1998
- December 31, 2005
In the period between 1998 and 2005, the rotation of the Earth sped up slightly, reducing the need for the leap second. The main force causing the Earth's rotation to slow down is the tidal force exerted by the moon, although movement of rock in the Earth's core and weather patterns also modify the Earth's rotational velocity.