A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time (UTC) time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks. To keep the UTC time scale close to mean solar time, UTC is occasionally corrected by an intercalary adjustment, or "leap", of one second. Over long time periods, leap seconds must be added at an ever increasing rate (see ΔT). The timing of leap seconds is now determined by the International Earth Rotation and Reference Systems Service (IERS). Leap seconds were determined by the Bureau International de l'Heure (BIH) prior to January 1, 1988, when the IERS assumed that responsibility.
When a positive leap second is added at 23:59:60 UTC, it delays the start of the following UTC day (at 00:00:00 UTC) by one second, effectively slowing the UTC clock.
Reason for leap seconds
Leap seconds are necessary partly because the length of the mean solar day is very slowly increasing, and partly because the SI second, when adopted, was already a little shorter than the current value of the second of mean solar time. Time is now measured using stable atomic clocks (TAI or International Atomic Time), whereas the rotation of Earth is much more variable.
Originally, the second was defined as 1/86400 of a mean solar day (see solar time) as determined by the rotation of the Earth around its axis and around the Sun. By the middle of the 20th century, it was apparent that the rotation of the Earth did not provide a sufficiently uniform time standard, and in 1956 the second was redefined in terms of the annual orbital revolution of the Earth around the Sun. In 1967 the second was redefined, once again, in terms of a physical property: the oscillations of an atom of caesium-133, which were measurable by an atomic clock. But the solar day becomes 1.7 ms longer every century due mainly to tidal friction (2.3 ms/cy, reduced by 0.6 ms/cy due to glacial rebound).
The SI second counted by atomic time standards has been defined on the basis of a history going back to the former standard time scale of ephemeris time (ET). It can now be seen to be close to the average second of 1/86400 of a mean solar day between 1750 and 1892. The current SI second was defined in 1967, as 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This number first arose from calibration of the caesium standard by the second of ET: in 1958, the second of ET was determined as the duration of 9,192,631,770 ± 20 cycles of the chosen caesium transition, (while at about the same time, and with the same caesium standard, the then-current mean length of the second of mean solar time (UT2) had been measured at 9,192,631,830 cycles). Later verification showed that the SI second referred to atomic time was in agreement, within 1 part in 1010, with the second of ephemeris time as determined from lunar observations. Time as measured by Earth's rotation has accumulated a delay with respect to atomic time standards. From 1961 to 1971, the rate of (some) atomic clocks was (for purposes of UTC) constantly slowed to stay in sync with Earth's rotation. (Before 1961, broadcast time was synchronized to astronomically determined Greenwich Mean Time.) Since 1972, broadcast seconds have been exactly equal to the standard SI second chosen in 1967.
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