"Although leap seconds caused by the need to compensate for the earth's rotation are extremely rare occurrences – the last whole second adjustment would have happened in 1820 had atomic clocks and NTP servers existed – there have in fact been 25 leap seconds for other reasons since the beginning of atomically-measured time in 1971."<p>The date for the definition by SI of the second using an atomic-clock reference is 1967:<p>"The SI second was defined in terms of the caesium atom in 1967, and in 1971 it was renamed International Atomic Time (TAI)"<p>1972 is the date the leap seconds have been introduced.<p>Since UTC has been initiated in 1961 (yes, even before the definition of the SI second in terms of caesium atom), there was an ongoing effort to synchronize the atomic-clock references with the solar time. The various methods for synchronizing the time were based both on changing of the length of the second and with time steps (see Stepped Atomic Time).<p>When the SI second was defined based on the atomic clock definition, it was decided that shifting the clock frequency to account to solar time synchronization was a mess and thus only time steps were used to keep the UTC in sync with mean solar time.<p>The steps were of 200ms; later it was recognized that this was also a mess and "In 1968, Louis Essen, the inventor of the caesium atomic clock, and G. M. R. Winkler both independently proposed that steps should be of 1 s only" [4]<p>Starting from 1972 these 1s corrections are the leap seconds.<p>The need for those corrections is caused by the irregularities in earth rotational speed.
Does anybody know what are these other reasons ?<p>1. <a href="http://en.wikipedia.org/wiki/Leap_second" rel="nofollow">http://en.wikipedia.org/wiki/Leap_second</a>
2. <a href="http://en.wikipedia.org/wiki/Atomic_clock" rel="nofollow">http://en.wikipedia.org/wiki/Atomic_clock</a>
3. <a href="http://en.wikipedia.org/wiki/Second" rel="nofollow">http://en.wikipedia.org/wiki/Second</a>
4. <a href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time" rel="nofollow">http://en.wikipedia.org/wiki/Coordinated_Universal_Time</a>