What is a Leap Second?

Leap seconds did not exist before the year 1972. At the start of January 1, 1972, UTC was redefined to be equal to UT1 and equal to TAI minus 10 seconds. The first true leap second was inserted at the end of June 30, 1972 when UTC was changed to TAI - 11 seconds. Each leap second changes the definition of UTC by exactly one atomic second to keep UTC within 0.9 seconds of UT1.


  • TAI: International Atomic Time (based on the SI second)
  • UT1: Universal Time derived from observations
  • UTC: Coordinated Universal Time
  • IERS: International Earth Rotation and Reference System Service
  • SI second: The duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium atom 133.

    There has been 24 leap seconds added in the 37 years that this method of coordinating time has been in use. International Atomic Time (TAI) is now 34 seconds fast of UTC. The earth rotates (with respect to the mean sun) a little slower than one revolution per 86400 seconds. The length of a day varies from about 86400.001 to 86400.003 seconds per day and does not have long term stability.

    This is a note I wrote (in California) the morning of 2005-12-31: At 3:59:60 PM PST this afternoon (2005-12-31 23:59:60 UTC) a leap second is going to be added to Coordinated Universal Time (UTC). UTC, adjusted for our time zone, is what we set our clocks to. If you have a radio that can receive WWV (5, 10, 15, 20, 25, 30 MHz) you can listen to when the leap second happens. I will be listening at home. The leap second will last for one second from 2005- 12-31 23:59:60 UTC to 2006-01-01 00:00:00 UTC.

    See the plot of leap seconds. Actually it is two plots in one. We have UTC - TAI as a function of time and UT1 - TAI as a function of time. UTC is adjusted in one-second steps to closely approximate UT1.


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    This page accessed times since Feb 5, 2006.
    Page created by: hjsmithh@sbcglobal.net
    Changes last made on Monday, 23-Mar-09 07:59:37 PDT