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Leap Second

January 2, 2009 - Michael Palmer
New Year's Eve at 7:00 PM our time the a leap second was added to atomic clocks worldwide. The event occurred at 23:59:60 (or near midnight) at Coordinated Universal Time (UTC) on December 31, 2008. This is the 24th leap second to be added since the first leap second was added in 1972. This is also the first leap second in three years.

A leap second is a second, as measured by an atomic clock, added to or subtracted from Coordinated Universal Time (UTC) to make it agree with astronomical time to within 0.9 second. It compensates for slowing in the Earth’s rotation and is added during the end of June or December.

The second is the base unit for modern time keeping. The second was previously defined based on the Earth's rotation, but because modern atomic clocks are more accurate than the Earth's rotation the definition was changed in 1967. A second is currently defined as being the duration of 9,192,631,770 periods/oscillations of radiation from a Cesium-133 atom at the ground state. This is where ground state refers to a cesium (or caesium) atom at rest at a temperature of 0 K (kelvin) (coldest possible).

UTC–TAI means the difference between the civil time (UTC) which is kept within 0.9 seconds from Earth's rotation and the International Atomic Time (TAI) which does not care about the Earth's rotation, but rather observations of the Cesium-133 atom. A difference of 33 seconds means that the Earth has slowed by 33 seconds compared with TAI since 1958 (when TAI and UTC were the same). The difference between UTC and TAI was defined as 10 seconds from January 1972 and the first leap second was added in June 1972.

So what did you do with your leap second this year?


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