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leap second 2005
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The first leap second was added to the UTC time system on the 30th June, 1972.



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Title: The Future of Time: UTC and the Leap Second
Authors: David Finkleman, Steve Allen, John Seago, Rob Seaman, P. Kenneth Seidelmann

Before atomic timekeeping, clocks were set to the skies. But starting in 1972, radio signals began broadcasting atomic seconds and leap seconds have occasionally been added to that stream of atomic seconds to keep the signals synchronized with the actual rotation of Earth. Such adjustments were considered necessary because Earth's rotation is less regular than atomic timekeeping. In January 2012, a United Nations-affiliated organization could permanently break this link by redefining Coordinated Universal Time. To understand the importance of this potential change, it's important to understand the history of human timekeeping.

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Leap seconds
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The first leap second was added to the UTC time system on the 30th June, 1972.

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Earth's rotation
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The summer solstice that falls this year on June 21 marks the longest day of the year in the Northern Hemisphere, sunlight-wise. Almost imperceptibly, however, Earth's daynight cycle --one rotation on its axis--is growing longer year by year, and has been for most of the planet's history.
Forces from afar conspire to put the brakes on our spinning world--ocean tides generated by both the moon and sun's gravity add 1.7 milliseconds to the length of a day each century, although that figure changes on geologic timescales. The moon is slowly spiralling away from Earth as it drives day-stretching tides, a phenomenon recorded in rocks and fossils that provides clues to the satellite's origin and ultimate fate

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Earth's twisted heart changes the length of the day

Fluctuations at the Earth's core subtly shift the planet's rotation by 0.4 milliseconds over a six-year cycle.
Nicolas Gillet of the University of Joseph Fourier in Grenoble, France, and colleagues, modelled fluid behaviour in the Earth's core based on measurements of fluctuations in the magnetic field.
The innermost region of the Earth's outer core periodically flows faster or slower, and this action "tugs" at the planet's magnetic field. Like an array of rubber bands, the field then pulls the region back towards its original position. But the effect ripples outward, changing the core's rate of rotation layer by layer.

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Leap Seconds
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Time is what prevents everything from happening at once.
Since a year is actually 365.24219 days long, a day is added to the end of February, every fours years (except for years which are both divisible by 100 and not divisible by 400) ) to keep the calendar year in synchronization with the four seasons. When a day is added, that year is referred to as a leap year.
In mankind's quest for order in the universe, we also have leap seconds to keep clocks in synchronisation with the rotating actions of the Earth.

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RE: leap second 2009
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Back on New Year's Eve you may recall, an extra second, known as a leap second, was added to the day to compensate for our slowing Earth. Leap seconds are added when necessary at the end of either June or December. The International Earth Rotation and Reference Systems Service (IERS) has since announced that there will be no positive leap second added this June.

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Leap second 2008
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The U.S. Naval Observatory, keeper of the Pentagon's master clock, said it would add the extra second on Wednesday in coordination with the world's atomic clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time, or UTC.
That corresponds to 6:59:59 p.m. EST (23:59:59 GMT), when an extra second will tick by -- the 24th to be added to UTC since 1972, when the practice began.

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RE: leap second 2005
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As 2008 turns to 2009 at the end of this month, an extra second will be added to every clock. But who decides exactly what time it is? Professor Brian Cox meets the man in charge of all our timekeeping - the world's director of time.
Time is something we all take for granted. Morning turns to evening; autumn drifts into winter and another year becomes history as the earth completes one more journey around the sun.

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