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'Leap Second' to Be Added on New Year's Eve This Year

Full story: <http://www.space.com/33361-leap-second-2016-atomic-clocks.html>

Revelers will get to celebrate New Year's Eve for a tiny bit longer
than usual this year.

A "leap second" will be added to the world's official clocks on Dec. 31
at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC),
which corresponds to 6:59:59 p.m. EST; the clocks will read 23:59:60
before ticking over to midnight. The goal is to keep two different
timescales in sync with each other.

The units of time had long been defined based on Earth's rotation
relative to distant celestial bodies. But that changed with the
invention of atomic clocks in the mid-20th century; scientists then
decided to base the second on the natural vibrations of the cesium atom.
[How to Build the Most Accurate Atomic Clocks (Video)]

These two timescales don't match up exactly, however. Measurements show
that, because the moon's gravitational pull and other factors are
gradually slowing Earth's spin, the rotation-based scale loses between
1.5 and 2 milliseconds per day compared to atomic time �� meaning the
two diverge by a full second every 500 to 750 days.

Leap seconds are a way to make up for this difference. Since 1972, the International Earth Rotation and Reference Systems Service (IERS) �� the organization that keeps track of time for the world �� has added 26 leap seconds to atomic clocks, with the last such insertion coming on June
30, 2015.

The aim is to keep the two timescales within 0.9 seconds of each other.

"We can easily change the time of an atomic clock, but it is not
possible to alter the Earth's rotational speed to match the atomic
clocks," officials with the U.S. Naval Observatory (USNO), which
maintains the Department of Defense's master clock, noted �� wryly, it would seem �� in a statement today (July 6).

While Earth's rotation rate is slowing, the effect is quite subtle.

"Confusion sometimes arises over the misconception that the occasional insertion of leap seconds every few years indicates that the Earth
should stop rotating within a few millennia," USNO officials wrote.
"This is because some [people] mistake leap seconds to be a measure of
the rate at which the Earth is slowing. The 1-second increments are,
however, indications of the accumulated difference in time between the
two systems."

When leap seconds are added, they are always inserted on June 30 or Dec.
31 of a particular year. In 1972, IERS officials called for a leap
second to be inserted on both dates.

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