> http://www.livescience.com/55312-leap-second-2016-atomic-clocks.html?utm_source=listrak&utm_medium=email&utm_campaign=20160708-lst > > <http://www.livescience.com/55312-leap-second-2016-atomic-clocks.html?utm_source=listrak&utm_medium=email&utm_campaign=20160708-lst> > > 'Leap Second' to Be Added on New Year's Eve This Year > > > A "leap second" will be added to the world's official clocks on Dec. 31, > 2016, to accommodate Earth's gradually slowing rotation. > > Credit: NASA > Revelers will get to celebrate New Year's Eve for a tiny bit longer than > usual this year. > > A "leap second > <http://www.space.com/29795-leap-second-tonight-atomic-clocks.html>" 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 ofatomic clocks > <http://www.livescience.com/32660-how-does-an-atomic-clock-work.html> 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) > <http://www.livescience.com/44602-how-to-build-the-most-accurate-atomic-clocks-video.html>] > > Report this Advertisement > > 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 > <http://www.usno.navy.mil/USNO/tours-events/2016_Leap_Second%20Press%20Release%20-%20Final.pdf> > 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. > > Follow Mike Wall on Twitter @michaeldwall <http://twitter.com/michaeldwall> > andGoogle+ <https://plus.google.com/u/0/108984047382030613667/posts>. Follow > us @Spacedotcom <http://twitter.com/spacedotcom>, Facebook > <https://www.facebook.com/spacecom> or Google+ > <https://plus.google.com/+SPACEcom/posts>. Originally published on Space.com > <http://www.space.com/33361-leap-second-2016-atomic-clocks.html>.
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