This year the Earth adds another 'leap second'

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This year, folks can delay their New Year's resolutions just a bit longer — one second longer, to be precise.

One "leap second" will be added to the very last second of the year on Dec. 31 in order to keep clocks in sync with the shifts in the Earth's rotation angle, according to an announcement from the International Earth Rotation and Reference Systems Service, the group responsible for recommending the addition of leap seconds to international time.

Leap seconds are a byproduct of more accurate timekeeping developed in the 20th century. Before then, time was typically measured by determining the position and movement of the Earth in relation to surrounding stars and planets. This is how civilizations have been keeping time at least since the ancient Egyptians first began using sundials.

But now we use atomic clocks to tell time. Atomic clocks are a complex technology that achieve an incomparably accurate measurement of time by harnessing the tendency atoms have to reliably jump between states of high and low energy only when exposed to a very specific frequency of radiation. That frequency is so specific, and the atomic behavior so reliable, it has become the basis for atomic time.

For example, in 2014, the National Institute of Standards and Technology debuted a new atomic clock that will only lose about a second of accuracy every 300 million years.

International Atomic Time is an average time calculated based on about 400 atomic clocks found throughout the world, including at the United States Naval Observatory.

Atomic time is combined with something else, called Universal Time (UT1), which is based on the rotation of the Earth. Together these make the international time standard known as Coordinated Universal Time (usually abbreviated as UTC).

The problem is that the length of a day on Earth varies, for a few reasons. The gravitational pull of the moon, sun and other planets cause atmospheric changes and changes to the distribution of weight on Earth that affect the planet's rotation. Tidal changes are an example of this — the moon is actually causing water to bunch up in certain places at certain times. Earthquakes, volcanoes or other geological phenomena can also affect the speed of the Earth's rotation. Sometimes the Earth spins faster, sometimes slower, but scientists have determined it is gradually slowing over time.

So the atomic clocks fall out of sync with the astronomical ones.

The IERS calls UTC a "compromise" between the measurements given by the atomic clocks and astronomical time. Part of this compromise involves adding leap seconds to close the gaps.

They were introduced in 1972, and 26 leap seconds have been added to time since then. The last one was added in January 2015.

A leap second added in 2012 reportedly caused some technical issues for many servers at several major tech companies, including LinkedIn, Reddit, The Mozilla Foundation, and others.

These companies reportedly made fixes to prevent the issue from happening again when subsequent leap seconds were added at later times. Emails sent to LinkedIn, Reddit and The Mozilla Foundation were not returned before publication.

Google found a few scattered problems after a leap second was added in 2005, and developed a fix for the issue they call the 'leap smear." Google adds a few milliseconds throughout the day to its server clocks, so by the time the change is made to UTC, the company's servers are already in sync.