Media playback is unsupported on your device Media caption Rebecca Morelle reports: "It's all because of a very slight wobble in the Earth's rotation"

A "leap second" will be added to this year's New Year's countdown to compensate for a slowdown in the Earth's rotation.

The extra second will occur as clocks strike midnight and a time of 23:59:60 will be recorded, delaying 2017 momentarily.

It will only affect countdowns in countries using Greenwich Mean Time, which includes Britain.

The change is required because standard time lags behind atomic clocks.

A leap second last occurred in June 2015 and this will be the 27th time it has occurred.

The National Physical Laboratory (NPL) - responsible for the UK's national time scale - uses the atomic clock to provide a stable and continuous timescale.

Along with other clocks across the globe, it provides the world with its coordinated universal time.

NPL senior research scientist Peter Whibberley said: "Atomic clocks are more than a million times better at keeping time than the rotation of the Earth, which fluctuates unpredictably.

"Leap seconds are needed to prevent civil time drifting away from Earth time.

"Although the drift is small - taking around 1,000 years to accumulate a one-hour difference - if not corrected it would eventually result in clocks showing midday before sunrise."

Atomic clocks use the change of electron energy levels to tell the time.

The time created by the clocks is used in GPS location devices and is used to control the wave frequency of television broadcasts.

The International Earth Rotation and Reference Systems Service in France tracks the Earth's rotation and announces the need for a leap second.</s>Credit: CC0 Public Domain

The year 2017 isn't coming as soon as you think. In fact, it will be exactly one second late.

On Dec. 31, 2016, the international time keeping community will tack an additional second, known as a leap second, on to the last minute of the year.

As midnight approaches, the official atomic clocks that keep Universal Coordinated Time will mark the time as 23h 59m 59s, followed by the leap second 23h 59m 60s. Jan 1 will continue as usual, beginning with 0h 0m 0s.

Unlike leap years, leap seconds are not a regular occurrence. Instead, they are decreed by the International Earth Rotation and Reference Systems Service, or IERS, in Paris, which measures the Earth's rotation and compares it with the time kept by atomic clocks.

You may not realize it, but in our modern society we are governed by two types of time.

The first, known as astronomical time, is based on how long it takes Earth to make one complete spin on its axis. Through most of human history this type of time was measured by the rising and setting of the sun. Today, however, scientists keep track of it by aiming a network of radio telescopes at a distant quasar.

Atomic time, on the other hand, defines a second as exactly 9,192,631,770 oscillations of a cesium-133 atom. This is what determines the time that displays on a computer or cellphone.

But these two types of time do not always line up.

In part, that's because the Earth does not keep perfect time.

Duncan Agnew, a geophysicist at Scripps Institution of Oceanography in San Diego, explains that movements within the Earth's liquid core can cause the spin rate of our planet to speed up or slow down.

And the U.S. Naval Observatory reports that over the last 40 years the Earth has generally run slow compared with atomic time, at an average of 1.5 to 2 milliseconds per day.

To keep the two types of time from veering too far away from each other, the IERS calls for a leap second whenever it appears there will be more than a 0.9 second difference between astronomical time and atomic time.

Since 1972, there have been 26 leap seconds added in intervals varying between six months and seven years.

For the record, the IERS could also say that a second should be subtracted from atomic time, but so far that has never been necessary.

Although it's no big deal for most of us to adjust to an additional atomic second in our year, it is a much bigger pain for people who run computer networks. In the past, tech companies like Google, Reddit and LinkedIn have all run into problems because of the addition of a leap second.

Therefore, the future of the leap second is being hotly debated.

After all, even if the difference between astronomical time and atomic time grew by one second every year, in 100 years the gap would be only less than two minutes. In 1,000 years, it would be off only by less than 17 minutes.

But for now at least the leap second stands. And although the second before midnight Universal Coordinated Time technically corresponds to 3:59:59 p.m. Pacific Standard Time, you might still consider marking the occasion of the extra second on Dec. 31 by adding a zero to your traditional New Year's countdown like this:

"... three, two, one, zero." And then "Happy New Year!"

©2016 Los Angeles Times

Distributed by Tribune Content Agency, LLC.</s>To maintain accuracy, NASA said its missions, including the Solar Dynamics Observatory, or SDO, which continuously watches the sun, will join official clocks around the world in adding a leap second to the final minute of 2016.

On December 31, 2016, official clocks around the world will add a leap second at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC).

Clocks do this to keep in sync with Earth's rotation, which gradually slows down over time. When the dinosaurs roamed Earth, for example, our globe took only 23 hours to make a complete rotation.

In space, millisecond accuracy is crucial to understanding how satellites orbit.

"SDO moves about 1.9 miles every second," Dean Pesnell, project scientist for SDO at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement.

"So does every other object in orbit near SDO. We all have to use the same time to make sure our collision avoidance programs are accurate. So we all add a leap second to the end of 2016, delaying 2017 by one second," Pesnell noted.

The leap second is also key to making sure that SDO is in sync with the Coordinated Universal Time used to label each of its images.

Designed to help us understand the sun's influence on Earth and Near-Earth space, SDO was launched in 2010.

It has a clock that counts the number of seconds since the beginning of the mission.

To convert that count to UTC requires knowing just how many leap seconds have been added to Earth-bound clocks since the mission started.

When the spacecraft wants to provide a time in UTC, it calls a software module that takes into consideration both the mission's second count and the number of leap seconds -- and then returns a time in UTC.</s>NASA missions, including the Solar Dynamics Observatory (SDO) which watches the Sun, will join timekeepers around the world to add a leap second to its clocks, just before midnight on New Year’s Eve.

Clocks will add the extra second to keep in sync with Earth’s rotation, which gradually slows down over time.

When the dinosaurs roamed Earth our globe took only 23 hours to make a complete rotation. In space, millisecond accuracy is crucial to understanding how satellites orbit.

“SDO moves about 3 kilometres every second. So does every other object in orbit near SDO,” said Dean Pesnell, project scientist for SDO at NASA’s Goddard Space Flight Centre in the US.

“We all have to use the same time to make sure our collision avoidance programs are accurate. So we all add a leap second to the end of 2016, delaying 2017 by one second,” said Pesnell.

The leap second is also key to making sure that SDO is in sync with the Coordinated Universal Time (UTC) used to label each of its images.

Read More : NASA astronauts to conduct spacewalks for power update

SDO has a clock that counts the number of seconds since the beginning of the mission. To convert that count to UTC requires knowing just how many leap seconds have been added to Earth-bound clocks since the mission started.

When the spacecraft wants to provide a time in UTC, it calls a software module that takes into consideration both the mission’s second count and the number of leap seconds – and then returns a time in UTC.</s>New Delhi (ABC Live): Indian Standard Time : Council of Scientific & Industrial Research (CSIR) -National Physical Laboratory (CSIR-NPL) is the custodian of Indian Standard Time (IST) and has the responsibility for realization, establishment, maintenance and dissemination of IST through an act of Parliament.

A “Leap Second” is added every now and then to Coordinated Universal Time (UTC) in order to synchronize clocks worldwide with the Earth’s ever slowing rotation. UTC consists of a time scale that combines the output of more than 300 highly precise Atomic clocks worldwide, including the one at CSIR-NPL. Atomic clocks are very accurate and are stable within 1 second over a period of millions of years. On the other hand, the Astronomical Time known as Universal Time (UT1) refers to the Earth’s rotation around its own axis and determines the length of a day. However, the Earth’s rotation around its own axis is not regular, as sometimes it speeds up and sometimes it slows down, due to various factors including the moon’s gravitational Earth-braking forces that often results in ocean tides. As a result, Astronomical Time (UT1) gradually falls out of synch with Atomic time (UTC), and as and when the difference between UTC and UT1 approaches 0.9 seconds, a “Leap Second” is added to UTC through Atomic clocks worldwide.

Since 1972, 36 “Leap Seconds” have been added at intervals varying from six months to seven years. 37th “Leap Second” will be added to UTC at the midnight of December 31, 2016 in the countries within this time zone. However, countries in other time zones will have “Leap Second” inserted according to their longitude. As the “Leap Second” is added simultaneously all over the world at UTC 23:59:59 on December 31, 2016, implying that in India the “Leap Second” will be inserted at IST 05:29:59 on January 1, 2017 (IST being five hours and thirty minutes ahead of UTC). In order to follow IST, the clocks need to be adjusted after the insertion of “Leap Second”. Those utilizing CSIR-NPL time dissemination services (NTP, Teleclock etc.) need not worry as they will receive the corrected time post the insertion of “Leap Second”. The “Leap Second” adjustment is not so relevant for normal everyday life; however this shift is critical for applications requiring of time accuracies in the nanosecond e.g. astronomy, satellite navigation, communication networks etc.

A “Leap Second” explanatory meeting will be held on December 31, 2016 at 23:30 Hours at the Main Building Reception area of the CSIR-National Physical Laboratory.

The detailed information about the IST is available on NPL website http://www.nplindia.org/time-frequency-and-electrical-electronics-metrology-division</s>To the time-poor of the world: take heart, for 2016 is a generous year.

Not only were you granted a leap day on 29 February, you will soon score a New Year’s Eve countdown bonus, a leap second, to hold off 2017 for a final sip or regret.

Whereas leap years add a day to align the calendar with the seasons, leap seconds align our everyday clocks with the Sun’s position in the sky, that is, with the Earth’s rotation.

Currently our planet takes roughly 86,400.00183 seconds (on average) to turn, instead of the expected 86,400 seconds you get by multiplying 24 hours by 60 minutes by 60 seconds. This may not sound like a great difference, but it amounts to a full second every 18 months.

If left unchecked, it would become noticeable over time, and ultimately become problematic.

How did we get into this awkward situation? Why not just define a second so that there are exactly the right number? This sensible idea was tried in 1874, but hit a snag: the Earth keeps changing.

In terms of today’s standard SI second (defined via atomic physics), the above discrepancy is due to the fact that the day is losing about 0.0015 seconds per century, due largely to tidal friction.

Not only that, it also changes quite erratically due to mass redistribution. For example, it is slowed by oceanic thermal expansion due to global warming, just as a playground spinning seat slows, via the conservation of angular momentum, when you place your body farther from the centre.

Leap seconds are used to make sure our usual timekeeping system, Coordinated Universal Time (UTC), never gets more than 0.9 seconds away from the Earth-tracking alternative, Universal Time (UT1).

But unlike leap years, leap seconds cannot be calculated centuries in advance. Because the Earth moves erratically, it must be observed closely, and leap seconds scheduled on an as-needed basis.

In UT1, seconds actually vary in duration, being stretched and compressed to match the Earth’s variations. In UTC, all seconds are standard SI seconds, which is much simpler, but it means that if you want to slow down or speed up UTC, there is no alternative but to jump.

All the leap seconds so far have been “positive”, meaning that an extra second is inserted, corresponding to jumping the clock back, and so slowing it down.

Time’s up for the leap second?

The leap second system has been with us since 1972. It represents an important chapter in the entangled history of civilian timekeeping, and of the definition of the second itself. Its days, however, may well be numbered.

For a number of years, support has been growing within the International Telecommunications Union, the standards body governing leap seconds, to abolish it.

The chief reason is complexity. Simply put, hardware and software can and do get things wrong. And the potential impacts are serious, from failures in navigation leading to collisions, to erroneous financial transactions, computer crashes and the inability to specify UTC times reliably into the future, because the leap second times are not yet known!

Because UTC jumps back at a leap second, effectively the second before the leap is repeated. Managing such “time travel” is inherently complex and error prone, so much so that in many cases the recommended action is to simply shutdown the system and restart it after the leap.

A dramatic illustration of the problem can be found in the internet. All computers have software clocks that generally rely on communication with time servers over the network to synchronise to UTC. Network timekeeping is a core internet service, and at its heart are the Stratum-1 servers, which have direct access to reference hardware such as atomic clocks.

We collected data from around 180 such servers around the world during the June 2015 leap second event, and assessed them from two points of view.

First, the clocks themselves: did they jump cleanly and sharply exactly as required?

Second, at the protocol level, that is with respect to the messages the servers send to the computers that rely on them: did they inform them properly of the upcoming leap?

Overall, we found that, at most, 61% of the servers were performing correctly. Many of the servers are well known and highly utilised, potentially impacting thousands of clients, possibly resulting in security vulnerabilities.

An expanded experiment is currently underway for the 2016 event, involving almost 500 servers, including from the widely used ntppool project.

This is part of a broader network timing project at UTS led by myself together with Dr Yi Cao, which aims to refashion the global system, and in particular to make it scale in a trusted way to the Internet of Things.

Finally, we must point out that leap seconds occur simultaneously across the globe, and it can’t be midnight everywhere.

Thus, as I confirmed with Dr Michael Wouters, responsible for Australia’s reference time at the National Measurement Institute, for us it will occur at 11am AEDT on January 1, 2017. Save the last sip till then.

This article was originally published on The Conversation. Read the original article.</s>The U.S. Naval Observatory announced last July that a leap second will be added to official timekeeping on December 31, 2016. That means your day and year – and everyone’s day and year – will officially be one second longer.

Leap seconds have been added 26 times since 1972. They’re inserted at the end of the last day of either June or December. The leap second will be added to the world’s clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC) on December 31. This corresponds to 6:59:59 p.m. Eastern Standard Time, when the extra second will be inserted at the U.S. Naval Observatory’s Master Clock Facility in Washington, DC.

The extra second is added to our official timekeeping mainly to keep our increasingly electronic world in sync. The most recent such leap second was added on June 30, 2015, and the one before that was June 30, 2012.

Why do we need a leap second? Isn’t the length of our day set by the rotation of the Earth? Like the ancients who insisted that all motion in the heavens must be perfect, uniform and unvarying, many of us today assume that the Earth’s rotation – its spin on its axis – is perfectly steady. We learned, correctly, that the sun, moon, stars and planets parade across our sky because the Earth turns. So it is easy to understand why we assume that the Earth’s rotation is precise and unwavering. Yet Earth’s rotation does not stay perfectly steady.

Instead, compared to modern timekeeping methods such as atomic clocks, the Earth is a notoriously poor timepiece. Not only is Earth’s spin slowing down, but it also is subject to effects that cannot even be predicted well.

If you have ever been to the beach, you will be familiar with the main reason our planet is slowing down. That reason is ocean tides. As our planet rotates, it plows past the great watery bulges (raised mostly by the gravitational interaction of the Earth and moon), which serves to slow it down much like a brake on a rotating wheel. This effect is small, actually very small. According to calculations based on the timing of ancient astronomical events (eclipses), the Earth’s rotation has slowed down by about .0015 to .002 seconds per day per century.

That in itself is not much, and is not enough to justify adding a “leap second” every few years, as has been done since 1972. The length of a day today is almost imperceptibly longer than the length as the same day last year. In the 1800s, a day was defined as 86,400 seconds. Today it is 86,400.002 seconds, roughly.

The discrepancy comes by comparing the Earth’s daily rotation relative to astronomical objects (which shows the planet slowing down), to a extremely high precision atomic clock (which is accurate to about a billionth of a second per day).

The Earth is slowing down, very slowly. It takes about 100 years for Earth’s rotation to add just 0.002 seconds to the time it takes Earth to spin once on its axis. What happens, though, is that the daily 0.002-second difference between the original definition of a day as being 86,400 seconds builds up.

After one day is it 0.002 seconds. After two days it is 0.004 seconds. After three days it is 0.006 seconds and so on. After about a year and a half, the difference mounts to about 1 second. It is this difference that requires the addition of a leap second.

The situation is not quite that clear cut, however. The figure of 0.002 seconds per day per century is an average and it can, and does, change. For example, you might recall that the Fukushima earthquake in 2011 resulted from displacements of portions of the Earth’s crust that actually speeded up the Earth’s rotation, shortening the day by 1.6 millionth of a second! While that is not much, keep in mind that such changes are cumulative, too.

Other short term and unpredictable changes can be caused by a variety of events, ranging from slight changes in the distribution of mass in the Earth’s molten outer core, to movement of large masses of ice near the poles, and even density and angular momentum variations in the Earth’s atmosphere.

The bottom line is that the actual variation day to day is not always plus 2 milliseconds. According to a U.S. Naval Observatory document, between 1973 to 2008, it has ranged from a plus 4 milliseconds to a minus 1 millisecond. Over time, that could necessitate a negative leap second, signifying an increase in the Earth’s rotation speed, but since the concept was introduced in 1973, this has never been done.

This all may seem pretty esoteric and unimportant, but not to the telecommunications industry.

We’ll say here that everyone thinks a leap second is a good idea. The International Telecommunications Union (ITU), a United Nations body that governs some global issues related to time, has been contemplating leap seconds for some time. They considered abolishing the practice, but in November, 2015 – with delegates from more than 150 nations meeting in Geneva – the ITU announced it had decided not to dump the leap second, at least not right now. The ITU said:

So they’re still thinking about it!

Consider the ITU’s situation. Telecommunications relies on precise timing, and the addition of a leap second forces many systems to be turned off for a second every year of two. To get all such systems in a global industry cycled on and off in sync can be a major headache. Consider also that the global positioning system (GPS) does not use the leap second system, which causes further confusion. Many in the industry feel that the periodic addition of a “leap second” to keep the to measurements in step is cumbersome and wasteful.

Although dropping the idea of a leap second would be a convenience for telecommunication and other industries, in the long (very long) run, it would cause clocks to get out of synch with the sun, eventually causing 12 p.m. (noon) to occur in the middle of the night, for example. At the current rate of change in Earth’s rotation rate, it would take about 5,000 years to amass just a one-hour difference between the Earth’s actual rotation rate and the atomic clock.

How, you may ask, do we even measure such small changes in the Earth’s rotation? Historically, astronomers (such as those at Britain’s famed Royal Greenwich Observatory near London) have used a telescope to watch a star pass through their eyepiece, crossing an imaginary line called the meridian. Then they time how long it takes for the Earth to bring that around star back around to cross the meridian again. This is highly accurate for everyday purposes, but for scientific use it is limited in accuracy because of the wavelengths used and the murkiness of the atmosphere.

A much more accurate method is to use two or more radio telescopes separated by thousands of miles, in a technique called Very Long Baseline Interferometry. By carefully combining the data from each of the telescopes, astronomers effectively have a telescope thousands of miles in size, which provides much greater resolution (detecting fine detail) and measurement of position. This allows them to determine the planet’s rotation rate to an accuracy of less than a thousandth of a second. They do not observe stars, however, but very distant objects called quasars. The NASA video below will tell you more …

Bottom line: A leap second will be added to the clock on December 31, 2016. Leap seconds have been added every so often since 1972. The last one was June 30, 2015. The International Telecommunications Union (ITU), a U.N. body that governs some global issues related to time, has considered abolishing the practice of inserting a leap second into official time-keeping. But the ITU decided in 2015 to defer a proposal to dump the leap second until 2023. Stay tuned, timekeepers!</s>People hoping for a speedy end to 2016 are going to have to wait a second – literally.

Clocks worldwide will pause for a "leap second" on Saturday. At 11:59 p.m. and 59 seconds Coordinated Universal Time (6:59 p.m. EST), international timekeepers will insert an additional second, marking the time as 11:59 p.m. and 60 seconds before midnight on New Year's Day.

Here are some things to know about the time change:

1. The time will automatically update on devices that adjust time automatically, such as cell phones and other electronics. The federal government also accounts for leap seconds in its GPS navigation messages, according to the Department of Homeland Security.

There's no need to worry about clocks that get manually adjusted either -- the time change is so small, it's likely to go unnoticed.

2. The extra second is necessary to keep time in sync with the Earth's rotation. Official international time, UTC, is based on both precise atomic clocks, which provide timekeepers with the "ticking rate," and astronomical time, based on the position of the sun and stars, according to the National Institute of Standards and Technology.

"Atomic clocks are more than a million times better at keeping time than the rotation of the Earth, which fluctuates unpredictably," said Peter Whibberley, a senior research scientist at the UK's National Physical Laboratory. "Leap seconds are needed to prevent civil time drifting away from Earth time. Although the drift is small – taking around a thousand years to accumulate a one-hour difference – if not corrected, it would eventually result in clocks showing midday before sunrise."

3. Leap seconds are not uncommon. Saturday's extra second will be the 27th leap second added to the clock since timekeepers introduced the measure in 1972, according to the National Institute of Standards and Technology.

The last leap second occurred on June 30, 2015.

4. The need for leap seconds is unpredictable. The organization tasked with maintaining global time, the Paris-based International Earth Rotation and Reference Systems Service, announced this year's leap second over the summer in one of its bi-annual bulletins.

Timekeepers are only able to tell with a few months notice whether a leap second will be necessary because of the unpredictable nature of the Earth's rotation speeds.

"The only thing we do know is that the rotation rate will slow down over the long term because of gravitational forced from the moon and sun," the NIST said in a news release.

5. The time addition is not without its critics owing to the fact that leap seconds can cause major problems. A leap second added to the end of the year in 2012 caused a software glitch that interrupted internet services for a few hours, according to NIST.

Other critics believe people would be better served by sticking to the regularity of atomic time, according to NPR.

Last year, the United Nations announced it would delay its decision on whether to do away with the leap second until 2023, NPR reported. The organization said that "further studies are required on the impact and application of a future reference time-scale," before the decision can be made.</s>Not only did we get an extra day for the leap year, 2016 is also adding a leap second on December 31.

In modern timekeeping, this would make 2016 one of the longests years.

Timekeepers announced the additional second back in July. On Dec. 31, 2016, at 11:59 p.m. and 59 seconds Universal Time (1:59 p.m. in Hawaii), the next second will become 11:59:60. This extends 2016 by a second.

NASA explains that clocks do this to keep in sync with Earth’s rotation, which gradually slows down over time. In space, millisecond accuracy is crucial to understanding how satellites orbit.

The U.S. Naval Observatory adds that the tides’ interaction with the moon and other factors, including warmer, denser waters from El Nino, cause Earth to take longer to go full circle each day.

The most recent leap seconds were added on June 30, 2015 and June 30, 2012.

Leap seconds aren’t without controversy. In fact, the scientific community is not in agreement with some believing that leap seconds should be abolished.

According the timeanddate.com, a decision on this topic was deferred to 2023.

If you were curious, the longest year was 46 BC, the last year of the pre-Julian Roman calendar. Julius Caesar added two extra leap months, 80 extra days, to “recalibrate” the calendar to fit his reform to align the calendar with the seasons. This resulted in a year with 445 days.

Dr. Markus Kuhn (University of Cambridge) lists the following arguments against leap seconds:
• Leap seconds could cause disruptions where computers are tightly synchronized with UTC.
• Leap seconds are a rare anomaly, which is a concern for safety-critical real-time systems (e.g. air-traffic control concepts entirely based on satellite navigation).
• Astronomical time (UT1), which is defined by Earth’s rotation, is not significant in most people’s daily lives.

His arguments in favor of leap seconds include:
• There have been no credible reports about serious problems caused by leap seconds.
• Some computerized systems that work with leap seconds are costly to modify (e.g. antennas that track satellites).
• Computer errors caused by leap seconds can be avoided simply by using International Atomic Time (TAI) instead of Universal Coordinated Time (UTC).
• Desktop computers and network servers have no trouble coping with leap seconds.
• Humankind has defined time by the Earth’s rotation for over 5000 years – this tradition should not be given up because of unfounded worries of some air-traffic control engineers.</s>(NBC News) Leap seconds happen occasionally to keep clocks in sync with the slowing of the earth’s rotation and to make sure they are as accurate as possible. This year’s leap second was announced by the international earth rotation and reference systems service, that’s the group responsible for adding leap seconds to international time. The time keepers said, “We can expect the clocks to read 11:59:60 instead of 11:59:59.”

The leap second has the potential to cause chaos for computers that can’t deal with a 61-second minute. The most recent leap second happened just last year, on June 30th.
Authorities add a leap second on December 31 to bring UTC more inline with mean solar time.