A sad announcement for everyone who wishes 2016 to get over quickly – the year will last a second longer, as a ‘leap second’ will be added to the world’s clock on New Year’s Eve. In layman’s term, December 31 will be a second longer due to an addition of a ‘leap second’ by the timekeepers. We all are well aware of Leap Year, but the Leap Second and that too in the not so happy year, 2016 is making people sigh worldwide. The extra second will be inserted at the US Naval Observatory’s Master Clock Facility at 23:59:59 Coordinated Universal Time, which corresponds to 5:29:59 am Indian Standard Time on January 1. The extra second or the leap second which will make the year 2016 longer than usual is to keep all our clocks in sync with the Earth’s rotation.

Not the first time, a leap second has been added in a year. Leap seconds which started in 1972, on 27 occasions out of 44, this process has taken place. An extra second is added as a one-second adjustment occasionally done to keep Coordinated Universal Time (UTC) time of day close to mean solar time or UT1. The Earth technically takes a slightly longer time than 24 hours to complete a full rotation (86,400.002 seconds, to be exact) and hence a ‘leap second’ is added over the years, making them longer than the average duration of a year.

How often is a leap second?

Since 1972, 26 additional leap seconds have been added at intervals varying from six months to seven years, with the most recent being inserted on June 30, 2015. Before the first time in 1972 when a ‘leap second’ was added to the year, UTC was 10 seconds behind International Atomic Time (TAI). The leap second added on December 31, 2016 will be the 26th time it has been added to clocks in history. It means that the rotation of the Earth has slowed 26 seconds since the early 1970s. After the insertion of the leap second in December 2016, the cumulative difference between UTC and TAI will be 37 seconds.

Why do we have leap seconds?

The one-second increments are indications of the accumulated difference in time between the two systems – Coordinated Universal Time (UTC) and International Atomic Time (TAI). Measurements show that the Earth, on average, runs slow compared to atomic time, at about 1.5 to 2 milliseconds per day.

Confusion sometimes arises over the misconception that the random insertion of leap seconds every few years indicates that the Earth should stop rotating within a few millennia. This is because some mistake leap seconds to be a measure of the rate at which the Earth is slowing. The decision as to when to add a leap second is determined by the IERS, for which the USNO serves as the Rapid Service/Prediction Center. These data are generated by the USNO using the technique of Very Long Baseline Interferometry (VLBI). VLBI measures the rotation of the Earth by observing the apparent positions of distant objects near the edge of the observable universe.

These observations show that after roughly 500 to 750 days, the difference between Earth rotation time and atomic time would be about one second. Instead of allowing this to happen a leap second is inserted to bring the two time-scales closer together.

When was the last leap second added?

The last time when an extra second ‘leap second’ was added was on July 1, 2015. The “leap second” in the year 2015 meant that the last minute of June 2015 had 61 seconds in it. Before July 1, the previous occasion when a ‘leap second’ got added was on July 1, 2012.

When was the first leap second added?

The first time when an extra second or a leap second was added to a year was on January 1, 1972. Historically, time was based on the mean rotation of the Earth about celestial bodies and the second was defined in this reference frame. However, the invention of atomic clocks set a much more precise “atomic” timescale and a second that is independent of Earth’s rotation. In 1970, international agreements established a procedure to maintain a relationship between Coordinated Universal Time (UTC) and UT1, a measure of the Earth’s rotation angle in space.

The International Earth Rotation and Reference Systems Service (IERS) is the organisation which monitors the difference in the two time scales and calls for leap seconds to be inserted in or removed from UTC when necessary to keep them within 0.9 seconds of each other.

To create UTC, a secondary timescale, International Atomic Time (TAI), is first generated; it consists of UTC without leap seconds. When the system was instituted in 1972, the difference between TAI and UTC was determined to be 10 seconds.

Many call this process of tinkering with time too confusing as the irregularity of additional leap seconds causes problems for computer networks. Adding leap seconds have significant impact as they can cause loss of synchronisation within communication networks, financial systems, aviation industry and many other applications which rely on precise timing. This is one of the most important reasons why leap seconds are announced six months in advance. Well, ignore the negative side and try enjoying another second in 2016 and wait patiently for another blink’s time to welcome the New Year 2017! (Inputs from PTI).