Why 2016 will be longer by a second? Know all about the 'leap second'

People will have to wait for a second longer to welcome the year 2017 as 2016 will be a bit longer. This is because of a ‘leap second’ that timekeepers will be adding to the world’s clocks on New Year’s Eve.

The leap second will be added at the US Naval Observatory's Master Clock Facility in Washington, DC at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC). The time corresponds to 5:29:59 am Indian Standard Time on January 1.

UTC is computed in Paris, France, at the International Bureau of Weights and Measures. 

Historically, time was based on the mean rotation of the Earth relative to celestial bodies. The second was defined in this reference frame.

However, the atomic clocks, that were invented later, defined a much more precise ‘atomic’ timescale and a second that is independent of Earth's rotation.

A procedure to maintain a relationship between UTC and UT1, a measure of the Earth’s rotation angle in space was established by international agreements in 1970.

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, is monitored by the International Earth Rotation and Reference Systems Service (IERS).

In order to create UTC, a secondary timescale, International Atomic Time (TAI), is first generated; it consists of UTC without leap seconds.

In 1972, when the system was instituted, the difference between TAI and UTC was determined to be 10 seconds.

26 additional leap seconds have been added since 1972. They have been added at intervals varying from six months to seven years. The most recent one was inserted on June 30, 2015.

The cumulative difference between UTC and TAI will be 37 seconds after the leap second will be inserted in December.

Sometimes, there has been confusion over the misconception that the occasional 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 one-second increments are, however, indications of the accumulated difference in time between the two systems.

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.

Measurements show that the Earth, on average, runs slow compared to atomic time, at about 1.5 to 2 milliseconds per day. 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.

(With inputs from PTI)