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Leap Second was Added to Final Minute of 2016
Jan 03, 2017

2016 was last a second longer as a ‘leap second’ added to the world’s clocks on New Year’s Eve by timekeepers. The extra second was inserted at the U.S. Naval Observatory’s Master Clock Facility in Washington, D.C., at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC) which corresponds to 5:29:59 a.m. Indian Standard Time on January 1.

Historically, time was based on the mean rotation of the Earth relative to celestial bodies and the second was defined in this reference frame. However, the invention of atomic clocks defined a much more precise ‘atomic’ time scale 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.
  • In order to create UTC, a secondary time scale, 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.
  • 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.
  • After the insertion of the leap second in December, the cumulative difference between UTC and TAI was 37 seconds.
  • 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.
  • 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. 

What is Leap Second?

Two components are used to determine Coordinated Universal Time (UTC):

1.International  Atomic Time (TAI):
A time scale that combines the output of some 200 highly precise atomic clocks worldwide, and provides the exact speed for our clocks to tick.

2.Universal Time (UT1), also known as Astronomical Time, refers to the Earth's rotation around its own axis, which determines the length of a day. 

When the difference between UTC and UT1 approaches 0.9 seconds, a leap second is added to UTC and to clocks worldwide. 

By adding an additional second to the time count, our clocks are effectively stopped for that second to give Earth the opportunity to catch up with atomic time.

Upcoming leap seconds are announced by the International Earth Rotation and Reference System Service (IERS) in Paris.

Leap Second Syncs Indian Time with Earth’s Spin 

A ‘leap second’ was added to the Indian clock at 5:29.59 hours on January 1 to synchronise with the Earth’s rotational clock. Adding a second barely has an impact on the daily life, but it does matter in the fields of satellite navigation, astronomy and communication. 

The Earth and rotation around its own axis is not regular, as sometimes it speeds up and sometimes it slows down due to various factors, including earthquakes and moon’s gravitational forces. As a result, astronomical time (UT1) gradually falls out of sync 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.

Extreme Precision

Adding the leap second to the Indian clock was done by the NPL under the Council for Scientific and Industrial Research. The NPL, one of the oldest laboratories in the country, has five atomic clocks and nearly 300 such pieces exist across the globe. 

Atomic clocks are so precise that the margin of error in its functioning is just of a second in 100 million years. The leap second adjustment is not so relevant for normal everyday life. However, this shift is critical for applications requiring time accuracies in the nanosecond, which are critical in the fields of astronomy, satellite navigation, communication networks.

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