December 31, 2016 will be one second longer, says US-based Naval Observatory’s Master Clock Facility.
A “leap second” will be added tomorrow, December 31. The ‘extension’ will be done at 23 hours, 59 minutes, and 59 seconds UTC (Coordinated Universal Time).
WHY? Why would a leap second be inserted into our clocks? The modification is due to atomic clocks bringing in more precision in how we determine time. Originally, the rotation of the planet was used to calibrate time, and thus do we define the ‘second’ as we know it—something which the advent of atomic clocks changed: we can now base time on a more precise scale as the second does not have to depend on the rotation of the Earth anymore. So, we have to keep the difference between the two in check. Scientists worldwide have, thus, devised a way to synchronise a measure of the planet’s rotation angle (UT1) with UTC.
The responsibility thereof falls under an organisation named International Earth Rotation and Reference Systems Service (IERS) which monitors the discrepancies between the two time scales. This is done by adding or removing leap seconds from UTC so that the difference does not exceed 0.9 seconds.
UTC itself relies on a secondary timescale, the International Atomic Time (TAI). This is first generated, and it is basically the UTC without leap seconds. Ever since the setting up of the system, 26 leap seconds have been inserted at intervals of 6 months to 7 years; the last one was done last year, on June 30. With the one scheduled for today, the UTC and the TAI will only differ cumulatively by 37 seconds.
Now, why is there a difference between atomic time and Earth’s rotation? This is due to the fact that the planet is decelerating in its rotation as opposed to atomic time; it runs slow at around 1.5 to 2 milliseconds daily, such that the difference between rotation time of the Earth and atomic time will become 1 second after 500 to 750 days—therefore, a leap second is added to reduce the ‘distance’ between the two time-scales.