News Item

That leap second

A leap second will be added to the last minute of 2016 . .

Leap seconds are added periodically to compensate for irregularities in the Earth’s rotation in relation to Coordinated Universal Time (UTC), the world reference time, to ensure that it remains within 0.9s of Mean Solar Time (the mean time of the Sun crossing the prime meridian - UT1). GMT is basically UT1, which thus should always be within a second of UTC. A leap second was added most recently on 30 June 2015.

Hence, the custodians of world time - the International Earth Rotation and Reference Systems Service (IERS) - has announced the introduction of the time step, to occur at the end of December. So UTC will be retarded by 1.0s, resulting in the sequence of UTC becoming:
- 2016 Dec 31 23h 59m 59s
- 2016 Dec 31 23h 59m 60s
- 2017 Jan 01 0h 0m 0s

The difference between UTC and International Atomic Time (TAI) - TAI-UTC - is currently +36s (ie atomic time is ahead due to the slowing down of Earth rotation). From 1 January 2017 it will become +37s. The times were aligned (ie the time-difference was zero) on 1 January 1958.

UTC and all time-scales based on it will be affected by this adjustment, but GNSS timing will not be physically changed. For GPS, for instance, the leap second correction is contained within the navigation message: before the leap second GPS-UTC = +17s (ie GPS has to subtract 17s from its own time to display UTC). From 1 January 2017, this correction becomes 18s. GPS time was aligned to UTC on 6 January 1980. All other GNSS use similar offsets, depicted in navigation messages.

Not everyone applies the leap second as a discrete step - for instance:
- Google uses a 'leap-smear' by minutely changing the frequency source for its Network Time Protocol (NTP) over a 24-hour period, finishing up with a full second inserted; so it has no problem with retaining leap-seconds.
- and Big Ben is also 'smeared' over 24 hours - the Great Clock is always maintained to within ±1 second of GMT. Weights, including old pennies, sit on a shelf on the pendulum rod; adding or removing weight on this shelf changes the length of the pendulum's centre-of-gravity and hence regulates the clock. A penny added moves the clock forward two-fifths of a second in 24 hours and vice-versa. Halfpennies and farthings are used for very fine adjustment.

A proposal to drop leap seconds was put to the ITU World Radiocommunication Conference (WRC) in November 2015, but the conference decided that further studies were required on its impact. So leap seconds will remain for the time being - a report will be considered by WRC-23. Until then, UTC will continue as normal.

The UK's and Institute's position is that the leap second should remain, retaining the link between UTC and Mean Solar Time forever.

  • 28 December 2016
  • RIN

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