Its 2017 already in half of the world and while those of you who are still in 2016, you will have to remain in the year 2016 for a second longer thanks to a leap second being added to the last minute of the outgoing year.
The primary reason behind adding leap seconds is to align our everyday clocks with the Sun’s position in the sky, that is, with the Earth’s rotation. If we look at the current rate of turning of our Earth, it takes roughly 86,400.00183 seconds (on average) instead of the expected 86,400 seconds you get by multiplying 24 hours by 60 minutes by 60 seconds. While the difference is not huge and wouldn’t mean a lot to average Joe, it does amount to a full second every 18 months and over a period of time the difference would become massive and problematic for a number of fields including astronomy.
This is not something that can be rectified by redefining the second to a precise number and the reason behind this is that Earth keeps changing. Tidal friction causes loss of about 0.0015 seconds per century and not only that the Earth changes quite erratically due to mass redistribution. For example, it is slowed by oceanic thermal expansion due to global warming, just as a playground spinning seat slows, via the conservation of angular momentum, when you place your body farther from the centre.
This is where leap seconds come in as they ensure that our time keeping system, Coordinated Universal Time (UTC), never gets more than 0.9 seconds away from the Earth-tracking alternative, Universal Time (UT1).
Leap seconds can’t be calculated centuries in advance owing to the ‘erratic’ motion of the Earth and scientists need to schedule leap seconds on a need basis. In UT1, seconds actually vary in duration, being stretched and compressed to match the Earth’s variations. In UTC, all seconds are standard SI seconds, which is much simpler, but it means that if you want to slow down or speed up UTC, there is no alternative but to jump.
All the leap seconds so far have been “positive”, meaning that an extra second is inserted, corresponding to jumping the clock back, and so slowing it down.