For people who can't wait for this year to end, here is the science behind the Earth's leap second.
By Darryl Veitch
To the time-poor of the world: take heart, for 2016 is a generous year. Not only were you granted a leap day on February 29, you will soon score a New Year's Eve countdown bonus, a leap second, to hold off 2017 for a final sip or regret.
Whereas leap years add a day to align the calendar with the seasons, leap seconds align our everyday clocks with the Sun's position in the sky, that is, with the Earth's rotation.
Currently our planet takes roughly 86,400.00183 seconds (on average) to turn, instead of the expected 86,400 seconds you get by multiplying 24 hours by 60 minutes by 60 seconds. This may not sound like a great difference, but it amounts to a full second every 18 months. If left unchecked, it would become noticeable over time, and ultimately become problematic.
How did we get into this awkward situation? Why not just define a second so that there are exactly the right number? This sensible idea was tried in 1874, but hit a snag: the Earth keeps changing.
In terms of today's standard SI second (defined via atomic physics), the above discrepancy is due to the fact that the day is losing about 0.0015 seconds per century, due largely to tidal friction.
Not only that, it also 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 center.
