At the stroke of midnight New Year’s Eve, something strange happened. The next tick of the clock was not the start of 2017, but in fact an extra second added to 2016, 11:59:60pm UTC (6:59:60pm EST), Coordinated Universal Time. This is known as a leap second, but why do we have it?
The leap second was introduced in 1972 to account for differences between Earth’s rotation speed and atomic clocks. Earth does not rotate in exactly one solar day (86,400 seconds), but instead can shift by a millisecond (a thousandth of a second) or so. Atomic clocks, on the other hand, have no such discrepancy and count each day as 86,400.
The result is that, over time, the two can go out of sync. So to account for the difference between International Atomic Time (TAI) and Universal Time (UT1), a leap second is occasionally added in either June or December.
Adding a leap second is decided by the International Earth Rotation and Reference System Service (IERS) in Paris, France, and generally happens when the difference between UTC and solar time is more than 0.9 seconds. The leap second on December 31, 2016, was the 28th since 1972.
When leap seconds are added is dependent on a number of factors. Earth’s rotation is slowing due to the gravitational pull of the Moon, but it can also be sped up by earthquakes and the shifting of glaciers. In theory, leap seconds could also be subtracted from UTC to amend the difference with solar time, although this has never occurred.
Leap seconds should not be confused with leap years. The latter are the result of Earth’s year being not exactly 365 days long, instead being about 365 days and a quarter (365.24 days to be exact). To account for this, we add an extra day to February every four years, except in rare circumstances.
2016 actually had the somewhat unwanted honor of being both a leap year and having a leap second added, making it particularly long. Fortunately for most of us, we didn’t notice the leap second at all, as clocks on our smartphones and computers automatically added the second. And some places, like Google, use a “leap smear”, where the leap second is spread out over several hours.
Who knew time could be so complicated.
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