I hope you’re enjoying 2016, because it’s going to be even longer than your regular leap year. The International Earth Rotation and Reference Systems Service (IERS) has announced that on December 31 a leap second will be added to the world’s clocks at 23:59:59 Coordinated Universal Time (UTC), or 18:59:59 Eastern Standard Time.
Time has been traditionally measured by looking at how fast the Earth rotates with respect to astronomical objects. But using Earth’s rotation to keep time is just not very accurate. The invention of atomic clocks led us to more precise timings, and UTC employs exactly that.
Nevertheless, we want to keep the two measurements, UTC and the mean solar time (UT1), close to each other. The changes are small, but we don’t want them to drift too far apart from each other. So once in awhile, the IERS inserts a leap second, either at the end of June or at the end of December. Leap seconds provide the necessary small adjustment to keep UTC and UT1 within 0.9 seconds of each other.
The leap second was introduced in 1972 (the longest year ever with a leap day and two leap seconds), and since then 26 leap seconds have been introduced at intervals between six months and seven years. The most recent one occurred on June 30, 2015.
To work out the difference in Earth's rotation speed, the U.S. Naval Observatory uses an astronomy technique called the Very Long Baseline Interferometry (VLBI), which looks at far away radio sources with high precision. Based on these observations, there's currently the need for a leap second every couple of years.
Leap seconds can be added or subtracted from the UTC, but so far they have only been added. This has caused some confusion, with people thinking that Earth will eventually stop rotating. The Earth’s rotation actually varies a lot, and since the 1970s, it has been accelerating, so we have had to insert leap seconds at a lower rate.
This means that 2016, for better or for worse, is here to stay a little longer.