What is a Leap Second?
“Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that,” said Daniel MacMillan of NASA’s Goddard Space Flight Center in Greenbelt, Md.
There are actually two components used to determine Coordinated Universal Time (UTC):
- International Atomic Time (TAI): A time scale that combines the output of some 200 highly precise atomic clocks worldwide, and provides the exact speed for our clocks to tick.
- Universal Time (UT1), also known as Astronomical Time, refers to the Earth’s rotation around its own axis, which determines the length of a day.
When the difference between TAI and UT1 approaches 0.9 seconds, an order is issued to add a leap second worldwide by the International Earth Rotation and Reference System Service (IERS) in Paris, France.
Atomic Time Too Accurate
The reason we have to add a second every now and then, is that Earth’s rotation around its own axis, is gradually slowing down, although very slowly.
Atomic clocks, however, are programmed to tick away at pretty much the same speed over millions of years. Compared to the Earth’s rotation, the atomic clocks are simply too accurate.
How Often Are Leap Seconds Added?
Before the first leap second was added in 1972, UTC was 10 seconds behind Atomic Time.
From 1972 to 1999, leap seconds were added at a rate averaging close to one per year. Since then, leap seconds have become less frequent. This June’s leap second will be only the fourth to be added since 2000. (Before 1972, adjustments were made in a different way.)
Scientists don’t know exactly why fewer leap seconds have been needed lately. Sometimes, sudden geological events, such as earthquakes and volcanic eruptions, can affect Earth’s rotation in the short-term, but the big picture is more complex.
So far, a total of26 leap seconds have been added. This means that the Earth has slowed down an additional 26 seconds compared to atomic time since then.
However, this does NOT mean that the days are 25 seconds longer nowadays. The only difference is that the days a leap second was added had 86,401 seconds instead of the usual 86,400 seconds.