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What's a Leap Second

Previous leap second occurred in Washington DC on Tuesday, June 30, 2015 at 7:59:60 PM.
UTC time was June 30, 2015 at 23:59:60.

Every now and then a leap second is added to Coordinated Universal Time (UTC) in order to synchronize atomic clocks with the Earth's ever slowing rotation.

Illustration image
Atomic clocks are slightly too accurate.
Leap seconds are added to realign atomic clocks with the Earth's rotation.
©iStockphoto.com/kvkirillov

Atomic Time vs. Universal Time

Two components are used to determine Coordinated Universal Time (UTC):

  1. 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.
  2. 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.

The Science Behind Leap Seconds

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. So far, a total of 26 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.

36 Seconds Difference

The previous leap second was added on June 30, 2015 at 23:59:60 UTC. The difference between UTC and International Atomic Time (UTC-TAI) is now 36 sec.

Check our Time Zone News for updates about leap seconds.

Topics: Timekeeping, Clocks, History

In this Article

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Leap Seconds

  1. What's a Leap Second?
  2. How Leap Seconds Work
  3. How Long is a Day on Earth?
  4. The Future of Leap Seconds
  5. Intl Atomic time (TAI)

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