Leaping into the New Year with a second

In order to synchronize clocks worldwide with the Earth’s ever slowing rotation, an extra second is added to UTC. The next leap second will be added on December 31, 2016 at 23:59:60 UTC. The difference between UTC and International Atomic Time (TAI) will then increase from the current 36 seconds to 37 seconds.

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 26 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. 

Atomic Time Too Accurate

The reason we have to add a second now and then is that Earth’s rotation around its own axis is gradually slowing down, although very slowly.

Atomic clocks, however, tick away at pretty much the same speed over millions of years. Compared to the Earth’s rotation, atomic clocks are simply too consistent.  A normal day has 86,400 seconds, but in the atomic time scale 1 second is not defined as one 86,400th of the time it takes Earth to rotate around its axis but rather as the time it takes a Cesium-133 atom at the ground state to oscillate precisely 9,192,631,770 times.

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 UTC and UT1 approaches 0.9 seconds, a leap second is added to UTC and to clocks worldwide. By adding an additional second to the time count, our clocks are effectively stopped for that second to give Earth the opportunity to catch up with atomic time.

Upcoming leap seconds are announced by the International Earth Rotation and Reference System Service (IERS) in Paris, France.

“More Precise” than Earth

The advantage of the Cesium-133 atom for a second’s definition is that it is extremely precise: atomic clocks deviate only 1 second in up to 100 million years. On the other hand, the Earth’s rotation, which is expressed by the time standard UT1, is far less reliable. It slows down over time, which means that days get longer. On average, an Earth day is about 0.002 seconds longer than the daily sum of the 86,400 seconds measured by the atomic clocks. This makes for a discrepancy between TAI and UT1 of around 1 second every 1.5 years.

Leap seconds are added to our clocks (UTC) so this discrepancy does not get too large over time and the time we use is synchronized as much as possible with the Earth’s rotation. Before the difference between UTC and UT1 exceeds 0.9 seconds, one second is added to UTC. This means that the time difference between TAI and UTC amounts to an integral number of seconds because whole seconds are added, while the time difference between UTC and UT1 is always less than 0.9 seconds.

Days Grow Longer

Not only do days become longer, but the rate at which day lengths increase also grows over time – but only by about two thousandths of a second per century, according to Dr Bruce Warrington, from Australia’s National Measurement Institute (NMI). This means that at the moment days are 0.002 seconds longer than the sum of 86,400 seconds measured by atomic clocks; in 100 years, they are expected to be 0.004 seconds “too long”.

Source: Time and Date

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