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What Is a Leap Second?

Every now and then, we add a leap second to our clocks to synchronize them with the Earth's slowing rotation.

In Washington DC, the previous leap second occurred on Saturday, December 31, 2016 at 6:59:60 pm.
UTC time was December 31, 2016 at 23:59:60.

Illustration image

Atomic clocks don't take into account the Earth's slowing rotation.

©iStockphoto.com/kvkirillov

Next Leap Second

Latest update: In July 2020, the International Earth Rotation and Reference Systems Service (IERS) announced that there would be no leap second in December 2020. There was no leap second added in June 2020 either.

The next possible date is June 30, 2021. However, the Earth's rotation has been speeding up lately, so it is becoming increasingly unlikely that any leap seconds will be added in 2021. In any case, we will publish the date of the next leap second here as soon as it has been officially announced.

Why Do We Need Leap Seconds?

We add leap seconds from time to time to ensure our clocks reflect the Earth's rotation speed as accurately as possible. The speed at which our planet rotates around its axis fluctuates daily, and it slows down very slightly over time.

By adding an extra second to the time count, we effectively stop our clocks for that second to give Earth the opportunity to catch up.

Adding a leap second creates a minute with 61 seconds—and a day with 86,401 seconds instead of the usual 86,400.

About One Second Every 1.5 Years

An average Earth day is about 0.002 seconds longer than 24 hours. The difference grows to one second in about 1.5 years. However, the Earth's rotation speed fluctuates constantly, so the actual frequency of leap seconds can vary.

Added on June 30 or December 31

Leap seconds are traditionally added either on June 30 or on December 31. They occur at the end of the day, just before the clock ticks over to midnight.

On a digital clock, a leap second reads 23:59:60 or 11:59:60 pm.

Illustration image

An atomic clock in Greenwich, U.K.

©itimeanddate.com/Steffen Thorsen

The Science Behind Leap Seconds

Coordinated Universal Time (UTC) is the world's time standard, and each time zone is defined by being a certain number of hours or minutes ahead or behind UTC.

At its core, UTC follows International Atomic Time (TAI). This time scale combines the output of some 200 highly precise atomic clocks maintained in laboratories around the world.

Atomic Clocks Are Too Accurate

The extraordinary accuracy of atomic time has one decisive disadvantage: it does not accurately reflect the length of a day on Earth. Due to the slowing of Earth's rotation, most days are very slightly longer than 24 hours.

Meanwhile, atomic clocks tick away at pretty much the same speed over millions of years. Compared to the Earth's rotation, atomic clocks are simply too consistent.

Earth's Lag Is Kept Below 0.9 Seconds

The average speed of Earth's rotation is measured by Universal Time (UT1). When the difference between UTC and UT1 is predicted to reach 0.9 seconds within 12 months, a leap second is added to UTC and clocks worldwide. In other words, our clocks are always kept within a second of the average length of a day.

In practice, the discrepancy is usually considerably smaller than 0.9 seconds when the leap second occurs. It is generally kept between 0.4 and 0.6 seconds.

The International Earth Rotation and Reference Systems Service (IERS) in Paris, France, is responsible for monitoring the Earth's rotation and deciding when a leap second is to be inserted.

Long-exposure shot of a tree and night sky with star tracks caused by the Earth's rotation.

Star tracks caused by the Earth's rotation, made visible in this long-exposure shot.

©iStockphoto.com/river34

How Far Are UTC, UT1, and TAI Apart?

Because whole seconds are added, the time difference between TAI and UTC amounts to an integral number of seconds, while the time difference between UTC and UT1 is always less than 0.9 seconds.

27 Seconds Added So Far

UTC was already 10 seconds behind TAI before the first leap second was added in 1972. Since then, the Earth has slowed down an additional 27 seconds, and a total of 27 leap seconds have been added (see table).

This means that UTC is currently 37 seconds behind TAI (see UTC and TAI clocks).

Negative Leap Seconds

Leap seconds can be positive (1 second added to the schedule) or negative (1 second omitted). So far, all leap seconds have been positive. However, if the Earth's rotation keeps accelerating, as it has done in 2020, the IERS may decide to announce the world's first negative leap second at some point in the future.

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).

At the moment, days are 0.002 seconds longer than the sum of 86,400 seconds measured by atomic clocks; in 100 years, each day is predicted to be 0.004 seconds longer on average.

Topics: Timekeeping, Clocks, History