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Now Wait a Second!

Sammie Chaffin on Unsplash

Count three seconds.

One Mississippi, Two Mississippi, Three Mississippi. You know without looking at the tick, tick, ticking clock how long a second is.

Scientists don’t.

Okay they do, but it turns out that measuring a second is a lot harder than it seems.

A second used to be defined as 1/864000 of a day. Every day has 24 hours. Every hour has 60 minutes. And every minute has 60 of those one Mississippi seconds.

The time units were created to define one cycle of Earth’s rotation, one day. People used sundials, water clocks, mechanical clocks, and then digital clocks to keep track of the turning world.

For a long time, those clocks were good enough. But as people needed more exact times, scientists developed atomic clocks, where each second is measured carefully and ticked off as it goes by.

Unfortunately for the clocks, Earth does not rotate in a perfectly even way. Earth slows down just a little all the time, because slowing down is what older planets do. Earth also occasionally speeds up just a little because of hurricanes and earthquakes and volcanic eruptions.

So sometimes, the times on the standard atomic clock and the actual rotation of the Earth are different. It’s a tiny difference, but it is a difference, nonetheless.

It’s the job of the International Earth Rotation and Reference Systems Service
(IERS)— a group whose name takes longer than a second to say — to decide when a second should be added or subtracted to account for the difference. When the world is slightly slower than the atomic clock, the team adds a leap second, either to June 30 or December 31 of a year. If the world is slightly faster, the team could subtract a leap second, though so far they haven’t done so.

That second re-syncs the atomic and natural clocks. Since 1972, the team has added 27 leap seconds to the world’s clocks.

Scientists and engineers at IERS used to think these leap seconds were important for the satellites that make your phones work, as well as those satellites used making your Zoom calls go through. What’s more, planes land on the schedule set on the times set by these clocks. Rockets take off. It is important, they said, to keep the satellites in sync so that we on Earth aren’t in a muddle.

But those thoughts are changing.

Adding a leap second is a lot of work, and there are a lot places where things can go wrong. Many people think that extra second should just take a flying leap. There are other ways of keeping satellites and the natural clocks in line. For example, telecommunication satellites could run a computer program that automatically corrects the satellite’s time to the natural time. So the satellite might say it is 11:59:59, but the almost instantaneous computer program runs, and we see it as 12:00:00. Some satellites already do this.

Which way is best? It will be a debate that will last more than a second. In fact, the team at IERS is already planning to meet other scientists in November of 2023 and maybe decide if they should keep or abolish that extra moment.

In the mean time, there are still a few chances for a leap second. It just depends how the world turns.

Learn More

Using Quasars to Measure the Earth


Leap Seconds 101


No leap second for December 31, 2021


Leap Seconds


What is a Leap Second?


Why Do We Need a Leap Second?


The Future of Leap Seconds


Negative Leap Second


How Do We Measure Time?


How Measuring Time Shaped History


Measurement of Time


Physics of Time


Bill Nye – Measuring Time

Scientists Redefine How We Measure Time