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7 things to know about tonight’s leap second

Like Taco Tuesdays? Well, thanks to a leap second, this one just got a little longer.

Right before the clock strikes midnight tonight Greenwich Mean Time (that’s 7:59 p.m. in New York), an extra second will be added to unify our perception of the solar day with our scientific standard for timekeeping.

While the average person may not notice the addition, here are seven interesting facts about the leap second:

1. A century ago, time was less accurate than it is today.

Since the dawn of humanity, people have kept time by the sun, which eventually spawned the concept of a day, or Mean Solar Time. So with 24 hours per day, 60 minutes per hour, 60 seconds per minute, that’s 86,400 second per day, or so they thought. In the 1920s, engineers developed pendulum and quartz clocks to verify time, upon which they learned that a second didn’t last a second. Scientists learned that gravity from the moon and sun tugs on the Earth, slowing our daily rotation. The result: each solar day is stretched by two milliseconds.

Scientists estimate that a solar day hadn’t lasted exactly 86,400 seconds since 1820 or so.

2. In 1967, the second became constant.

Later on in the mid-century, scientists developed atomic clocks, which used vibrating Cesium atoms to provide a steady time measurement. This invention became the basis for the current standard tracking our days, Coordinated Universal Time (UTC) With atomic clocks, a person can keep accurate time whether they’re in Michigan, on Mount Everest or on Mars.

3. I’ll never let you go, solar time.

In the end, the atomic second is slightly shorter than the solar second, but people still adhere to the 24-hour, 86,4000-second solar day.

Enter the leap second.

4. Since 1972, 26 leap seconds have been added.

The International Earth Rotation and Reference System Service (IERS) is tasked with keeping the UTC in sync with the Mean Solar Time. As the difference approaches 0.9 seconds, the service will report the date for the next leap second. This year’s announcement came in January.

5. Extreme weather and earthquakes can change Earth’s time.

Because time depends on the Earth’s rotational speed, seconds can be inserted or removed depending on factors speeding or slowing the planet’s spinning rate.

While the moon and sun are the main factors slowing time, Mother Nature could also play a part. During El Nino years, the rotation of the Earth “may slow ever so slightly” because of persistent strong winds, increasing the length of a day by as much as one millisecond.

If large enough, seismic shocks can either set the planet forward or backward by a fraction of a second. Scientists say the 8.8 magnitude earthquake that struck Chile in 2010 may have shortened the day by 1.26 millionths of a second. However so far, leap seconds have never been subtracted from the official clock, only added.

6. Leap seconds can cause HUGE problems

While it may not affect people’s daily lives, even such a tiny difference can have devastating consequences on computer and telecommunications systems. Most famously, the issue arose in 2012 when a leap-second brought down popular websites Reddit, LinkedIn, and Yelp. It also caused the delay of more than 400 flights on Amadeus airline reservation system.

7. Countries might kill solar time for good

The U.S, France and Germany are the leading countries advocating for a permanent reliance on the atomic clock. Currently, scientists are testing an atomic clock that would not lose a second for the next 15 billion years! However, other countries like Canada, China and the UK support keeping the leap-second to keep time in line with everyday life.