Tonight, just before the clocks roll over from 23:59:59 to 00:00:00, they will tick 23:59:60. That’s your “leap second” that you can spend starting a sneeze, blinking four times, or making a hug slightly more awkward. It’s our way of fighting the gradual slow-down of Earth’s spin.

Each day, technically, is 86,400 seconds long. Once humanity established what a day actually was — based on the rising and setting of the Sun — we split it into hours, minutes, and seconds. Twenty-four hours multiplied by sixty minutes per hour multiplied by sixty seconds per minute gives you 86,400 seconds.

If that calculation sounds too simple, that’s because it is. When we had to firmly establish how long a second actually was, we decided to go atomic. Officially, a second is how long it takes an atom of the element cesium to vibrate 9,192,631,770 times. It’s complicated, but accurate. Our atomic clocks based on vibrating cesium are accurate to within one second in 1,400,000 years.

A distributed series of 70 such cesium clocks, operated the U.S. Naval Observatory in 18 temperature-controlled vaults, is what a “master clock” averages for Coordinated Universal Time (UTC), the global standard.

But time it takes the Earth to make a full rotation — our original basis for a day — is slowing down.

Using a worldwide network of stations that collect radio signals, scientists track the rotation of the Earth and calculate the time it takes our home to make a complete rotation. Right now, the average length of a day is 86,400.002 seconds long. The gravitational battle between the Earth, Moon, and Sun, as well as weather systems like El Nino and even earthquakes are to blame for this slow trend towards a stop.

See the problem? NASA estimates that the true length of a day (based on spin alone) hasn’t been exactly 86,400 seconds for almost 200 years. As the Earth slows down, every once in a while we need to add a second to our atomic time (UTC) in order to keep the two in sync. If we let them diverge, in a few decades our atomic-based days will end before our rotation-based days do.

Tonight’s extra second will only be the fourth added to our clocks since 2000, and it’s hard to know when the next second will be needed. Even though our spin tracking is very precise — three millionths of a second — something like a large volcanic eruption can quickly alter the Earth’s rotation, and therefore our clocks.

And a leap second isn’t without consequence. Since they can’t be reliably predicted, leap seconds are primed to cause havoc in our increasingly digital and precise world. Writing at Slate, David Wolman calls the leap second a “ticking time bomb,” already responsible for past outages on sites like Reddit and Gawker, and airline scheduling/tracking mistakes, among other problems.

Later this year in Geneva, Switzerland, those in charge of keeping our planet’s time will rule on the future of the leap second. Should we keep basing our days on the spin of our home, as the first humans did, or should we give in to the surgically precise vibration of atoms? Maybe that’s something to think about for a brief moment during cesium 9,192,631,770 vibrations after midnight.

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IMAGE: NASA