For some, the end of 2016 can’t come soon enough. But they’ll have to hold on for an extra second longer on New Year’s Eve thanks to the ocean, of all things. There are many forces that work on our big blue marble—the ocean tides being one of them. And this ebb and flow actually slows the Earth’s rotation by about 2 milliseconds per day.
"The earth's rotation changes on many different timescales because of all sorts of different factors," NASA physicist Richard Gross tells Joseph Stromberg for Vox. "Basically, anything that substantially changes the distribution of mass throughout the earth will do it."
For millennia, humans measured time based on astronomical events, such as the passage of the sun through the sky. But 50 years ago, the world’s official timekeepers switched to a more standard measurement: the amount of time it takes for a single atom of cesium to vibrate, Blaine Friedlander reports for The Washington Post. One second is about 9,192,631,770 vibrations.
While that change made official clocks more accurate, the Earth doesn’t rotate according to the human definition of time. That means in order to keep our universal clocks accurate, timekeepers add a second to the clock every 500 days or so—and this time, it’s lining right up with New Year’s Eve, Brad Plumer and Joseph Stromberg report for Vox.
As Peter Whibberley, a researcher with the U.K.'s National Physical Laboratory, explains:
Atomic clocks are more than a million times better at keeping time than the rotation of the Earth, which fluctuates unpredictably. Leap seconds are needed to prevent civil time drifting away from Earth time. Although the drift is small —taking around a thousand years to accumulate a one-hour difference—if not corrected, it would eventually result in clocks showing midday before sunrise.
Leap seconds might seem small, but they can cause some big problems. In recent years, they’ve caused issues with time-based systems like airline reservations and GPS, Colin Dwyer reports for NPR. Even financial systems and stock markets, which have become increasingly reliant on smaller and smaller fragments of time, can be affected by adding a single second.
“By adding a leap second, we’re essentially stopping our atomic clocks to let the Earth catch up,” U.S. Naval Observatory astronomer Geoff Dwyer tells Friedlander.
Even so, unless you’re paying extremely close attention, you likely won’t notice the added second. Just think of it as 2016’s last gift to the world as it makes way for next year.