Unlike leap years — which deal with Earth's 365.24-day orbit by adding an extra day to the calendar every four years — leap seconds exist because Earth's rotation periodically changes speeds, which makes it an inherently inaccurate way to keep track of the time.
Leap seconds were introduced 40 years ago in response to atomic clocks, which are the most accurate timepieces known. Since atomic clocks rely on atomic radiation rather than sunlight to tell time, they inevitably outpace the gradually lengthening solar day. A leap second, therefore, simply "pauses" all atomic clocks whenever they start getting too far ahead. In fact, we're adding another one at the end of this year.
But the U.S. and several other U.N. countries support scrapping the leap second, since it handicaps our most precise clocks to accommodate our relatively imprecise planet. "This would be an important decision because the problem of introducing the leap second would disappear, and we would have a more steady time than we have today," official Vincent Meens of the U.N.'s International Telecommunication Union (ITU) tells the Associated Press.
According to Judah Levine, a physicist at the U.S. National Institute of Standards and Technology, "most of the people who operate time services favor discontinuing leap seconds," including operators of cellphone networks, financial markets, airports and spaceports. "The main problem is that the leap second is usually implemented by stopping the clock for one second," he tells the AP. "However, the world doesn't stop."
The world may not stop, but it is getting slower. A new historical survey of hundreds of eclipses, some dating back to the 8th century, shows that the rate of the Earth's rotation is slowing down slightly. The reason why is complicated, but this analogy about riding a bicycle around a merry-go-round sums it up well. On average, planet's day has increased by roughly 1.8 milliseconds over each of the last several dozen centuries.
That study means that it will be 3.4 million years before a minute gets added to the average day. Those extra milliseconds may be tiny time shifts, but they are precisely why timekeepers have been adding the leap second since 1972.
What's the downside of ditching the leap second?
Not everyone wants to nix leap seconds, though, which became clear during the 2015 ITU meeting in Geneva. China warned the move could make it hard for astronomers to compare long-term data, while the U.K. said it could render Greenwich Mean Time (GMT) meaningless. Canada also had reservations, and even some U.S. officials seemed hesitant. "Leap seconds are an inconvenience to the telecommunications people, but there are many other users of time who should be considered," said Ken Seidelmann of the U.S. Naval Observatory.
Without the leap second, atomic clocks would outpace the solar day by about 90 seconds per century, or roughly one hour every 4,000 years. And after a few dozen millennia, that means atomic clocks would say it's afternoon before the sun has risen.
"We have to have some means of making a correction, but at the moment no one knows how that's going to be done" if leap seconds are eventually abolished, Peter Whibberley of the U.K.'s National Physical Laboratory tells CNN.
But, speaking to the BBC, Whibberley expressed relief the decision is now delayed. "This result achieves the U.K.'s aims of securing much broader debate and understanding of the consequences of ending the link between UTC [Coordinated Universal Time] and solar time before a final decision is taken," he says. Delegates still plan to discuss the issue at another ITU meeting, but no formal decision is expected until 2023, the journal Nature reports.
And until then, we'll just keep adding them as we need them
In fact, the next one will be added on Dec. 31, 2016, according to the AP.
On Dec. 31, at 11:59 p.m. and 59 seconds Universal Time (6:59 p.m. Eastern Time), the next second will become 11:59:60.
And that means the new year won’t start for another second. Enjoy the extra moment.
Editor's note: This story was originally posted in 2012 and has been updated with more recent information.