The Economist explainsWhy time may be running out for the leap second
AT MIDNIGHT on June 30th 2015, the super-accurate clocks provided by America's National Institutes of Standards and Technology (whose time-keeping can be seen by anyone at www.time.gov) did something rather odd. For precisely one second, the time they displayed was 23:59:60. That 61-second minute was not a glitch. Instead it was a leap second, an extra second inserted deliberately by the time lords of the International Earth Rotation and Reference Systems Service, designed to fractionally extend the day. It was the 26th such intervention since the world's standards bodies agreed to the idea in 1972. But now the leap second may be running out of time. Its fate was most recently discussed in November 2015 at a conference in Geneva, run by the International Telecommunications Union. Many of its member countries want the leap second abolished. But why? And why was it introduced in the first place?
Leap seconds exist to stop two different ways of time-telling from drifting too far out of sync. The first is simple, ancient and intuitive, and based on the rotation of the Earth, under which each complete rotation (or, equivalently, each of the sun's apparent trips through the sky) counts as one day. The second is new, alien and much more accurate. It uses atomic clocks to count up seconds, 86,400 of which add up to a day. The Earth's rotational speed varies over time as its hot, molten innards churn (see chart). Besides those short-term, random fluctuations, it is also slowing down, thanks to the spin-sapping orbit of the moon. That means that, over time, the two ways of telling the time lose sync with each other.
Whenever they get too far out of whack, a leap second is added to reunite them. But many people (especially the tidy-minded types who run national standards organisations) dislike the leap seconds’ hackish nature. The variability of the Earth's rotation means that leap seconds must be added as and when they are required, rather than adhering to a tidy schedule. And critics argue that as the world becomes more computerised, the risks of adding leap seconds grow. Computers and leap seconds do not always mix: in 2012, the booking systems of several airlines struggled to cope with the change, as did those that run FourSquare, a social networking site.
Traditionalists retort that such worries are overblown, and point out that, despite worries about everything from internet servers to bank infrastructure, the addition of the June 2015 leap second passed without a hitch. Besides, abolishing the leap second would mean that the two forms of time-keeping begin to diverge irretrievably, although the effects (such as the sun and the atomic clocks disagreeing about whether it is noon) would not be obvious for decades at least. At the Geneva conference, the delegates were split: Australia, America and China were all in the abolitionist camp; Britain, Russia and many of its ex-Communist satellites favour the status quo. In the end they decided to delay a decision until 2023. The leap second survived, at least for the time being.
Update: This blog post has been amended to remove the news peg.