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12 January 2015updated 09 Sep 2021 2:18pm

June’s leap second will be a kind of deliberately-induced Millennium Bug

Sometimes the Earth doesn't spin as fast as it should - and that can give programmers a headache.

By Ian Steadman

When the human ability to measure things becomes iteratively better over time, it can lead to some counter-intuitive changes in how certain everyday things are defined. Think about one “second”: it doesn’t take too much to work out how long it takes for the Sun to move from noon one day to the next, and then divide that up into 24 hours, and each of those hours into 60 minutes, each of 60 seconds. That’s a second – 1/86400th of a day.

Except, of course, it isn’t that easy, because the Earth has the frustrating habit of not spinning at a constant speed. Day-to-day, the Earth’s spin is affected by a range of factors – from the angle of rotation to its distance from the Sun – which can cause a “day” to be as much as 20 seconds longer or shorter than other days in the year. The most obvious solution would be to take an average of the length of all of the days over the course of a year, and that’s how the second was defined at a meeting of the International Committee for Weights and Measures (CIPM) in 1956. The second, the meeting concluded, was equal to 1/86400th of the average tropical (as in, on the equator) day, as measured in 1900.

Problem solved? No, because the Earth’s rotation is still slowing down over the long term at an unpredictable rate – thanks to tidal forces from the Moon and Sun, and internal geological activity like earthquakes – meaning that the average day of today is almost two milliseconds longer than the average day of 1900. This was apparent decades ago, which is why the CIPM met in 1965 to redefine the second again, in terms that didn’t rely upon an unchanging Earth. Instead, a second became “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom”. 

(Put more simply: whenever an electron orbiting the nucleus of an atom drops down to a lower orbit, it releases electromagnetic energy which, like a radio wave, resonates at a specific frequency. This is true of all atoms – it’s what radioactivity is, as unstable atoms rapidly decay – but caesium is particularly good for timekeeping because of its stability and reliability, so measuring this energy release anywhere in the universe should, in theory, give the same result every time. And the CIPM decided that a second would be defined as 9,192,631,770 cycles of this energy.)

Defining a second with this kind of accuracy is absolutely necessary to almost every aspect of modern technologically-aided human existence. Both the SI units for metre and ampere are defined with reference to the second, and there are innumerable scientific, engineering, business and cultural projects which rely on precise timekeeping to work worldwide. This is what makes leap seconds so tricky.

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The issue of the Earth slowing down over time doesn’t go away just because the second isn’t defined as relative to its spin. Instead, it’s meant that since 1965 the length of an atomic day has remained static, while the average length of a solar day has increased. The solution, since 1972, has been leap seconds – on a pre-determined date, Coordinated Universal Time (UTC) logs the 59th second of the 59th minute of the 11th hour as having lasted two SI-defined seconds, not one. Just like a leap year adds an extra day to the calendar every four years because the planet takes 365 and a quarter(ish) days to orbit the Sun, a leap second brings UTC back into line with where the Earth’s midnight “should” be (but meaning UTC moves further and further out of sync with International Atomic Time (TAI), which remains unchanged). We’ve had 25 leap seconds so far, and the 26th is due to take place just before midnight on 30 June, 2015.

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Since the slowing of the Earth’s rotation happens inconsistently, it’s impossible to know more than about six months in advance whether a leap second will be necessary – so there’s no way to automatically program them into many computer systems, like leap days. The International Earth Rotation and Reference Systems Service (IERS, which is in charge of monitoring the Earth’s spin for exactly this reason) was only able to announce this most recent leap second last week, for example. That creates some significant problems for programmers, and indeed, it might be better to think of every leap second as a tiny kind of Millennium Bug.

When it comes to dates, computers are – how to put this – not clever. The Millennium Bug was a simple problem caused by machines storing dates as two digits instead of four, meaning the year 2000 rolling over to “00”, indistinguishable from 1900. In the event it had very few negative effects, but whether that’s because hundreds of billions of dollars was spent worldwide fixing it advance or because it wasn’t actually a serious threat remains a topic of debate in the computing world.

In June it’s unlikely that we’ll see leap seconds causing planes to crash, but when the last leap second was added in 2012 it did cause Australian airline Qantas’ computer booking and checking-in systems to go down for several hours. Amazon’s cloud services – which are used by a massive number of multinational companies – were hit, as were many open source programs running on servers worldwide, taking down some big sites like reddit and Gawker. Nothing disastrous, and nobody was hurt, but it still seems an issue worth considering – or maybe not, since Linux creator Linus Torvalds thinks “99.9 per cent of all users will never need or want to care” about it because it has such little real-world significance.

Getting rid of leap seconds and letting UTC drift, unmoored, from Earth-time, is a popular stance among some scientists who dislike the possibility for confusion between UTC and TAI. Yet the general consensus (particularly among politicians, who have some sway on this) is that time – as understood in the day-to-day sense – needs to keep its connection to the day-and-night cycle, with concepts like “noon” and “midnight” forming part of a shared human experience of time. That said, that hasn’t stopped some very strange time zones being decreed which ignore that same “natural” conception of time, so there’s every chance this could change in future.