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8 May 2014updated 09 Jun 2021 11:14am

The pros and cons of leap seconds

The slowing pace of the earth’s spin means that occasionally we have to add on a second – but should this practice continue?

By Michael Brooks

The British science minister, David Willetts, wants your input on an issue you’ve probably never even thought about. The question, in essence, is this: would you care if, in 800 years’ time, the sun was at its highest point overhead at 1pm, rather than today’s 12 noon?

There’s an international scientific kerfuffle over this. It is prompted by the changing pace of the earth’s spin. The moon and sun pull on our planet, slowing its rotation and giving us an ever-lengthening day. The effect is tiny – adding less than two-thousandths of a second per day – and it is not consistent. Sometimes, the rotation even speeds up for a while. We’re not sure why but we think it is because interactions between earth’s liquid iron core and the rocky mantle that surrounds it can exert an accelerating effect. Ocean currents also seem to speed up the pace at which the

world turns. In the long term, though, we know the days are getting longer. As a result, occasionally, to keep our clocks in sync with when we expect sunrise and sunset to occur, we have to add a “leap second”.

It sounds easy but it’s not. For 14 years, countries have been debating whether the practice of adding a leap second should continue. Shoehorning an extra second into the clocks of computer programs can create software glitches that have widespread effects. In 1998, for instance, the insertion of a leap second caused a mobile-phone blackout across the southern United States because different regions were suddenly operating with time differences outside the error tolerances. Then in 2012 an airline’s booking system went belly-up for hours after a leap second insertion. The US department of defence has argued vociferously that the leap second compromises the “safety and reliability” of certain systems; scaremongers talk about missiles and air-traffic control systems going awry in some such future adjustment.

One solution to this is to let our clock readings gradually drift away from any association with the position of the sun in the sky. After all, who cares?

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Well, you – perhaps. Britain is one of very few nations that have battled to keep the leap second. Most countries are happy to let the clocks drift away from “solar time”. The reason for Britain’s reticence is largely to do with ministerial gut feeling about our sense of cultural heritage: the time of day has always been linked with the position of the sun in the sky and why should we abandon that just because some programmers can’t do their job properly? In April, the UK government launched a public consultation to find out what you think (full disclosure: I am on the consultation’s oversight committee checking that the process is fair and frank).

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There are potential issues with abandoning the leap second. Human beings have always lived by sunrise and sunset; our biology responds to rising and fading light levels. Without leap seconds, or some other adjustment of time, noon in the year 4000 will occur in total darkness. Also, the sun’s position in the sky plays a role in the timing of certain religious observances. Whether the link to the numbers on a clock face matters in these instances is as yet unknown, hence the consultation. Can we justify dropping the leap second – and maybe redefining “noon” – just because of computer programming problems?

On the other hand, some will argue that we cope with time zones and daylight saving time; why would we care about a second every few years? That’s for you to answer, if you care enough to bother.