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No giant leap for mankind

Digital clock graphic showing a leap second

Notice anything strange? On a normal day, the clock would go from 23:59:59 to 00:00:00. But when we have a leap second, there is an extra second added, which would make the display read “60”, before going on to 00:00:00. It has been calculated that a leap second will not be needed in June 2013.


THE WORLD BODY that regulates our planet’s timekeeping system, has announced that there will not be a leap second added at the end of June this year.

Leap seconds are added from time to time (no pun intended) in order to keep two different time systems in sync – the time measured by the spin of the Earth, and that kept by atomic clocks.

The time measured by the Earth’s spin is the one that has been used for thousands of years. One rotation of the Earth – a day – is broken down into 24 hours, with each hour having 60 minutes and each minute having 60 seconds. That means there are 86,400 seconds in one day.

Or, to put it another way, the length of a second is 1/86,400th of one day. So what we call one second, is entirely dependent on being that fraction of the length of one rotation of the Earth.

The problem is, the Earth makes for a pretty rotten clock. Its rate of rotation is not constant – it speeds up and slows down over the course of a year, and from year to year is gradually slowing down. The overall rate of slowing is about 0.002 of a second per day, per century. That means if you measured the length of the day today, and then came back in 100 years, a day in the future would be 0.002 of a second shorter.

It doesn’t sound like much, and for centuries this was just fine. Hardly anyone needed to make measurements of time with a precision greater than one second. And those who did, limited themselves to perhaps tenths or hundredths of second, easily achievable with precision timepieces.

Here’s a fun little video that shows how scientists can tell the Earth is a poor clock:

But by the middle of the 20th century, the requirements for measuring time had become far more stringent. Our technology – computers and communications – plus sciences such as astronomy and physics, demanded better precision and regularity.

And so atomic clocks were invented. Atomic clocks keep extraordinarily precise and regular time. Since the late 1960s, our time system has been based on these atomic clocks, which are located in scientific institutions in many parts of the world.

The problem is, with atomic clocks keeping precise, regular time, but with the Earth gradually slowing down, the “natural” time of the day according to Earth’s rotation gets out of sync with atomic time. Earth falls behind. So every now and then, the powers-that-be decree that a one second delay needs to put into the atomic clock system to let the Earth catch up. And that’s what a leap second is.

It’s a bit like a parade of troops being ordered to march on the spot for a moment, to let the stragglers catch up.

Leap seconds can be added (or subtracted) at the end of June and December each year. But they’re not always necessary, and some years there aren’t any leap seconds at all. Since 1972, they’ve been added in June only 10 times; in December, 15 times. (On only one of those occasions, 1972, was a leap second added in both June and December).

Incidentally, most developed countries have their own institutes for maintaining standards in measurement, be it length, mass, electrical properties, time, and so on. In a nod to Dr Who, the person responsible for time at each institute is known as that country’s Time Lord. You can listen to (or read the transcript of) an interview I did a few years ago with Australia’s Time Lord, Dr Bruce Warrington, here: The Science Show

More information:

Wikipedia entry on leap seconds

Story by Jonathan Nally. Digital clock graphic courtesy Wikimedia Commons.

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