Why we need a leap second added to our clocks

  • February 5, 2017

2017-02-05, ABC.NET/Dr Karl Kruszelnicki

Dr Karl Kruszelnicki 

I talked about what would happen if the Earth stopped spinning. Luckily for us, that’s not going to happen. What we do know is that the rotation of the Earth is gradually slowing down. So as a result, and because our atomic clocks are so accurate, the timekeepers have to add in an extra second every year and a half.

In the past, this extra second has made parts of the internet crash, and will almost certainly do that again in the future.

Way back in the time of the dinosaurs, the Earth spun a bit faster on its axis—so a day was about 23 hours long.
Some 4,000 years ago, the Egyptians counted time by simply dividing the daylight into 12 hours. Of course, Cairo was about 30 degrees from the Equator, so the amount of sunshine varied from the shortest day to the longest day by almost four hours. But back then, it was close enough.

By the 14th century, the public pendulum clocks in European city squares were accurate enough to display the hours, but not yet accurate enough to show the minutes.

By 1927 we had quartz clocks, which in our time are accurate to about half-a-second per day. In 1948 we had the first atomic clock, and by 1960, the even more accurate hydrogen maser clock.

Today, a hydrogen maser clock is accurate to about one ten-thousandth of a millionth of a millionth of a second each year. This kind of accuracy is more precise than the spin of the Earth itself.

The spin of the Earth speeds up or slows down by a tiny amount each day. This day-to-day variation is typically caused by weather systems on either side of the Andes mountain range.

The Andes are like a huge sail extending all the way from the Equator halfway to the South Pole. So a high or low pressure system on one side of the Andes can, in the short term, speed up or slow down the spin of the Earth.

Earthquakes also can cause short-term changes—for example, the Fukushima earthquake (and tsunami) shortened the length of the day by 1.6 millionths of a second.

But in the long-term, the spin of the Earth is slowing down. That’s mostly because of the tides running into the land masses, and their friction on the ocean floor. But there are other factors, such as the movement of the Earth’s crust relative to its core.

Way back in the time of the dinosaurs, the Earth spun a bit faster on its axis, so a day was about 23 hours long.

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Taking all these factors into account, after a century has passed, on average, the day is about 1.4-1.7 thousandths of a second longer. Multiply this tiny amount by the number of days in a year-and-a-half, and you’re close to a second. That’s why, counting the 44 years from 1972 to 2016, the timekeepers have added in 27 leap seconds.

As always, there’s swings and roundabouts (or advantages and disadvantages).

The astronomers are happy with adding leap seconds and adjusting the atomic clocks, because at Greenwich in the UK, when the atomic clock reads 12 midday, the Sun will be at its highest point.

But other people are unhappy: people such as the bankers, the military and everybody else who depends on having only 86,400 seconds in each day.

So on December 31, 2016, just before midnight, the atomic clock had an extra second added. At 11:59pm, the display showed 58 seconds, then 59 seconds, and then—instead of ticking over to 0 seconds on the first of January, the next day—it went to 60 seconds. That extra second was the leap second.

Nothing much happened in 2016, because people were ready for it. But not so long ago, in 2012, inadequate preparation for the leap second caused crashes on the websites Reddit, Foursquare and LinkedIn—and also in Amadeus, the international airline reservation system.

So in Australia, far from where the leap second was added into the master atomic clock, passengers flying Qantas couldn’t get onto their planes.

In a typical second, 4.3 people are born, 1.8 people die, and 2.5 million emails are sent. So the next time a leap second gets added, try to use that extra second wisely—and don’t blink, or you might miss it.