June 30, 1992

Article at Reuters

The day the Earth stands still – for a second

By Wilson da Silva

SYDNEY – Good news if you’re hard-pressed for time – on Wednesday an extra second will be added to clocks around the world.

That’s because the Earth’s rotation is slowing, and will eventually slow so much that a day will be 27 hours long rather than the current 24. The bad news is this will not happen until around the year 500 million AD.

The extra time, known as a ‘leap second’, will be added to atomic clocks in laboratories and satellite ground stations around the world at 0000 GMT on Wednesday, according to Australia’s National Measurement Laboratory in Sydney.

It will be coordinated by the International Earth Rotation Service at the Paris Observatory. Under its direction, the world’s scientists and timekeepers will simultaneously pause their super-accurate atomic clocks for one second to bring the instruments into line with the slowing Earth.

“Atomic clocks have an absolutely regular rate over an amazing amount of time,” said Standards Liaison Officer Glenda Sandars on Tuesday.

“It would take 300,000 years before two atomic clocks running in tandem would be out by one second.”

The Earth has been slowing ever since it came into being 4.5 billion years ago. No-one knows the length of a day then, but scientists have estimated that, 850 million years ago, a day was 20 hours long.

Billions of years in the future, the Earth’s rotation will slow to a crawl and a day – or one complete rotation of the planet – will be as long as a year, or one complete orbit around the sun, said Professor Kurt Lambeck of the Australian National University in Canberra.

“But that is so far in the future it is not worth thinking about,” he added.

Super-accurate timing is needed for a host of modern functions, such as telephone transmission of large packets of information, satellite reception of television signals and the study of sub-atomic particles.

Atomic clocks are accurate enough to measure one million millionth of a second, so a difference of one full second can matter a great deal.

One second is measured by the frequency of vibration produced by the radioactive element Caesium 133 on its transition between two specific energy levels, said Steve Nason of the Australian and Overseas Telecommunications Corp.

Atomic clocks worldwide will be synchronised with the Paris centre via international telephone links, a method accurate within a billionth of a second, he added.