Earth's slow spin throws atomic clocks out of sync

By Daniel Dasey
July 24, 2005
The Sun-Herald

Blink and you'll miss it.

The international authority governing time has announced 2005 will be extended by an extra second to correct a clash that has developed between the world's atomic clocks and the earth's rotation.

While the super accurate clocks, used as the official measure of time since the 1970s, have been steadily counting out the seconds and minutes, the earth's rotation is less consistent and has slowed slightly.

The International Earth Rotation and Reference Systems Service said a leap second, the first to be added to the calendar since 1998, would be observed on December 31 at midnight.

Official timepieces will read 23.59.60 before flashing to 0.00.00 for the end of 2005.

Local time authorities last week said Australia would make the adjustment, but that there was debate about whether the leap second was necessary.

However, there was consensus the world's clocks needed to run on time.

"We need a stable and very clear long-term definition for time so we can accurately plot the order and time interval between events," said Peter Fisk, general manager of Physical Metrology at the National Measurement Institute.

"That's important in all walks of life, but particularly in science and technical activities."

The National Measurement Institute is responsible for establishing and maintaining Australia's units and standards of measurement and for co-ordinating the national measurement system.

Dr Fisk said atomic clocks were far more accurate at monitoring the passing of time than traditional systems based on astronomy.

Using the natural, regular oscillations of an atom, usually cesium, they are accurate to within about a second in a million years.

Clashes arise between the length of a year as measured by atomic clocks and the actual length of a year on earth because the earth rotates at an inconsistent speed.

Dr Fisk said factors such as changes in the distribution of the earth's mass, conditions in the earth's core and the effect of the moon's gravitation could alter the length of a year.

He said there was debate about whether leap seconds were necessary.

If the discrepancy between atomic and astronomical time were not corrected, after thousands of years they would be out of sync by hours. The sun could eventually rise at midnight.

However, in the shorter term some involved in time keeping regarded leap seconds as an administrative nuisance.
