Horologists Add A Leap Second

Horologists around the world carried out one of the strangest operations of their profession on Saturday night - they held back time.

The last minute of June 30, 2012 was 61 seconds long with a "leap second" added to compensate for the erratic movement of the Earth.

The brief halting of the second hand compensates for the creeping divergence from solar time - the period required for Earth to complete a day.

Our planet takes just over 86,400 seconds to carry out a 360-degree revolution.

But it wobbles on its axis and is affected by the gravitational pull of the Sun and Moon and the ocean tides, all of which brake the rotation by a tiny fraction of a second.

As a result, Earth gets out of step with International Atomic Time, known by its French acronym TAI, which uses the pulsation of atoms to measure time to an accuracy of several billionths of a second.

To avoid solar time and TAI moving too far apart, the widely used indicator of Coordinated Universal Time (UTC) is adjusted every so often to give us the odd 86,401-second day.

The adjustments began in 1972. Before then, time was measured exclusively by the position of the Sun or stars in relation to the Earth, expressed in Greenwich Mean Time.

"Today, time is constructed, defined and measured with atomic clocks that are infinitely more stable than astronomical time," explained Noel Dimarcq, the director of the SYRTE time-space reference system at the Paris Observatory.

"This allows us to ensure that everyone on Earth is on the exact same time."

Every time the time discrepancy becomes too big, the International Earth Rotation & Reference Systems Service announces a "leap second" - usually several months in advance.

The extra second is added to UTC, also known as Zulu time, only ever at midnight, either on a December 31 or a June 30.

The vast majority of the world's seven billion people are likely to be indifferent to Saturday's change.

But every time a second is added, the world's computers need to be adjusted, a costly practice that also boosts the risk of error.

High-precision systems such as satellites and some data networks will have to factor in the leap second or risk provoking a calculation catastrophe.

For this reason, rocket launches are never scheduled for leap-second dates.