Horologists around the world on Saturday will carry out one of the weirdest operations of their profession: they will hold back time.

The last minute of June 20, 2012 is destined to be 61 seconds long, for timekeepers are to add a “leap second” to compensate for the wobbly movements of our world.

The ever so brief halting of the second hand will compensate for a creeping divergence from solar time, meaning the period required for Earth to complete a day. The planet takes just over 86,400 seconds for 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 sliver of a second.

As a result, earth gets out of step with International Atomic Time(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 Earth, expressed in Greenwich Mean Time(GMT) or its successor UTI. This will be the 25th intervention to add a “leap second” to UTC.

Tai is kept by several hundred atomic clocks around the world., measuring fluctuations in the atom of the chemical element caesium that allows them to divide a single second into 10 billion smaller bits. Every time the discrepancy between Tai and UTI becomes too big, the International Earth Rotation and Reference systems service jumps into action and announces a “leap second”. 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.
Time-catching is as irregular as the Earth’s rotation itself. The last three adjustments were in 2008, 2005 and 1998. The year 1972 saw two additions, followed the next seven years by a second every year.

Every time a second is added, the world’s computers need to be manually 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.