… well, longer by 1 second that is. A leap second will be added on Saturday 30 June 2012 at 23:59:60 UTC (Coordinated Universal Time).
Leap seconds are occasionally inserted to keep UTC close to the mean solar time. By tradition, our clock time is related to the position of the Sun in the sky, which is in turn determined by the rotation of the Earth. But the Earth’s rotation rate is slightly variable, depending on factors like tidal friction and other processes which cause major mass redistribution. So Coordinated Universal Time (UTC) has been defined in order to keep the apparent position of the Sun in the sky, and hence our concept of the day, in line with International Atomic Time (TAI).
UTC is defined as being an integral number of seconds different from TAI so as to keep the Earth’s rotation linked with the day. This number of seconds is altered by one each time that the Earth’s irregular rotation has produced a difference amounting to a second.
These leap seconds are added (or subtracted) at either the end of the year (31 December) or halfway through the year (30 June).
The most recent leap second was added on 31 December 2008, and the one prior to that was on 31 December 2005.
Back on New Year’s Eve you may recall, an extra second, known as a leap second, was added to the day to compensate for our slowing Earth. Leap seconds are added when necessary at the end of either June or December. The International Earth Rotation and Reference Systems Service (IERS) has since announced that there will be no positive leap second added this June.
Since the system of adding leap seconds was introduced back in 1972, a total of 24 leap seconds have been added. The three most recent being at the end of December 1998, 2005 and 2008.
The Earth’s rotation is monitored by a variety of means, including satellite laser ranging (SLR). The British facility is located in the old solar dome at Herstmonceux and is operated by NERC (Natural Environment Research Council).
In a nutshell, a laser beam is fired at a satellite to which a retro-reflector has been attached. By measuring the time it takes for the beam to return, it is possible to measure where the base station is relative to the satellite and therefore detect fluctuations in the Earth’s rotation as well as tidal movements of the land (due to the pull of the Moon). Interestingly, the solar building was not only the first of the new domes at Herstmonceux to become operational following the move from Greenwich; it is the only one where active research is still carried out.