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.