Hold on a second; this is the year's longest. And it matters

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Despite appearances, today is the longest day of the year. Whereas every other day simply has 24 hours, today has a total of 24 hours and 1 second.

Due to the Earth's slowing rotation, alert insomniacs awake at 1am this morning would have heard six pips, rather than five, before the long pip at the hour. The extra second was added on at the same time across the world, so that satellite systems and high-speed data links would not find themselves out of step, depending on their time zone.

Without such coordination, navigation equipment could become catastrophically misaligned, leading airplanes to fly far off course. Happily, the adjustment also means that the millennium will occur on time.

While the need for the extra "leap second", added on at 0000 GMT, might not have been obvious with older timepieces, modern atomic caesium clocks are so accurate - to less than one second in 300,000 years - that they can detect the varying rotational speed of our planet, caused by the interaction of tidal "friction" from the gravitational pull of the sun and Moon, and the fluid composition of the Earth's core.

The International Earth Rotation Service, based at the Paris Observatory, determines from time to time that an extra second must be added to the time. Today's was the 21st leap second to be introduced since the caesium- based "Universal Coordinated Time" (UTC) was adopted in 1972.