You may not notice but Tuesday we will be a second longer.
The Universal Time czars at the International Earth Rotation and Reference Systems Service (IERS) will add an extra second at the very end of the day June 30. The "positive leap second" will be introduced just after the 23:59:59 mark Tuesday, minutely delaying the onset of July.
Leap seconds can be added at the end of December or June to correct minor desynchronizations between Universal Time (UT), which is defined by the Earth's rotation, and International Atomic Time (TAI), which is the weighted average of about 200 atomic clocks in laboratories around the globe.
"Earth's rotation is gradually slowing down a bit, so leap seconds are a way to account for that," said Daniel MacMillan of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Many other factors influence the time of day, including earthquakes.
Leap seconds were first introduced in 1972 to compensate for variations in the Earth's speed. Atomic clocks are more accurate and consistent than the Earth's rotation, which can pull the UT and TAI a tad out of whack when it slows. Adding an extra second to the Universal Time gets them back in synch.
From 1972 through 1999, leap seconds were added at a rate averaging close to one per year. Since then, leap seconds have become less frequent. This year's leap second -- planned for back in January -- will be only the fourth to be added since 2000.
The last leap second came at the end of June 2012, and though it surely went by unnoticed in most people's lives, it did cause the disruption of several websites whose server clocks were suddenly off the mark.
"In the short term, leap seconds are not as predictable as everyone would like," said Chopo Ma, a geophysicist at Goddard and a member of the directing board of the International Earth Rotation and Reference Systems Service. "The modeling of the Earth predicts that more and more leap seconds will be called for in the long-term, but we can't say that one will be needed every year."
Situations like the website disruptions could convince the International Telecommunication Union, a specialized agency of the United Nations, to abolish the leap second during an international meeting in November.
In the meantime, enjoy that added second.