If you feel that every second in your life needs to count for something, you may want to bookmark this evening as you’re about to get a whole 1 000 milliseconds more than your bargained for. The reason for this is that an extra second, or leap second, is being added to the clocks to counter the earth’s slowing rotation.
“Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that,” explained Daniel MacMillan of NASA’s Goddard Space Flight Center.
By using the Coordinated Universal Time, or UTC, a day consists of 86 400 seconds. This is calculated by atomic time, “based on extremely predictable electromagnetic transitions in atoms of [the element] cesium.” The atomic clock is accurate to one second in 1 400 000 years.
So where does the extra second come in? It takes earth 86 400.002 seconds to rotate a full cycle, and by adding those up through an entire year, it equals to almost a second – but it’s not that easy.
“The time standard called Universal Time 1, or UT1, is based on VLBI measurements of Earth’s rotation. UT1 isn’t as uniform as the cesium clock, so UT1 and UTC tend to drift apart,” MacMillan said. “Leap seconds are added, when needed, to keep the two time standards within 0.9 seconds of each other. The decision to add leap seconds is made by a unit within the International Earth Rotation and Reference Systems Service.”
This will only be the fourth leap second added since the turn of the century, and the leap second on June 30, UTC will move from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1.
Even though leap seconds have becomes less frequent, scientists don’t exactly know why less of them have been needed in past.
“In the short term, leap seconds are not as predictable as everyone would like. 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,” said Chopo Ma, a geophysicist at Goddard.
Do you have to do anything with your watch come 12pm tonight? Probably not, but if one second is a huge deal to you, just push it back one second at midnight.
[Image – Charlie Fripp/Astronomical clock in Prague, Czech Republic]