You probably won’t notice, but you have a little bit more time on your hands today. Just before the clock strikes midnight, an extra second will slip in.
The unusual move is necessary to realign what we think of as a day — the planet’s full turn on its axis — with real time.
“The alternative is to just allow the time, which is measured by atomic clocks, and the time measured by the rotation of the Earth to get more and more out of sync,” said Chopo Ma, a scientist at NASA’s Goddard Space Flight Center.
The issue, Ma said, is that despite appearances, Earth’s rotation rate is not uniform. Anything that redistributes mass on the globe — including winds and how ice is stored — can speed the spin up or down, although the overall trend is to slow.
“The solar day is going to get longer, and that’s one of the reasons why leap seconds in the more distant future would become more frequent,” said Ma.
By convention, whenever the solar day and atomic time diverge by more than 0.9 second, a leap second is added, either at the end of June or December. Since 1972, when the chronological fix was first put into place, timekeepers have added 26 of them, including today’s.
The change can be difficult for computers to handle. In 2012, the last time a leap second was inserted, glitches disrupted websites, including Reddit. For this reason, the United States would like to do away with the system. The ultimate decision, though, is up to an international organization.
So, for now, relish your extra second.