The day will officially be a bit longer than usual on Tuesday, June 30, 2015, because an extra second, or “leap” second, will be added.
“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 in Greenbelt, Maryland.
A day lasts 86,400 seconds. However, the mean solar day, the average length of a day, based on how long it takes Earth to rotate, is about 86,400.002 seconds long. According to NASA’s Goddard Space Flight Center, that’s because Earth’s rotation is gradually slowing down a bit, due to a kind of braking force caused by the gravitational tug of war between Earth, the moon and the sun.
Scientists estimate that the mean solar day hasn’t been 86,400 seconds long since the year 1820 or so.
This difference of 2 milliseconds, or two thousandths of a second, less than the blink of an eye, hardly seems noticeable at first. But if this small discrepancy were repeated every day for an entire year, it would add up to almost a second. In reality, that’s not quite what happens. Although Earth’s rotation is slowing down on average, the length of each individual day varies in an unpredictable way.