NASA explained that time on Earth will be a second longer on June 30 as scientists will be adding the extra second, also known as the "leap" second, to match the pace of Earth's rotation.
A typical day has 86,400 seconds. But a solar day, computed based on the time it takes for the Earth to complete a single rotation, is about 86,400.002 seconds long. That extra 2 milliseconds per day sums up to a second per year.
"Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that,” Daniel MacMillan of NASA’s Goddard Space Flight Center in Greenbelt, Maryland said in a news release.
NASA further explained that scientists started accounting for the solar day after observing that days had become longer than usual due to changes in Earth's atmosphere. For instance, El Niño can cause atmospheric changes that could make Earth's rotation to slow down by 1 millisecond.
The atmospheric changes due to different factors disrupt the gravitational pull between the Earth, the moon and the sun.
Scientists have two options on when to insert the leap second, either June 30 or December 31. This year, they decided to have it on June 30 when the clock will be moved from 23:59:59 to 23:59:60, and then to 00:00:00 on July 1. To synchronize all clocks, many systems would be turned off for one second.
Scientists at the International Earth Rotation Service in France are responsible for adjusting Earth's time when needed. The decision to apply the leap second on June 30 was announced as early as January this year by the Paris Observatory. The first leap second was added in 1972, and this year will be the 26th time it has been added to clocks in history.
So how would the leap second affect our daily lives? In 2012, the leap second caused the software of Mozilla, Reddit, Foursquare, Yelp, LinkedIn and StumbleUpon to crash, as HNGN previously reported. These companies were using Java, which sync their times with the atomic clocks, but they weren't able to program the software to recognize the extra second.
"In the short term, leap seconds are not as predictable as everyone would like,” Chopo Ma, a geophysicist at Goddard and a member of the directing board of the International Earth Rotation and Reference Systems Service, said the news release. “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.”
The United States proposed to remove the leap second as it causes precision issues in various navigation and communication systems. This proposal was rejected by Britain as the country wants to maintain the sync between the sun and the atomic clocks. The International Telecommunication Union is expected to release a decision late 2015.