Why June 30 Will Be 1 Second Longer
Credit: Arina P Habich | Shutterstock.com

2015 is not a leap year, but it does have a leap second, set to take place Tuesday (June 30) at 7:59:60 p.m. EDT (23:59:60 GMT).

"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 statement.

Ask any person the length of a day, and she'll say 24 hours, which equates to 86,400 seconds. But the time it takes for Earth to rotate on its axis relative to the sun, called a mean solar day (or the average length of a day) is roughly 86,400.002 seconds. This happens because Earth's rotation is slowing down, thanks to a kind of braking force caused by the gravitational tug of war among Earth, the sun and the moon, researchers at NASA said.

Because of these planetary forces, a mean solar day likely hasn't lasted 86,400 seconds since about 1820, NASA scientists said. [The Mysterious Physics of 7 Everyday Things]

Two milliseconds might not sound like much, but it adds up to almost a second over the course of a year. However, in reality, it's much more complicated. Earth's rotation may be gradually slowing down, but individual days can also vary in unpredictable ways, the researchers said.

Many factors can affect the length of a day. For instance, seasonal and daily weather changes can influence the length of a day by several milliseconds every year, as can oceanic and atmospheric tides and variations in the atmosphere, oceans, groundwater and ice storage. Even the cyclic climate phenomenon called El Niño — associated with a weakening of the tropical Pacific trade winds and a strengthening of the mid-latitude westerlies — can slow down Earth's rotation, adding a millisecond onto a day, the researchers said.

These factors can change day length by redistributing mass and momentum within the different parts of Earth, said Chopo Ma, a geophysicist at Goddard and a member of the directing board of the International Earth Rotation and Reference Systems Service. "As mass or momentum is redistributed, the rotation rate [of Earth] can change just like the spin of a skater changes when the arms are moved in or out," Ma told Live Science in an email. 

Scientists record how long Earth takes to fully rotate each day by using a method called very long baseline interferometry (VLBI). Researchers developed VLBI in the 1960s to look at quasars, incredibly bright galactic centers created by matter falling onto a supermassive black hole. But researchers soon realized that because quasars barely move, the bright objects could act as reference points.

Now, VLBI, which relies on radio dish signals around the world, helps scientists determine how Earth is moving relative to the quasars, according to NASA.

Universe Time 1 (UT1) is based on VLBI measurements of Earth's rotation. Because UT1 isn't constant, it drifts apart from Coordinated Universal Time (UTC), which is based on precise atomic clocks. Researchers like to keep UT1 and UTC within 0.9 seconds of each other, so they add a leap second when needed, usually on June 30 or Dec. 31. [Keeping Time: 5 of the Most Precise Clocks Ever Made]

On a normal day, the clock moves from 23:59:59 to 00:00:00 the next day. But on June 30, the one minute will have 61 seconds, and the clock will move from 23:59:59 to 23:59:60 before it reaches 00:00:000 on July 1. But most systems just turn off for that one second, NASA said.

Computer challenges?

In the past, leap seconds have caused problems for computer systems, often because scientists don't know about the extra second until the last minute (figuratively speaking). For instance, the extra second can create glitches galore for stock traders, computer programmers and airline companies unless their systems are prepared for the change.   

The leap second added in 2012 caused problems for Reddit, LinkedIn, Gizmodo and FourSquare, Live Science reported in January.

"In the short term, leap seconds are not as predictable as everyone would like," Ma said. "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."

Scientists added about one leap second every year from 1972 to 1999, but leap seconds have become less frequent since then; this June's extra second is only the fourth since 2000.

It's unclear why fewer leap seconds are needed nowadays, but sudden geological events, including earthquakes and volcanic eruptions, can affect Earth's rotation, at least in the short term, NASA scientists said.

However, the leap second may be short-lived.

"The leap second is an irregular and currently unpredictable occurrence. The addition of a second at a particular time simultaneously around the world disrupts the even flow of time epochs," Ma said, adding that, in 2012, the leap second led to computer issues. "Those advocating the ending of leap seconds wish to avoid the possible problems and somewhat ad hoc solutions now in place."

There are several proposals to shelve the practice, but the decision won't be made until late 2015 at the earliest, according to NASA. The International Telecommunication Union, an agency of the United Nations that addresses issues in information and communication technologies, will make that call.

Follow Laura Geggel on Twitter @LauraGeggel. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.