Tuesday, June 30, will be the longest day of the year by exactly 1 second.
In order to keep timing with Earth's slowing rotation, one second will be tacked on to the end of the day. The leap second is a way of balancing Coordinated Universal Time with the gravitational tug of war between Earth, the moon and the sun, NASA said.
While many celebrated the "longest day of the year" on Sunday, June 21, as summer officially kicked off, the length was referring to the number of daylight hours, not solar time.
A typical day consists of 86,400 seconds in UTC time. However, each solar day is about 86,400.002 seconds long. To make up for the difference, the leap second was introduced.
Still, figuring out when a leap second will be needed can be unpredictable. Factors such as El Niño, seasonal and weather variations, as well as deviations in oceans, groundwater and ice storage can alter the length of each solar day.
"In the short term, leap seconds are not as predictable as everyone would like," said Chopo Ma, a geophysicist at Goddard and a member of the directing board of the International Earth Rotation and Reference Systems Service. "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 leap second was first introduced in 1972 and added about once a year until 1999. Tuesday's leap second will be just the fourth since 2000.
NASA said the lack of needed seconds since the turn of the century cannot be fully explained, but sudden geological events, such as earthquakes and volcanic eruptions, can affect Earth's rotation in the short-term.