Why June 30 is one second longer

Lori Grisham | USA TODAY Network

Show Caption Hide Caption June is one second longer this year Scientists will add a second to Tuesday, June 30 in order to keeps the world's clocks in sync with the earth's rotation.

Short on time? Don't worry. Tuesday you'll get an extra second.

A "leap second" will be added on June 30 at midnight Coordinated Universal Time (or 8 p.m. EDT). Leap seconds are added occasionally to help keep the atomic clock synced up with the earth's rotation.

"The earth's spin isn't uniform, it's not constant," Nick Stamatakos, the chief of earth orientation parameters at the United States Naval Observatory, told USA TODAY Network.

Because of the earth's inconsistent speed, scientists in the 1950s created an atomic clock to keep precise track of time. However, as the earth's rotation sped up and slowed down, the atomic clock continued steadily ahead and the two time indicators grew farther apart.

To fix that inconsistency, scientists then created the UTC (Coordinated Universal Time) to help bridge the gap between the earth time and the atomic clock. However, the atomic clock continues to race ahead, so at least once every 10 years scientists add an extra second to the UTC to keep them closer together. It's particularly important for things like GPS navigation, Stamatakos said.

"The GPS will tell a user where his location is relative to the GPS spacecraft," he said. In order to do that, the GPS spacecraft has to know time very accurately and that time has to incorporate the earth speeding up and slowing.

"If you don't know the time accurately you can mistake where you are and your speed," according to Stamatakos.

"Having these leap seconds every so often is like having a mini Y2K every couple years," Stamatakos said. In advance, many people working with computer systems have tested them to see how they will deal with the extra second, he said.

The stock market is sometimes impacted because computers are so fast now that trading can happen in a second or less.

"Some U.S. exchanges will pause around the leap second as a precaution or will halt after-hours trading beforehand," according to MarketPlace's Mitchell Hartman.

"In every-day life, this extra second has practically no importance. However, in every field where exact time is needed (e.g. astronomy, navigation, spaceflight, but also computer networks for stock markets or energy supply, and much more) this second is of great importance," Wolfgang Dick, a spokesman for the International Earth Rotation and Reference Systems Service, the organization that maintains global time, told USA TODAY Network in an email.

What will you do with your extra second?

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