Why June 30 will be one second longer

Traditionally, the summer solstice – June 21 in 2015 – is considered the longest day of the year.

But this year, June 30 will be one second longer.

That’s because a so-called “leap second” is being added. This will compensate for an infinitesimal slowing of the Earth’s rotation in the past two centuries.

“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, told ScienceDaily.com.

A new report from NASA/Goddard notes that our entire system of time is based on the idea that a single day lasts 84,000 seconds. In reality, though, the average day is approximately two one-thousandths of a second longer. Over 365 days, that’s an annual slowing of about .73 seconds.

So every now and then, one of these “leap seconds” is added to keep our clocks on pace with our planet.

Why does this happen?

It turns out there are a few factors that can, in fact, alter the rotation of the Earth. Just orbiting in a sector of space that also include the Sun and the Moon accounts for some of it. The ongoing gravitational give-and-take tends to exert a minor, miniscule amount of drag.

Persistent atmospheric variations – El Nino for example – can turn the trick as well, NASA/Goddard says. And an enormous earthquake is capable of hurling enough lateral force through the entire planet to ever-so-slightly adjust our rotational speed.

But the slow-down isn’t constant. Today’s leap second is only the fourth that’s been added since 2000. From 1972 to 1999, they were added almost annually.

“In the short term, leap seconds are not as predictable as everyone would like,” Goddard geophysicist Chopo Ma told ScienceDaily.com. “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.”

All of this is imperceptible if you happen to be living on Planet Earth. The celestial clockwork appears constant and unchanging enough for all of us to set our clocks and plans by it. But nothing is truly constant. Adjustments need to be made, and today is one of those days.

But if someone is hurrying you, and you feel like you need one more second?

Today, you get it.