Tuesday, June 30, will last 86,401 seconds

Jun 27, 2015 07:00 GMT  ·  By

Once every 4 years, the month of February packs a grand total of 29 days instead of just 28. We call that a leap year. Interestingly, there is also such a thing as a leap second, which is basically an extra second that scientists every once in a while add to a carefully chosen day. 

This year, it will be Tuesday, June 30, that will be one second longer than any other day of the year. Specifically, June 30 will last 86,401 seconds and not 86,400, as is customary for run-of-the-mill days.

The leap second will be added to next week's Tuesday at 11:59:59 UTC (Coordinated Universal Time) on the dot. What this meas is that, rather than switch to a brand new day, the atomic clocks that scientists rely on to keep track of time will also show 11:59:60 UTC.

What use are leap seconds anyway?

It was back in 1972 that time keepers got into the habit of adding leap seconds to certain days. They've since added a total of 26 leap seconds, the last in 2012.

The reason some days need be made to last 86,401 seconds instead of just 86,400 is because otherwise atomic clocks might become out of sync with Earth's rotation.

Since about 1820, the Earth's rotation around its axis has been slowing down as a result of gravitational interactions with the Moon and the Sun, scientists explain.

Hence, the average length of a day is no longer 86,400 seconds but 86,400.002. The leap seconds that are regularly added to years are meant to make up for this discrepancy.

“Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that,” explains Daniel MacMillan of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

But for this extra second, the Coordinated Universal Time measured by atomic clocks could over the years become so out of sync with Earth's rotation that it would show noon instead of midday.

Keeping tabs of Earth's movements

Scientists have long been aware of the fact that our planet does not sit still but instead circles the Sun while also spinning around its axis. In recent years, it was discovered that Earth also wobbles just a bit.

As detailed in the video below, specialists rely on a technique called Very Long Baseline Interferometry to accurately document just how long it takes our planet to complete a full rotation around its axis.

The technique allows them to precisely pin down Earth's orientation in space at any given moment and use this information to establish the exact duration of solar day, i.e. the length of a day as determined by how long it takes our planet to complete a full rotation.