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The leap second, deep space and how we keep time

Eliza Mills Jan 23, 2015

The leap second, deep space and how we keep time

Eliza Mills Jan 23, 2015

You may have heard on the news or the internet about a very, very small leap that’s coming. It’s the leap second. Yeah, not year. Second.

On June 30th, all clocks around the world will add one second to their time. Why? The short version is that it brings clocks into sync with the movement of the earth.

For centuries we’ve measured time based on how the earth revolves around the sun. But the way our planet moves is weird. Earth wobbles, and sometimes solar time gets strays from the much more precise, human-made measurement: atomic time.

The solution? Every few years add or subtract a second on atomic clocks to bring everything back into alignment. The Paris Observatory, home to the International Earth Rotation and References Systems Service, determines when a leap second will be added — when atomic time is .6 seconds or more off from solar time on earth. Since 1972, there have been 25 leap seconds.

To most people, a leap second is a pretty small thing. But for machines and computers, it can cause big problems… when all of a sudden there’s an extra second in a minute, a computing clock’s entire world can be thrown out of whack. In 1998, the leap second caused cell phone outages. In 2012, it lead to crashes on websites that ran on Linux and Java — Mozilla, Reddit, Foursquare, Yelp, LinkedIn, and StumbleUpon all reported leap second-related issues. 

Some critics worry about problems with navigation, or energy. But it turns out that if you prepare for a leap second, it’s fairly manageable. Google planned for the 2012 second with a workaround that it called a “leapsmear,” introducing the leap by the millisecond so that it wasn’t even noticable. Apple seems to have fared similarly well.

And for NASA, a company for which time, down to the second, and even the nanosecond, is crucial, the leap second is no big deal. At NASA’s Jet Propulsion Laboratories in California, researchers spend a lot of time thinking about, well, time. Robert Tjoelker, co-investigator on NASA’s Deep Space Atomic Clock Mission, says that their master clock system is “carefully designed to accommodate the leap second.”

Their enormous master clock in California, one of three around the world, even has a box to mark the leap second. Once the leap second is initiated, the clock will add time, nanosecond by nanosecond, so there’s no change in the timing system’s continuity.

“Atomic clocks on the GPS satellites have to be very reliable, and have to be very accurate, because you’re building a navigation system that is used everywhere on the earth,” Tjoelker says, “similarly, in space, we could not track our spacecraft if we didn’t accommodate the leap second.”

In unexplored areas of deep space, like the places the deep space atomic clock will eventually go, these second, and teeny, tiny fractions of seconds, make a big difference. 

“There are no signposts when you travel in deep space, and so the only mechanism, really, for navigation, is sending signals back and forth between the spacecraft.”

NASA wants to launch atomic clocks into deep space to improve their navigation systems and communications between earth and space. The deep space atomic clock that they’re building at JPL is advanced enough to speed up communications times by double, because instead of sending signals back and forth, it will send them directly from the spacecraft to earth.

And even that clock, one of the most sensitive, accurate clocks in the universe, will account for the leap second. 

There’s still some pushback about the leap second, but Tjoelker says that as long as changes to universal time are transparent and widely known, there shouldn’t be any problems. It’s not necessarily the leap second throwing computer systems off, it’s preparedness. 

Tjoelker says that for his team at JPL, whether or not the leap second is abolished doesn’t matter too much, as long as everyone is aware. “This was an arbitrary definition based upon international agreements, it is just a construct of time, just as time zones are, daylight savings time… the actual offset of time is relative and the main goal of time synchronization is that everybody’s clocks are ticking together.”

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