How Do We Really Know What Time It Is?

​A second before midnight on June 30, an extra second will be added to the Earth's official clock by the International Earth Rotation and Reference Systems Service. These "leap seconds," added every few years, are part of a long-running effort to create a universal time of day.

"What time is it?" is a simple question, but despite what your iPhone says, the answer always seems up for debate – by scientists, at least.

Because the Earth rotates at an inconsistent rate, efforts to link time to night and day have been frustratingly elusive, affecting flight schedules, ship navigation, stock markets and even Yelp! reviews. As technology increasingly manages our daily lives, a method of global timekeeping has become more crucial than ever.

October 1840 

Railway Time Introduced

Teddington, England

For the majority of human existence, humans kept track of time based on the position of the Sun relative to the observer. This, however, makes scheduling between distant points difficult, since landmark events like sundown happen at different times across the globe.

England's railroad companies spearheaded a drive to unify local times, pegging the time to the sun's position in the sky as observed from one specific place: the Royal Observatory in London. By 1947, this was designated Greenwich Mean Time (GMT), and was the first attempt to come up with a universal marker for time across the world.

1955

Atomic Clock Created

Teddington, England

Pegging time to Earth's rotations worked well enough, for a while. But the Earth doesn't rotate at a consistent rate: It slows by 1.4 milliseconds every century, and tectonic movement introduces additional variation.

These slight rotational inconsistencies became problematic with the rise of commercial flight and other navigational innovations, as they required much more precise timekeeping than trains did.

So scientists developed a technique for measuring time based on the vibrations of the atomic element cesium instead. This "atomic clock" was completed in 1955, and it kept time with astonishing consistency: Over the course of several centuries, it would theoretically gain or lose only a single second.

1967

Sun Abandoned As Marker Of Time

Gaithersburg, MD, US

The National Institute of Standards (which later added "and Technology" to its name) eventually adopted cesium as the standard timekeeping device for the world. They called it Coordinated Universal Time (UTC), and it replaced GMT as the global standard.

This was a radical redefinition of what constituted time. With the NIST's decision, the unit of a second was defined in relation not to the movement of celestial bodies, but the vibrations of an atomic element.

1972

Leap Seconds Introduced

Maryland, US

There was just one problem with UTC: It completely disrupted the navigational techniques used at sea. Sailors calculated their position and direction using a formula that combined traditional GMT with the current position of the sun and stars. Because UTC was wholly divorced from celestial movement, it made sea navigation much more difficult.

And so a compromise was introduced: Whenever UTC drifted more than nine tenths of a second away from GMT – every couple of years or so – one second would be added to the UTC. This allowed navigators to enjoy the precision of atomic time while maintaining the planetary consistency of solar time.

JULY 1, 2012

Leap Seconds: Disaster In The Making?

Everywhere

When a leap second was added to UTC in 2012, Yelp!, LinkedIn, Gawker and other major websites suddenly crashed. Qantas Airways' booking system failed, and more than 400 flights were delayed.

Since the Earth's rotational inconsistency is irregular, leap seconds must be added at irregular intervals — manually. That involves turning off the atomic clock for one second every couple years: If a computer has to carry out an operation during that second, it can crash.

JUNE 30, 2015

Settling It Once And For All?

Geneva, Switzerland

With the media spotlight on the 2015 leap second, there's been talk of scrapping them. Computer systems are steadily growing more reliant on atomic time, prompting concerns that future "leap-second bugs" could cause security vulnerabilities, flight accidents or stock market crashes.

The growing movement against leap seconds would abolish them and return to unaltered UTC time. This wouldn't pose the same problems for nautical navigation as it did 30 years ago since most sailors now use GPS, not the night sky, to find their way at sea. It would, however, cause the official time to drift further and further from the setting sun.

But maybe that's OK, because there's no such thing as a perfectly precise, 100% reliable timekeeping device. Our approximations may have improved, but they're still just approximations.

Want more?

Further reading

The boffins at NASA explain Leap seconds.

The Leap Second: Its History and Possible Future — Metrologia.

A Brief History of Timekeeping — BBC.

Frequently Asked Questions helpfully provided by the National Institute of Standards and Technology.

History of the Atomic Clock — Science Museum.

The Time Regulation Institute by Ahmet Hamdi Tanpinar.

This post originally appeared on Timeline.