SciTech

Ready for the June 30 leap second?

An employee at the Electric Time Company plant in in Medfield, Mass., cleans the face of an 84-inch Wegman clock. At the close of June 30, clocks internationally will be set back one second. The leap second will align timepieces with atomic clocks, which are pegged to oscillations of a cesium-133 atom, instead of Earth rotation.
An employee at the Electric Time Company plant in in Medfield, Mass., cleans the face of an 84-inch Wegman clock. At the close of June 30, clocks internationally will be set back one second. The leap second will align timepieces with atomic clocks, which are pegged to oscillations of a cesium-133 atom, instead of Earth rotation. 2008 AP FILE PHOTO

If you were hoping to find a little more time for yourself or your loved ones in the new year, you are in luck: 2015 is going to be exactly one second longer than 2014.

The “leap second” was decreed by astronomers at the International Earth Rotation and Reference Systems Service in Paris who measure Earth’s rotation and compare it to the time kept by atomic clocks. It’s the 26th time this has happened since atomic clocks started governing our time.

“It’s not like the leap day, which everyone knows about years in advance,” said John Lowe, who works at the National Institute of Standards and Technology in Boulder, Colo. “It’s not predictable. Nobody knows when the next one will be.”

The extra second will be tacked on to the final minute of June 30. On that day, the official atomic clocks that keep Universal Coordinated Time will mark the time as 23h 59m 59s, followed by the leap second 23h 59m 60s. July 1 will continue as usual, beginning with 0h 0m 0s.

The international timekeeping community has two ways of measuring the passing of our days. The first, known as astronomical time, is based on how long it takes Earth to make one complete spin on its axis. Scientists keep track of this by aiming a network of radio telescopes at a distant quasar.

Atomic time, on the other hand, defines a second as exactly 9,192,631,770 oscillations of a cesium-133 atom. This is what determines the time that displays on your computer or cell phone.

When the 13th General Conference on Weights and Measures came up with atomic time back in 1967, it was designed to be in sync with astronomical time. But it hasn’t always worked out that way.

“Atomic clocks keep uniform time scale, but the Earth does not,” said NASA geophysicist Richard Gross, who studies the planet’s motion at the Jet Propulsion Laboratory in La Canada Flintridge.” The speed of the Earth’s rotation varies and changes.”

Why Earth’s speed varies

Large weather systems and atmospheric winds can exert enough force on Earth’s surface to cause it to slow down or speed up by thousandths of a second over a single season, Gross said. Meanwhile, movements of molten rock in the planet’s core can affect the speed of its rotation by four-thousandths of a second over a period of decades.

“Anything that causes a large-scale change in the movement of mass around the planet can change the Earth’s rotation,” he said.

Large volcanoes and earthquakes may also cause Earth to speed up or slow down, Gross said, but not by enough to be detectable. In 2011, he calculated that the magnitude 9 quake that struck near Japan may have shaved about 1.8 millionths of a second off the calendar.

In addition, as Earth spins on its axis, it wobbles with the jerky motions of a poorly balanced car tire. That wobbling often increases in winter, when ice and snow build up on the mountains of the northern hemisphere, where the bulk of the planet’s land mass is concentrated.

“It’s like a skater with her arms out,” Lowe said. “That buildup slows the Earth down, and when it melts, it’s like the skater has pulled her arms back.”

As Earth’s rotation has sped up and slowed down, the atomic clock has been ticking away at a steady rate. And that rate has been slightly too fast.

Second glance

It takes 1 second...

for an average person’s heart to beat once.

It takes a 10th of a second...

for the blink of an eye.

It takes a 100th of a second...

for a stroke of lightning to strike.

It takes 1 billionth of a second...

for a typical home computer to execute one software instruction (2 to 4 nanoseconds)

science.howstuffworks.com

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