Interesting read on leap seconds and atomic time (plus a great Jeopardy answer "9,192,631,770 oscillations of a cesium-133 atom")
By Deborah Netburn LOS ANGELES TIMES
Today, the world will receive the gift of time: a single, extra second known as a “leap second.”
At that moment, the official atomic clocks that keep Universal Coordinated Time will mark the time as 23 hours,
59 minutes, 59 seconds, followed by the leap second at 23 hours, 59 minutes, 60 seconds. Wednesday will
continue as usual, beginning with zero hours, zero minutes, zero seconds.
Leap years come like clockwork every fourth calendar year, but leap seconds are less predictable. Today’s leap
second is the 26th time that an extra second has been tacked on to a day since atomic clocks began governing our
time in 1967.
The extra second is designed to keep astronomical time in sync with atomic time.
The international timekeeping community has two ways of measuring the passing of our days. Astronomical
time is based on how long it takes Earth to make one spin on its axis.
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 a computer or cellphone.
When the 13th General Conference on Weights and Measures came up with atomic time in 1967, it was designed
to be in sync with astronomical time. But it hasn’t always worked that way.
While atomic clocks keep nearly perfect time, Earth does not.
All kinds of events affect the speed of the Earth’s rotation, including large weather systems, atmospheric winds
and even the changing of the seasons.
John Lowe, of the National Institute of Standards and Technology, explained that 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.”
It is not clear that leap seconds are really essential. Even if the difference in astronomical time and atomic time
grew by one second every year in 1,000 years it would be off by less than 17 minutes.
