But the atomic era demanded more exact timekeeping, and the world began doing business by UTC in 1972.
But the two time scales aren't quite in sync. After eons in space, the Earth spins a bit slower each year due to tides and internal processes that create a gap between the two scales.
The International Earth Rotation and Reference Systems Service (IERS) monitors this difference and periodically inserts a leap second to keep the two in tandem.
Leap Second Pains
The difference between atomic and rotational time is tiny—only an hour or so every thousand years.
But that doesn't mean it's unimportant.
The leap second causes a host of timekeeping issues, because no clock can accommodate an extra second.
Instead, clocks are traditionally stopped at 23:59:59 for one second, so that the year's last second is transmitted twice—but life does not halt during that interval.
Elisa Felicitas Arias, of the International Bureau of Weights and Measures in Paris, said the leap second causes many problems.
"It might seem stupid to say that you have a difference of only one second.
"But for the stock exchange one second is important. For an airport one second is important. For global navigation satellite systems the difference of a second is unacceptable."
Navigation systems work by measuring the time it takes a signal to travel between a known satellite location and a receiver.
Such systems require extreme precision on the level of nanoseconds, or billionths of a second.
"In one billionth of a second light travels about one foot," said Dennis McCarthy of the U.S. Naval Observatory in Washington, D.C.
"So for navigational accuracy [even a] billionth of a second can be important."
Cell phone networks have blacked out in past years when their timekeeping got out of sync because of failure to observe the leap second, said Judah Levine of the National Institute of Standards and Technology in Boulder, Colorado.
The electric power grid is also vulnerable.
"Companies share power and it's very important that all generating stations are running at the same frequency," Levine said. "That's a very serious issue."
Goodbye, Leap Second?
Arias of the Paris bureau is part of a working group that's arguing to abolish the leap second.
"The leap second was created in 1972 because there was a need to have a time scale somehow linked to rotational time," she said.
"At that time celestial navigation was common and people needed to have time linked to the rotation of the Earth."
"Today we don't need to the leap second for navigation because the GPS system exists for finding directions in the sea, or anywhere on Earth."
(Related: "An Extra Day for Everyone—Lobbying for Leap Year Status" [February 24, 2004].)
But if the leap second is abolished, it could cause problems for astronomers. Complex adjustments may be needed for research that has long relied on a strong connection between clocks and the sun.
For now, the leap second will continue to cause headaches.
"It can become super-duper confusing," said National Institute of Standards and Technology's Levine.
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