The 61-Second Minute, But Blink and You’ll Miss It

By HARVEY MORRIS

NASA, via Associated Press Slowing down: Earth seen from NASA’s Suomi NPP satellite.

LONDON — The world’s timekeepers were preparing to adjust their clocks at midnight on Saturday to add an extra second to the day.

If there is something slightly unsettling about the concept of a 61-second minute, that is probably because it inserts an element of cosmic uncertainty into the popular notion of measuring time.

Some governments and scientists do not like the concept of “leap seconds” either — this will be the 25th since the system was introduced in 1972 — but their objections are entirely rational. They argue that these sporadic adjustments to the clock present a potential source of catastrophic failure for the world’s computer networks.

The adjustment is needed to reflect a slowing in the Earth’s rotation which gradually prolongs the solar day. The day has not been exactly 24 hours long since 1820. When dinosaurs roamed the planet, the day was only 23 hours long.

The slowing of the earth did not matter much as long as time was measured in accordance with the average rotation of the Earth relative to other celestial bodies. Modern atomic clocks, however, are based on a consistent signal emitted by electrons within an atom. They are accurate to within about one second in 200 million years.

That gave rise to two systems of timekeeping: International Atomic Time (TAI), defined by an international network of atomic clocks; and Universal Time 1 (UT1), successor of Greenwich Mean Time, an astronomical definition of time based on the position of the Sun in the sky.

Under an internationally recognized system, determined by scientists at the Paris Observatory, atomic clocks are adjusted every 18 months or so to keep the two measuring systems from diverging more than one second. That prevents the time on atomic clocks from eventually moving ahead of the perceived pattern of night and day.

One challenge is that the slowing of the Earth is irregular, influenced as it is by the gravitational pull of the sun and the moon and by unpredictable changes in the atmosphere and in the planet’s molten core.

As a consequence, the scientists charged with measuring the Earth’s rotation determine the timing of leap seconds only six months in advance.

It is a system that may not survive over time. As my colleague Kenneth Chang has written, the United States is the primary proponent for doing away with the leap second.

U.S. experts argue that “the sporadic adjustments, if botched or overlooked, could lead to major foul-ups if electronic systems that depend on the precise time — including computer and cellphone networks, air traffic control and financial trading markets — do not agree on the time.”

Reporting on an international conference of the United Nations’ telecommunications agency, Ken wrote, “Britain, along with Canada and China, would like to keep the current system, arguing that, in the 40 years that leap seconds have been gracefully inserted in our midst…there have been no problems to speak of, and the worriers have greatly exaggerated the potential for havoc.”

For the average clock-watcher, there’s not much to worry about. On social media sites, most posters were offering humorous suggestions about what to do with the extra second.