Amazing Science

Historical list of leap seconds since 1972

Unlike the better-known leap year, which adds a day to February in a familiar four-year cycle (with a few well-defined exceptions), the leap second is tacked on once every few years to synchronize atomic clocks — the world’s scientific timekeepers — with Earth’s rotational cycle, which, sadly, does not run quite like clockwork.

The United States is the primary proponent for doing away with the leap second, arguing 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.

Since the 1950s, the world has run on two sets of clocks. One is the ticking of atomic clocks, defined by the precise frequency that electrons jump around in atoms. The other is based on the traditional notion of a spinning Earth.

If the leap second is stamped out, the astronomical definition of time will diverge from what is dictated by the atomic clocks, about a couple of thousandths of a second a day, growing to a minute over the course of a century, and someday — thousands of years from now — noon will strike at sunrise instead of when the sun is overhead. The problem is a distinctly modern one.

Only a few centuries ago, people set their watches by the clock in the town square, and the time in each town was different from the next. That mattered little, since there was no need or ability to communicate with anyone elsewhere in the world. Railroads changed the situation, creating the need to set cross-country schedules. This in turn led to the creation of time zones, synchronizing time across large swaths. But the length of a day and a second remained tied to the rotation of Earth.

In 1967, the nations of the world changed things around, creating a new definition of a second based on atomic clocks and pegged to the length of an astronomical day in 1900. But Earth, like any spinning top, has slowed down since 1900, and the time between sunrise and sunset has grown longer. Atomic clocks now run slightly ahead of what is defined in the sky, and, starting in 1972, leap seconds have been added to keep the two sets of clocks synchronized.

A panel of experts at the International Telecommunication Union, an arm of the United Nations, began discussions eight years ago, but could not come to a consensus to keep or get rid of them. The United States and Britain have been butting heads over the issue over most of that time.

“What it decided was to give the baby to the higher levels,” said François Rancy, director of the union’s radio communication bureau. Discussions are continuing between the United States and Britain, and Mr. Rancy said he hoped the two could finally come to a consensus in the hours before that agenda item is reached. But if necessary, the proposal to eliminate leap seconds would be put to a vote of the delegates. In a poll conducted by the union last year, only 16 nations expressed an opinion. Thirteen would abolish leap seconds. Three wanted to keep them. If approved, the recommendation would still have to be ratified by a larger meeting next month.

Regardless, there will be one extra second to enjoy this summer. The change would not take effect until 2018.