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Time is running out for the leap second

To the world’s timekeepers, the leap second is a kludge, a bane, a pain in the little hand. Now they’re proposing to ditch it

Time is running out for the leap second
Representative image


Roughly every four years, an extra day gets tacked onto the end of February, a time-keeping convention known as the leap year. The practice of adjusting the calendar with an extra day was established by Julius Caesar more than 2,000 years ago and modified in the 16th century by Pope Gregory XIII, bequeathing us the Julian and Gregorian calendars. That extra day is a way of aligning the calendar year of 365 days with how long it actually takes Earth to make a trip around the sun, which is nearly one-quarter of a day longer. The added day ensures that the seasons stay put rather than shifting around the year as the mismatch lengthens.

Humanity struggles to impose order on the small end of the time scale, too. Lately the second is running into trouble. Traditionally the unit was defined in astronomical terms, as one-86,400th of the mean solar day (the time it takes Earth to rotate once on its axis). In 1967 the world’s metrologists instead began measuring time from the ground up, with atomic clocks. The official length of the basic unit, the second, was fixed at 9,192,631,770 vibrations of an atom of cesium 133. Eighty-six thousand four hundred such seconds compose one day.

But Earth’s rotation slows ever so slightly from year to year, and the astronomical second (like the astronomical day) has gradually grown longer than the atomic one. To compensate, starting in 1972, metrologists began occasionally inserting an extra second — a leap second — to the end of an atomic day. In effect, whenever atomic time is a full second ahead, it stops for a second to allow Earth to catch up. Ten leap seconds were added to the atomic time scale in 1972, and 27 more have been added since. Adding that extra second is no small task. Moreover, Earth’s rotation is slightly erratic, so the leap second is both irregular and unpredictable. Fifty years ago, those qualities made inserting the leap second difficult. Today the endeavor is a technical nightmare, because precise timing has become integral to society’s highly computerised infrastructure.

“What was before just a way of measuring the flow of time is today essential for transportation, location, defense, finance, space competition,” said Felicitas Arias, former director of the time department of the International Bureau of Weights and Measures, known as BIPM from its French name and based outside Paris.

The process of squaring these two time scales has become so unruly that the world’s time mavens are making a bold proposal: to abandon the leap second by 2035. Civilisation would wholly embrace atomic time; and the difference, or tolerance, between atomic time and Earth time would go unspecified until timekeepers come up with a better plan for reconciling the two. A vote, in the form of Resolution D, is expected at a meeting in Versailles, France, of the bureau’s member nations. “From a technical point of view,” said Patrizia Tavella, the current director of BIPM’s time department, “all the colleagues all over the world agree that we have to do something.”

Farewell, heavens

If the resolution passes, it would sever the timekeeping of atoms from the timekeeping of the heavens, probably for generations to come. The change would be indiscernible for most of us, in practical terms. (It would take a few thousand years for atomic time to diverge as much as an hour from Earth time.)

But the second is a huge amount of time in the technology of the internet. Cellphone transmissions, power grids and computer networks are synchronised to minuscule fractions of a second. High-frequency traders in financial markets execute orders in thousandths and even billionths of a second. By international law, data packages related to these financial transactions must be time-stamped to that fine level of precision, recorded and made traceable back to Coordinated Universal Time, the universally agreed-upon standard managed by the timekeepers at the BIPM.

Every additional leap second introduces the risk of confusion: that some digital networks won’t implement the change correctly, won’t know precisely what time it is with regard to the other systems, and will fail to synchronise properly. The leap second is a dollop of potential chaos in a soufflé that demands precision. For that reason, discarding the leap second has wide support from nations across the world, including the United States.

The result of the vote is not a foregone conclusion, however. The fate of the leap second has long been the stuff of high diplomatic drama, designated one of just four “hot topics” at the BIPM. Getting Resolution D on the agenda has involved more than two decades of study, negotiation and compromise to resolve the issue. “It should have happened 20 years ago, and if not for political manoeuvring, it probably would have happened 20 years ago,” said Judah Levine, a physicist at the National Institute of Standards and Technology, or NIST, in Boulder, Colorado. He is co-chair with Tavella of the BIPM committee that discusses hot topics, and he helped draft the resolution.

Russia, for instance, has tried to delay a shift away from the leap second because doing so would require extensive alterations to its GLONASS satellite system, which incorporates the extra second. As a result, the resolution has been phrased to postpone any change until 2035. The United Kingdom, historically and emotionally tethered to the astronomical standard, enshrined in Greenwich Mean Time, has been reluctant to commit publicly.

The sacred and the compromised

Time is fraught with emotion. Consider the messy debates that have erupted recently over whether to keep daylight saving time.

Last month, Mexico’s Senate voted to end the practice but only for parts of the country not sharing the border with the United States. In March, the U.S. Senate voted to permanently keep daylight saving time, but the motion is stalled in the House. In the European Union in 2018, Parliament voted to keep clocks the same year-round but has been flummoxed by how to do so or which setting to choose.

The leap second, though less visible to the public, also elicits strong opinions. The Vatican, for instance, has argued for keeping the leap second, on existential grounds.

Time “is a constant reminder of our mortality,” wrote the Rev. Pavel Gabor, an astrophysicist and the vice director of the Vatican Observatory Research Group in Tucson, Arizona, in “The Science of Time,” published in 2017. “And perhaps because of this we want to believe that our time, our lifetime, somehow corresponds to the eternal cosmic cycles.”

Alanna Mitchell is a journalist with NYT©2022

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