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Published Nov. 14, 2022
Updated Nov. 15, 2022
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 computerized 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 B.I.P.M. from its French name and based outside Paris. “Time is ruling the world.”
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. Civilization 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 on Nov. 18 at a meeting in Versailles of the Bureau’s member nations.
“From a technical point of view,” said Patrizia Tavella, the current director of B.I.P.M.’s time department, “all the colleagues all over the world agree that we have to do something.”
https://www.nytimes.com/2022/11/14/scie ... econd.html
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